GD & T

Is a method (systems of symbols) for defining a parts geometry

it goes beyond the form description based simply on tolerance dimensions as directed under rule #1

A basic dimension is a theoretical perfect size, location or orientation dimension.

2h

12
Varies Basic Dimension

These symbols are used when dimensioning.

2h h Counterbore 0.5h h Dimension Origin 1.5h 2h 30 Conical Taper 0.3h Square 15
h

h 90 h 0.6h Countersink h 60 Depth (or deep)

0.5h

h
h

( )
2h

1.5h

0.3 h Arc Length

Places or By

Radius

(
12
Diameter

Reference

Slope 2.5h 0.8h

ST

1.5h

Varies

Basic Dimension

60 Statistical Tolerance (dimensional)

Spherical Diameter

Spherical Radius

SR

Controlled Radius

CR

These symbols are used when dimensioning.

h 2h 90 h h 0.6h Counterbore 0.5h 0.5h Countersink 60 Depth (or deep) h h

h Dimension Origin
h = height of lettering

2h

30
Conical Taper Square

These symbols are used when dimensioning

1.5h 1.6 Arc Length 2h

.3h

15 1.5h

( )

(
X
Places or By

Reference 2.5h

Slope

12
Varies

0.8h

ST
60

1.5h

Basic Dimension

Statistical Tolerance (dimensional)

R
Radius

Diameter

SR
Spherical Radius

S
Spherical Diameter

CR
Controlled Radius

A feature control frame (the backbone of GD & T) modifies a parts geometry.

Geometric tolerance(characteristic) symbol (position)

Diameter symbol Numeric tolerance (feature tolerance)

Modifier

0.2

A B

2h

Tertiary datum Secondary datum with modifier Primary datum

Two additional examples of feature control frames modifying a parts geometry.

Numeric tolerance Diameter symbol Geometric tolerance (perpendicularity) 0.08 Numeric tolerance Geometric tolerance (flatness) (A) 0.05 A

Modifier Datum
(B)

Geometric characteristic(tolerance) symbols are categorized in two ways.

Geometric Characteristic Symbols
Symbol Description Straightness Form Flatness Type of tolerance

Circularity
Cylindricity Profile line Profile surface Angularity Perpendicularity Parallelism Position Concentricity Symmetry

Individual features Individual or related features

No datum or datums needed

Profile

Depending on the situation

Orientation

* *

Runout Total

*Either filled or unfilled

Runout

Runout Circular

Location

Related features

A datum or datums are required

GD&T geometric characteristic symbols illustrated.

0.6h 2h 1.5h Parallelism h Flatness

1.5h

Straightness

1.5h

1.5h

1.5h

0.8h

Concentricity

Circularity

Modifier

GD&T geometric characteristic symbols illustrated. 2h 1.2h

h 0.5h Symmetry 60 1.5h 1.5h h

Cylindricity 2h

Position 2h h

h Profile surface

All round

Profile line

GD&T geometric characteristic symbols illustrated.

3h 1.5h 2h Perpendicularity 30 1.5h Angularity 0.8h

* 0.6h
Between

0.8h

45 1.1h

1.5h

0.6h Runout Circular

Runout Total

* Filled or unfilled

Table lists GD&T geometric characteristic symbol modifiers.

Modifiers
M L

Maximum Material Condition Least Material Condition Projected Tolerance Zone Free State Variation Tangent Plane Statistical Tolerance (geometric) Between

P
F T ST

* *Filled or unfilled

Modifiers
Maximum material condition, MMC (M) Least material condition, LMC (L)

It should be taken literally.

It should be taken literally.

The geometric feature or size is as large as it can be. In the case of a hole, as small as it can be.

The geometric feature or size is as small as it can be. In the case of a hole, as large as it can be.

Rule #1:
Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent of which variations in its geometric form, as well as size, are allowed.
ANSI Y14.5 1994

When only size tolerance is specified the objects form can vary within the stated size limits.
12.2 12.2

12.2 11.8

12.2 11.8 11.8

(B)

11.8 12.2

(A)
at 12.2 MMC must be perfect form
at 11.8 MMC must be perfect form

12.2

(C)

11.8 11.8 for entire length External dowel plug

11.8 11.8 for entire length Internal hole

A cylinder can have a variety of shapes yet stay within the limits of size.

Max

Max

Max

Min

Min

Min

(A)

(B)

(C)

The rectangular prism can vary in shape as long as it stays inside the volume of the limits of size. 22 20
18 16 16 14 9 7 8 (A)

11 9

(B)

Checking the size limits envelope: A ring gage and plug gage are used to check the geometric form of a pin and hole.
8.0 7.8 16.0 15.8 Ring gage 8.0 MMC (A) Checking geometric form with ring gage 7.0 6.8 7.0 LMC 7.8 LMC along entire length of dowel

Plug gage

6.8 MMC

(B) Checking geometric form with plug gage

Standard Stock Item

Items whose geometry are already controlled by established industrial or government standards
bars, sheet stock, tubing or structural shapes

Datums
A datum is an exact surface, line, point ,axis or cylinder from which measurements are taken.
Ex. A surface plate or a polished slab of granite (simulated datum). 3 points define a plane. Datum feature is the surface of the part in contact with the simulated datum.

A height gage measures the height of an object from the simulated datum surface of a surface plate.
Dial face Probe

Up

Down Measured height Simulated datum (surface plate)

Datum feature Note: the datum feature rests on the simulated datum. the height is measured from the simulated datum and not from the datum feature.

Degrees of freedom (12) allow movement in two directions along each axis and rotation about each axis (clockwise and counterclockwise).

+Z

-X -Y

+Y

+X
In order to measure geometric features part motion must be restricted.

-Z

A datum reference frame consists of three intersecting planes at 90 to each other.

The part can still move in the positive X, Y and Z directions part motion must be restricted What if we temporarily clamped the part? Measurements can be taken from the simulated datums.

Datum surfaces must be indicated on the drawing

Varies H 2H H

60

2H for single lettering 4H for double lettering

Filled or unfilled

(A) H = height of lettering

E F 16 G

(B) Applications

The datum symbol is applied to solid cylinders.

14
-or- 14

The datum symbol is applied to holes.

12 0.1
M

G H

N M 6 P 6 12

Note: letters I, O and Q are not used to indicate datums because they may be confused with numbers one(1) and zero(0). Double letters can be used e.g. AA, BB etc.

Order of Datums: Primary datum S has 3 pts of contact, secondary datum T has 2 pts and tertiary datum U 1pt of contact.
2X 6 0.2 0.1 M S T U

(A)This drawing symbology

U 6 10 12
1 pt of contact 3 pts of contact

T S

(B) Means this

2 pts of contact

The order of the datums is critical!

2X 6 0.08 0.1 M V W X

X (1 pt)

V (3 pts) W (2 pts)

X (1 pt)
True height True width

V (3 pts)

X (2 pts)
False width

False height

V (3 pts)

W (2 pts)

W (1 pt)

Correct inspection procedure

Incorrect inspection procedure

The datum axis is formed by two intersecting planes.

8 8

4X 40.5 0.5 G H

Datum axis Intersecting planes perpendicular to G Primary datum G

24
H 8 G (A) This drawing (B) Means this

The plug gage establishes the datum axis.

C 10.80.1

(A) This drawing

Simulated datum cylinder C - largest that fits into hole

Datum axis Plug gage

(B) Means this

Datum feature C

The smallest circumscribed cylinder establishes the simulated datum and the datum axis.
D

(A) This drawing

190.5

Simulated datum D Datum feature D Datum axis

(B) Means this

The simulated central datum plane is established by the center plane of the largest block that fits into the groove.
12.2 11.8 0.4
Z Y Z

A) This drawing
Y

24.2 23.8

B) Means this

Simulated datums at maximum separation Central datum plane Datum features

The simulated central datum plane is established by the center plane located by the two blocks at minimum separation.
18.2 17.8 0.6 B A B

A) This drawing

B) Means this

Simulated datum planes at minimum separation Central datum plane