Drills, Taps

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and Dies

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ARGUS BOOKS
 ' I Ci C) ). ( IJ, I

Argus Books Limited Contents

1 Golden Square

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London W1 R 3AB
England

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AUTHOR'S PREFACE 6

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ACTON LIBRARY

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SECTION 1 Twist Drills 7
H!GH STREET

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LON DON W3 6NA SECTION 2 Drills of Other Kinds 24
IIIII IIIE

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SECTION 3 Drill Sharpening 31
©Argus Books Ltd 1987

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SECTION 4 Drill Chucks 38
All rights reserved. No part of this publication may be

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reproduced in any form, by print, photography, microfilm SECTION 5 Screw Threads 44
or any other means without written permission from the
SECTION 6 Taps and Dies- General
publ isher. //b SECTION 7 Hand Taps
54

57
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ISB N 0 85242 866 9
SECTION 8 Tapping Drills 63
STOCK]
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[ RESERVE SECTION 9 Screwing Dies 71

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SECTION 10 Tap and Die Sharpening 78

APPENDIX A Dri IIi ng Forces 81

Phototypesetting by Photocomp Ltd., Birmi ngham APPENDIX 8 Dimensions of BA Tap Shanks 82

Printed and bound by A. Wheaton & Co. Ltd., Exeter
APPENDIX C Tables 83

INDEX 102
 SECTION 1

Author's Preface
TWIST DRILLS
In preparing this book I have taken a choice of tapping drill will be made

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somewhat different approach from pre- somewhat easier and that you will find
vious volumes on the same subject. that tap breakage is drastically reduced!
First, I have given a great deal more However, this leads to one consequence;

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attention to drills. After all, we drill a you will, I hope, soon realise that there there is a zero rake cutting face and a
score or so of holes for every one which is no "correct" tapping drill for any Types of Drill clearance on the opposite face. If used

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is tapped! But there is another reason. t hread - you must make a choice, and Many thousands of years ago some as a hand tool t he shape wou ld be as in

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The apparently simple twist drill is, in t his means that the tables of drills may Bronze Age warrior found that he could Fig. 2b, w ith what is in effect a negative
fact, a very complex tool indeed; even appear to be somewhat more volumin- drill holes in ti mber by using his spear rake on both edges. These tools are

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now its cutting action is not fully under- ous than you are accustomed to. I have with a twisting motion. In later times a very effective in wood, especially that of
stood, but enough has been discovered tried to make this choice as easy as similar tool was made specifically as a Fig. 2a in hardwood. The point gives a

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in recent years to give a better apprecia- possible for you- for 90% of your work drill, and this type is still used in wood- good start, and used with the hand-rest
tion, and to reveal facts which will a single size will serve. But my own turning. Fig. 1 shows some, made by on the lathe the directional control is
enable us both to drill better holes and experience shows that f or the other Holtzapffel about 150 years ago and st ill good, t he tool cuts freely, and quite

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to extend the life of the tool. I hope too 10% - and it is almost always in this in service. The edge is formed as in Fig. deep holes can be drilled provided the
t hat what I have written will encou rage region t hat tap breakage occurs - a 2a and as the blade is tapered in thick- chips are cleared frequently.

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readers to treat their twist-drills with the sensible use of the tables will improve ness a fairly sharp point resu lts. These When used for metal, however, this
respect they deserve! matters for you considerably. are intended for use in a lathe, so that type is too weak. The point soon col-
The second part of t he book, dealing
with the tapping operation, is also quite
//b
Finally, I have left out all consideration
of screw-cutting in the lathe - this in
spite of the fact that almost all the male
the direction of rotation is uniforn:' and lapses, and the curved profile causes

new, and is the result of work which I

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carried out some years ago. I cannot threads I cut with a die are partially
be the only one to have been struck screwcut first. But there is another book
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by the fact that in Industry the main by the same publishers dealing with
preoccupation is with tap sharpening, this in detail, and I see no point in
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whereas the jobbing worker, the model "vain repetition "! In any case, there are
engineer, and the amateur generally, is enough problems in the use of drills,
beset with problems of tap breakage! taps and dies without extending the
The analysis of the effects of depth book to deal w ith lathe-work as well.
of thread engagement on fastener I hope t hat you w ill derive as much
strength, which I can only summarise enjoyment from reading this book as I
in these pages, showed that the prob- have in the writing of it - and that my
lem lay not with the tap, nor yet with good friends in the tap and die industry
any lack of skill on the part of the user, wil l not be too upset at seeing a drop in Fig. 1 Spear-point
but rather with the choice of tapping their sales! drills for use in the
drill. I hope that after reading this the T. D. W Westmorland, July 1986 ,.the.

6 7
 SIMILAR FLUTE ON OTHER SIDE

~r--------------,~
~~r----~c:~~~~~~~~~~~~~~~~~~~~-7~-/~~~T~b~~
\J-------~ud----------------------1-
\ / ~CLEARANCE
A A EDGES
Fig. 6 The straight fluted drill.

penetrates. There is space for chips Woodworkers had, of course, used

within the flutes, but not enough to hold auger-type drills for a very long time.

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Left, Fig . 2 The spear point (a) For unidirectional rotation. all the swart, so that for holes more These, with their helical grooves or
(b) For use as a hand-too/. than about 1 V2 diameters deep it is flutes, "wound out" the chips and al-
necessary to w ithdraw perio dically to

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Above, Fig. 3 A home-made spade drill. lowed very deep holes to be drilled
clear the chips. The defect of the design quite easily. Such drills could not (at
SECTI ON AA meter. However, the spade drill is still is that the centre of the drill has a w eb, that time) be used for metal, as t hey

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used today at the two extremes of size - so that the actual point has a very un- w ere not stiff enough either to accept
~ (b)
v ery smal l and very large - and we shall favourable cutting ang le - Fig. 7. We the much higher thrust or to resist the
difficulties. The spear point was, as a deal w it h these later. shall be looking at t his aspect of the much greater torque applied compared

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result, developed until the Spade form To be su re of dril ling to size and cutting act ion later; suffice to say at this with that in woodwork. They were, in
evolved- Fig. 3. The profile is shown in keeping t he hole straight t he fluted drill stage t hat apart from t he very point, effect, a f lat p iece of tool-steel twisted
Fig. 4, and again, this may be arranged w as developed during the 19th century. which acts like a punch, t hese edges do

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into a helix. However, it was not long
for unidirectional rotation as at (a) or for This, Fig. 6, was, in effect, a cylindrical cut, even t hough t he rake angle is ex- after t he introduction of the straight-
reversing use at (b) . The latter is found p iece of tool-steel (hence the term "drill tremely negative. The main parts o f the flute drill that m achinery was developed

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in the very small sizes used by watch- rod") with two flutes milled along part cutting edges offer a nominally zero t o permit this f lute to be cut (or forged
makers, w ho drive the drill with a bow of the lengt h. The cutting edges were rake. Later refinements of the straight and then twisted) in helical form, and

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and bit-holder. Fig. 4a is still to be found formed by grinding a cone with a small flute dri ll led to t he cylindrical su rface the present day " tw ist drill" was born.
in some blacksmiths' shops, as they are relief angle - known as the " relieved being relieved, to reduce fri ction, and to The point form is exactly the same as

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very easy to make, and I show one in cone" -so t hat the actual cutting edge a tapered web, so that t he stiffness was that of t he straight flute drill; the helix
use in Fig. 5. If carefully made, with was very similar to that of Fig. 4a. The maintained with a thin ner web at the angle provides positive rake, and the
equal lips, hole size is fairly w ell con-
trolled, but due to the relat ively weak
shank and the absence of any guiding
//b
drill is much stiffer than the spade type
and has the great advantage that the
cylindrical part guides the drill as it
actua l point. edges of the f lutes are again relieved,
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action directional control is uncertain -
this is especially so with smaller drills,
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say below about 10mm (3/ain.) dia-

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120°
--...-\-==:
(o)

E=<:::
(b)
VIEW IN DIRECTION
OF A RROW "A"

Fig. 4 Cutting edges of the spade drill. Fig. 5 A spade drill in use.

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 Fig. 8 Comparison POINT
between straight-
flute and twist drills.
'r~NTANGLE
• iii
. -- ·- .

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so that the guiding action is almost as wide variety of types, but before con-

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good. Compared with the straight-flute sidering these it will be as well to look at
drill, however, the twist drill is much the cutting action, so that the reason for
less stiff, and more prone to wander so many can be understood.
NOMINAL RAKE ANGLE

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when drilling deep holes. Fig. 8 shows

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The Drill Point (HELIX ANGLE AT PERIPHERY)
the comparison between the two types.
The twist drill is now almost the univer- There is far more to a drill point than

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sal tool, but those who still have straight meets the eye! First, to avoid confusion Courtesy B.S.!.
flute drills available will use these in later, let us "name the parts". Fig. 9 ENLARGED VIEW
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preference to twist drills for brass, shows the parts of the drill as a whole,
gunmetal, and similar materials. As we the upper diagram being a taper shank Fig. 10 The significant elements of the point of a twist-drill.
and the lower a straight shank drill. The

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shall see later, a d eveloped form of the
spade d rill, with a special, very stiff, recess shown on the latter is usually of the names are self-descriptive; the ance angle of a lat he tool. The effect of
shank, is used for large diameter holes. found only on drills above 6mm or '!4in. "heel" is the tail end of the helix, and

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this at the centre of the web is that
Twist drills themselves now come in a diameter, and not always then. Most the "land" is the narrow part of the instead of forming a conical point we
body which bears on the sides of the find a chisel edge which is like the roof

DRILL
HEEL //b LIP hole to give guidance. The "lip" is the
cutti ng edge. The "lead" is the same
of a house- there is no "point". Have a
look at a largish drill- a new one, or one
thing as t he pitch of a screw thread, and
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which has been properly ground. Note
AXI S results in a helix angle. Here is the first that the line of this chisel edge lies at an
Important thing to notice. The lead is angle to the line across the two lips or
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BODY CLEARANCE FACE the same right across the drill section, cutting edges. This is the chisel edge
1-4-- - -SHANK - --1--- - - -- - - BODY - - - - - - ---.j but as the diameter is reduced as we get angle, and it is important that this be
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to the axis of the drill, so the helix angle correct. Fig. 10a also shows the usual
~------FLUTE LENGTH - -- - *"1
will diminish. If there were no web shape of the flutes, and you can see
between the flutes this angle would how easily a straight-flute drill can be
become zero at the centre. made, given a suitable milling cutter. At
Fig. 10 refers specifically to the point 10b is shown the all-important point
the drill. At (a) we huve the end view. angle, the maintenance of which pre-
the point were a simple cone it would sents difficulties for those without proper
1--- - - LEAD OF HELIX - - - an actual point at the centre.
[ sharpening jigs. It is VITAL that this
- - -- - - -- - OVERALL LEN GTH - - - - - - - - -- - --1 the two halves of the conical part point angle be exactly equally disposed
given a relief - see 10c - which either side of the drill axis. Similarly, the
Fig. 9 Naming the parts of a twist drill. Courtesy B.S./. the same function as the clear- two lip lengths must be equal if oversize

10 11
holes are t o be avoided. (These matters effective rake angle is different - a alter the point angle, the cone relief edge w ill actually form a chip. At the
will be gone into again when we come matter we shall look at later. (and hence the lip clearance), the chisel very cent re of rotation the point acts like
to drill sharpening.) Note also the spot Now, there is no need to sit up all edge angle and the lip length. A lot of a rotating punch. A contributory factor
marked outer corner. This is the most night "learning the names of the parts"! variables. It is surprising how well drills to the cutting effectiveness of this part
vulnerable part ofthe drill point and the But it w ill help to have these diagrams put up with them all! of the drill point is that the heat pro-
spot where wear first occurs in normal to refer to later. In the meantime, looking duced in this negative rake area softens
use. at Figs. 9 and 10 you will see that many The Cutting Action the workpiece slightly. On the other
Now look at Fig. 10c. Notice how the of the dimensions- or "parameters", to The action of the drill edge is often hand, with work-hardening materials like
conical relief provides a clearance behind use the 'i n' word - are fixed by the likened to that of a knife-tool in lathe some classes of stainless steel the slight-
the cutting edge or lip, and that the maker. There is nothing you can do to work, but the apparent similarity is mis- est relaxation of feed pressure will
hel ix ang le provides the rake to the alter the helix ang le (t hough you can leading. Look at Fig. 11a which shows harden up the workpiece locally and in

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cutting edge. When t hese figures are alter its effects) or the flute depths. But the end view of the point. We have many cases result in destruction of the
quoted they refer to the angles at the as soon as you start using t he drill you al ready noticed that the rake angle chisel edge of t he drill.

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outer corner (see 10b) - as already w ill wear the chisel edge, t he outer formed by the twist in t he drill will Fig. 12 shows some remarkable illus-
observed, the rake angle diminishes corner, the lip and, after extensive use, diminish towards the centre - from P to t rations of the ch ip formation, which are
along the length of the lip towards t he the drill diameter across the lands. R on the drawing. However, t his view sketched from photographs made du r-

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drill axis. Note that the rake angle is When y ou resharpen the drill, whatever shows that the ang le at which the edge ing research by Mr W. A. Haggerty of

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marked "nominal". This is because the means are used, you have the power to meets the work also changes across the the Cincinnati Mach ine Tool Co. in 1961 .
length of the lip. This lip is displaced These w ere, in fact, micrographs, ob-

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from the centre of rotation of the drill by tained by photographing sections of the
half the web thickness, w/2, so that material after an almost instantaneous

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A while at P the approach is very nearly at stop to the action of the drilling rig. In
right angles to the lip, at R it is about each case the direction of cut is from
40°. This means that the rake angle w ill

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8 left to right, and it will be seen that even
be reduced still further- in fact, at R the as close as 0·010in. t o the centre of
effective rake is negative, the transition

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+to• rotation of the drill t here is som e sem-
occurring (with the normal-flute helix) blance of a "chip" being formed. The
at Q , about half-way between P and R. chisel edge does cut - provided the
0 to ' I
~ ! 25'7.
'
50%
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- - + - - --
75%
-<
t00% TI P
Fig. 11b shows this effect, which is
exactly the same as that foun d when a
axial force on the drill is sufficient. The
main problem in this region is the
boring tool is set above centre-height
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FIG llo
I limited space through which the chips
on the lathe. can escape. They are often found to be
-to• On the chisel edge, however, the "wiped" out of the cutting region, simply
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A. RAKE ANGLE MEASURED AT

RIGHT ANGLE TO LINE OF LIP
epproach of the edge remains almost because they have been forced th rough
6. EFFECTIVE RAKE ANGLE AS constant - provided, that is, that the a very slim aperture.
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-20° MEASURED ON LINE OF ACTION drill is p roperly grou nd. The rake here is The relevance of these factors to t he
SEE ARROWS ON FIG llo.
lbout 56° negative, which appears to be drill user is that the sharpness of the
In almost impossible cutting situation. chisel edge is just as important as that
-30' However, it must be remembered that of the lips. Further, if the chisel edge
drilling the axial forces are VERY angle (Fig. 10) is not correct then the
greater than those in turning*, cutting action will be impaired; the drill
Fig. 11 (a) Showing how the
angle of approach varies
across the lip (see text).
-40l provided that this axial force is
intained this apparently ineffective
forces w ill be increased and hence the
local heating, with the end result that
(b) The change in rake the chisel edge may be destroyed com-
-so·~
FIG lib
across the lip of a twist (After C. J . O•fonl, TronsASHE vol 77 1955) pletely. The integrity of the chisel edge
drill. Appendix 'A', page 81. of the drill is as important as that of the

12 13
 +1 2• Fig. 12 Sketches taken from material when an alteration of t he point
micrographs of the chip angle may be justified. However, the
formation at various points on clearance angle (see Fig. 10) also has an
the cutting edge of a drill. effect, and we will consider both of
(After W. A. Haggerty)
these angles in this respect later.
More important than the actual angle
is its symmetry. It will make very little
0 . 020" 0.030"
difference if the total included angle is
115° or 125°, but if one lip lies at 5JO to (a) (b)
the drill axis anq the other at 61 ° - still
providing 118° total -the effect can be Fig. 14 (a) Effect of unequal angles.

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serious - even a d ifference of one (b) Lips of unequal length.
degree will have an effect. See Fig. 14a.
The flatter lip 'C' is doing all the cutting tai nly run off line, and w ill have a very

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and will, as a result, suffer excessive rough finish.
wear. Further, the thrust is all concen-

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trated on this lip, tending to drive the Clearance Angle Fig. 15. At first sight it
r--

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0. 5" ~ ··- point over sideways. An oversize hole is might seem that provided there is a
probable and if the difference in angle is clearance all would be well, the more so
Other aspects of point geometry

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remainder of the point, and b ecomes of more than slight there is a risk that the as the clearance measured at the peri-
paramount importance when drilling The point angle A change in the point drill will run off course. The situation is phery of the drill increases towards the
exaggerated if the lips are of unequal

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work-hardening materials. A final point angle - normally 118° - does more than centre. However, feed rates in drills can
to note is that the surface of the cone make the drill more or less "pointed". It length as well - almost unavoidable if be q uite high and if the clearance angle
which provides the clearance for the lip affects the shape of the lip as well. See the point angle is askew. As shown in is insufficient the depth of feed during

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- t he main cutti ng edge- is in fact the Fig. 13. Drill designers arrange the Fig. 14b, the hole is definitely oversize. partial rotation of the drill could exceed
rake surface of the chisel edge, over shape of the flute so that at the "general The corner ' D' cuts away the metal left the dimension (a) and the drill w ou ld

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which the chips must flow. The finish of purpose" angle of 118° the lip or cutting by the short lip 'E', with consequent risk rub. In addition, the chip cut by t he
t his area is often neglected, as users edge is a straight line. Reducing the in- of this corner failing prematurely, and chisel edge, shown shaded at (b), has
feel that there is" obviously" no need for
a good f inish on a surface which appar-
//b
cluded angle as at (b) produces a back-
wards curving lip, and increasing it
the hole will be oversize, almost cer- no route of escape except along the
FACE
ently is not in contact, but careful dress- gives one which curves forwards - (c).
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ing of the wheel before grinding - or For almost all materials the straight lip
even a touch from an oilstone on larger gives t he best results, but there may be
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drills- can improve the performance of occasions w hen a lot of work has t o be
t he chisel edge quite markedly. done on either very soft or very hard
ht

CHISEL EDGE ANGLE

a

ROTATION L1 P CLEARANCE
ANGLE

Fig. 13 Effect of point

angle on lip shape for a 16 Clearance angle. The arrow (c) shows the path of the chips leaving the chisel cutting
POINT ANGLE 90° POINT ANGLE 118° POINT ANGLE 150" normal helix drill. st "b". See text.

14 15
 is to be drilled in an awkward material. also provides the "auger" effect w hich will have a slightly slower helix. In
However, the following are the usually helps t o bring the chips out of the hole almost all cases, quick helix drills will
recommended figures. and also curls the chips so that they normally be supplied with wide f lutes
take up as little room as possible. (The and smaller lands on the body than on
flute volume is considerably less than the standard drill. Quick helix drills are
Material Point Clearance also used even with steel for drilling the
that of the metal rem oved.) As always, a
angle angle long oil-holes in engine crankshafts,
compromise is necessary between the
various requirements. though such are usua lly provided with a
Mild Steel 118° 10°-12° The St raight-flute drill offers zero thicker web than standard and always
Tough Steel 130° 12° rake and no auger effect. Chips must have a rather "special " point.
Brass, Bronze 118° 15° be cleared by "woodpecker" action - For general use the reader is advised
AI. & alloys 100° 15° to stick to the standard helix and stand-

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Fig. 16 Compound drill point for drilling repeated withdrawal of the drill. But the
cast iron. Copper 100° 15° lips are very strong and the body of the ard point on drills until " trouble" makes
Plastics 90° 10° drill is much stiffer than that of the twist a change imperative. There is, however,

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arrow (c). If the lip clearance angle is Medium Cast Iron 118° 10°- 12° drill. Such drills (Fig. 6) are rather one exception to this. The received
too small then these chips will be difficult to find amongst tool-dealers wisdom when drilling brass is to stone
wedged, will probably weld to the drill For softer grey cast iron and malleable the lip to reduce the rake to zero. There

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these days, but they are the ideal for

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face, and the point will be damaged. For iron a compound point is often used. brass. is a tendency when drilli ng brass and
general purposes the clearance angle, See Fig. 16. The outer 25-30% of the lip Next comes the Slow helix drill with a
measured at the periphery of the drill,

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is ground to an included angle of 90°, helix angle of about 22% 0 • Fig. 18. This

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should be from 10° to 12° - though keeping the same clearance angle as for provides some auger effect without
again, it may be desirable to alter this the standard drill. This provides the unduly weakening the lip compared

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for some materials. The chisel edge strong chisel edge ofthe standard grind, with the straight-flute type. In "produc-
angle shown in Fig. 15 should be about but gives more favourable cutting action tion" work on the copper alloys, brass

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130°, and this angle is a good indicator on the lips. Some researchers recom- especially, such slow helix drills are "SLOW"
that the drill has been correctly ground. mend a fu ll 90° point as for plastics, but manufactured with larger flute spaces

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the compound point is favoured because than normal, but for "jobbing" work the
Choice of angles standard drills are easily altered. normal section is quite satisfactory.
The standard point will meet all normal
work, and it is hardly worth altering it
unless a considerable number of holes
//b
The application in recent years of
twist drills used in electric hand tools
for drilling timber has resulted in the
The Norm al helix is about 40°, an
angle which has been found by experi-
ence to cover almost all the jobbing
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development of a point with a flat end - worker's requirements. The lip is not
Fig. 17. This "Brad Point" is ground with quite as strong as that of the straight-
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about 15° clearance angle and the chisel flute or slow helix, but the axial pressure
edge formed into a projecting bradawl- It less and less power is needed to drive "NORMAL"
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like centre; such drills have very thin the drill. The faster helix gives much
webs at the point, tapering to a larger better chip clearance and, on steel
thickness at the shank. This type of drill •pecially, a better " curl" to the chip.
has found applications in drilling soft Quick helix drills w ith an angle of
sheet metal, but woodworking drills 45-50° were originally developed
have soft temper and should not be use with the light alloys, and provide
used. ch better chip clearance, especially
deep holes. The weaker lip is of less
The Helix Angle rtance wj th these soft materials. "Q UICK"
Fig. 17 Brad-point twist drill for timber or We have seen that the helix angle are also used on copper, though
soft sheet metal. determines the cutting rake. This angle Its ordered specifically for this metal Fig. 18 Helix angles normally available.

16 17
simi lar alloys for the drill to "walk into" the case by buying a second drill for There is little price difference between
the workpiece - indeed, with brass it normal materials, I suggest that, instead, Jobber's and Stub drills- the latter are
has been known for the chuck to be ~-;;;:,~
you consider investing in a slow-spira l very slightly more expensive in the
~~.
pulled off its taper when the drill breaks (or even a straight-flute, if you can find sizes up to %in. - but the Long type are
through. The reason for this is complex, one) reserved for copper alloys only. about 2% times the price of Jobber's
but it will be appreciated that these They are more expensive, but will last a and Extra Long are very expensive
metals are relatively soft and, more long time. indeed! In cases where extra length is
important, soften up even more when To sum up, the standard helix angle needed to get at an awkward spot it is
hot- as at the chisel edge of a drill. The will serve for almost all materials, and almost always possible to drill the end
downward resistance to the feed at this even for the exceptions will give satis- of a piece of mild steel rod (about 1 V2 to
point is therefore much reduced. The factory results if care is taken.ln produc- twice the drill size in depth) and soft

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shear force is quite high, and if you look tion work the SLOW helix is used for solder the drill in place. See Fig. 21. Soft
at the point of the "normal" drill on Fig. Brass, Beryllium-copper, Manganese solder will be quite adequate up to
18 you will appreciate that as the chip

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steels, h ard Plastics and for stone- about 3/,sin. dia. if the drill shank pene-
passes over the lip there will be a drilling. The QUICK helix would be used trates about 2 drill diameters; above . .............
--. ~
component of force acting downwards. for Aluminium and its alloys, Nimonics, about 5/1s in. I use low temperature
--

.
This can, in some circumstances, exceed Titanium al loys, Magnesium alloy and brazing alloy (BSAg 1 or Ag2). This is

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the axial resistance to feed, and the drill in the endgrain of hardwoods. For all quite safe on a high-speed steel drill.
then "walks into" the metal. other materials the normal helix would For extensions where the hole itself is Fig. 20 Various lengths and helix angles of

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To prevent this by stoning a flat on be employed. deep it is possible, with care, to braze twist drills.
the drill lip is quite sat isfactory - it is not the drill onto a shank smaller t han the

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really necessary to have a very wide hole diameter - Fig. 22. The shanks of increases in thickness towards the shank.
flat, either. But the after-effect is that Drill Lengths Fig. 19. H.S.S. drills are relatively soft and can This means that a conventionally ground
you will either have to regrind the drill, The usual straight shank drill is the be drilled or machined reasonably well, drill point will have a chisel edge of

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or buy another, for use on normal Jobber's, shown at (b), and derives its using a low cutting speed and plenty of excessive length. In converting such
materials. Regrinding the drill takes name from the fact that it was used in cutting oil. However, in using an exten- broken drills it is advisable to use one of

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time, needs a proper drill-grinding jig, "jobbing" workshops, where frequent aion drill of this type it is imperative that the methods shown on page 34 - either
and unless you are well practised in drill changes of drill size were needed. (In frequent wit hdrawals be made to clear resorting to point-thinning or using the
sharpening can resul t in a faulty point.
Further, we have already noted that the
//b
production work a taper shank drill is
used, avoiding the need for a chuck.)
chips. The drill will certainly wander off
course otherwise.
four-facet method of grinding (see page
34). For use in the lathe, however,
web of the drill gets thicker as you These drills are designed for holes from It is, of course, possible to transform where a combination centre drill ("Sio-
s:
approach the shank, so that in due about 4 to 10 diameters deep. The broken jobber's drills into stub type, but combe Drill") is used to start the hole,
course the chisel edge gets wider and majority of holes - especially tapped I would again emphasise that the web this may not be necessary.
tp

wider. If, on the other hand, you meet holes - are less than this, and the Stub
drills, (a), designed for holes from 2 to 4 21 Drills lengthened to
; s'sc .. ; • ; :
ht

diameters deep, have a number of an awkward place,

advantages. They are much stiffer and m~triiUil•a into
extension-
in many cases do not need any "start" 1
in the way of a centre-punch or prelimin-
b ~ ary drill point. I use nothing else for
tapping-drills. For deeper holes the Long
series is used, and will cope with depths
0 ~----' of from 5 to 12 diameters. There is
available an Extra long series, for holes
Fig. 19 "Long': "Jobber's" and "Stub" up to 30 diameters deep, but these are
length drills. "Extra long" are also available. seldom needed!

18 19
 Acton L~ ·
H igh Street. W . )

Drill Diameters
The largest twist drill I have ever seen
-e.g. 3·26mm for No. 30 - and it will
cost you almost twice as much as the
recommended a lternative of 3·25mm -
preferred sizes for both Imperial and
Metric drills for straight-shank types
only, but covers all lengt hs from stub
•
Drilling speeds
The recommended drilling speeds found
was between 5in. and 6in. diameter, 0·01 mm or 0·00039in. smaller. So, today to extra-long. The difference in price in Production Engineering reference
but for such holes today a Spade Drill there are two ranges of drills only which between these and intermediate sizes books are designed to achieve the most
(see page 26) would be used. Stock comply with BS 328; the Imperial, or depends on demand- it can be as much economical production rate, taking into
size twist drills nowadays go up to "inch" sizes, and the millimet re range . as 30% or as little as 1 0% - sometimes account the cost of sharpening and
about 4in. The smallest drill listed in a " Number and Letter" drills are still nil, for a "pref erred Imperial" d ri ll is a replacing the drills, and allowing for the
current catalogue is 0·0001 in. (yes; supplied in U .S.A., but even there they non-preferred size in the metric range fact that the drills will be sharpened by
1110,000 inch!). though the price of such are mostly I.S.O. metric equivalents. and vice-versa! (Table I gives metric skilled toolmakers on proper machines.
would send your bank manager into The Inch range works to intervals of conversion.) On the other hand, they also assume

m
fits - even more so for the special 1
/64in., though decimal sizes can be that the drill is in use continuously. For
drilling machine needed! In regular supplied if the quantity is large enough. the jobbing worker and model engineer
Number and Letter Equivalents

co
jobber's straight shank lists the range is The metric range provides very close quite d ifferent considerations apply -
from 0·20mm (about 0·008in) up to intervals i ndeed at the smaller end, We have to work to many old drawings chiefly the wish to avoid having to
20 mm, and for taper shank types from there being about 600 sizes in the on which all drill sizes are given by resharpen for as long as possible! In

.
2·4mm (3/32in.) up to 4in., with from No. jobber's range between 0·20mm and number or letter, many older practi- addition, almost all commercial drilling

fo
1 to No. 6 Morse tapers. "Micro Drills" 20mm. tioners still use them even on new is done under power downfeed, and
can be had ex-stock from 0·05mm dia. drawings and, of course, most users this makes a great deal of d ifference.

in
up to 1·45mm dia. in steps of 0·05mm still have stocks of such drills, with t he For manual drilling (or, as we used to
at quite reasonable prices; these have a need to replace the odd one now and call it in the works, "pin-drilling"!) the
Preferred sizes

rs
uniform shank diameter of 1 mm up to again. Table Ill shows both the recom- feed control is very variable and in
0·80mm, and 1·5 mm shanks from there In order to rationalise this very wide mended alternative and the "exact" selecting the speed a great deal does
choice both the British Standards Insti-

le
upwards. My current drill catalogue is a (with in 0·005mm or so) equivalent, depend on your own feel ings about
shade over one inch t hick, and the tution and the International Standards together with the decimal sizes in inches. "feeding". Indeed, a very good rule is
ordinary jobber's drill section has eight Organisation (I.S.O.) have listed a range You will see that the B.S.I. recom- that the speed should be selected so

oi
double-column pages. There is a drill of drills which considerable consulta- mendations are seldom more than that both you and the drill "seem to be
for every size hole! tion suggested would meet all normal 0 ·01 mm, or, at worst, 0·02 mm, (0·00039 comfortable", with reasonable chips
Years ago there were three families of
drills; Imperial, starting at !f64in. and
//b
requirements. However, it must be
emphasised that these preferred sizes
to 0·00078in.) different from the "exact"
figure. The difference is larger with the
emerging steadily from the drill. There
is no EXACT, single, "correct" speed for
rising by 1/64-in. steps upward, Metric, are directed to the machinery designer, llrger sizes, but even t here not sufficient any type of drill; even a change in the
s:
starting at about 0·025 mm and in steps in an attempt to limit the number of to be significant. In a few cases the cutting oil used will make a difference.
of perhaps 0·01 mm in the smaller sizes, different sizes he calls for on the draw- me metric size is equated to two So, use your judgement, and don't
tp

to 0·25mm for the larger drills, and the ing. The user is in a different position, numbers: e.g. No. 59 and 58- hesitate to depart from "the book" if
Morse "Number and Letter" series, run- for if the drawing calls for (e.g.) "7 mm difference between the drill you find that you get better results . ....
ht

ning from No. 80 at 0·0134in. up to ream" normal workshop practice may ers is also small. Having said that, you do need some-
letter Z at 0·413 in. diameter. This last call for a non-preferred size. Similarly, it in U.S.A., where " Number" thi ng to give a guide as to where to
series was very much u sed, though is very likely that tapping sizes will b e are still quoted, can use the table start, and to that end I show overleaf a
there were disadvantages; the differ- non-preferred drills. All sizes are avail- rpret metric sizes in terms of their set of speeds which I have found will
ence between (e.g.) No. 77 and 76 is able, of course, but the normal tool rd. For others, I suggest that the give reasonable results, when using the
0·005in ., but between 76 and 75 only dealer may stock only the B.S.I. list. It 1 mm to 6mm x 0·1 mm will serve lubricants suggested on page 76. Use
0·002in. This series was declared should be noted, however, that the s just as well as the old system. these as a guide.
obsolescent nearly 20 years ago at time classical " Metric Drill Set", 1 to 6mm all, it is no great matter if t he d rill ~ It is often suggested t hat very small
of writing, and is now discontinued. (or 1 to 10mm) in steps of 0·1 mm is 14in. diar:neter instead of 0·113in. drills- below, say, 2mm - should be
True, if you order a "number drill" you made up entirely of "preferred " sizes, 33) or0·116in. (No. 32) -even if the run " as fast as you can". This is true
will be supplied, though it will be metric which is helpful. Table II shows the cuts to size exactly! when faced with the ordinary drilling

20 21
 Drill dia., mm. 2 4 6 8 10 12 This table serves to emphasise that with drills of 10-12mm upwards. If a
in. 0·08 0·16 0·24 0·32 0·39 0·47 the tip cutting speed used in conven- reaso nable feed rate is maintained our
tional machining is not relevant to the machines just do not develop enough
Material drilling operation. With the smaller power. It is better to drop the speed and
d rills especially- 3mm and below- it keep the feed-rate up, so that reason-
Aluminium, Dural, Tufnol - - 4000 3100 2300 1900 pays to run a little slower when d rilling able chips are formed. The worst thing
Brass, Freecutting M.S. - 3400 2500 1900 1450 1250 holes more than 3 diameters deep. to do to a drill is to allow it to rub -
Bronze, Grey C.l., BDMS, 4600 2500 1700 1260 1000 800
At the other extreme, lack of power remember, the chisel edge can only cut
Gunmetal, Phos. Bronze
Mall. Iron, Monel metal. Silver 3600 1700 1150 880 660 550
prevents the use of "proper" speeds if the axial force is large enough.
steel, Stainless steel
Hard cast-iron 1800 1000 650 500 400 330

m
Dri ll dia., mm. 14 16 18 20 22 24

co
in. 0·55 0·63 0·71 0·79 0·87 0·98

Material

.
fo
Aluminium, Dural, Tufnol 1600 1400 1250 1070 1000 900
Brass, Freecutting M.S. 1050 900 800 700 640 560
Bronze, Grey C.l., BDMS, 700 600 540 480 420 400

in
Gunmetal, Phos. Bronze
Mall. Iron, Monel metal, Silver 480 420 400 380 350 200

rs
steel. Stainless steel
Hard cast-iron 280 240 220 200 180 160

le
If the machine has a limited number of speeds, choose t he next LOWEST to that given in the

oi
table.

machine, with a top speed around and this can set up enough stress to
3000 rpm, but there a re machines which
can run much faster. Here the use of "as
break it. //b
The makers of one brand of special
fast as possible" can, in fact, blunt or HSS Micro-precision drills suggest that
s:
even break small drills. It has to be the highest speed should be used at
remembered that t iny drills are very about 1 mm dia., and then be sharply
tp

likely to bend a t rifle under the axial reduced BELOW this figure. The table
load, even though that may be very below gives a few examples, the speed
ht

small. The drill then starts to "whirl" being in thousands of rpm .

Drill dia., mm 0·2 0·4 0·6 0·8 1·0 1·2 1·4 1·6 1·8

Aluminium 6·5 12·5 16·0 18·5 20·0 20·0 19·5 18·5 17·5
Brass 6·0 10·7 14·0 15·5 16·0 15·8 15·2 14·4 13·2
GM, Bronze 2·5 5·5 8·0 9·7 10·3 11·0 10·7 10·0 9·2
Steel {EN81 4·0 5·8 8·0 8·3 8·8 8·8 8·0 7·8 7·2
C.l. 2·1 3·6 5·1 6·0 6·5 6·2 5·8 5·4

22 23
 diameter in inches as well as in m illi-
SECTION 2 metres. Note that type 'A' can be had
tingle-ended, but the dimensions are
exactly the same. I have included in the
table the old, obsolete, 1950 "I mperial "
alzes as well. These should be defined
by the BS No.- BS1, BS5, etc. Incident-
ally, these drills can be had in body
DRILLS OF OTHER KINDS d iameters up to 25mm, but few of us
have chucks that size!
When used to produce lathe centre-

m
holes the penetration for types 'A' and
'C' shoul d be such that the diameter of

co
the tapered hole is between 75% and
86% of the body diameter. With type 'B'
Rather an odd title for a section heading, tapered centre-hole this procedure gives the guard recess should be 85-90% of

.
but I can think of no other which fits! a good start, but care must be taken to the body diameter. The drill should be

fo
There are literally hundreds of "other avoid drill chatter when starting if the discarded if the pilot length, '1', is
kinds" of drill, mostly for special pur- reverse is the case. ground so much that it falls below 75% f
0..

in
poses, but some made in quantity. Type '8' is a refinement. The second of the original length, otherwise there is
However, I propose to limit myself to 120° taper forms a recess which will rlak of the centre-point bearing on the

rs
those most likely to be used by model protect the centre-hole proper. This bottom of the pilot hole. This cannot
engineers, amateur mechanics, and the type should be used on workpieces n with type 'C'. c
small jobbing shops. where the centre-hole will be needed

le
during its service life. Type 'C' is a
The Combination Centre Drill very refined centre-drill. It produces Spad e-bit drill

oi
Fig. 23 The "Siocombe" or Combination
This is sometimes called a "Siocombe" a hole in which the lathe centre bears have already noticed this one, in centre drill. (a) The normal type. (b) Type
drill, Slocombe being the original pro- with line contact and, in general, gives 4 and 5, page 8, but a few words producing a guard recess. (c) Precision type
ducers. See Fig. 23. Type 'A' is that
commonly used. As the name implies,
//b
a more accurate "run" to the workpiece.
Further, the friction is less in service
not be amiss. There are two types
you can make yourself, and the
producing a curved profile to the centre hole.

its purpose is t he formation of centre- and there is a reduced risk of over- rnrnrn<>rcial type. I use the former necessary, reheated and straightened. I
s:
holes in the ends of workpieces for use heating the lathe centre. On the other uently, usually when I want a then turn it to the shape needed in the
in the lathe between centres. The former hand, if the tail-stock exerts too much a bit larger than my chuck will hold, lathe, using a point angle of 120°, the
tp

way of producing such centres was first pressure the poppet may have to be smaller than one of the three Morse diameter about V1s in. oversize. After
to drill a pilot hole and then follow with adjusted more frequ ently as the w ear shank drills that I own. Some are cutting to length the tool is annealed at
ht

a 60° countersink. The invention of the will be faster. It is really for use on light crude, but nowadays I use 1/2 in. 76o•c - "blood-red" - and t hen finally
"combination drill" has saved billions precision work, and I use this type only steel. The end is heated to yellow shaped, leaving j ust a shade on for
of man-hours! (Earlier still t hey used on my Lorch precision lathe - and use ooo•c) and flattened. Take care - grinding. Harden from 79o•c (cherry
the "square centre"; I still have one, but no other type on it. go on hammering if the metal red) and temper as required for the
very seldom use it.) The combination Table IV (page 88) gives the dimen- below bright red. The width is material on which it is to be used. The
drill is also used as a starter for twist sions of the current standard drills - Increased and the section thin ned. cutting edges are ground as shown in
drills in the lathe, and someti mes in the though 'C' is designated as " TypeR" in than forge heavily if t he drill is t o Fig. 24. For use on steel etc. I form a few
drilling machine also, but it is not a BS and ISO sta ndards. Note that the best just over %in. I would probably nicks as seen at 'b'; these break up t he
necessary adjunct, just a useful one. designation to use is the body diameter, to 3fe in. stock, but experience chips and make the work easier for the
Provided that the twist drill is smaller though the pilot diameter is often used. that the larger the shank the drill; note that the nicks in the two lips
in diameter than the diameter of the I have given the approximate pilot The end is cleaned up and, if must be at different radius.

24 25
 first saw adopted in my place of work Fig. 25 a Watchmaker's
about 50 years ago, when dri lling long "Foret" - a m inia ture
spade drill, shown fitted

--- holes in large connecting rods.

The main feature of these drills -
apart from the spade itself- is the very
in the typical
archimedean drill-
stock.
stiff holding bar. This, coupled with t he
narrow cylindrical land, helps to prevent
the drill from wanderi ng. In fact , they old days these were used with a drill- (1 mm ) but anything between No. 6 and
produce very straight holes, with a stoc k and bow, givi ng a back and forth No. 15 will do. This " pop" is quite
good finish. However, they MUST be rotation, but nowadays a small archi- enough to start small drills - up t o
used with copious lubrication (indeed, I medean drill-stock is employed. Fig. about4mm or so-and provides a more

m
use force-feed even on my Fig. 24 type). 25a. Indeed, apart from the flatter point accurate "spot" for the subsequent use
The bits are interchangeable, though, of angle and harder material they are very of a centre-pu nch w hen t hat is needed.

co
course, a different arbor would be used similar to the " Fretwork Drills" once They are, of course, drills in t heir own
for a 5 in. cutter compared with a 2 in. very common . They can be had right right, and provided t he hole is less than
Carbide tips are available. A variant of down to 0·1 mm dia., and are numbered about 4 diam eters deep there is no

.
this type has a f lat cutting end for flat- In "tenths" - e.g. a No. 12 w ill be problem with chip clearance.

fo
bottoming holes. 1·2 mm dia. and a No. 5% is 0·55 mm.
Fig. 24 Point form of a home-made spade On the face of it it m ight seem that The shanks are normally u niformly 2m The Core Drill

in
drill. home-made drills of this type would be dia. for use in collets, but the smaller These drills are designed for the open-
quite practicable, but there are a few ones may be found with 1 mm shanks. ing out of smaller holes, especially

rs
The commercial type of spade drill is snags. First, I could not imagine that To cater for the increasing use of small those cored out in cast ings or rough
a different kettle of fish. See Fig. 25. anyone cou ld drill a (say) 2 in. hole in motor-driven bench drills some makers punched in plate. The two main features

le
Used for holes of 1 in. diameter upwards a 3% in. centre-lathe - or even in a drill - now supply these litt le drills ground for (Fig. 26) are that they have a much
in steel, cast iron, bronzes and brass, ing machine with but a % HP motor! unidirectional rotation; if used in a bow thicker web and hence are m uch stiffer
Second, it is vital that the spade be

oi
especially the more difficult types, but or archimedean drill-stock these cut in than a twist drill, and t hat they have
not for the stickier aluminium alloys, accurately centred on the arbor, other- one direction only, but as they cut three, rather than two, flutes. They have
which may form false edges to the lips. wise the drill will hole oversize. How- faster, no t ime is lost. no chisel edge and cannot be used to
There is, of course, no auger effect, so
that "woodpecker" action is essential , could be devised!
//b
ever, no doubt a "poor man's version" These little fellows are very hard and
fast-cutting, in brass especially, and
drill a hole from scrat ch. There must be
an existing hole which is larger tha n the
have a very fine point. I use them a lot, diameter across t he web. The reason
s:
sometimes w ith fluid washouts, but
increasingly this problem is being got The Watchmaker's "Foret" so much for drilling as f or preparing for using three flutes can, in part, be
over by inverting the drilling operation This is, in effect, a tiny spade drill of the drill. Once the position of the hole is seen from Fig. 27, which exaggerates
tp

so that the chips fall out- a procedure I type shown in Fig. 4b, page 8. In the out, the point of the foret is the effect to make it clearer. If an
d along one of the marked-out ordinary twist drill is used to enlarge a
ht

SHANK TO until is felt to engage with the hole by more than a very small amount

~~==========::::J
line. The drill-stock is then held a " lobed " hole is almost certain to be
ht and a few strokes will make a formed. On initial contact one lip will,
,L SU) HOLDER spot-hole. I use a No. 10 drill unless a jig-bush is used and centring is

Fig. 25 Commercial
fTTj#HjiJ form of spade drill.
The holder is ohen The 3-f/u te core-drill
EN D V IEW OF BLADE
drilled for coolant with a standard
EN LARGED SECTION ON AA supply to the tip.

26 27
 ~,,;/9 -====------~---""'---_-_-_-_-_-_-_-_-----"~
0

The half-round-+
...
DESIRED SIZE

~--------------------~~ -
Fig. 27 The "lobing" action of a normal twist-drill when enlarging a hole.
drill cuts only on its front edge, which "woodpecker"; t he chips usually clear
absolutely precise, make a deeper con- devised for use with special collets. The has relief as shown. Provided that the well enough. If the hole must be deep -

m
tact than the other. A twist drill is use of a chuck "adds to t he lack of finish on the body is good such drills above 5 diameters - t hen it is best to
very flexible so that the point will tend rigidity" . Core drills are available from will work to reamer size and finish, but drill in stages, using O-bits both times.

co
to wander, and the second lip rotate about 8mm (5/,sin.) dia. up t o 50mm they MUST be started by drilling a hole The first can be woodpeckered, but t he
about the f irst, as at 'a'. This second lip (2in.) and are sold in "reaming sizes"- of the required size, of depth equal to second will take full depth, as the chips
then digs in, and the drill point rotates from 0·2 to 0·4mm below "preferred around half the diameter. These drills will be much narrower. O-bits with a

.
about it, at 'b', causi ng the other to dig sizes" -as well as in the sta ndard dia- are usually m ade up from silver steel, very small cross-clearance, say V2°, can

fo
in about 1/Jrev. later. This process con- meters. However, u sed alone they pro- but the ground finish is not always be used to flat-bottom holes if a dead
tinues, ('c') as it is self-sustaini ng, and a duce a hole finish considerably better good enough and if used direct from f lat face is not essential.

in
" lobular" hole results. A test bar of the than will a normal twist drill. As the the ro d, the grinding marks shou ld be
drill diameter may "fit" the hole but name implies, they can be used very polished out. I prefer to machine down The Half-round drill

rs
only "where it touches" -there will be successfully to open up rough cored f rom larger stock, polish to give per- This type seems to be almost unknown.
gaps all round. cast holes. In the absence of a proper haps 0·0002 in. clearance in the hole, It is, in effect, a O-bit with a point, Fig.
The three-flute drill avoids t his risk, core-drill, a stub drill w ill be less likely and t hen file o r machi ne the flat. It is 29. They are commercially available

le
first, because it is very much stiffer, and to cause lobing, being much stiffer t han u sual to make the dimension 'h'=0·51 0 f rom 0·35mm (about No. 80) up to
the point tends to "stay put". Second, a jobber's pattern. If t he latter is used, - i.e. about 0 /100 more t han the half- 12 m m and 112 in., both metric and imper-

oi
there are no "opposite flutes", so that then each step should remove only a diameter. Both the top and f ront faces ial. They will, with care, drill to 10
the risk of chatter is reduced.ln addition, very small amount of metal. ahould be got up with an oilstone, diameters deep, leave a reamer f inish,
the point geometry is slightly different,
a change made possible by the fact that The O-bit
//b taking care to keep both quite flat. The
bit is sharpened on ly on the f ront face.
and unlike the O-bit need no start hole.
Woodpecker operation may spoil the
the drill does not have to remove its full This type of drill is really a guided Cutting speed is best somewhat below finish and in p roduction work an air-jet
s:
diameter of metal. boring bar - Fig. 28. The body is very that of a twist drill of the same diameter. is used on sizes above about 6 mm to
These drills are usually made with slightly less in diameter than the desired The feed-rate should be kept up, and to remove chips. They are not normally
tp

taper shanks; there is a straight shank hole - a " running fit" to it - and is ensure a good finish it is wise NOT to used on steel or cast iron. For best
version, but NOT for use in a chuck; it is flattened on one side so t hat there is
ht

provided with a small tang, and is just over half a circle remaining. The

30 Astepped
drill
thin
If has
Fig. 28 The D-bit. a single flute.

28 29
 Fig . 31 The Conecut drill in action.
SECTION 3
the price of a jobber's twist drill, and are
available f rom about 0.5mm up to
10mm dia.

The " Conecut" drill

This is a relatively recent introduction.
It is, in effect, a single-flute drill, and
it is designed for cutting clean holes
DRILL SHARPENING
in sheet m etal. They can be had in a
plain conical shape, typically running
from 3-14mm, 6-20mm, 16-30mm dia.,
designed for opening out existing holes,
or in the form seen in my il lustration,
with the diameters in fixed steps, which From w hat has gone before it is evident normal helix.) To grind a drill by ha nd
can start from a centre-pop, again with that sharpen ing a drill means more t han calls for considerable ski ll and experi-
a range of maximum and minimum just p utting an edge on it . See Fig. 32. ence, and t his can only come from
diameters. They are v ery effective in- The two lips or cutting edges 'a' must practice. You must expect to " get things
deed. That shown in Fig. 31 (a " UN I- be of the same length, and the point w rong" for quite a while - or at least,
BIT") has six steps, cutting from 3/1s in. Ingles 'b' must both be correct and "get things not quite right"!- but it wi ll
dia. to 1hin. dia. in steps of '/,sin. It is equally disposed about the drill axis. help if you practise without a grinding
.drilling tough steel sheet, and the limit The clearance 'c' must be sufficient. The w heel to start with . Seek out the largest
results cuttin g speeds should be about of thickness is about 'la in. in mild steel. ng edges of the lips must be level u nused drill you have- m ine is 31/64 in.,
25% less than for normal drills and The Unibit is available i n various sizes at 'd ' -this will follow auto- a hole size I seem never to have used.
feed-rates about 50% more. They can , up to 34mm (13/s in.) maximum di a- cally if 'a' and ' b' are correct. And Find a vertical smooth metal surface
of course, be made from carbon steel meter, but those with maxima above chisel edge ang le must be correct, and cover this with marking blue. Offer
stock (silver steel or drill rod), but it is about 18mm (%in .) need a p ilot hole 'e'. (The f igu res shown are those for a the drill point and practise th e motion
more important that the point be t ruly slightly smaller than the smallest step; 8" point with 10-12° clearance angle, needed evenly to coat the drill point
central and that the h alf-cone be pro- the usual way of dealing with this is to
b= 118°
perly relieved w ith clearance. Bought use the next size down Unibit and to
commercially they cost about four times enlarge with the larger one.
e

(a 8. b MEASURED ALONG LIP)

V I EW AT X V I EW AT Y y

The critical elements needing attention when sharpening a twist-drill.

30 31
with blue. You will find that you have to to the correct angle and clearance. You special-purpose drill-grinding machines edge on by rubbing t he point on a sheet
(a) hold the drill at the correct angle; (b) must now p ractise further to get both (cost ing thousands of pounds) became of silicon carbide paper, maintaining
gently raise and lower th e left hand (the sides even. Th is wil l take you quite a un iversal, but has none of t he complica- t he angles by the peculiar motion of its
one holdi ng the point end); (c) move w hile, but persevere! It is extremely tions of vernier angle settings, micro- "wheels". It works, and wit h care can
t he right hand slightly up and down and doubtful t hat you will ever get a drill meter feed devices etc. found on such. work qu ite well enough for me to prefer
side to side at the same time; and (d) absolutely correct with off-hand grind- Properly used it will give a point very it to off-hand grinding when I am in too
slightly rotate the d rill wit h the right ing like this, but you should not have a near to perfection provided that it is much of a hurry to set up my Potts or
hand - all these motions occurring great deal of trouble in achieving a set up and used correctly. Follow the the Ouorn. But it is used only on the
simultaneously! t olerable point-so long as you practise. assembly instructions exactly, so that it drills I keep for use in the hand or
Practise this until you can get a reason- Remember, the aim is to take off the is presented to the grinding wheel in electric portable drill-drivers. In which
able cover of blue on the point. Then minimum amount necessary to renew precisely the fashion intended by the connection I would urge readers NOT
take an old drill and try it on the the cutting edges of lip and chisel point. designer. The main problem in use is in to use their "workshop" drills, and
grindstone. Note that you must use the I always fi nish my drills with a medium the accurate setting of the drill point to especially not their tapping drills, for
flat side of the wheel, not t he periphery. fine India oilstone- you will remember the stop in the jig. This must be done D.I.Y. odd jobs. Small drills are easily
Yes, I know that "everyone says" that t hat the clearance faces to t he lips form very carefully. It would be a w aste of my bent, and larger ones can suffer badly
you should never do this, the reason t he rake faces to the chisel edge, and t ime to go through the instructions as, f rom both chisel edge and land wear
being that wheels are not very strong should, therefore, be smooth. although I have a Potts which I have when used with portable drilling engines.
sideways. But the force used in drill- Far better than off-hand grinding is to used for nearly 30 years there are other However, even "second best" drills
grinding is and shou ld be very slight use a drill grinding jig. Fig. 33 show s the similar devices available and each has should be kept sharp, and a cheap drill
and I can assure you that no harm will well-known "Potts", marketed since Mr its own setting m ethod. I think I should sharpener, used with care, is well worth-
come. The ideal wheel is, of course, a Potts' death by Waking Precision M odels add that when properly set up these jigs while.
cup wheel, but few bench grinders will Ltd. (There are others of similar d esign.) often "look wrong". Don't be alarmed-
accept one. After a while you will find It is a reduced version of the man-sized just check through the instructions and The "Four-facet" point
that you can get a reasonable approach j igs formerly used in industry, before make sure that you have not made a This type of point grindi ng has become
mistake, and then press on! increasingly popular followi ng the intro-
duction of numerically controlled mach-
•other Devices"
ine tools, (though it has been in use
There are, of course, many devices since about 1905), as it is more or less
other t han these. Some are basically self-starting and needs no preliminary
Identical to the Potts and not to be dis- centre-hole. The difficulty for the ama-
dained. After all, Mr Potts based his teur user is that it is almost impossible
design on the more elaborate industrial to produce the shape by off-hand grind-
Others often appear to be crude, ing, and an error in the profile is much
do serve their purpose- the restor- more serious than in the case of the
of t he edge of drills used f or "Do-it- relieved cone point which we have
projects etc. The handyman is considered so far.
om concerned with accuracy to the Fig. 34 shows the arrangement. There
hundredth of a millimetre or with is a double clearance angle; the primary
performance, and for this class of angle, 'A', gives the major clearance
it is far more important that the and corresponds to that produced in a
should be keen than that it should relieved cone drill. The secondary angle,
rm exactly with the design geo- 'B', is the cutting edge clearance. This
Such cheap gadgets are much secondary clearance is carried right to
than unskilled off-hand sharpen- the centre of the drill, so t hat there is a
Fig. 33 The Potts drill sharpening Indeed, I confess t hat I have one, point on the chisel edge instead of a
fixture for use on the bench grinder. entirely of plastic, which puts an f lat; I have shown an end view of this

32 33
 PRIMARY CLEARANCE Fig. 35 Reducing the chisel edge
by point-thinning the web.
/ PRIMARY
CLEARANCE ANGL E
Ll P........_
-1 SECONDARY
CLEARANCE
ANG LE
in Fig. 35. Two narrow grooves are of course - the edge of a normal
ground along the line ofthe chisel edge, grinding wheel, even if a sharp corner,
B ENLARGED SECTION ON "AA" and this reduces it almost to a point. At has too great an angle. To do the job
the same time the acutely negative rake properly needs a grinding jig, and Fig.
angle is reduced, thus providing better 36 shows a set-up designed and built by
cutting conditions. On smaller drills -
UP say below 10mm Wa in.) -little advant-

•
age is gained, and the grinding opera-
tion involved becomes tricky. However,
VIEW ON POINT for drills between about 8 mm and
ON "BB" ON "CC" 12 mm the increase in web thickness
ENLARGED SECTION AT DRILL POINT as the drill becomes shorter due to
Fig. 34 The "Four-facet" drilf point. regrinding can become a nuisance, and
in these cases point thinning may have
type of grind in Fig. 34, and you can acy of angle-setting for both point and to be resorted to when about half the
see the difference. If used with stub clearance, and so on, apply equally to usable length has been ground away.
drills this grind completely obviates the the four-facet and relieved cone points. Point thinning is always used on drills
need even for centre-punching (pro- 1 would emphasise this again: neither above 12mm (V2in.) when shortened in
vided that· you can position both drill the use of a drill-grinding machine, a this way.
and work sufficiently accurately) and on "Potts" jig, nor a "Quorn" will guarantee I have found considerable benefit
jobber's length drills a marked reduc- a good point to a drill unless the when using large (above 12mm, or
tion in the tendency for the drill to instructions are carefully followed and Y2 in.) drills in the lathe, the reduction in
wander from (e.g.) the centre-pop will the drill properly set up in the holder. thrust making the difference between a
be observed. There is, in addition, evi- "struggle" and almost normal drilling.
dence that in deep hole drilling the drill Point thinning But with anything much smaller there is
runs much straighter. On larger drills (say above 12-15 mm little benefit and below 10mm I never
So far there appears to be no simple dia.) other considerations apply, notably thin the point, even on well-shortened
drill-grinding jig on the market which the need for strength to resist the high drills. Thinning is a very delicate opera-
can produce this grind, and the mach- torsional and thrust forces. It is doubtful tion, and if not done exactly right the
ines used in industry cost rather a lot of whether the four-facet grind offers any last state is worse than the first. It is
money! However, the Quorn tool and advantage in these larger sizes. How- Important that the "nicks" cut in the
cutter grinder can grind the four-facet ever, to provide the necessary strength chisel edge should be identical in length
point (but can't grind the relieved cone the web of the drill is quite large - and preferable that they be the same
without modification) and full instruc- perhaps 3mm ('loin.) on a 15mm (o/sin.) depth also, otherwise the drill w ill cut
t ions are given in Prof. Chaddock's book drill- and the consequently wide chisel large and probably run off line as well.
" The Quorn Tool & Cutter Grinder", edge can cause problems both in start- There is also a risk of accidentally
from Argus Books Ltd. All the considera- ing and when drilling. To overcome this touching the lip cutting edge. A thin Fig. 36 The Bradley point thinning fixture.
tions about equality of lip length, accur- the point is usually "thinned" as shown aaucer-type grinding wheel is needed, (Courtesy /an Bradley Esq.).

34 35
Mr lan Bradley. The wheel is about drill tends to jam it is worth while Fig. 37 A home-made box of
stub-drills.
100mm (4in.) dia. and as you can see showing it to your micrometer to see if
has been dressed to a very thin edge. the land wear is excessive.
The drill can be set accurately in the
holder and clamped up, after which the Drill Storage
depth of the thinning groove can be From what has gone before it is evident
adjusted and, finally, fixed when the that the bad old days when drills were
holder is rocked towards the wheel. No jumbled together in a tin box are, or
doubt those who have a Guorn could should be, abandoned to the mists of Fig. 38, below. Set of drills
devise a similar arrangement. I would memory! Quite apart from the incon- Tmm - 6mm X 0· 1mm in metal
NOT, however, advise any but the most venience when trying to find the right storage case.
skilful reader to attempt to thin the size I hope that what I have written w ill
point of any drill less than about 12 mm have shown that a drill is a complex
(% in.) dia. working freehand on the cutting tool; the integrity of the cutting
bench grinder. It is worth noting, when edges and the finish of the relief faces rack inside which opens up to present harm, and a drop of rust-preventative
considering point thinning of a much- deserve our best care and attention. To the drills in a convenient way. These are oil will preserve them. (Shell "ENSIS"
used drill, that the diameter across the this end, for many years die-cast stands a bit more expensive but, over time, grade 254 is excellent, as it possesses
lands is tapered, being largest (to the have been available in which the drills save their cost in keeping drills free water-repellent properties.) My few very
nominal diameter) at the point. This stand up like soldiers, each in its correct from dirt (which can cause corrosion) large taper shank drills I just put away in
taper is typically 0·02 to 0·08 mm in hole. In the best of these the holes were and easily accessible. They have the a rack, taking care that they don't bump
100mm (0·001 to 0·004in. in 4in.) length sized so that no drill will fit into the next added adva ntage to those who have a anything, but wrap them if I know I am
This is to prevent binding in the hole. smaller hole (the small "number" drills large range of drills in that the boxes not going to use t hem fo r a week or so.
Clearly, a worn and shortened drill will being <Jn exception) but were easy to can be stacked. Fig. 38 shows a set of
be undersize on the diameter. remove when needed. This is an excel- metric drills 1 to 6mm x 0·1 mm. Natur-
lent method, the only objection being ally, it is important always to put the
Land Wear that the tools are not protected from drill back when finished with -this is no
This last fact leads to another. The lands dust. After a time oily dust collects in more than normal workshop hygiene -
on the flutes (see Fig. 9) guide the drill, the holes and these have to be cleaned otherwise you have empty boxes or
and also establish the diameter of the out. but I have kept most of my drills blocks and loose drills all over the
hole which will be cut- assuming that in this fashion for 40 years. Such place.
you have ground the point correctly! stands can, of course, be home-made, Spare or unused drills are best wrap-
Even in ideal circumstances these lands using boxwood or some similar timber ped in oiled paper and stored in wooden
will wear, but as soon as the point which does not absorb moisture. (Avoid or card boxes, though I have used strips
geometry is less than perfect there is mahogany like the plague; this wood of corrugated· packing-paper, soaked in
considerable extra load applied to one can take up moisture sufficiently to rust oil, and rolled the drills up within the
or other of the lands. This increases the the drills solidly into the block in time). corrugations. For t he very smallest sizes
wear rate. Sooner or later the lands at Fig. 37 shows my range of tapping drills 1 mm and below (Nos. 60 to 80) -I use
the point end will wear down to a (all of "stub" length) in their box, which stiff paper, 80 to 100gm typing
smaller diameter than that further up has a turned lid to keep them clean. A , oil-soaked, in which the drills are
th e drill. Then, when drilling a hole little care may be needed to ensure that like pins or needles, and then rolled
deeper than usual these unworn pa~s there is a slight clearance in the holes, Write the sizes on the paper before
will rub on the sides of the hole and, 1n and on the smallest ones I enlarge them it, though! One solution to the
the worst case, actually jam the drill in slightly with a taper broach. ll'nt'l"''rn of small drills which I have
the hole. There is no cure, other than More recently drill cases of metal was to keep them in glass or
shortening the drill back to beyond the with a cover lid have become available, c pill-tubes, point upwards, with a
worn land faces. So, if you find that a which are fitted with a sort of display stopper. The points can come to no

36 37
 jaws, or the modern keyless types of for a given torque on the chuck key the
which the ALBRECHT is typical. (The jaws can exert considerably more force
SECTION 4 keyless types used on hand-drills are on the drill shank. Indeed, for smaller
quite different, by the way, and though drills hand-tightening may suffice. They
they are quite adequate for their pur- are, of course, more expensive, but well
pose, they are not suitable for serious worth considering by those who expect
drilling.) many and frequent changes of drill size.
The Jacobs type is shown in section
in Fig. 39. It comprises a body, 'a', Keyless Chucks
DRILL CHUCKS machined to accept three jaws, 'b'. Fig. 40 shows the Albrecht design of
These are circular in section with helical keyless chuck. The jaws are of tee
teeth cut in the upper end, the lower section at the back face, and are guided
end being formed into the gripping in tee-shape grooves. Their movement
jaws. The teeth engage with a split ring, is controlled by a thrust-plate working
'c', which is a tight press-fit to the against the inner end, and this is oper-
shank. The collet directs the drill truly, adjusting sleeve 'd'. The inner face of ated by rotation of the screwed sleeve
It is no use having a perfectly formed
the drive being transmitted through the the split ring is cut with threads match- seen in the illustration. The thrust is
drill point if the drill itself is not held
two flats so that there is no risk of ing the teeth on the jaws, and both the carried on a ball-bearing. The chuck
both firmly and true. In "Production"
slipping. (On the Morse taper, however, ring and jaws are hardened. When the exerts enough force on the jaws to
work, as I have said, a Morse taper
the tang is there to ease drill extraction, sleeve 'd' is turned by the chuck key enable drills to be held securely by
shank is almost always used, though
and plays no part in the driving.) (which engages with the teeth on the hand alone, no key being required, even
for smaller sizes in special machines
The majority of readers will, however, lower edge) the jaws are forced down on the largest size which accepts drills
straight shank drills may be held in
use drill chucks, both in the lathe and in or retracted as required. The combina- up to 16mm (%in.) dia. The smallest
collets. Even here special drills may be
the drilling machine. I cannot emphasise tion of the wedging action of the slop-
used, having a straight shank terminat-
too strongly the need for a good quality ing jaws and the magnification pro-
ing in a tang -two f lats at the end of t he
chuck. The duty is onerous in the extreme duced by the screw action of the split
-the thrust on even a 1/4-in. drill can be ring provides a very strong grip indeed
100 lbf (50 Kg f) or so and if you work out on the drill shank. The chuck shown in
the torque you will find that t his puts a Fig. 39 is arranged for fitting to a taper
considerable tangential load on the arbor (the taper is an international
chuck jaws as well. If the grip is not standard) but they can also be had w ith
adequate then the drill must slip. Dam- a threaded socket - mainly for use on
age to the drill shank may put it out of portable drilling machines. There are, of
true and will almost certainly set up course, other makers than Jacobs, but
burrs, and, of course, wear on drill the principle is the same except for
chuck jaws is just as serious as wear on details.
the jaws of a lathe chuck. There is a variation in the design -the
There have been many forms of chuck led "Super-chuck" which has a
devised over the years, but the majority fitted between the split ring
of users now employ the well-known the body, to take the reaction when
JACOBS type with key-tightening of the chuck is t ightened. This means that

Fig. 39 The Jacobs drill-chuck. (A) Body. (B) Jaws. The Albrecht keyless chuck. The design
(C) Split threaded ring, force fit into (D) Adjusting even large drills to be gripped firmly when
sleeve. (Courtesy Jacobs Mfg. Co. Ltd.) (Courtesy Jacobs Mfg. Co. Ltd.)

39
38
 of pedesta l drill with 'h HP motor will Fig. 42 Dismantling and
cope wit h 12 mm (V2in .) and t he Jacobs re-assembling a Jacobs chuck
No. 34 is app ropriat e; either can be using a stepped driver-ring in
used in t he lathe tailsto ck when fitted the bench vice.
w ith t he appropriate arbor.

Chuck Troubles
Like any other mechanical device chucks
are subject to wear, and in due course
some remedial action will be necessary.
The first problem is: removing the
chuck from the arbor. The Jacobs taper
(which is an international standard
Fig. 41 Removing a chuck from its arbor for chuck fitting) is, like the Morse, a
using a pair of w edges. "wedging" taper and after som e years
in place the arbor may 'be extremely
ranges are noted for their precision, and difficult to remove. The solution is
the "0 to 3mm" does mean that it will simple. Look at Fig. 39 again. You will Where the chuck is mounted on an need either a vice wit h about 4 inch
hold a drill of almost zero diameter! The see that both at the bottom of t he taper arbor greater t han No. 2 M orse there open ing or a mandrel press. Look again
very smallest has a calibrated sleeve so socket in the body 'a' and behind the will be a shou lder on the arbor. In this at Fig. 39. The ring 'c' is a force fit in the
that the chuck can be set to the drill jaws there is a recess w ith a "d ril l point" case a pair of thin " folding wedges" can sleeve 'd', but only over its own w idth.
diameter before loading ; there can be end. The metal is quite soft here, and a be used (see Fig. 41) which can be As soon as the sleeve has moved about
few who have not struggled with a hole can be drilled t h rough so that the squeezed together using the vice - or a 10mm the ring is slack and al l m ore or
V2mm (No. 76) dri ll - som ehow they arbor can be removed by using a brass hammer if you must. Th is is the only less falls apart. To dismantle you must
always want to go into t he spaces or copper dri ft. However, a better way is way possible fo r t he A lbrecht type of f irst make a ring - and I suggest a
between jaws instead of in the centre! to dri ll and tap t his hole, so that a ck, as the internal construction does stepped ring as shown in Fig. 42, w hich
With this calibrated Albrecht t here is forcing screw can be used. Chucks up allow the drilling of a centre hole. w ill serve for reassemblin g a lso. After
nowhere for the drill to go except in the to 8mm capacity (5/,o in.) may be tapped
adj usting the sleeve so that the jaws are
proper place ! 6mm or % in. BSF. From 8 to 12mm
about half-extended set the chuck up on
As to chuck capacity, my largest Jacobs capacity (up to V2 in.) M1 0 or 3/a in. BSF jaws tend to wear most at the point, the smaller diameter of the ring and
w ill accept 12 m m (V2in .) drills at the is suitable. The problem for many is that in time the drill is held only at apply pressure on the jaws. On chucks
upper end, and I have no difficulty that they have no other chuck in which end of the shank. This not only of any age the pressure required w ill be
holding down to 1 mm (No. 60) - it is to hold t he drill! But all is not lost. Set the drill to run out of true at the large, and it helps if the vice is tightened
now fairly old, and could hold even the chuck in the tailstock, jaws fully point but can also cause slippage and then the movable jaw given a good
smaller when new. Albrecht type chucks retracted, and grip the drill in the lathe excessive force is used on the bump w ith a rawhide mallet. As soon as
are expensive, so that I use only the 3-jaw self-centring chuck; then drill in key. Damage to the drill shank the ring has started to move, the rest of
small 0-3 mm size (this is not the very the normal way. I now drill and t ap and, of course, t he jaws w ear t he travel is fairly easy.
smallest) but wish I had one going up to chucks when new, having learnt the more. Contrary to supposition, The faces of the jaws can be trimmed
6mm (V4in.) for use in the Lorch lathe! It lesson some 30 years ago. In this case it ntling of these chucks is quite with a medium India slipstone until the
does depend a great d eal on the class of is best to hold the chuck by the body - ble and the fitting of new jaws beari ng face is of uniform width full
work done in the shop; those concerned away from the chuck key holes- in th e time to time is normal practice. In length. This is "better t han nothing " but
with small scale models, or clock and lathe 3-jaw and drill down the taper case of t he Albrecht type it is treat ed t his way the jaws w ill no longer
instrument w ork, would not need any- hole. This way there is less risk of chip s ble to send for the leaflet of hold drills at the smallest limit of the
thing much larger tha n the Jacobs No. getting into the jaw-ways. Even so th e ntling instructions, and I will say chuck's range, and there is no certainty
32, taking up to 10mm (3/oin .) and the chuck must be washed out thoroughly than that no special tools are that all three jaws are even ly worn or
very small Albrecht, but the usual type afterwards. The Jacobs keyed chucks do stoned. New j aws are relatively cheap

40
41
and it is better to replace them. A Chuck-drop (A little thin oil will do no harm.) The opening, and some means of bolting
replacement split ring will come with This is the term I have invented for final and most serious fault is, of course down to the table. It is an advantage if
the new jaws. In re-assembling - after the situation where the chuck comes a burr on the mandrel nose of the the j aws have vee-grooves, both across
cleaning the inside of body and sleeve, free from the arbor whilst d rilling. As drilling machine (which can be caused and vertical, for holding round stock - it
andre-greasing- it is vital that the jaws remarked earlier, it can happen when by too enthusiastic use of the wedges in can be a nu isance to have to f it up vee
be f itted in their correct guides. In som e drilling brass at a high feed -rate just Frg. 41) or on the drill arbor due to bad blocks. M ost important, the base should
cases t he jaws are numbered, in others when t he dri ll b reaks th rough if pre- storage.
be soft. With the best will in t he world
they have one, two or three little grooves cautions have not been ta ken. But it and even if soft packing is set below th~
set in the recess at the end of the can, occasionally, become an epidemic. workpiece, sooner or later a drill will
thread. The split ring should be g reased
The Drilling Vice
There can be two causes. On a new be run right through, and if the vice
and then assemb led over the j aws - it arbor- o r even a new drilling machine- It is necessary to hold the work as well b?dY is hard the point will be destroyed.
will only go on with the two halves it can be a mismatch of tapers. This as the d rill, and though often enoug h it Frg. 43 shows one good com mercial
correctly mated - and t hen the sleeve should be checked using marking b lue can be hand-held on the machine table type- the " NIPPY" - and a very rough
can be pressed on again, using the and if a good fit is not indicated the new (always with a block of wood or soft home-made one which has, neverthe-
forcing ring as before. This job is not component should be sent back for metal between), this is not the most less, served well for 20-odd years. It
difficult, and full detailed instructions replacement. (This applies to a new prudent of procedures. Especially when lacks any holding-down facilities but is
(the above is just an outline) come with chuck, too, though an error here is breaking through the drill can exert a so heavy t hat this is seldom needed !
the spare parts. And to allay any u n- unlikely.) A much m o re likely cause is considerable twisting force and with There are many drilling vices o n the
necessary worry - those ch uck jaws dirt. The two surfaces, inside and out, smal l workpieces will cat ch hold, with market, and the only ones to avoid are
which I have HAD to replace have all must be scrupulously clean if the chuck consequent and sometimes serious dam- those made of die-cast light alloy; it is
seen 15 to 20 years' service! is to be secure- and for it to run truly . age to the hand. A vice is an essential. bad enough to have the workpiece
However, it need not be a "machine spinning round, but if t he drill picks up
vice" of the type used when milling. th~ vice as well, damage to operator,
Soft, renewable jaws are preferable to drrll, and even the machine is very
hard ones, and the main attributes probable! "Mass" is an advantage in all
should be a secure grip, fairly wide vices.

Fig. 43 Drilling vices. A commercial type {the "Nippy") is at the front, with a massive
home-made one behind. The Nippy has a cast iron base but the jaws are hard.

42 43
 THREAD ANGU: 45<> THREAD ANGLE70° THREAD ANGLE 45c
SECTION 5 PITCH=D/8 PITCH=D/8 PITCH=D/16

SCREW THREADS
CORE DIA CORE DIA CORE DIA
0.730 0.82D 0.87D
Fig. 44 Comparison of various thread geometries.

Before dealing with taps and dies it may or by hand for amateurs. Both fastener same diameter and have the same having sharp crests and roots for "attach-
be as well to have a few words about and attachment threads are, almost thread form, but (c) has a much f iner ments" - mandrel nose, chuck accessor-
the threads they produce, as there is universally, of vee-form. The others pitch (i.e. more threads/inch) and (c) ies and the like- and a "shallow" thread
quite a number of different thread forms may be square, trapezoidal, or vee- clearly provides a greater core area. On of about 60° angle, still with sharp
available. A screw is, in effect, a "contin- form. the other hand, the torque magnification crests and roots, for his screws and
uous wedge" wound round a rod, and Fig. 44 shows some of the basic provided by the screw is much greater- bolts. He initiated the screwcutting taps
this feature has been known for many considerations relating to the vee-form in effect, we have a wedge of much and dies over a period and was, so far
centuries. However, it was not used for thread. Look at (a) and (b) first. Both are finer slope and the tightening stress in as I can ascertain, the first to set up a
fasteners (nuts and bolts etc.) until the same diameter and the same pitch, the bolt will be greater. In addition, the " master system". His methods need not
relatively recently, for with the element- but have different thread angles. It is thread requires much greater accuracy concern us, but having produced an
ary machinery of a few hundred years immediately clear that (a) has a much in both form and dimension - a small accurate tap and die for any particular
ago the simple taper cotter was easier smaller "core" diameter on which to error will be much more serious. There size this was used ONLY to make a few
to make and use - indeed, all the blast carry the load, so that one might assume is also more risk of "cross-th reading" in "master" taps and dies. These were
tuyere pipes on modern blast furnaces that (b) is preferable. However, though assembly, too. So, again a comprom ise used in turn to make the "workshop"
are attached this way, not by nuts and (b) provides a greater core area the must be made between the relative taps and dies used in production, so
bolts; much quicker when a tuyere flatter slope of the vee means (i) that the attributes to get the best all-round per- that the masters had little wear. How-
water-jacket has to be changed. Nowa- wedging action will increase the effect formance. The various standard thread ever, when wear was found, then a new
days the screw thread can be used for of friction compared with (a); this in forms we use all effect this compromise master could be made from the "origin-
(a) Measuring devices; (b) Traversing, turn means that for the same spanner in different ways, and, of course, we ator" tap or die -these might be used
as in the lathe leadscrew; (c) Attach- effort the fixture w ill be "less tight". choose one or the other in making our only once every ten years or so. It is true
ments, where components screw into Further, (ii) there will be greater risk of personal compromise between the that his diameter/pitch combinations
each other (e.g. boiler fittings) and (d) bursting the nut. Not something w e types. may strike us as "queer"- %in. x 9.45tpi
as Fasteners - studs, nuts and bolts. think about much these days, but it was and 0·36 in. dia. x 19·89tpi are examples
The first two are always screwcut in not uncommon in the days of wroug ht Standard Thread Forms -but it must be realised that (a) he was
order to obtain the necessary degree of iron, and even today when screwing The first engineer to standardise thread- concerned only with standardisation
precision- or possibly thread ground or into soft brass so f lat a thread could forms was John J acob Holtzapffel, who, within his own works and, (b) this was
mil led. Type (c) are, preferably, always cause trouble. The "best" thread form IS early as 1798 was working on this in the days when the inch was divided
started by screwcutting on the male is, therefore, a compromise between problem for use on t he lathes he made into 1/12th, '/,44th etc., going down by
thread even if finished with a die, but large core area and reasonable thread - lathes which, at that time, were well '/12th each time from the foot. (And, of
fasteners are, almost universally, made wedging action. Now look at (c) and ahead of anything else available. He course, there were no less than 19
with machine dies in "Production" work, compare this w ith (a). They are the ,eaoplted a "deep" thread of 50° angle, "standard lengths" to the foot in Europe

44 45
at the time!) These thread types and truncation of t he thread at the root on Fig. 46 Terms and
standards were in use right up until the the bolt; in addition, to reduce risk of definitions relating
damage to the threads at the sharp to Vee-shaped thread-
last lathe was sold in 1925, though quite
early on- about 1810- he changed to a point (a weakness of the Holtzapffel
standard) this was to be rounded off.
forms.
- r --r
MINOR (COREl EFFECTIVE
uniform 10 threads/inch for all feed- OfW ETER DIAMETER
screws. (A few tiny ones were 20 tpi.) See Fig. 45. This truncation has, as we (PITCH OIA}

Joseph Whitworth' s approach was shall see a little later, an important IJI,. \ .,1,.\ A. IMA:\t"--::....=-==.- - -·_L -
quite different. He was aiming at a effect both on the tapping process and
"Universal" standard, so that a bolt on the behaviour of the threaded pair in
made in Birmingham would fit a nut service.
made in Bristol. At that t ime (c.1840) In addition to proposing a standard BASIC
TRIANGLE
the situation was chaotic. Not only did for the threads Whitworth suggested a
different makers use different thread- standard form for the nuts so that (a)
forms, pitches etc., but even within the the number of threads in engagement
same firm it was not uncommon to find
that nuts would fit only the bolts they
were made w ith, as anyone who has
was such that the various methods of
failure were more or less in equilibrium
(remember, he was working with cast I
I
f
D(PTH OF
1: :~CAO
dismantled an engine from that period and wrought iron, not steel), and (b) so
will have found out. He wrote to all the that spanners made by one firm would I
major builders of engineering mach-
inery and collected details of their
fit all other nuts. The proportions were
that the nut height should be equal to .---
1- \)
I
the bolt diameter, the across-corner RO<iT
threads. He averaged the pitches so I ROOT RADIUS IROOT TRUNCATION I'
found at V4, %, 1 and 1% in. diameter, dimension should be about twice the
working out at 18, 12, 10, and 6tpi bolt diameter, and hence the across- ~-- ____!'ITCI_, - ____../
when rou nded off, and used these as a f lats dimension was around V3x the
sort of "scale" from which to suggest bolt diameter= 1·7320. These various
pitches suitable for the other sizes - proposals were presented in a paper ran down to this size, and rose by Whitworth Form
and, I suspect, keeping in mind the pitch before the Institution of Civil Engin- intervals of V64 in. up to V4 in.) Nowadays This is shown in Fig. 47, and applies
of the leadscrews of his own screw- eers in 1841 and, after some decades, the fine instruments are catered for by to British Standard Whitworth (BSW),
cutting lathes! He found that the mean became almost universally adopted, in the BRITISH ASSOCIATION (BA) thread British Standard Fine (BSF), British Stan-
of the thread angles was about 55°, and this country at least. and watchmakers either by BA or the dard Pipe (BSP), the British Standard
he took this as his suggested standard. However, even in 1841 there were PROGRESS system. Conduit Thread, and to the "Model
This was quite a steep angle, and to some who found that the diameter/ Engineer" standard threads as well. The
compensate for the loss of core area pitch combinations suggested by Whit- Current Thread Forms thread angle is 55°, and this gives a
which would result he proposed the worth were too coarse in the smaller Before dealing with specific forms it is height to the basic triangle of 0·96 x
sizes - 1/2in. and below - especially as well to be familiar with the terms pitch. The depth of the thread itself is
those concerned with instrument work, used. Fig. 46 shows the definitions and 0·64P, quite a lot less than the triangle
where brass and steel (high carbon is more or less self-explanatory. M ost of height owing to the truncation, giving a
steel, that is) were the normal materials. these apply equally to the "nut" - core diameter equal to D-1·28P (D is
Most of the threads proposed then are female thread - and the bolt or male the nominal bolt diameter) and this
now forgotten, but almost all had much thread, but you will not be surprised to applies to all the threads shown. In
steeper angles- 45°, for example- with learn that the "crest" of the bolt mates passing, this factor shows the virtue
greater or less degrees of rounding. with the "root" of the nut and vice- of a truncated form, as a sharp vee of
Whereas y,sin. Whit. was 60tpi, a versa! This drawing shows a thread 55° would leave a core diameter of only
Fig. 45 A thread form truncated by radii common figure for instrument work w as with radiused crest and root, but some, D-1·92 P.
at root and crest. 100tpi. (Whitworth engineering threads we shall see, have flats instead. The diameter/pitch combinations are

46 47
 CREST BASIC CREST CREST BASIC CREST
TRUNCATION TRIANGLE RADIUS O·•l"l P TRUNCATION TRIANGLE RADIUS o. IBP
I 0. 167P

1 . 136P

L__j_
BRITISH ASSOCIATIO N

(SA) THREAD FORM

I~~------'-.c.;..;::o.-"----
ROOT RADIUS IBp

....,. PITCH

B.S.W, B.S .F, B.S . P. AND M. E. THREAD FORM

Fig. 48 The British Association (B.A.) thread profile.

Fig. 47 The Whitworth thread profile. be replaced by the ISO (International British Standard Cycle Thread
Standards Organisation) series, as will This thread form is now obsolete, as
given in the t ables at the end of the was essentially metric, but the BA stan- the Whitworth and similar threads. How- m etric forms are almost universal, but
book; the "Fine" thread, BSF, has now da rd is, in fact, defined in inches. The ever, the BA series is very useful, and as there must be tens of millions of
almost entirely replaced the original thread angle is 47%0 , and is v ery heavily will be with us for a long time yet. cycles to the old standard I give the
standard Whitworth except when a stud rounded at both crest and root - the
is screwed into cast iron or a light alloy, width of the rounded part is 0·236 P CREST BASIC CREST P/ 6
TRUNCATION TRIANGLE RADIUS
when the stud will be BSW at that end compared with but 0·167P on the Whit-
and BSF at the nut end. BSF nuts are, in worth . Hence both the depth of thread _____!_
I _ _
0. 1 92P~.,
. I •
general, one size down from BSW - e.g. and t he flank height (which carries the P/ 6
a 1/2 in. BSF nut will have an across-flats load) are less for a given pitch than in
d imension corresponding to a '/,sin. the case of Whitworth form. This thread
BSW n ut. has t he ONLY "rational" diameter/pitch
combination. The pitch of each size is DEPTH OF
British Association (B.A.) Form 0·9 x the pitch of the larger one; that for THREAD
0. 533P
This, Fig. 48, is a development of an No. 0 BA, the largest, is 1 mm; No. 1 is
older thread, the THURY, developed on 0·9mm; No.2 is 0·9X0·9=0·81 and so
the continent to suit horological and on - all being "rounded" and expressed
fine instrument work. Here the problem normally in inch units. The Diameter of MINOR OR
was that steel screws, often hardened, a BA screw is derived from the pitch , CORE DIA
were screwed into brass plates and
were, for their size, pulled up very
D = 6P1' 2 . However, there is no need
to work it out, as the figures are given I R00T RADIUS
P/ 6
D- 1. 065P MAJOR OR
NOMINAL DIA
tightly. A shallow angle was needed to
prevent distortion of the femal e thread
in Table VII on page 91! The core dia-
meter is D- 1·2P. This thread is now
~-- PITCH
t
" BICYClE " THREAD FORM
in the softer materials. The Thury thread "obsolescent" and w ill, in due course,
The British Cycle Thread. Most cycle threads are now metric.
48
49
 PERMITTED ROUNDING AT ROOT bolt, and that "permitted rounding" 60° I. S. 0. (METRIC)
--+~~-- - - - --#-=t-MAJOR
ensures no interference at the root of
DIA the nut. It is not dissimilar to t he ISO a.
I()
standard shown later, but is, of course, 0
specified in " Inch " units. The core dia- 0
meter is given by D- 1·226P but some
users round t he root with consequent
reduction of core diameter. This form is
now the American standard, but is
obsolescent in Great Britain, where the
ISO form is now standard. There is a
i
-,-------J~~-------...,....c-- MAJOR
DIA number of different series of diameter/
I

~~---
PERMITTED pitch combinations: Coarse (U NC), Fine
ROUNDIN G
T- 0.1 OSP (UNF), Extra Fine (UNEF), and several
o BOLT constant pitch series which are denoted
by the pitch/UN - thus 32UN or 26UN
etc. Deta ils of UNC and UNF threads are
given in Table XI on page 95.

The ISO Thread

~---0-L_____ _ +I----- ---~-- I
- - ------ - - - - -- AXIS
This form was devised by an internationa l
committee of the various National Stan-
dards Institutions, including B.S.I. All of
1 ..
PITC'"',
J
Fig. 50 The Unified thread form. the thread forms so far considered have
disadvantages - either in the difficulty
form in Fig. 49. It has a 60° angle, and is of maintaining tolerances, or accuracy
t runcated fairly heavily - used on thin of form, or service problems, and in
tubes it is essential to reduce the core some cases are prone to manufacturing
as little as possible- the core diameter difficulties in h ig h volume production. ~J~
is D-1·065P. A useful Model Engineer The Unified system met some of these, -a. ~

I=-i
Pipe thread if taps and dies can be though not all, and in any case was ~ ::;::
found of suitable sizes! based on inch units. Even in Great Ill :.::
Britain it was not too popular. The ISO 0 5
Unified Thread
~
thread has a 60° angle - Fig. 51 -which :z
This thread was introduced about 40 can be generated, so that accurate form
years ago in an attempt to "marry" can be achieved without reference t o -a.-+-....---
American and SAE (Society of Auto- gauges or measuring equipment. The ~ -~"<(' 6
-'----~...J·L'--'-~ --1--~-
motive Engineers) thread standards to crest of the male thread (bolt) is nor-
the Canadian and British usage. It differs mally flat, but if rounded the rounding a::.
' - - -- --

little from the original "American Nation- must be inside the flat form; and the
al" standard, and in most cases is root of the bolt has a fairly generous
interchangeable. Fig. 50 shows the form, radius, giving better fatigue resistance
and it will be seen that the "nut" has a
considerable flat on the crest so that it
and avoiding stress raisers. The crest of
the female thread (nut) has a wide fl at,
t
can never "bottom " in the root of the which can never "bottom" on the root The lS. 0. (International Standards Organisation) thread form. It is metric in dimensions.

50
51
 Fig. 51 a Showing
The MODEL ENGINEER'S METRIC
the fit of a mating Constant Pitch Series ATIACHMENTTHREADS, Table X, were
l pair to "perfect"
l Of these the industrial standard most drawn up in 1982 by a committee of
"' I.S. 0. profile. The
~ flanks only are in useful to model engineers is the BRASS eminent model engineers appointed by
u contact. THREAD, of 26tpi -Table VI. This was the publishers of Model Engineer. The
> used mainly on thin-walled brass tubing object was to devise a series of threads
~ for the old gas-brackets, and was more which would both accord with the I.S.O.
"' fitted to the t hi n piping than the corres-standard and at the same time meet the
ponding "Gas" thread, intended for special needs of model makers of all
thicker w rought iron and, later, steel branches. It had taken nearly 50 years
tubes. The constant pitch enables differ- for the Whitworth form M.E. threads
ent sized pipes to be coupled together to be agreed and adopted, and it was
by screwing the adaptor coupling well felt that although it might be some
onto one pipe and then bringing it time before these (M.E.) screwing tools
forward onto the other. Unlike the B.S.P. became very expensive it would be as
of the bolt; the tolerance system (in meter and pitch available with each (British Standard Pipe) thread, the pitch well to have a metric standard which
"production" runs) is such t hat tool system. The aim is to maintain a more is the same for all sizes, whereas B.S.P. manufacturers could w ork towards. The
wear on eithe.t: mating thread cannot or less constant angle of thread in any has varying pitches in the smaller sizes series was found acceptable to the
cause trouble here. The root of the nutgiven set - Coarse or Fine. The use of a (Table XIII). It is superseded by the British Standards Institution and is,
is generously rounded outside the majorfiner pitch for a given diameter does, as I.S.O. 1 mm pitch series. in fact, published by BSI as P.D.6507/
already noted, provide a larger core
diameter of the bolt so that interference The MODEL ENGINEER threads, of 1982. There are three standard pitches,
area. In addition, because the fine pitch
is not possible here either. This has an Whitworth form, were introduced after 0·5 mm, 0·75 mm and 1·0 mm, with some
important consequence in two respects. involves a smaller circumferential angle many decades of debate and confusion overlap of diameters in each set. The
to the thread, a given spanner torque
First, the O.D. of the tap w ill be greater about 40 years ago; in this case the 1 mm pitch series is particularly useful
will apply a greater tightening load in
than the nominal diameter of the thread, need was both for a finer pitch than for large scale workers, as it forms part
a point to remember when checking the bolt. There is no difference in the then currently available, and for a con- of the I.S.O. 1 mm series, ru nning right
taps with a micrometer! Second, as we strength of the actual thread, no matter stant pitch which could be used in up to 20mm dia.
how fine it may be, but clearly the
shall see later, it affects the choice of making small f ittings. Originally 40tpi It may be some years before this
smaller the thread section the more
tapping drill. Perhaps the greatest virtue was used for sizes up to %in. and 32tpi standard comes into use, but it will be
of the ISO thread, however, is the fact important it becomes to avoid dimen- above this. However, the 40tpi range there when needed and it is to be hoped
sional error and the more serious is any
that it is possible to make a very close- has been slowly extended upwards, that designers will use it, and so avoid
fitting threaded pair with absolutely NOdamage to the crests. Anyone who has end can be had (at a price) up to % in. the disastrous confusion which existed
risk of interference at the crests and dismantled an old telescope, where the IIIIAm.,•tPr, There- is great risk both of in former times. It is worth mentioning
threads are very fine, w ill realise the
roots. See Fig. 51 a. This is very import- IH'n•ct<>-1'hreading and of bad fits above that the same working party drew up
ant, and nowhere more so than in difficulty that can be experienced just t %x 40, and it is only in rare cases
recommendat ions for the hexagon sizes
model engineering, where we are work- due to thermal expansion! There is a even this should be necessary. for use with I.S.O. threads in models -
ing with very small screws. The load isfinal point; "classical" engineering prac- recently one enterprising manu- those used for industrial purposes are
taken entirely on the flanks. As can betice is to have between five and eight rer has re-introduced t he 60-tpi far too clumsy - and P.D.6507 gives
seen in Fig. 51, the minor diameter of threads engaged in any threaded pair - in diameters from V•ein. up to details of these also, as does the "Model
the nut is given by D-1 ·082P, but the bolt and nut, or whatever. When using ln. These are very usefu l for adjust- Engineer's Handbook" **.
core diameter of the bolt is D-1·226P. attachment threads into relatively thi n purposes and if one end of a rod
members the use of a finer pitch w as ICrewed 60tpi and the other 40tpi
Diameter/Pitch combinations obligatory. So, the choice of fine, med- handy "differential" adjustment is
With the exception of the B.A. thread ium or coarse pitches is a question of ble. These threads are given in *"'The author was the Secretary to the Work-
and the British Standard Pipe Th read purpose and design rather than man u- VI. ing Party which drew up these standards.
there are various combinations of dia- facture or strength.

52 53
 Even though very special furnaces are out a little more of the total depth. Once
SECTION 6 used and close control is kept over both fully engaged (in the case of a tap, when
hardening and tempering there is some it is one diameter deep) the load in the
risk of distortion. The tolerance allowed shank and in the workpiece is much
on the straightness has to take into greater than would be the case if the
account the manufacturing methods, as thread had been screwcut full depth
does that on the thread form. With with a sing le point tool, due to friction.
some cheap taps especia lly quite a In H.S.S. ground thread taps and dies
TAPS AND DIES- GENERAL marked bend in the shank may be
found. High-speed steel taps and dies
of any size- above, say, 8mm (SAs in.)
the teeth have circumferential relief to
on the other hand, are made from reduce this, but the torque is still con-
hardened blanks. The hard barstock is siderable. We shall cover this further
first ground to diameter, the thread is when we come to the selection of
then formed by thread grinding and tapping drills.
then finally sharpened in the flutes. A more important point is that with
Before dealing with these in detail a few that a tap can get hot it may get at least Thus the whole of the thread must be ductile materials (worst of all with bright-
general points are worth discussing. "well aired" in such service! Similar co-axial with the shank, the thread form drawn steel) there is far less room for
The most common question asked is considerations apply to automatic screw- wil l be to a finer tolerance (thread- the chips. Though theoretically the tap
whether there is any advantage in using ing machines making male threads, grinding wheels are continuously moni- or die is designed to provide ample
High Speed Steel against the cheaper though these use, as a rule, special die- tored and automatically reformed when space in the flutes or clearance holes,
Carbon Steel for both taps and dies. The chasers rather than the button dies needed) and - of importance to those curly chips do tend to jam up - see Fig.
first point to note is that industrial used in the hand die-stock. For hand who have Ouorn cutter grinders -the 52. The rule is to make only half a turn
needs are very different from those of work the hot-working feature of high male or female centre at t he business forwards f ollowed by half a turn back-
the amateur or even the jobbing work- speed steel is of no relevance. Carbon end can be relied upon when resharpen- wards to break off the chips; if this
shop. In industry hand tapping is very steel is, in fact, markedly harder than ing becomes necessary. is not done with all ductile materials
rare and the use of a die by hand almost H.S.S. and this is an advantage in hand For jobbing work, therefore, a H.S.S. sooner or later you w ill find that the tap
non-existent. Tapping is done with tap- work. tap or die will cut more accurate threads, or die cannot be moved either way -
ping machines which have a delicate More important than the material is but will not be as hard as one of carbon though the problem is not so great with
torque control to avoid over-twisting, the way these taps and dies are made. steel and (under hand-use conditions) is dies as, with luck, the chips may curl
and w hich automatically reverse the In the case of carbon steel the tap or die likely to blunt sooner. H.S.S. is margin- right out of the clearance holes. With
rotation to withdraw the tap on reach- is made either by screw-cutting or from ally better in its resistance to chipping free-cutting materials which do not pro-
ing the preset depth. These machines a master tap or die, fluted, and THEN of the cutting edge. I have found no duce this type of chip the problem is
work fast, and are working all day long, hardened, the edges finally being sharp- advantpge either way over the problem reduced and with "screwing brass" the
so that while it may seem inconceivable ened by grinding the faces in the flutes. of tap breakage but if a tap does break thread can be cut continuously. When
then there is no comparison - carbon making male threads in the lathe free-
TAP steel is far easier to remove as it can, in cutting mild steel and most non-ferrous
CUTTING
DIRECTION the last resort, be softened with quite metals can be threaded with a die from
moderate heat! H.S.S. is, of course, the tailstock under power, but unless
more expensive. bright-drawn mild steel is first normal-
ised the threads are likely to tear.
One final point which applies to split
Cutting Action dies (see page 72). All dies are designed
REVERSE TWIST The cutting action of the teeth of both to cut the thread to full depth in one
CHIPS
CHIP BROKEN OFF taps and dies is the same as that of a pass. The split feature is intended to
CURLING IN FLUTE
single-point screw-cutting tool, but the allow the dies - especially those of
Fig. 52 Showing how the chips in the tap or die flute may be freed by reverse rotation. teeth are so arranged t hat each takes carbon steel - to be adjusted to size.

54 55
Whilst it is permissib le pract ice to take a too great, t hen it needs sharpening, and
fine second cut t o adjust the size of the this should be attended to. Frankly, I SECTION 7
thread to gauge, the habit of "roughing prefer to use sol id dies, but these are
down" with the die open, followed by a usually obtainable only in H.S.S. with
"finishi ng cut " should be. avoided. This g rou nd threads. Kept sharp, t_hey pro-
causes u nnecessary w ear. If it is found duce a thread of the correct s1ze every
t hat t he die has to be run t hrough twice time -especially important with threads
because the effort with a single pass is below 3 mm (5BA).
HAND TAPS

Fig. 53 is a generalised illustration of a taps m ay be cut w ith thread relief, but

typical hand tap. The body is threaded this is seldom found on carbon steel cut
and has from two to five flutes cut thread taps. Such relief eases the cutting
longitudinally, depending on size, though loads - reduces friction- but on the other
2-flute taps are usually for use on tapping hand a tap without relief is better guided
machines only. Depending on the shape by t he threads already cut. Unless a lot
of the fluting cutter the rake of the cutting of work is to be done in difficult mater-
face may be straight or curved, as seen in ials - work-hardening stainless steel, for
the top RH figures. For production work example (or uranium!) - it is not worth
these rakes are carefully specified accord- the extra cost to ask for this feature.
Ing to the duty, but this need not concern Small taps w ill have male centres at the
the jobbing worker. As already remarked, point and may have a similar point at the
RAKE( STRAI GHT RAKE
SI ZE O F CUTTIN G LAND CUTTING (CURVED CUTTING FACE)
FA~E
.~ H:~~TE:t
SQUARE FACE)

. . } T A P CENTRE
WEB \..RAKE
THIC KNESS ANGLE
THREAD RELIEF (RADIAL)
LENGTH OF SQUARE INTERNAL
CENTRE

c~~~~
[
SH.HA~N~K~--~Effi~~IDHffiW~~~7--+~PO~J~NT
- DIA DIA

LENGTH OF
--S
HANK --
. 1.. LENGTH OF
THREAD EXTERNAL
- - - OVERALL LEN GTH
CENTRE

Fig. 53 Tap nomenclature. Courtesy B.S./.

57
56
 ENGLISH NAME. U. S NAME t here may be 10 to 15th reads cha mfered

E~~
off. The " Seconds" tap has a shorter

TAPER TAPER
) taper of about 16° included angle, whilst
the " p lug " o r " bottoming" tap is shorter
still, about 45"-50° included, which
means that in effect only a single thread

~~?
is affected. (Note that in USA the term
" Plug tap" is ap plied to the British
SECOND PLUG " Seconds" type; it is b etter to use the Fig. 55 A "Nut Tap" showing the threaded nuts feeding along the shank.
term " Bottoming"). Occasionally one

~~ (
will find a bottoming tap with no lead at to use them for blind holes - the use of They are intended only for through
all, presumably with the intention of the taper tap is alm ost imperative. holes.
carrying the thread right to t he very The Nut Tap, Fig. 55, is intended for There are many special taps of one
PLUG BOTTOMING bottom of a blind hole. There is little short production runs making nuts, and sort or another, and these turn up both
Fig. 54 The shape of taper, second and plug point in trying to d o this and the cutting as can be seen in t he photograph the at club j umble sales and on the surplus
or bottoming taps. action of the leading thread of such a shank is reduced to less than the core market. They are best left alone, or kept
tap is so poor that the leading (cutting) diameter of the thread, so that when as curiosities. Almost all of them will be
squared end, though smal l H.S.S. taps edge soon blunts or, w orse, chips off. tapped the nuts can run ov er this part. machine taps, intended for use with
are held, during manufacture, in collets The taper tap is intended for making That shown has a squ are on the shank, tapping machines; t hey may be to
on the shank. Larger taps may have the first cut in through holes - it is but this has been ground on to convert special or unusual tolerances, or hav e
either male or female centres at the useless on a blind hole. The threads it to a hand tap. They are strictly machine obscu re rake angles intended for awk-
threaded end. Male centres often get in even on the parallel part are undersize, taps, having a plain shank with a single ward materials and which will jam up if
the way when tapping shallow blind and it must t herefore be followed by the flat for holding in special collets. The used on steel or brass. The "common or
holes; before grinding it off remember seconds t ap. This w ill give a full thread lead is longer than a seconds hand tap garden" hand tap, whether of H.S.S. or
that you will then lose means of holding if taken beyond the taper, and there is and usually the flute is of special form carbon st eel, w ill meet almost all needs.
the t ap at both ends when resharpening no need to follow it with a bottoming to give free cutting at high speed. Many The only " specials" I have needed over
in the Quorn or sim ilar m achines. tap. For blind holes, however, the taps sold as " surplus" are of t his type half a century of making screwed holes
The ent ry end is chamfered w ith a seconds tap is used first, followed, only and very usef ul they are, too. However, have been f or odd pitches or to reach
taper lead, and this " lead" is intended to if necessary, by t h e bottomin g tap if the they may need resharpening if th ey awkward places, and I w ill deal with this
st art the tap i nto the hole. As seen in seconds does not take the thread deep have come from a production work- latter problem in a moment!
Fig. 54, t here are three lengths of taper. enough. For general purpose work the shop. You MAY find that n uts made
The longest has an included angle of seconds tap will cover almost all needs. with such a tap can be fitted only with Tap wrenches
about so and the end is smaller in The main point of the taper is that it difficulty onto a bolt; y ou have picked This is an unfortunate term, as the last
diameter than the usual t apping drill; reduces wear on the seconds and, in the up a tap to Class 1 tolerances, too close thing you should do to a t ap is to
case of very small taps, reduces the risk for normal work! "wrench " it! The term arose, of course,
of breakage as there is less load on the Spiral Point Taps have the flutes in the days when a half-inch thread was
seconds t ap when it takes its turn. formed into a helix (a "spiral" tap would regarded as "small". The important
= Occasionally you may meet with taps
which are made with flats rather than
get you nowhere very f ast!) with the
intention of feeding the chips forwards
thing is to have means of turning the
~ NUO-o•U N •-
tap smoothly, with even torque so t hat
11111111 I~ flutes in very small sizes (below 12BA out of the f lutes. This certainly makes t here is no bending action, and of a size
or 1·5 mm). These have either a square for easier tapping. but there is a price to which "matches" the size of the thread.
pay. The chips will be fed to the bottom
·--!!ii£!!:::::!!::::::!!ii!;jii!J or triangular section and, clearly, operate
with considerable negat ive rake on the
cutting edges. These are suitable only
of the hole and will not remain in the
flutes. This means that you will not be
In the old days this would be a bar of
flat steel with square holes in it to fit
various size tap shank. A barbarous
Fig. 54a Typical taper, seconds and plug or for brass, and even there must be used able to reach the bottom of a blind hole affair; I have seen them w ith holes
bottoming taps. with great care. It is almost impossible without f irst blowing out all the chips. suitable for 10mm (3/ain.) taps down to

58 59
 Fig. 57 Showing how a pin-vice
may be used to reach an
inaccessible hole. The tap
is 4BA.

Fig. 56a The so-called "American" type tap wrench; this one will accept 'hin. squares.

3mm (5BA) all on the one bar - asking progressively smaller down to about
for trouble. 5mm or 3/,e in. The lowest is a chuck
Fig. 56 shows various acceptable type, for taps from 4 mm or 5BA down
types. At 'a' is a large one, accepting up to 2mm or 8BA. One point to note: in
to 12mm (% in.) squares, say 22mm or the upper three t he tap square should
7
/ain. taps. It is about 40mm (15i n.) long be set in the hole so that the locking

smallest pin-vice though, and I have

one with jaws which will hold down to
Ill
18BA. In most cases enough grip is had
f rom the fingers, but it is important NOT
--
..#" - . . ., .....
---~.... ----
~~~"'
Fig. 56b Three bar-type
wrenches; the largest will
to turn it using the knurls at the nose, or
this will unscrew and release the tap.
accept a 12 mm or '/2-inch Whit.
« t Ei I.:; tap. Below, a small chuck-type
Finally, in Fig. 571 show how the shorter
taps of small size may be caused to
to take up to 4BA. reach down into awkward places. This

T
is a larger pin-vice than that in Fig.
56 and is holding a 4BA tap (3·6mm)
with one of the medium size (Fig. 56b)
wrenches on the end of the vice.

Keeping taps square to the work

and st rong in proportion. This type of screw bears on a corner, not on the flat. This is a perennial matter for discussion!
wrench is sometimes known as the The largest wrench would be improved Like many others, I was taught to use a
"American", but the one shown is British if the circular hole were made more like small trysquare once t he taper had
to the last knurl! In Fig. 56b I show four t hat of t he lower type. Finally, at Fig. 56c started to cut and to ease it one way or
types, the largest at the top for taps up I show the "wrench" I use on very small the other to rectify any "slantendicular-
to 12mm (V2in.) dia. and the two below taps - 12BA or 1·5mm. It is not my ity". Once the taper tap runs square, of
course, the others will follow. Many
devices, simple and complex, have been
described to e n~ure squareness. All
need making and many need setting I
up. For those who do a great deal ~
of tapping into blocks t he "George Fig. 58 The George Thomas Staking Tool,
Thomas" Staking and Tapping Tool (or seen being used to guide an BBA tap.
Fig. 56c A pin-vice, used as a tap-wrench for smaller taps. That shown is 12BA. " Pillar Tool") is the answer (Fig. 58) but (Courtesy of G. H. Thomas Esq.)

60 61
 it has to be admitted that it is a major
piece of equipment and hardly worth SECTION 8
while unless there are other uses for it
-it is, after all, a STAKING tool. But it
will handle taps from 3fs in. x40tpi down
to 16BA, a range not to be despised!
However, my own arrangement is effec-
tive for 85% of my work, shown in Fig.
59. The workpiece is held square in the
vice on the drill ing-machine table and
TAPPING DRILLS
the tap guided by t he drill-chuck which
is finger-tightened until the tap can just
be turned. The drilling machine spindle
is, of course, locked. The wrench is
fitted to the shank and most of my taps
now have a small flat ground in a At first sight it w ould seem that all we can. Tap the hole equally careful ly. Now
convenient place. The same arrange- have to do is to drill a hole the same size try the drill to see if it will still enter the
ment is used with the tailstock dri ll- as t he core diameter of the thread and hole. If you have drilled in steel or any
chuck in the lathe. It is effective and has then use the tap. This is far from being ductile material you w ill f ind it just
the merit of costing nothing- you must the case. First, the cutting forces are so won't go in, and even with brass it may
have a d rill chuck before you can tap a great that large taps would be almost be a stiff fit. The reason is t hat even with
hole, anyway! The one limitation is that immovable and sm all taps too weak to a sharp tap the cutting forces tend to
it cannot be used with taps which are stand the torque. But there is another extrude the metal into the vacant space
not reasonably straight, but this applies factor - one which you can try for at the root of the tap threads. If the hole
Fig. 59 The author's method of guiding taps. to any other method. (The operation of yourself. Choose any size tap about 3 or is core diameter size there will be
A similar method can be used in the lathe " Murphy' s Law" has ensured that my 4mm (say 3/ls x 32) and use a drill which nowhere for this metal to go; it w ill be
(see text). best-cutting V4 x 32 tap has a badly is about three-quarters of the O.D. of compressed and the friction on the tap
distorted shank!) the screw. Say 3.6mm for 3/•sX 32. Drill will, even at this size, be sufficient to
carefully - make sure that the drill is break it. This means that w e MUST use
properly form ed; use a new one if you a tapping drill which is larger t han the
MALE THR EAD( BOLT)
s' s:•:•·•·•·•·• :·:-
\e.,.,_----.~-~~
· • · ··· -· o'
..:::::•::::::: {••:;:::;:_ ·:F
. :l?i~~::::: :\\._ : : : .
·\ .:• · · ·f; ;rr ·· \.•·•·: . .
b

··:: \:::•i:::;:} FEMALE·v~·~~AD (NUT)

Pig. 60 A threaded pair shown in full (left) engagement and (right) in partial engagement.
Note that only the flanks of the threads are in contact in both cases.

62 63
 core diameter, and quite a bit larger at is any risk of the tap breaking. Further, 0·59X, at 60% it is 0·43X, at 55% about the core area in tension, and no less
that. The result will be what is known as the taps they use are all really sharp- 0·3X and if we went down to 50% than five times as strong in resisting
"Partial Engagement" of the threads. they are taken out of service as soon as thread engagement it would be only crushing of the flanks. (This assumes
Look at Fig. 60. On the left is shown a their performance begins to fall off. 0·22X. For the jobbing worker and the the full nut height and reasonably well
mating pair- nut and bolt - with full How long is it since you sharpened any amateur this is clearly a serious con- made threads, of course, correspond-
engagement of the threads. Apart from of your taps? I think you will find that if sideration, for whereas an industrial ing to the coarsest commercial fit.) The
the unavoidable clearances the male they were put into a tapping machine firm will buy taps in dozens (or tens, nut, or female, thread is even stronger
thread (bolt) fills the female thread they might well start slipping straight these days) and always have plenty in in shear, by the way, if it is made from
completely. On the right hand thread, away! stock, you, like me, may have only one the same material as the male thread.
however, the male thread is complete Now look at Fig. 61. This shows the of each size- AND no tool-dealer within One reason why we can use mild steel
but the female thread in the nut does relative torque required to drive a tap at 30 miles from whom to get a new one ! nuts on high tensile bolts! However, the
NOT fill the male thread completely. various % t hread engagements. (It is . So, does it matter if we use a low point I am trying to make is that all we
The ratio al b, expressed as a percent- drawn for Unified Thread taps, by the engagement figure? do when we use a low thread engage-
age, is called the "% thread engage- way; the effect will be more pronoun- ment is to change the ratio of shear to
ment". On the left of Fig. 60 this ratio is ced for BA or Whitworth form threads.) Strength of Partially Engaged
tensile strength, and we should have to
100%, on the right about 75%. This You can see that at 75% engagement Threads
lose two-thirds of the shear area before
latter is about the normal for power- the risk of tap breakage is 2·3 times that Look again at Fig. 60. The first thing to the strengths become equal.
driven machine taps in industry, and is at 60%; at 80% it is three times as great, notice is that we gain no benefit from In Fig. 62 I have analysed one of the
the basis for most of the tables pub- but if we went down to 50% the risk 100% thread engagement anyway. The most common threads w e use - t he BA
lished by tap and die makers, B.S.I., etc. would be halved compared with 60%. threads not only do not, but MUST form. I have, in effect, sliced off the nut
But remember what I said earlier; these Or, put the other way round, if we call NOT, touch on the radiused crests and threads at intervals of 5% of the total
machines have sensitive torqu e limiting the risk with industrial thread engage- roots; if they do they will jam. The load height and worked out first, the actual
devices, and they s lip well before there ment as 'X', then at 65% the risk is is, in fact, carried ONLY on the flanks- flank engagement and then the "strength
I f rom A to B on the left hand side. The ratio", assuming that bolt and nut are of
! ELATIVE TOI!OUE TRUE depth of engagem ent is not al b the same material and that the nut is
ON TAP but DEIBA. This actually works out one bolt diameter tall. You w ill see from
at 82% on the diagram instead of the table on the right of the diagram
the nominal figure of 75%. Now con- that there is NO POINT in using a thread
sider what happens if the threaded pair engagement above about 82% - you
2l fails. First, the bolt may snap off across gain nothing in strength but the torque
the core diameter. This is, in fact, the curve on Fig. 61 goes off the chart. At
basis on which the bolt will have been 60% thread engagement the strength
designed. {I use the term "bolt" to ratio is 2·5 - i.e. the threads are 2%
Indicate ANY male thread, whether it be times as strong as the core diameter of
a fitting, adjusting rod or actual nut and t he bolt. NOTE THAT THIS IGNORES
bolt.) Or the thread may " strip". This ANY EXTRUSION EFFECT- the actual
means that it will shear across the line thread engagement will always be about
BC on full engagement, or OF with 5% more than the nominal, more in
partial engagement. The third method some materials, especially BDMS.I have
of failure might be in crushing of the shown the results in graphical form in
flanks- along AB.or DE. Fig. 63, and have added a second curve
Fig. 61 Graph showing
I won't bother you with the mathe- which shows the comparison between
how the tap torque (and
hence the risk of tap matics or the "materials science" of the the nominal thread engagement and
breakage) increases as ng; suffice to say that at 100% en- the engagement of the load-bearing
so 60 70 80 90 the depth of thread ent the bolt thread is three times flanks.
% THREAD ENGAGEMENT engagement increases. strong in thread shear as it is across What this all means in practice is that

64 65
 Fig. 63 Graph showing <(
(a) true % flank engagement 0
and (b) relative shear/tensile 0
strength of a bolt based on u
the conventional figure 300 :c <
derived from the drill size, ...0
BA thread form. z
290 ~

280 =w

270 ::
3
0
?f
260 ~
~ <{

0I 0
<{
250 ~
.....
..... ;::
;;;;
0
<{
0
t;;
240 0
() :c
z "'
"' <(
G
0:::
c.. "'
~ "' STD B. A . THREAD
230
~
<{
.... ~
~ 0
-"' :::::> ~
80"'
N
~
u
...
"
~
z
220
<{
I'
1-
0 ~I
0 a::
z<(
,__
~
50 60 70 80 90 :a"'
0 :::>: CONVENTIONAL THREAD ENGAGEMENT% BASED ON DRILL SIZ E
Cl
~
"'
I 0""
1-
simply by going down from the conven- apply to those of Whitworth form. In the
1 I
u
1-
a:: tional 75% (or more) thread engage- case of ISO metric threads the situation
0 ment to 65% we HALVE the risk of tap
0.... is even more pronounced. Look again at
breakage with a loss of no more than Fig. 51 , page 51. You will see that the
7% of the shear strength of the thread. tap has a greater diameter than the
If we go down to 60% the tap breakage male screw thread. If we calculate the
W e.: risk is reduced still further and the loss "nominal engagement " of the threads
->
1- w
w
,__
shear strength is still only 12W'/o- in the ordinary way, based on t he bolt
~::;;
~
u.. -
<( still 2V2 t imes as g reat as the tensile thread height, the tap will actually "see"
~ 0
ngth of the bolt itself. I have for over a greater engagement. Thus, a "nom-
years used a general f igure of 60% inal" engagement of 65% will provide
~
I-
threads below 2·2mm (78A), 65% about 74% of flank height, but the tap
w I
those between 2·2 and 6mm (% in .) will f ind itself w ith about 76% "cutting
0
u
()
f-111,--------- ~ ----t-~ between 70 and 75% for larger engagement". It is even more import-
:.: <>- ••rnetel'!'; w ith no ill-effects at all - and ant with this type of thread form to
I- z~
I
() ~0 tap breakages have fallen d ramatic- examine t he balance between t he risk
u.. ,,
;;:; Indeed , below 8BA I am now of t ap breakage and the strength of the
f-olic------; § ---------~ to sharpen taps; previously they t hread at reduced engagement.
cz ~a:
"""'
to get broken before they got
~0 Other considerations
II II
calculat ion has been done for t he We have so far assumed that the hole
thread; identical consideratio ns drilled is to proper size. But take a look

66
67
again at Fig. 62. In the case of a 6BA harm. In all t hese difficult materials the The sizes between bold vertical lines and though S-type nuts may appear to fit
(2·8mm) thread the difference of 5% sharpness of the tap can be crucial and are those which I use. They are stocked M-type bolts t he thread mating will be
engagement represents a hole size dif- this is nowhere more so than i n the case by several Model Engineers' materials poor. The two types should be kept
f erence of only 0·0006in.- 15 microns. of work-hardening grades of stainless suppliers. separate.
It is highly unlikely that anyone can drill steel. There is also, in all the tables except Finally, for those concerned to replace
to that sort of tolerance using old job- t hose covering metric threads, a column BA screws with their ISO counterparts I
ber's length drills resharpened by hand. Tapping Drill Tables (See pages for "General Purpose" tapping drills. have given a rough comparison in Tables
I have for many years used nothing but 89 to 101) These are all in the "old tradition" -
stub length drills for tapping - few There are two approaches to select-
y111 (IS~ standard) and IX (ISO fine pitch)
fractional or Morse number and letter
1n the fmal column. These comparisons
tapped holes are of any depth. They are ing tapping drills, and I have tried to drills. These will provide a thread engage- are based on the thread 0.0., though in
kept in a special box (see Fig. 37, page cover both in these tables. The first is ment somewhere between 60 and 75%. most cases the core area is about the
35) and I use them for no other pur- to decide on the thread engagement But if you want to work closer, just look same, too.
pose. I strongly recommend this prac- needed and then look up the size of up the metric size of drill needed and
tice though, of course, within reason; drill necessary. The second is to have a then use Table Ill, page 87, to find the Jointing threads
the possible error ceases to be of standard set of dril ls and use a table to nearest number or letter drill. I have not Many practitioners have the idea that if
significance with threads above 5mm find out what engagement each will done this for the metric threads because they use almost 100% thread engage-
or % in. diameter. give. The second procedure is covered if you A RE going metric with taps and ment they can rely on making a f luid-
W hen tapping thin material, threaded by Ta ble XV (page 99) where I have dies you should do the same with your tight joint in the threads. This is just not
right through, the risk of tap breakage is list ed the modern standard metric set drills. Finally, at the head of each table true, not with tap-and-die cut threads
reduced, as even on the seconds t ap the running from 0·1 mm to 10mmx0· 1 mm. there is a formula which will enable you anyway. The commercial class fits even
full -thread cutting edges wil l not engage Thus, if you want to tap 7BA, look down to calculate t he drill size should you need when using ground thread taps and dies
until the leading threads have emerged the BA column and you will see that any dept h of engagement for some always leave a clearance on the flanks
from the underside. Further, we now 2·0m gives 82% and 2·1 mm 65% thread special reason.
and at the roots of the threads, and even
have fewer threads in engagement, and engagement. Take your pick! The tables for ISO METRIC THREADS when, as in full-size pipe-fitting, taper
although the strength in shear is seldom Alternatively, say, for BA threads use are rather different in one respect. We t~ reads are used and pulled up really
a consideration in such work it is worth Table VII, page 91 . This gives t he FULL have already seen that the 0.0. of the tap t 1ght, some form of jointing has to be
aiming at rather deeper thread engage- details of the thread and then a choice of is greater than that of the male thread. used. It is far more effective to concen-
ment. I usually use between 5% and drills for different th read engagement. This means that if a drill is selected to trate on cutting a good, clean, thread
10% more than I would normally - Those shown in bold type are the British give (say) 70% of the male thread en-
profile and then to use an appropriate
agai n, note that Fig. 62 shows that for Standard "preferred sizes", but ALL t he gaged it will provide about 80% of the jointing compound. In the special case of
BA threads there is no point in going sizes shown are available in jobber's flank height and the tap will actually boiler stays this is even more the case.
above 80% nominal. drills. Stub drills in the smaller sizes do •see" about 82% when it is cutting. So, in Copper is liable to tear if too small a
Certain mat erials are easier on taps have a more limited choice, but some case the column headings are based tapping drill is used and, in addition it
than others, and cast iron (the softer sense of proportion must be maintained ! flank engagement and I have shown has, in the annealed state after brazing, a
grades), cast aluminium, and free- The difference between 1·89mm and square brackets the engagement seen greater tendency to "extrude" than most
cutting brass ("screwing brass"), can 1·90m m for a 60% engaged 8BA thread the tap. You will see that the difference metals. If a 60 to 65% drill is used the
be worked at least 5% higher engage- is hardly worth bothering about! (Four- but as it is there I have thought it threads will be well formed for strength -
ment t han those I suggest on page 65. TENTHS of a thou !) If the "words and give you the data.
and you will be "caulking " with soft
On the other hand, st ainless steel, m usic" call for a 70% No. OBA thread small point about the threads in solder or, better, brazing alloy to make
chromium alloy steels, and manganese (look at t he table) t hen t he use of a VIII. This includes the ISO Miniature the seal anyway.
bronze should be worked between 50% 5·1 m m or 5·2mm drill w ill not bring the threads to B.S. 4827 - t he'S'
and 55% only. I have found t hat monel world t o an end. Indeed, if the 5·1 mm is as well as the standard ISO range. Clearing drills
metal can be a bit tricky, and some old it may well drill 5·2mm anyway. But I t hat although S1.0 and S1.2 have I have heard criticism of some published
grades of Germa!l (nickel) silver can have given the size of drill in ALL the me pitch as t heir M1 .0 and M1 .2 drill tables to the effect that they do not
cause trouble. Both are strong materials tables which is the nearest available to Ients they are NOT interchange- give the "right" size of clearing drill for
and 5% less engagement will do no provide the depth of engagement shown. M-type nuts will not fitS-type bolts, each size of thread. This is a puzzler! A

68
69
number OBA thread is 6mm 0.0., so that even if they have been jig-drilled, other-
a drill 6·05mm dia. will give a clea ring wise you will spend a lot of time trying SECTION 9
hole. But so will a 7 mm- or even 10. It to get it (the cover) on.
all depends on what you want the clear-
ance FOR. Looking at published draw- Conclusion
ings over many y ears my own view is Tapping dril l sizes as used in industry
that designed clearance holes are usually are not appropriate to jobbing work,
too small for the application. Even in especially in small scale. Even with 50%
the old days I always used one, and
often enough two, sizes of drill larger
thread engagement the loss of shear
area is unimportant unless the actual
SCREWING DIES
than that called for on the drawing for number of threads engaged is small
such things as cylinder covers, valve- {less than four or five). Between 60%
chests and the like. However, the criticism and 65% is adequate for almost all
had to be met, but I have tackled it applications, and these proportions will
another way! In each table you will find reduce the risk of tap breakage very The earliest form of commercial tackle The Button Die. This is the universal
the O.D. of the thread given in milfi- considerably. On the other hand, in for making screws and male threads style these days, except for threading
m etres. This means that you can select small scale work the difference of 5% was the Screwplate, Fig. 64. At 'A' is the large diameter pipes. There are two
your clearing drill directly if you have t hread engagement is very small, so "Warri ngton" pattern used by clock- types. The SOLID die is shown in Fig.
metric drills and only need to refer to that it is very important that drills cut to makers, but that at 'B' is the more usual 66. For the smaller sizes, when the die is
Table Ill (page 87) if you are still stuck size. Stub drills, especially if ground pattern. The "Warrington" has a num- less than 25mm or 1 in. dia. there are
with number and letter drills. But keep with four-facet points (see page 34) are ber of gauge-holes for checking the size either three or four cutting edges, but
in mind the job that the hole has to do; far superior to jobbers for tapping work. of the wire before screwing in the for larger dies of 25mm (1 in.) O.D.
if t here is but a single bolt to fit, then Finally, if at all possible- and I u rge you match ing die-hole. That at ' B' carries upwards there may be five or more.
quite a small clearance - 0·05mm - to consider "doing without" something roughing and finishing die-holes. For These dies cannot be adjusted for wear,
should serve. But if you are goi ng to else to MAKE it possible - keep your small screws the blank would be driven and so long as they are sharp will cut to
have to wangle a cover over ten or a tapping drills for that purpose only, and through using the screwdriver slot or size. However, for jobbing and amateur
dozen studs allow quite a bit more, try not to use them for anything else. hexagon head, but for longer threads
the tool was rotated using the handle.
About the middle of the 19th century
the die-stock with interchangeable dies
was introduced ; Fig. 65 is from the
Britannia catalogue of 1890. The prin-
A
ciple is_easily followed from the draw-
ing; the two half-dies were set in the
recess and screwed up t ight - the
pinching screw is NOT for adjustment. 66e7,!7p &! t7 cJ CJ Cl I'
-;;;;;:;;;;;;;;:::;::aiQ
ccr .o eT d eJ 0 r.J' CJ CJ
(The apparent ovality is due to the way
were made - the actual threaded B
is circular.) By fitting a pair of vee-
r-nl!lpe~a dies the same tool could be
to hold taps. Even in those days Fig. 64 Illustration of 19th century
tools were expensive - a set screwplates taken from an early textbook.
10\olerma 1/a in. to 3/ain. Whitworth cost Upper, the " Warrington pattern, with gauge
holes for the screw-wire. Lower, typical
at a time when a fully equipped
"Engineer's Screwplate" for smaller sizes,
centre-height screwcutting lathe with roughing and finishing holes to
ld be had for £25! each size.

70
71
 Fig. 65 Set of
stocks and dies,
taken from a
Britannia Co's
catalogue of 1890.

Fig. 67 Modern split dies, ranging from %in. Fig. 68 The "Pratt & Whitney" type die,
up to 1'12in. O.D. normaffy fitted in a coffet holder.

the thread and off again, removing there is one fundamental difference. In
burrs and damaged threads. For this all but exceptional cases a die must cut
purpose they are very effective indeed. a thread full depth. There is some
use the SPLIT DIE is more common - nearly 50 years old or more and should But that is all. I don't wish to suggest extrusion effect as in the case of the tap,
though, as explained earlier, many users be treated with reserve, as they may that it is impossible to originate a and this means that there is consider-
are changing to the solid type. A range have had a great deal of use. However, thread on a b lank with them, but it is not able force tendi ng to expand the die.
of split dies is shown in Fig. 67, and it the small 5/sin. dia. dies are still made by any means easy. The cutting edges The result is that a split die will, even if
will be seen that the largest - 1% in. dia. and these can be fitted into collet are of different shape - usually with adjusted to dead size before use, tend
- has only four cutting edges. This is holders, as shown in Fig. 68. These quite a large negative rake - so that to cut large, and the more use the die
non-standard. collets will not fit the present standard they "shave" rather than cut. In addition has had the greater the effect will be.
Such split dies clearly need means of die-holders - they are 0·877in. O.D. it is almost impossible to get an even The die-holder of the normal pattern
adjustment. At one time they were - but a 13/,sin. holder can easily be torque with a single lever as provided just has not the strength to resist these
available with an adjusting screw within adapted. The collet is hardened and the by the spanner. They can be extremely forces. The problem is much less with
the die itself, but sheer cost has driven pinching screws lie below the circum- useful to those who restore ancient the solid die, but an eventual price has
them from the market; any found as fe;ence. A useful feature, seen in the machinery, and as they cost only half to be paid; once it has become at all
"surplus" or at sales are likely to be top left-hand view, is that the front of the price of a regular split or solid die blunt the torque necessary to turn the
the collet has a guide to direct the perhaps worth investing in for this die becomes excessive - in larger sizes
25mm workpiece axially to the threading teeth. type of activity. In fact, it is actual ly it ca n become unusable. Split dies in
In all other types of die it is necessary to to use a dienut rather than an the "Pratt & Whitney" type of collet, Fig.
16mm and 20mm DIA
45~
1---
45° have means of adjustment in the die- ry die, as there is less risk of 68, are very nearly as good as solid dies
holder and to adjust each time the die is ng the shape and fit of the bolt. but will cut large when worn, as the
used. We will come to this point later. owever, for general machine-shop use forces are resisted only by small screws
have little application, and the few in the wall of the collet.
Dienuts I have are used only on the rare The obvious way of mitigating this
These are intended for the restoration , ..,,,,.,,.·J• s when th'e moon is seen to be problem is to machi ne the workpiece
of damaged threads on bolts, and at slightly undersize to allow for the
first sight appear to be no more than a extrusion effect when this is possible,
solid die with a hexagon outline to fit a and I follow this practice when neces-
spanner. They are simply screwed onto cutting action of a die is similar to sary, especially when threading stain-
Fig. 66 The modern solid pattern button die. the bolt and worked along the length of for a tap, already discussed, but less steel valve-rods and the like. The

72 73
 for the small 5/s in. size. You can see that screw is at fau lt; if with one particular recesses be accurately coaxial w ith t he
exact amount to take off is indetermin-
there is a larger screw standing at right die, then scrap it, for it means that the mandrel bore. For years I was plagued
ate, but I find that between 5% and 10%
angles; this is to prevent the die from groove at the split is faulty. This trouble with a commercial unit with an error
of the t hread height works very well.
rotating, though it can also expand the cannot occur w ith a solid die - another here and poor threads were the norm. It
This may only be 0·002 to 0·004in. but it
die if the latter is slack in the holder. On of their advantages. My second illustra- was only a few thousandths of an inch,
does make a great d eal of difference.
eit her side, not quite at 45° to t his, is a tion, Fig . 70, shows the TAILSTOCK DIE- but it did matter! The best plan is to
More important is to keep dies sharp,
pair of small grub-screws intended to HOLDER, for use in the lathe. At the rough drill the bore, then machine out
and I deal with this matter on page 78.
close the die to its correct size. One of bottom is a taper shank mandrel which the die recesses leaving on a machining
Unlike the case of the tap, chips can
the holders is shown back-to-front so f its the tai lstock. Above is the double- allowance. You can bore out the centre
escape quite freely f rom a die and so
that you can see that there is a shoulder ended die-holder, for '3f,s and 1 in. size to a smooth fit on the mandrel (or,
long as the die is cutting cleanly it is
at the bottom of the recess. The die dies. There is a hole bored through to fit better, ream this hole and make the
possible to thread long rods in th e lathe
MUST sit flat on this otherwise it will the mandrel, and it is very important mandrel to suit) and one die recess at a
at quite high speeds with a suitab le
present to the workpiece askew and cut that this be a smooth slide fit- neither single setting. Then set the body on the
lubricant. However, when cutting bright-
a fau lty thread . tight nor sloppy. These may be bought, normal taper mandrel between centres
drawn mild steel, drawn bronze, copper
A similar fault can be caused by but are easy to make - if you need one and, wit h a ti ny boring tool, true up t he
or stainless steel it may be necessary t o
improper use of the centre, locating, then you must have a lat he! The machin- bore at the other end.
use the reverse rotation technique to
screw. If this is tightened down too hard ing is quite straightforward except for The st andard dimensions of dies are
break up the chips. The taper lead-in at
there is a risk of f riction at the point one point; it is VITAL t hat t he die as under:
the mouth of a die is fairly steep and
with the deeper threads - say above displacing one half of the die relative to
6mm or 1/a in.- and a "used" die some the other - only with a split die, of Thread O.D., Thickness, Tolerance
t ea ring may b e experienced with a course. I have had this happen to such Type in. in. on O.D.
continuous cut. The answer, again, is to an extent t hat the die simply stripped
sharpen the die and, perhaps, use a the workpiece down to the core dia- All BA * 13/16 Va + 0/- 0·005
heavier duty cutting oil. meter! True, it was a fine thread, so that
only a small displacement was needed, Whit. Form
Die-holders but I have traced malformed threads to 3/ 1s or less % Va +0/-0·005
Fig. 69 shows fou r of different sizes. this cause in a nu mber of cases. If it 1/15-Va 13/16 Va +0/-0·005
That at the top is for 1-5/,sin. dies, next happens regularly with one die-holder '/a-3/a 1 3fs + 0/ -0·005
f or 1 in., then 13/ ,sin. and at t he bottom then it is probable that t he point of the s;16-3fs 1-5/16 7
/16 +01- 0·006
3/a-% 1'12 V2 +01- 0·006
%-1 2 % + 0/-0·008

I. S. 0. Metric
e M1-M2·5
M3-M4
16mm
20mm
5mm
5mm
+ 0/-0·12mm
+ 0/- 0·12mm
M4-M6 20 mm 7m m +0/-0·12mm

-=-------
--.
M7-M9
M10-M11
25mm
30 mm
9mm
11mm
+0/-0·12mm
+ 0/-0·15mm

., M12-M14
M16-M20
M22-M25
38mm
45 mm
55mm
14mm
18mm
22mm
+0/-0·15mm
+ 0/-0·15mm
+01- 0·2mm

*BA dies below No.2 may be found % in. dia., and some makers supply
Fig. 69 A range of die-
holders, from 16/t6in. (top)
Nos. 0, 1 and 2 at 1 in. dia.
down to % in. dia.

75
74
There is some overlap in " Whitworth carry the wide stocks of lubricants found Fo r general tapping I have foun d that any f orm of cutting oil on very small
Form" (SSW, BSF, M.E. etc.) die dia- in industria l concerns. I have found t he the aerosol spray of Rocol "RT D" oil is taps is that the chips t end to clog easily
meters and if buyin g dies individually it following to be helpful: very useful, as it can be injected down in the flutes, and it is worth while
is prudent to specify t he diamet er. Dies A lu minium and Machine oil and the hole wit h little difficulty. The same having a soft brass wire brush with
sold in sets will use the overlap to alloys paraffin, 70/30. additive is available from some suppliers which to clean them after wit hdrawing
reduce the number of sizes to a mini- Cast brass, GM, Light machine o il or as a special tapping and die-cutting oil. the tap for chip clearance. The smaller
mum. I find that a tailstock die-holder bronze neat cutting oil. It is not cheap, but a little lasts a long taps, of course, should not be anointed
for •3/,sin. and 1 in. dies meets 85% of Ditto, forged, Neat cutting oil. t ime. The main problem with the use of with heavy oils.
my requirements. rolled or drawn
Phos. bronze, Cutting oil with
drawn Rocol additive.
Lubrication of Taps and Dies Copper Machine oil and
Cast iron needs no lubrication, as paraffin, 50/50.
the chips are powdery and any lub rica- Magnesium alloy Cutting oil 10%,
t ion will make an abrasive paste. The light machine
graphite in the iron will provide adequate oi l40%,
"slipperiness". A lmost all other mater- paraffin 50%
ials will benefit from the use of Steel, Mild Neat cutting oil;
a lubricant. Unfortunately "the authori- Stainless Cutti ng oil with
t ies" differ very widely in their recom- Rocol additive;
mendations! M ost, of course, are con- Alloy Neat cutting oil.
cerned with machine tapping, w hich is Nickel silver Neat cutting oil.
not ap plicable to our work - and in any Silver steel Neat cutting oil
case the jobbing workshop is unlikely to with Rocol
additive.

Fig. 70 A tailstock
die-holder for use in the
lathe. Double-ended for
two sizes of die.

76 77
 sary to follow with the oilstone and then piece of expanded polystyrene packing
SECTION 10 to use the die on a piece of mild steel material to fit the bottom. This can be
stock to remove burrs after grinding. had free (and with a sigh of relief) from
Incidentally, on really large taps you most electrical goods shops, usually in
can use the same method, but let the moulded shapes to fit the appliances,
rotation of the wheel be such that this but sheet material up to about 20 mm
tends to take the wheel away from the thick is used too. Cut it w ith a hot knife.
cutting edges. Then find a few pieces of steel rod
TAP AND DIE-SHARPENING You may well find that resharpened
taps and dies do not perform quite as
about t he same size as the taps and a
small "cheese" the same size as the die.
well as new ones, for you cannot expect Heat this gently and rest it (or t hem ) in
to preserve the carefully formed rake the appropriate places on the poly-
angle which the manufacturer has set styrene sheet and you will have a series
up. But you will reduce considerably the of recesses in w hich the tools can sit in
risk of tap breakage. a few seconds. It just needs a little
Taps can be sharpened both easily and shape, No. 53, (the " Machinist's stone" ) experiment to find the right tempera-
accurately on the Ouorn tool and cutter which has a t apered pointed END, and t ure, that is all. I use this system for
grinder and full instructions are given in t his can be used in the same way. The Storage of Taps and Dies endmills as well as for taps and dies.
the "Ouorn Handbook" . Those who main problem with tiny taps is to keep The days of the "little tin box" are over! I have already shown, in Fig. 37, page
have no such facilities can, however, the cutting face at a reasonable angle, OXO comes in cellophane bags and 37, my box of tapping drills, home-
tackle the job by hand. The results will and I must confess that my own experi- those who still smoke cigarettes have to made, but still reasonable in appear-
not be perfect, but even an imperfectly ence shows that after the second sharp- make do w ith thin cardboard. In a way, ance. Fig. 71 shows a home-made tap,
sharpened tap is better than a blunt ening the tap is best thrown away this is a good thing, for a jumb le oftaps, die and dri ll box made some years ag o
one. rather than embark on a third attempt. drills and dies in a ti n box is hardly to house a small set of I.S.O. screwing
There are two ways. I use a medium Note that with almost all taps it will Cf!lcu lated to preserve the cutting edges. tackle. The box itself came from a
coarse India oilstone- you may have to be the first three or four threads that B'ut if tin has gone, various forms of jumble sale and needed new hi nges.
remove a relatively large amount of will have worn, and these should be plastic boxes have taken their place,
metal, and a fine stone is no good for looked at closely with a glass first. You and, fortunately, they can be had very
that. On larger taps I use a circular stone may find that a few strokes of the cheaply from watch- and clock-makers
about 6mm (% in.) dia., and f or smaller stone on these alone will mend matters. sundriesmen in a wide variety of sizes,
ones a tapered round stone which goes Finally, DON'T attem pt to sharpen a tap and, moreover, so designed as to sit
down to a point. With these, and lash- off-hand on the bench grinder, nor t o neatly on a shelf. However, even a
ings of oil, t he cutting face in the flute is convert a broken seconds tap into a plastic box will not prevent the taps
stoned, keeping the stone well down plug tap on the same tool! Even a p lug from rubbing their edges against each
into the fl ute so that there is no risk of tap has a taper lead-in! other or t he point of the tapping drill.
bevelling the crest of the th reads. For Dies are rather easier; all t hat is needed There is an easy solution for small taps.
sm all taps - perhaps 6BA and below - is a round slipstone - again, not too fin e If the bottom of the box lid is lined with
even the pointed stone is too large, and - applied through the clearing holes corrugated card, well-dried and then
for these I use a wedge-sha ped stone round the centre threaded portion. But soaked in anti-rust oil (Shell Ensis 254 is
known as the "Knifeblade", (rather like for larger dies a small grinding wheel in my preference, but there are others)
a gouge-slip but w ith one edge sharp) a high-speed flexible shaft u nit can be then t he drill and the taps at least can be
India shape No. 28. This can be applied used. You must, however, keep careful separated. Set t he taps so that adjacent
to the leading edges in the tiny grooves. control of the attitude of the wheel to ones are head-to-tail as an added pre-
The tap is held in a strong pin-vice prevent it from riding up and bev elling caution.
w hich is, in turn, held in the bench vice t he crests. The nearer t he wheel is to An alternative, using larger boxes of Fig. 71 Home-made case for storing taps,
for these small ones. There is another the hole size the better. I find it neces- w ood or even cardboard, is to cut a dies, and tapping drills.

78 79
 Into this is fixed a thin block of lignum and the contents are prevented from
vitae, but any good timber w ill do - impact either by the use of corrugated APPENDIX IAI
but not mahogany, which, as men- cardboard or dividers of some sort. The
tioned, absorbs moisture and w ill rust very small ones (8BA down) are housed
everything that touches it. The holes for in small plastic watchmaker's boxes in
the dies w ere excavated with a ' o/,sin. these drawers, partly for safety but
woodworking auger-bit, with finger-grip mainly because it is easier to get hold of
extensions taken out with a gouge. The them . In which connection a final hint
slots for the drills and t aps were made
with a ball-ended slot drill run very fast
may not be am iss. Cheap p lastic tweez-
ers are avai lable from Woolworths and
DRILLING FORCES
(about 2400rpm) in the milling machine similar places - a godsend when trying
but could easily have been done with a to extract a 14BA tap from a smal l box!
small gouge if you have one. The cross- It is worth spending a fair amount of
slots at the ends of these tool slots are time considering the st orage of t h ese
deeper and enable me to get the tap or tools. It takes very little to chip the edg e
drill out simply by pressing down on its of a small tap, and these are just the It has already been remarked that the of mild steel of about 28tonf/sq.in. UTS,
end. The lid of the box is filled with oil- ones which are the most difficult to axia l forces involved in the drilling hot rolled and in what appeared to be
soaked foam material - polyurethane, sharpen and even more difficu lt to operation are considerably greater t han t he normalised state. At each test the
probably - which had been used to pack extract if the chip causes them to break those found when roughing-down in drill was brought down and allowed to
something that came through the post. in the hole. The manufacturers have the lathe. There are many published form a predrill with the corners of the
It took half an afternoon to make, and is taken a lot of care, and spent megabucks figures but without exception they refer lips about v,6in. below the surface. The
just as go od as the boxes supplied, at on q uality control to provide you with to drilling operations with power down- drill was then fed at a "comfortable"
considerable cost, with commercial sets first-class tools, and the model you are f eed and using "perfectly " formed drill rate by hand and th ree observations
of screwing tackle. going to make w ith them will be equally points. Some years ago the author had made on each size (in different holes) of
However, most of my taps and dies well cared-for. A pity not to spend a occasion to make some measurements the axial force registered on the scale. A
are kept in an old (war-time surplus) set little time on looking after the taps and for another purpose, using hand feed final test was made on the larger drills
of cellu loid drawers above t he bench, dies in between? and drills which, while in reasonable when "hard driving " -feeding as hard
condition, were in what might be called as possible. The mean of the results
" normal workshop state". None were was as under.
blunt, but for the purposes of the trial
no " new" drills were used. Drill dia., Speed Force
The drilling machine was a Progress in. rpm lbf
No. 1 pillar drill, of 4 in. capacity fitted
with a Jacobs No. 34 chuck. Drive was
Vs 2580 33
by vee-belt f rom a V2 h p motor - there 3/16 2580 49
was no back gear- and 5-speed pulleys
% 1325 56
provided 2580, 1325, 1040, 630 and
o/1 6 1325 73
340 rpm. The measuring device was a
3fs 1040 98
substantial set of "bathroom" scales
'/,s 630 114
with a maximum capacity of 2801bf -
'h 340 134
very solidly maae of cast iron and with
an iron "platform". The deflecti<:m rate Hard driving
was high- about 420 1bf/inch - and the o/s 630 155
very large dial could be read to intervals 7/16 340 167
of 11bf w ithout difficulty. V2 340 210
The test piece was a large (151b) slab

80 DTO-f' 81
The maximum force which could be quently given by the author that, when
exerted was off-scale - estimated at drilling in the lathe with the drill held APPENDIX C
3201bf. in the headstock chuck, with the work
These figures may be of use as a on the saddle, the saddle should be
guide for those interested in building pushed forwards using the tailstock
their own bench drilling machines. poppet and NOT by the use either of the
They also emphasise the advice fre- rack handwheel or the leadscrew.
TABLES

APPENDIX B I Inches to millimetres and reverse. X Dimensions and Tapping Drills for
II British Standards Institution Metric Model Engineer Threads to
" Preferred" drill sizes. PD.6507.
Ill M illimetre equivalents of Gauge XI Dimensions and Tapping Drills for
Number and Letter size drills. UNC and UNF (Unified) Threads.
DIMENSIONS OF BA TAP SHANKS IV Dimensions of Combination
Centre-drills.
XII Dimensions and Tapping Drills for
Small U.S. Fine and Coarse
v Dimensions and Tapping Drills for Threads.
BSW and BSF Threads. XIII Dimensions and Tapping Drills for
VI Dimensions and Tapping Drills British Standard Pipe Threads
for "Model Engineer" Threads. (BSP).
Several designs of tap holders and
VII Dimensions and Tapping Drills for XIV Dimensions and Tapping Drills for
guiding devices involve a collet or pin-
British Association (BA) Threads. SAE Sparking Plug Threads.
vice type of grip to the tap. The follow-
ing dimensions have been taken from a VIII Dimensions and Tapping Drills for XV Schedule of Threads which can be
I.S.O. metric standard Threads. tapped with the standard set of
fairly wide selection of BA taps, includ-
IX Dimensions and Tapping Drills for Metric Drills, 1 mm to 10mmx
ing British, European and American
I.S.O. Fine Pitch series Threads. 0·1mm.
makes.

BA No. Shank dia., Square A-F,

in. in. General Note on Tapping Drill At the head of each table is given a
Tables formula which can be used to deter-
0 0·238/241 0·185/188 Drill sizes in BOLD type are B.S.I. "Pre- mine the tapping drill needed to provide
1 0·206/210 0·164/166 ferred" sizes, but ALL sizes shown in ANY desired degree of thread engage-
2 0·192/196 0·145/149 the tables are available. Drills above ment, but see the heading notes in the
3 0·160/163 0·129/133 14mm have Morse taper shanks. case of I.S.O. threads. The depth of the
4 0·1411143 0·112/114 The column marked "G.P. Tap Drill" male (bolt) thread can be found by
5 to 12 inc. 0·125/128 ) 0·103/105 shows the gauge number, letter, or subtracting the core diameter from the
'
fractional inch size which will give top diameter and halving the result.
between 60% and 70% thread engage-
A few examples of US manufacture ment.
were found to have the uniform dimen- CLEARANCE DRILLS may be ascer- The section on "Tapping drills" on
sion of smaller sizes corresponding to tained by noting the column giving the pages 63 et seq., and especially the
the dimensions of the 4BA size rather thread 0.0. in millimetres and adding notes on page 65 should be read before
than the more common 5BA. the desired clearance. using the tables for the first few times.

82 83
00
.J:>

TABLE I
INCH/ MILLIMETRE CONVERSION

Fractional Inch to Decimal and to Millimetres

inches mm inches mm inches mm inches mm
1/64 ·01 563 0·3969 17/64 ·26563 3.¥64 . 51563 49/64
6·7469 13·0969 ·76563 19·4469
V32 ·03125 0·7937 % 2 ·28125 7·1437 11/32 ·53125 13·4937 25/3 2 ·78125 19·8437
3/6 4 ·04688 1-1906 19/64 ·29688 7·5406 3 o/64 ·54688 13·8906 5 V64 ·79688 20·2406
V16 ·06250 1·5875 o/16 ·31250 7·9375 9/16 ·56250 14·2875 13/16 ·81250 20·6375
·07813 1·9844 2 V64 ·32813 8·3344 37/64 ·57813 14·6844 53/64
o/6 4 ·82813 21·0344
·09375 2·3812 1 VJ2 ·34375 8·7312 ·59375 15·0812 ·84375
:Y32 'o/32 27/32 21·431 2
·10938 2·7781 2 :Y64 • 35938 9·1281 39/64 5 o/64
%2 ·60938 15·4781 ·85938 21·8281
'/a ·1250 3·1750 3/a ·3750 9·5250 % ·6250 15·875 'Ia ·8750 22·2250
9/6 4 ·14063 3·5719 2 o/6 4 ·39063 9·9219 • v 64 ·64063 16·2719 57/64 ·89063 22·6219
o/32 ·15625 3·9687 '3/32 ·40625 10·3187 21/32 ·65625 16·6687 29/32 ·90625 23·0187
11/ 64 ·17188 4·3656 27/64 ·42188 10·7156 4 :Y64 •67188 59/64
17·0656 ·92188 23·4156
:Y,6 ·18750 4·7625 'I•• ·43750 11-1 125 •v,. ·68750 17·4625 'o/•e ·93750 23·8125
13/e• ·20313 5·1594 2o/64 ·45313 11·5094 4 o/64 ·70313 17·8594 61/6•·95313 24·2094
7/32 ·21875 5·5562 15!J2 ·46875 11·9062 23!J2 ·71875 18·2562 31/32 ·96875 24·6062
1% 4 ·23438 5·9531 3 V64 ·48438 47/64
12·3031 ·73438 18·6531 s:y.. ·98438 25·0031
v. ·250 6·3499 % ·500 12·6999 :y. ·750 19·0497 1 1·000 25.400

Decimal inch to mm.

X 1/ 1000 xv,oo X 1/1o INCHES + 10" + 20"
0 254·0 508·0
·0254 ·254 2·54 1 25·4 279·4 533·4
·0508 ·508 5·08 2 50·8 304·8 558·8
·0762 ·762 7·62 3 76·2 330·2 584·2
·1016 1·018 10·16 4 101·6 355·6 609·6
·1270 1·270 12·70 5 127·0 381·0 635·0
·1524 1·524 15·24 6 152·4 406·4 660·4
·1778 1·778 17·78 7 177-8 431·8 685·8
·2032 2·032 20·52 8 203·2 457·2 71 1·2
·2286 2·286 22·86 9 ---
228·6 482·6 736·6

Millimetres to Inches
V1ooo mm V1o mm millimetre + 10 mm + 20 mm + 30 mm + 40 mm +50 mm +60 mm + 70 mm +80 mm +90 mm
0 ·39370 ·78740 1·1811 1·5748 1·9685 2·3622 2·7559 3·1496 3·5433
·00039" ·00394" 1 ·03937" ·44307 ·82667 1·2205 1·6142 2·0079 2·4416 2·7953 3·1890 3·5827
·00079 ·00787 2 ·07874 ·47244 ·86614 1·2598 1·6535 2·0473 2·4410 2·8347 3·2284 3·6621
·00118 ·01 181 3 ·1 1811 ·51 181 ·90551 1·2992 1·6929 2·0866 2·4803 2·8740 3·2677 3·6614
·00158 ·01575 4 ·15748 ·55118 ·94488 1·3386 1 ·7323 2·1260 2·5197 2·9134 3·3071 3·7008
·00197 ·01969 5 ·19685 ·59055 ·98425 1·3780 1·7717 2·1654 2·5591 2·9258 3·3465 3·7402
·00236 ·02362 6 ·23622 ·62992 1·0236 1·4173 1·8110 2·2047 2·5984 2·9921 3·3858 3·7795
·00276 ·02756 7 ·27559 ·66929 1·0630 1·4567 1·8504 2·2441 2·6378 3·0315 3·4252 3·8189
·0031 5 ·03150 8 ·31496 ·70866 1·1024 1·4961 1·8898 2·2835 2·6772 3·0709 3·4646 3·8583
·00354 ·03543 9 ·35433 ·74803 1-1417 1·5354 1 ·9291 2·3228 2·7165 3·1102 3·5039 3·8976

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..... - ~ ~-
...... Ill ~ "'-- <tl
" ' :::r :I -..
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, .,o ~ rn0 = ~ ..... rn6
z m ~ rn 0"' I • I ~ Ill tree ~ -g O'Q:c a~ ~~»
I»
~

- ~» .... ~ • · - · 111 a~ -· ..... C)

-1
m <t1 3 ~. ~ ~ I ~. ;t !;l - c Cb- Ill
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0::::1 c "'::::I<tl 3 -~~ a;- =
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0
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Q) <tl <tl
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-
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<tl ~ g
ii' -· -i :I Cb ~
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.._ - <tl - · . 0 0
r;;. ~~;ao3N
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•
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<tl
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a~ < m :I _:::r -
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-< .... o· a~ 3 3 3 ::;: 0 - ..:... l> < ct~:::r- Q) Q) -Q) c ~» o· ~ 111 .g a~ ~»
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3 ..... 3 .~:: 0.... 9- -o ~~
l>r-
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c S 111 :::r -· ro -·
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< =!::. Q) 0 ~ 0
ct~-..O;jj,...o,... - o3 - ~l> 0 0 ..., N
~ u, 3
0 !;; Ill
ro~$. ~ ~ $. m o -<co- 111 m
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10
Millimetre equivalent s of Gauge Number aiid Letter Size Drills
ALTERNATIVE ALTERNATIVE ALTERNATIVE ALTERNATIVE
Gauge SIZES p auge Decimal SIZES fGauge Decimal SIZES GaugE SIZES
Decimal Decimal
No. or No. or No. or No. o
~quivalen ~quivalen ~quivalen equivalen
Letter B.S.!. Exact Letter B.S./. E t Letter B.S./. Exact Letter B.S.!. Exact .
Rec. Rec. xac Rec. Rec. 1
.
in. mm mm in. mm mm in. mm mm in. mm mm I

80 0·013 5 0·35 0·34 50 0·070 0 1·80 1-78 20 0·161 0 4·10 4·09 K 0·281 0 %2in. 7.14
79 0·014 5 0·38 0·37 49 0·073 0 1·85 1·85 19 0·166 0 4·20 4·22 L 0·290 0 7·40 7·37
78 0·016 0 0·40 0·41 48 0·076 0 1·95 1·93 18 0·169 5 4·30 4·30 M 0·295 0 7·50 7·49
77 0·0 18 0 0·45 0·46 47 0·078 5 2·00 1·99 17 0·173 0 4·40 4·39 N 0·302 0 7·70 7-67
76 0·020 0 0·50 0·51 46 0·081 0 2·05 2·06 16 0·177 0 4·50 4·50 0 0·316 0 8·00 8·03
75 0·021 0 0·52 0·53 45 0·082 0 2·10 2·08 15 0·180 0 4·60 4·57 p 0·323 0 8·20 8·20
74 0·022 5 0·58 0·57 44 0·086 0 2·20 2·18 14 0·182 0 4·60 4·62 Q 0·332 0 8·40 8·43
73 0·024 0 0·60 0·61 43 0·089 0 2·25 2·26 13 0·185 0 4·70 4·70 R 0·339 0 8·60 8·61
72 0·025 0 0·65 0·64 42 0·093 5 3
/32in. 2·37 12 0·189 0 4·80 4·80 s 0·348 0 8·80 8·84
71 0·026 0 0·65 0·66 41 0·096 0 2.45 2·44 11 0·191 0 4·90 4·85 T 0·358 0 9·10 9·09
70 0·028 0 0·70 0·71 40 0·098 0 2·50 2·49 10 0·193 5 4·90 4·92 u 0·368 0 9·30 9·34
69 0·029 2 0·75 0·74 39 0·099 5 2·55 2·53 9 0·196 0 5·00 4·98 v 0·377 0 3/ein. 9·58
68 0·031 0 'f,, in 0·79 38 0·101 5 2·60 2·58 8 0·1 99 0 5·10 5·06 w 0·386 0 9·80 9·80
67 0·032 0 0·82 0·81 37 0·104 5 2·65 2·64 7 0·201 0 5·10 5·11 X 0·397 0 10·10 10·08
66 0·033 0 0·85 0·84 36 0·106 5 2·70 2·71 6 0·204 0 5·20 5·18 y 0·404 0 10·30 10·26
z 0·413 0 10·50 10·49
65 0·035 0 0·90 0·89 35 0·110 0 2·80 2·79 5 0·205 5 5·20 5·22
64 0·036 0 0·92 0·91 34 0·111 0 2·80 2·82 4 0·209 0 5·30 5·31
63 0·037 0 0·95 0·94 33 0·11 3 0 2·85 2·87 3 0·213 0 5·40 5·41
62 0·038 0 0·98 0·97 32 0·116 0 2·95 2·95 2 0·221 0 5·60 5·61
61 0·039 0 1·00 0·99 31 0·120 0 3·00 3·05 1 0·228 0 5·80 5·79
60 0·040 0 1·00 1·02 30 0·128 5 3·30 3·26 A 0·234 0 ' %..in. 5·94
59 0·041 0 1·05 1·04 29 0·136 0 3·50 3·45 B 0·238 0 6·00 6·04 NOTE
58 0·042 0 1·05 1·07 28 0·140 5 %..in. 3·57 c 0·242 0 6·10 6·15 " GAUGE" designation drills
57 0·043 0 1-10 1·09 27 0·144 0 3·70 3·66 D 0·246 0 6·20 6·25 are not longer made. In this
56 0·046 5 3/s..in. 1·18 26 0·147 0 3·70 3·73 E 0·250 0 '!..in. 6·35 table " BSI Rec" shows t he
British Standard " preferred
55 0·052 0 1·30 1·32 25 0·149 5 3·80 3·80 F 0·257 0 6·50 6·53 size" drill.
54 0·055 0 1·40 1·40 24 0·152 0 3·90 3·86 G 0·261 0 6·60 6·63
53 0·059 5 1·50 1-51 23 0·154 0 " Exact" is within 0·01 mm
CX) 3·90 3·91 H 0·266 0 ''/s..in. 6·75
-..J
52 0·063 5 1·60 1·61 22 0·157 0 4·00 3·99 I 0·272 0 6·90 6·90 (0·00039") of the old gauge
51 0·067 0 1-70 1·70 21 0·159 0 4·00 size. " Preferred" drills are
4·04 j
- -
0·277 0 7·00 7·03 "hPl'ln~>r
 TABLE IV COMBINATION CENTRE (SLOCOMBE) DRILLS TABLE V TAPPING DRILLS

Current Standard t ypes See Fig. Below British Standard Whitworth Thread
Tapping d rill d ia.= D - (1·28P x E/ 100) inche s
All Dimensions in mm

Body Guard Body Length Pilot Length Point Rad. Dia. TPI Core Core Tapping Drill Dia. - mm G.P. Thread
Pilot
Dia. D Dia. "d' (Max.) (Max. ) (Max.) (Min.) Ins dia. Area 50% 55% 60% 65% 70% 75% Tap O.D.
Dia. "d"
mm mm mm mm mm mm ins sq. ins drill mm
Ins. mm
(approx.) "A" "B" "C" "A" ''B" "C" 'A"/"B" "C"
v,e 60 0·041 0·0013 1·32 1·29 1 1·25 1·23 1·22 1-18 1·25mm 1·587
4·0 3·15 2·12 33 37 33 1·9 3·0 33·5 29·5 '¥32 48 0·067 0·0035 2·05 2·0 1·98 1·95 1.92 1·88 5164 2·381
¥64 1·0 3·15
6·3 4·0 3·35 37 47 37 2·8 4·25 37·5 33·5 Vs 40 0·093 0·0068 2·78 2.74 2·7 2·65 2·60 2·58 No. 37 3· 175
Vte 1·6 4·0
32 0·116 0·0106 3·45 3·4 3·35 3·30 3.25 No. 30 3·969
8·0 5·0 4·25 42 52 42 3·3 5·4 42 38 o/n 3-2
o/64 2·0 5·0
-%2 2·5 6.3 10·0 6·3 5·30 47 59 47 4·1 6·7 47 43 3
l•a 24 0·134 0·014 4·09 4·0 3·95 3·86 3·8 3·75 No. 24 4·763
11·2 8·0 5·70 52 63 52 4·9 8·5 52 48 20 0· 186 0·027 5·55 5·45 5·35 5·30 5·22 5·13 No. 4 6·35
Vs 3·15 8·0 'I•
%2 4 ·0 10·0 14·0 10·0 8·50 59 70 59 6·2 10·6 59 53 o/,e 18 0·241 0·046 7·05 6·95 6·85 6·75 6·65 6·6 H 7·938
17·0 74 68
v. 6·3 16·0 20·0 16·0 13·20 74 83 74 9·2 ¥a 16 0·295 0·068 8 ·5 8·4 8·3 8·2 8·1 8·0 p 9·525
'l•e 14 0·346 0·094 9·9 9·8 9·7 9·6 9·5 9·4 v 11 ·113
V> 12 0·393 0·121 11·3 11·2 11·1 11·0 10·8 10·6 'I•• 12·7
Old (1950) Standard 51a 11 0·509 0·203 14·4 14·25 14·1 13·9 13-8 13·7 35fs· 15·875
Type " A" , but point at 118• :y. 10 0·622 0·304 17·4 17·25 17·1 16·9 16·75 16·6 2 V32 19·05
All Dimensions in inches 'Ia 9 0·733 0·422 20·5 20·25 20·0 19·84 19·75 19·5 26132 22·225

Ill "
1 8 0·840 0·554 - 23·2 23·0 22·75 22·5 22·3 29132 25·4
No. " d" ''D" Ill"
Max. Max.

851 ¥64 Va 1'h •164 BRITISH STANDARD FINE THREAD (Whitworth form)
852 'I•• 3l•s 1% % 2
2 5132
BS3 3132 '/4 Ta p ping Drill Dia.= D - (1·28P x E/ 100) inches
854 'Ia •I•• 2V• 3l•e
BS5 :y,. 7116 2Vz •132 Dia. TPI Core Core Tapping Drill Dia. - mm G.P. Thre ad
856 v. Sis 3'Vn ' %2 Ins d ia. Area 50% 55% 60% 65% 70% 75% Tap O.D.
ins sq. ins drill mm

'¥16 32 0·148 0·017 4 ·25 4·2 4·15 4·1 4·05 4·0 No. 20 4·763
% 26 0·201 0·032 5·72 5·66 5·6 5·54 5·47 5·4 '132 6·35
1 lr
Sl•e 22 0·254 0·051 7·2 7· 13 7·05 6·98 6·9 6·83 I 7·938
I.i
lr--1-~--- :Ys 20 0·311 0·076 8·72 8·64 8·55 9·47 8·4 8·3 Q 9·525
'I• a 18 0·366 0·105 10·2 10·1 10·0 9·9 9·85 9·76 26164 11-113
v. 16 0·420 0·138 11·7 11·6 11·5 11·4 11·3 11 ·2 29164 12·7
IJr o/e 14 0·48;3 0·224 14·7 14·6 14·5 14·4 14·25 14·1 "I·· 15·875
The "Siocombe" or Combination % 12 0·643 0·325 17·7 17·6 17·4 17·3 17·1 17·0 • o/6 4 19·05
centre drill. (a) The normal type. (b) Type 11 0·759 0·452 20·75 20·6 20-4 20·3 20·2 20·0 22·225
'Ia 51164
producing a guard recess. (c) Precision 1 10 0·872 0·597 23·75 23·6 23·5 23·1
23·3 22·9 2%2 25·4
t ype producing a curved profile to the
centre-hole. The General Purpose (GP) Tapping drills give between 60 and 70% thread engagement in
t his table.

88 It DTO-C 89

lL
 TABLE VI TAPPING DRILLS TABLE VII TAPPING DRILLS

Model Engineer and Special Threads, Whitworth Form British A ssociation (BA) Threads
Tapping drill dia.= D - (1·28Px E/ 100) inches Tapping drill dia.=D- (1·2Px E/ 100) inches

Dia. Dia . Core Core Tapping Drill Dia. - mm G.P. Tap BA O.D. Pitch TPI Co re Core Tapping Drill Dia. - mm G.P. BA
Ins mm d ia . Area dri ll No. mm mm Approx. Dia. Area Drill No.
Ins Sq. ins. 60% 65% 70% 75% 80% 85% No./ins Exact mm mm 2 60% 65% 70% 75% No.

40 TPI Series Thread Height 0·016"=0-406mm

Va 3-175 0·093 0·0068 2·70 I 2·65 2·60 2·58 2·55 2·48 No. 37
0
1
6·0
5·3
1·0
0·9
25·4
28·25
4·8
4·22
18·1
14·0
5·3
4·65
5·22
4·6
5·16
4·55
J 5·1
4·50
No. 5
No. 14
0
1
5/a, 2 4·7 0·81 31·35 3·73 10·9 4·1 4·0 3·95 3·9 No. 22 2
3·97 0·1 25 0·0123 3·5 3·45 3·40 3·35 3·3 0 3·28 No. 29
3/, . 3 4·1 0·73 34·84 3·22 8·14 3·57 3·5 3·45 3·4 No. 29 3
4·763 0·155 0·0189 4·3 4·22 4·20 4·15 4·10 4·07 No. 19 4 3·6 0·66 38·46 2·81 6·20 3·15 3·1 3·05 3·0 No. 31 4
'/32 5·556 0·187 0·0275 5·10 5·05 5·0 4·95 4·90 4·86 No. 9
5 3·2 0·59 43·10 2·49 4·87 2·78 2·75 2·7 2·65 No.36 5
V4 6·35 0·218 0·0373 5·85 5·80 5·78 5·75 5·70 5·66 I 6 2·8 0·53 47·85 2·16 3·67 2·44 2·4 2·38 2·35 No. 41 6
9/32 7·144 0·249 0·0487 6·65 6·63 6·60 6·55 6·50 6·45 G 7 2·5 0·48 52·91 1·92 2·89 2·15 2·1 2·08 2·05 No. 45 7
o/16 7·938 0·282 0·063 7·45 7·4 7·37 7·35 7·30 7·25 L
8 2·2 0·43 59·17 1·68 2·22 1-89 1·86 1·84
3fa 9·525 0·343 0·0924 9·05 9·0 0·95 8·90 8·85 8·83 s 9 1·9 0·39 64·94 1·43 1·61 1·62 1·6 1·57
1·8
1·55
No. 49
No. 52
8
9
32 TPI Series Thread Height 0·020"=0·508mm 10 1·7 0·35 72·46 1·28 1·29 1·45 1·42 1-4 1·38 No. 54 10
V4 6·35 0·21 0 0·0346 5·75 I 5·70 5·65 5·60 5·55 5·50 No. 2 11 1·5 0·31 81·97 1·13 1·00 1·28 1·26 1·24 1·20 3/64'1 11
9/32 7·144 0·241 0·0456 6·55 6·50 6·45 6·40 6·35 6-30 F 12 1·3 0·28 90·91 0·96 0·724 1-10 1·08 1·06 1·05 No. 57 12
s;,6 7·038 0·272 0·0581 7·35 7-30 7·25 7·20 7·15 7·10 K 13 1·2 0·25 102·04 0·90 0·636 1·02 1·00 0·99 0·97 No. 60 13
3fa 9·525 0·335 0·088 8·90 8·85 8·80 8·75 8·70 8·65 s 14 1·0 0·23 109·9 0·72 0·407 0·83 0·82 0·81 0·79 No. 66 14
'/•6 11 ·11 0·398 0·124 10·5 - 10·4 - 10·3 10·25 z 15 0·9 0·21 120·5 0·65 0·332 0·75 0·74 0·72 0·71 No. 69 15
'h 12·7 0·460 0·166 12·1 - 12·0 - 11·9 11·8(88%) 15!J2
11
16 0·79 0·19 133·3 0·56 0·246 0·65 0·64 0·63 0·62 No. 72 16
17 0·70 0·17 149·4 0·50 0·196 0·58 0·57 0·56 0·55 No. 74 17
26 TPI Series Thread Height 0·0246"= 0·625mm (Brass Gas thread) 18 0·62 0·15 169·3 0·44 0·150 0·51 0·50 0·49 0·48 No. 76 18
3/16 4·7634 0-138 0·015 4·0 3·95 3·89 3·82 3·76 3·70 No. 23
% 6·35 0·201 0·032 5·60 5·54 5·47 5·41 5·35 5·29 '/32
•;,. 7·938 0-263 0-054 7·19 7· 12 7·06 7·0 6·94 6·87 K
% 9·525 0·326 0·083 8·77 8·71 8·65• 8·59 8·52 8·46 1V32
'/•s 1,., 1 0·388 0·12 10·36 10·30 10·23 10·1 7 10·11 10·05 y
V2 12·7 0·451 0· 16 11·95 11 ·89 11 ·82 11·76 11·70 11 ·64 15/n
5fs 15·88 0·576 0·26 15·13 15·06 15·0 14·94 14 ·88 14·82 19!J>
% 19·05 0·701 0·39 18 ·30 18·24 18 ·1 7 18·11 18·05 18·0 23/32

60 TPI Series Thread Height 0·0107"= 0·27 1mm

v,.
o/32
1·588
2·38
0·041
0·083
0·0013 1 1·26
0·0054 2·06
I- 1·23 1·21
2·0
1·18
1·97
1·15
1·96
N.R. 1·25mm
N. R. No. 47
1/a 3·175 0·114 0·0103 2·85 2·82 2·80 2·75 - 2·71 No. 34
%2 3·967 0·145 0·0165 3·65 3·60 - 3·57 3·55 3·50 No. 28
3/16 4·763 0·177 0·0246 4·45 4·40 4·39 4·35 - 4·30 No. 17

N.R.=Not Recommended at the% engagement unless exceptional circumstances demand it.

90 91
 TABLE VIII TAPPING DRILLS
TABLE IX TAPPING DRILLS
I.S.O. METRIC THREADS
'M ' Designation to B.S.3643 and 'S' Designation to B.S. 4827 I.S.O. M etric Threads - Fine Pitch Se ·
and heading of Table VIII. nes, To B.S.3643. See Notes on page 50
IMPORTANT: See notes on psge 50
Tapping drill dia. for "M" designation= D - {1·083PxE/100): For " S " designation=D - ISO f ine threads should be designated by the prefix 'M ' and th 't h, t h US
ePIC M 2·0 X2.5.
{0·096PxE/100) where E=% Flank height.
Size Pitch Core Core
Column headings of "%" based on the f lank contact. (] indicates % of nut thread depth, Tapping Drill Dia. - mm
mm mm Dia. Area ~pprox
and () is% of bo lt thread depth.
mm mm 2 BA
60% 65% 70% 75% 80% Equiv.
Size Pitch Core Core Tapping Drill Dia.- mm B.A. (63) {53) (68) {57) [73) {62) (77) {66)
[82)(70·5)
mm mm Dia. Area Equiv-
1·0 0·2 0·755 0·52 0·87
mm mm 2 60% 65% 70% 75% 80% 85% alent 0·86 0·85 0·84 0·83
1·1 0·2 0·855 0·65 14
[63] (53) [68] (57) [73] (62) [77] (66) [82](70·5) [86] (75) 0·97 0·96 0·95 0·94
1·2 0·2 0·955 0·81 0·93
1·07 1·06 1·05 1·04
1·4 0·2 1·03 13
s 0·5 0·125 0·369 0·107 0·43 0·42 - 0·41 0·40
1·6 0·2
1·155 1-16 1·27 1·26 1·25 1·24 1·23 11112
s 0·6 0·1 50 0·432 0·147 0·51 - 0·50 0-49 0·48 Not 18
1·8 0·2
1·355 1·57 1·47 1·46 145 1·44 1·43 10/11
s 0·7 0·175 0·504 0·199 0·6 0·59 0·58 0·57 0·56 Recom- 17 1·555 2·04 1·67 1·66 1-65 1·64 1·63 9/10
s 0·8 0·20 0·576 0·261 0·69 0·68 0·67 0·66 0·65 mended 16
2·0 0·25
s 1·0 0·25 0·720 0·407 0·85 0·84 0·83 0·82 0·81 14
2·2 0·25
1·69 1·84 1·84 1·83 1·81 1·80 1·78 9
s 1·2 0·25 0·920 0·664 1·04 1·04 1·03 1·02 1·01 13
2·5
1·89 3·03 2·05 - 2·0 +- 1-+ 2-() 1·98 8
3·0
0·35
0·35
2·07
2·57
3·71
5·61
-
2·78
2·26 2·25 - 2·20 7
M 1·0 0·25 0·694 0·378 0·84 0·82 0·81 0·80 0·78 14 2·75 +- f-> 2·75 2·71 2·70
3·5 0·35 3·07 7·90 5/6
M 1·2 0·25 0·894 0·627 1·04 1·02 1·01 0·99 0·98 Not 13 3·26 3·25 3·23 3·20 4
M 1·4 0·30 1·033 0·838 1·21 1·19 1·18 1·16 1-14 Recom- 11/12
4·0 0·5 3·39
M 1·6 0·35 1·171 1·007 1·37 1-35 1·33 1·32 1·30 mended 10/11
4·5 0·5 3·89
9·8 3·70 3·65 - 3·6 3·55 3
M 1·8 0·35 1·371 1·476 1·57 1·55 1·53 1·52 1·5 9/ 10
I 5·0 0·5 4·39
12·8
16·1
4·20
4·70
4·15
4·65
- 4·10 4·05 3/2
6·0 4·62 4-60 4·57 2/1
M 2·0 0·40 1·51 1·79 1·74 1·72 1-70 1·67 0·75 5·08 22·0 5·50
1-65 1·63 9 5·48 5·43 5·40 5·35
8·0 1·0 6·08 31 ·3 0
M 2·2 0·45 1·65 2·14 1·9 1·88 1·86 1·84 1·81 1·79 8 7·35 7-30 7·25 7·20 7·15
M 2·5 0·45 1·95 2·99 2·20 2·18 2·15 - 2·10 2·09 7
M 3·0 0·50 2·39 4·49 2·70 2·65 2·62 2·60 2·58 2.55 5/6 ~~~~~~e=~~=~t~~ " % " based on flank contact. (] indicates %of tap engagement and {) is % of
M 3·5 0·60 2·77 6·00 3·10 3·05<- f-.3·05 3·00 2·95<- f->2·95 4
~ indicates
. that drill shown lies midway between the two col umns.
M 4·0 0·70 3·14 7·74 3·55 3·50 3·45<-t->3·45 3·40 3·35 3 BOLD f 1gures are British Standard "Preferred sizes".
M 4·5 0·75 3·58 10·06 4·00 3·97 3·95 3·90 3·85 3·81 3/2
M 5·0 0·80 4·02 12·7 4·50 4·45 4·40 4·35 4·30 4·25 2/1
M 6·0 1·00 4·78 17·9 5·35 5·30 5·25 5·20 5·15 5·10 0
M 8·0 1·25 6·47 32·9 7-20 7·10 7·05 6·95 6·90 6·85

M 10 1·50 8·1 5 52·2 9·2 9·0 8·9 8·8 8·7 8·6

M 12 1·75 9·86 76·4 10·9 10·8 10·7 10·6 10·5 10·4
M 16 2·0 13·55 144·2 14·7 14·6 14·5 14·4 14·3 14·2
M 20 2·5 16·95 225·6 18·4 18·2 18·1 18·0 17·8 17·7
M 24 3·0 20·33 324·6 22·0 21·9 21 ·7 21·6 21·4 21 ·25

Not e: Drills over 12mm dia. are taken from the taper shank l ist, thou gh straight shank drills
may be available up to 20mm dia. .
~ indicates that drill shown lies midway between the two columns.
BOLD figures are British Standard "Preferred sizes".

92
93
 TABLE X TAPPING DRILLS TABLE XI TAPPING DRILLS
M et ric Const ant Pitch Threads for M odel Engineers, to BS.PD6507-1982 Unified Coarse (UNCI and Fine (UNF) Threads
IMPORTANT: See Notes on page 53 Tapping Drill Dia.= D - {1 ·082P x E/100) based o n flank engagem ent.
Tapping dri ll diameter, based on depth of flank engagement, = D - (1·083PxE/100) (With rounded root to nut thread the tap engagement will be about 3°/.0 greater)

Size Pitch Core Core Tapping Drill Dia. - mm U.N.C. Core Core T.P.I. Tapping Drill Dia. - mm
Dia. Dia. Thread
mm mm Dia. Area Area O.D.
mm mm 2 60% [631 65% [68) 70% [73} 75% [77] 80% [82] ins. ins. ins 2 55% 60% 65% 70% 75% mm

3·0 0·5 2·38 4·45 - 2·65 - 2·60 2·58 % 0·187 0·028 20 5·60 5·50 5·45 5·4 5·31*
5/16 6·35
4·0 0·5 3·39 9·02 - 3·65 - 3·60 3·57 0·243 0·046 18 7·1 7·00 sj-95 6·90 6·80 7·94
4·5 0·5 3·90 11 ·95 4·20 4·15 - 4·10 4·05 % 0·297 0·069 16 8·60 8·50 8·40 8·35 8·25
7/ 16 9·53
5·0 0·5 4·39 15·10 4·70 4·65 4·62 4·60 4·57 0·394 0·096 14 10·0 9·92 9·85 9·75 9·65 11·1 1
5·5 0·5 4·90 18·9 - 5·15 - 5·10 5·06 'h 0·404 0·128 13 11·5 11·4 11·3 11·25 11·11*
%
12·70
6·0 0·5 5·39 22·8 - 5·65 5·63 5·60 5·56 0·512 0·206 11 14·5 14·4 14·25 14·1 14·0
%
15·88
0·625 0·307 10 17·5 17·4 17·25 17·1 17·0 19·05
4·5 0·75 3·58 10·10 4·0 3·97 3·95 3·9 3·86 7Ja 0·737 0·427 9 20·5 20·4 20·25 20·1 19·9 22·23
6·0 0·75 5·08 20·2 5·50 5-48 5·43 5·40 5·35 1 0·847 0·563 8 23·5 23·3 23·2 23·0 22·8 24·40
7-0 0·75 6·08 29·0 6·53 6·50 6·45 6·40 6·35
8·0 0·75 7·08 39·4 - 7·5 7·45 7·40 7·35 U.N.F.
10·0 0·75 9·08 64·7 - 9·50 9·45 9·40 9·35 % 0·205 0·033 28 5·80 5·75 5·70 5·65 5·60 6·35
12·0 0·75 11·08 96·4 11 ·51 11·50 - 11·40 11·30** 5/16 0·260 0·053 24 7·30 7·25 7·20 7·15 7·10 7·94
10·0 1·0 8·77 60·4 9·35 9·30 9·25 9·20 9·15 % 0·323 0·082 24 8·90 8·85 8·75 8·73*
7/.s 8·65 9·53
12-0 1·0 10·77 91 ·1 11·35 11·30 11 ·25 11·20 11·1** 0·375 0·11 0 20 10·30 ....._. 10·3 10·25 10·20 10·08 11-11
14·0 1·0 12·77 128·0 13·4 13·3 13·25 13·20 13·1** % 0·438 0·151 20 11·90....._.11·90 11·80 11·75 11·60 12·70
16·0 1·0 14·77 171 15-4 15·3 15·25 15·2 15·1** % 0·556 0·243 18 15·08* 15·0 14·9 14·8 14·75 15·88
18·0 1·0 16·77 221 17·4 17·3 17-25 17·2 17·1** 3/4 0·672 0·355 16 18·1 18·0 17·9 17·86* 17·75
7/a 19·05
20·0 1-0 18·77 277 19·4 19·3 19·25 19·2 19·1** 0·876 0·485 14 21-1 21·0 21·0 20·9 20·75 22·23
1 0·896 0·631 12 - 24·0 +--+24·0 23·81 23·5 25·40
Figures in brackets [) indicate nut t hread %engagement - i.e. the depth "seen" by the tap.
**These drills w ill p rovide about 84% flank engagement. Note: * mdicates a Morse number or fractional Imperial equivale nt d rill.

94
95
 TABLE XII TAPPING DRILLS TABLE XIII TAPPING DRILLS

Small U.S. National Fine and Coarse (N.F. and N .C.) Threads British Standard Pipe Threads (Parallel) Whitworth form.
Tapping Drill Dia.taken as 0 - (1·3P x E/ 100). See page 50. Bolt thread height 0·614P. Tapping drill diamete r= D - (1·28Px E/100)

# Dia. T.P.I. Core Core Tapping Drill Dia. - mm Thread Nom. O.D. TPI Core Core apping Drill Dia. - mm or inc G.P. Thread
No. ins. Dia. Area O.D. Dia. ins Dia. Area Tap O.D.
ins. ins 2 55% 60% 65% 70% 75% mm ins ins2 70% 75% 80% 85% drill mm
0--80 0·060 80 0·0447 0·0016 1·29 1·27 1·25 1·23 1·21 1·524 Ys 0·383 28 0·337 0·089 8·9 8·85 8·80 8·75 s 9·73
1-64 0·073 64 0·0538 0·0023 1·57 1·55 1·52 1·49 1·47 1·854 0·518 19 0·451 0·160 12·0 11·9 11·8 11·7 'o/Jz" 13·16
1- 72 0·073 72 0·0560 0·0025 1·60 1·58 1·55<---->1·55 1·51 * 1·854 0·656 19 0·589 0·273 15·5 3 9/64" 15·4 15·3 Jo/64" 16·66
2-56 0·086 56 0·0641 0·0032 1·86 1·83 1·80 1·77 1·75 2·184 0·825 14 0·734 0·423 19·4 19·25 19·1 3/4'1 3/4" 20·96
2-64 0·086 64 0·0668 0·0035 1·90 1·88 1·85* 1·82 1·80 2·184 1·041 14 0·950 0·709 24·75 31f32" 24·5 24·3 3 YJz" 26·44
3-48 0·099 48 0·0734 0·0042 2·15 2·10 2·07 2·05 2·00 2·515
3-56 0·099 56 0·0771 0·0047 2·20 2·18 2·15 2·10 2·07 2·515
4-40 0·11 2 40 0·0813 0·0052 2·40 2·35 2·30 2·26* 2·20 2·845
4-48 0·112 48 0·0864 0·0059 2·45 2·44* 2·40 2·35 2·30 2·845

5-40 0·125 40 0·0943 0·007 2·70 2·65<----.:2·64* 2·60 2·55 3·175

5-44 0·125 44 0·0971 0·0074 2·75 2·71* 2·70 2·65 2·60 3·175
6-40 0·138 40 0·1073 0·009 3·05* 3·0 2·95* 2·90 2·85 3·505
8-32 0·164 32 0·1257 0·0124 3·60 3·55 3·50 3·45* 3·40 4·166
8-36 0·164 36 0·1299 0·0135 3·66* 3·60 3·57* 3·5o.---.3·50 4·166

1G-24 0·190 24 0·1389 0·0152 4 ·05 4·0 3·91* 3·86* 3·80 4·826
1G-32 0·190 32 0·1517 0·0181 4·25 4·20 4·15 4·10 4·05 4·826
12-24 0·216 24 0·1649 0·0214 4·70* 4·65 4·60 4·55 4·45 5·486
12-28 0·216 28 0·1772 0·0233 4·85* 4·76 4·70* 4·65 4·60 5·486

Drills marked * are an exact equivalent to Morse " Number" drills.

96 97
 TABLE XIV TABLE XV INDEX OF l APPING DRILLS

This table is "drill-based" and shows the threads and % thread engagement which is
Sparking Plug Threads - SAE Standard. 60° Thread Form. provided by the classical metric drill set of 1mm to 1Omm x 0·1 mm steps.
Tapping drill diameter= D - (1·25P x E/ 100) **NOTE** Unless otherwise stated threads in the "Model Engineer" (M.E.) column are all
40 tpi.
Dia. Pitch Core Core Tapping Drill Dia.- mm
mm mm Dia. Area
mm mm 2 60% 70% 80%

10 1·0 8·75 60·1 9·15 9·10 9·0

12 1·25 10·44 85·6 11-10 10·9 10·75
Drill BA * * M.E.** BSW BSF ISO UNC/ UNF
14 1·25 12·44 121·5 13· 10 12·9 12·75
18 1·5 15·75 194·8 16·8 16·7 16·5
,.,
1·0 13 65%
12 70%
M1 ·2 73%

1·2 11 75% Vur 60 72% v.e72% M1 ·4 63% 0-80 77%

1·3 11 55% V•a 54% M1 ·6 80%
1·4 10 70% 1--64 68%
1·5 M 1·8 80% 1- 72 76%

1·6 9 65% 1- 72 55%

1·7 M2·0 70%
2--64 75%
1·8 8 75% M2·2 83%
2·56 65%
1·9 8 58% 3/32 73% M2·2 62%
2·0 7 82% %2-60 70% 3 /n 55%

2·1 7 65% M2·5 80% 3-56 70%

2·2 M2·5 60% 4--40 75%
2·3 4--40 65%
2·4 6 65% 4--48 65%
2·5

2·6 5 80% '/a 70% '/a 70% M3·0 75% 5---40 70%
2·7 5 70% 1/a 60% 1/s 60% M3·0 60% 5-44 65%
2·8 5 59% Va--60 70%
2·9 Va--60 52% 6-40 70%
3·0 4 75% M3·5 75% 6-40 60%

3·1 4 65% M3·5 60%

3·2 75%
5/32

3·3 %z 65%
3·4 3 75% o/32 70% %2 55% M4·0 80% 8--32 75%
3·5 3 65% %2 60% M4·0 65% 8--32 65%

3·6 3 57% o/Jz--60 65% 8--36 60%

3·7
3·8 :Y,&-26 77% 3
/1a 70% 10-24 75%
3·9 2 75% ¥1&-26 70% ¥18 62% M4·5 75% 10-24 66%
4·0 2 65% ¥·6- 26 60% /15 75%
3 M4·5 60% 10-24 60%

98 99
 Drill BA *"M.E.** BSW BSF ISO UNC/U NF Drill BA **M.E. ** BSW BSF IS O UNC/UNF

4·1 3
/1e 80% / 16 65%
3 10-32 70% 8·1 % 70%
4·2 /16 70%
3 /16 55%
3 10-32 60% 8·2 % 65%
4·3 3/16 60% M5·0 80% 8·3 % 60% % 75% 3/aUNC 73%
4·4 o/u~--60 65% M5·0 71% 8·4 % 55% % 70% %U NC 65%
4·5 1 75% M5·0 60% 12- 24 73% 8·5 3/s 63%
3/aUNC 60%

4·6 165% 12- 24 65% 8·6 ¥or26 74% %57%

4·7 1 55% 12- 28 65% 8·7 3/or26 64% M10 80% 3/aUNF 72%
4·8 12- 28 57% 8·8 3/or32 70% M10 75% 3/aUNF 63%
4·9 7/3280% 8·9 3
/or40 75% M10 70% % UNF 55%
5·0 132 70%
7 9·0 3/or40 65% M10 65%

5·1 0 7 5% 7
/32 60% 9·1
5·2 0 67% v. 71% M6 75% 9·2
5·3 0 60% v. 65% M6 65% 9·3
5 -4 v. -26 75% %57% %75% 1/.UNC 70% 9·4 7
/16 75%
5·5 V•-26 67% v. 53% % 67% 1/.UNC 60% 9·5 /16 70%
7

5·6 1/.-32 75% % 60% 1/.UNF 75% 9·6 7/le 65%

5·7 %-32 65%
1/.-40 6 5%
v.52% 1/.UNF 65% 9·7 7/16 60% 7/1eUNC 73%
5·8 1/.UNF 55% 9·8 7/16 55% 7/1s 7 2% 7/1sUNC 68%
5·9 9·9 7
/1e 65% 7/1aUNC 62%
6·0 10·0 7/ 16 60% 7/,sUN F 80%

6· 1
6·2
6·3
6·4 %r32 75%
6·5 % r32 6 5%

6·6 %> 70% 5/16 7 5%

6·7 5/1e 67%
6·8 5/le 58% •!1eUNC 7 5%
6·9 5/16 58% 5/ 1670% M8 80% s/1oUNC 70%
7 ·0 / ur 26 75%
5 5/1s 50% 5/ls 60% M8 70% e/1sUNC 60%

7·1 5fur26 67% /16 52%

5 M865% o/1sUNF 75%
7 ·2 o/ur32 75% o/1sUNF 65%
7·3 o/1or32 65% o/1sUNF 55%
7·4 5/1or40 65%

7·5

7·6
7·7
7·8
7·9
8 ·0 3/s 75%

100 101
 "Long" drills 18 Spade drills 8,25
Lubrication of taps & dies 76 Spear-point drills 7
Speeds for drilling 21
Metric conversion table 84/5 Tables 22
Metric drill equivalents 87 Spiral-point tap 59
Metric (ISO) thread form 50,53 Split die 72
Metric threads for Model Engineers 53 Storage, drills 36
Model Engineers thread forms 47,53 taps&dies 79
Micro-dril ls, cutting speeds 22 Straight-flute drills 9,17
Strength of threads 65
Index Number drills 20
metric equivalents 21,87 Tap shapes and types 58
ALBRECHT chuck 39 Dienuts 73 Nut tap 58 Tailstock dieholder 75
Angles, drill point 11, 31 Dimensions of centre-drills 88 Tapping drills, discussion 63
for various materials 16, 18 dies 75 Parts ofthe twist drill 10 Tapping tool 61
taps 82 Partial thread engagement 63 Tapwrench 59
BA thread form 48 Drill lengths 20 effect on strength 65 Thread engagement 64
Brad-point drill 16 Drill thrust forces 81 Pipe-thread forms 47 Thread forms 45
Brass, drilling 17 Drilling vice 43 Point angles 11 ,14, 31 definitions 47
Brass thread form 53 for various materials 16 Torque when tapping 64
Enlarging holes 27 33 TAPPING DRILL TABLES, Notes 83
BSF thread form 47 18 "fou r-facet"
Extra-long drills 34 BA 91
Button die 71 Extrusion effects, tapping 63 nt thinning
sharpening fixture 32 BSF, BSW 89
" Foret" type drill 26 20 BSP 97
Centre drills 24, 88 " d ri II sizes
Flute angles 17 "Drill Index" 96
Chip formation, drills 13 Four-facet drill point 33
taps 54 17 ISO metric coarse 92
Chisel edge 9 54 34 ISO metric fine 93
HSS taps & dies, choice Model Engineer series 90
Chucks 38 Half-round drill 29
Arbor removal 40 31 12 Model Engineer Metric series 94
Hand-sharpening of d rills 9 Sparking plugs 98
drop 41 Helix angle 16
jaw wear 41 Small UNC/UNF 96
Clearance angle 15 20 71 Regular UNC/UNF 95
I.S.O. standards 31 "Unibit" drill 30
Clearance drills 69 I.S.O. metric screw thread form 50
Conduit thread form 47 78 Unified thread form 50
Conecut drill 30 32
Jacobs chuck 38 30 Vice - drilling 43
Constant pitch threads 53 repair 39
Core drill 27 33 24
Jigs for drill-sharpening
Cutting action, drills 13 18 88 Watchmaker's drill 27
Jobber's length drills 17 Wear, chuck jaw 41
dies 73 J ointing threads 69
taps 55 71 drill land 36
Keyless chucks 39 22 Whitworth thread form 47
D-b it 28
Definitions, Twist drills 10 Land (of drill) 10
Hand taps 57 Letter drills 20
Screw threads 47 metric equivalents 21
Dieholders 74 table of equivalents 87

103
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