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INTRODUCTION TO CNC
What is manufacturing process ?
A sequence of operations and processes designed to create a
specific product .
The process of converting raw materials into a product .
Examples:
welding, casting, cutting, assembling, etc.
History
The first NC machines were built in the 1940s and
1950s by Prof. John T Parson.
CNC machine came into existence after evolution of
computer around 1980.
Modern CNC Machine are improving further as the
technology is changing with a variety of functions
according to applications.
In the early 1950s the Massachusetts Institute of
Technology developed a more advanced vacuum tube
computer called Whirlwind.
Cont…
To ensure that all U.S. military airplanes were manufactured identically after world war II the
US Air Force invited several top companies to develop and manufacture numerical control
systems which can handle the quantity and repeatability of machine.
In 1952, The first three axis, Numerically controlled, tape-fed machine tool was created. A
Cincinnati Milacron Hydro-Tel Vertical Spindle Milling (VMC) machine was retrofitted and
controlled by the Whirlwind Computer. The controller was equipped with optical sensors and
used a straight binary perforated tape to hold the instructions; The tape was read via a
mechanical feeding mechanism. In 1954, Numerical control was announced to the public, and
after three year the first Production NC machine was delivered and Installed.
Numerical Control Machine(NC)
Form of programmable automation.
Mechanical actions of machine tool are
controlled by program.
The program is in form of alphanumeric
data.
After a job is finished the program of
instructions can be changed to process a
new job.
Numerical Control Machine(NC)
Advantages Disadvantages
High investment cost.
Reduces non productive time
High maintenance effort.
Reduces manufacturing lead time
Part programming.
Greater manufacturing flexibility
Higher utilization of NC Equipment's.
Improves quality control
Reduced inventory
CNC Machine
To overcome the shortcomings of the NC machines CNC machines evolved.
CNC machines are the NC machines whose MCU is based on the micro computer
rather than the hardwired controller.
Elements of
Machine Control
CNC Part program
Unit
Machine tool
machines are
Features of Storage of more
Improved
In-process programming Adaptive control
CNC than one part compensation and operating adjustments
machines are: program features
Block diagram of CNC Machine
Elements of CNC machines
Elements of CNC machine tool essentially consists of the following parts:
Part Program
Program Input Device
Machine Control Unit (MCU)
Drive System
Machine Tool
Feedback System.
Part Program
A part program is a series of coded instructions required to
produce a part.
It controls the movement of the machine tool and on/off control
of auxilary functions such as spindle rotation and coolant
The coded instructions are composed of letters, numbers and
symbols.
E.g:
N10 G01 X5.0 Y2.5 F15.0
| | | | |
| | | | Feed rate (15 in/min)
| | | Y-coordinate (2.5")
| | X-coordinate (5.0")
| Linear interpolation mode
Sequence number
Program Input Device
The program input device is the means for
part program to be entered into the CNC
control.
Three commonly used program input
devices are punch tape reader, magnetic
tape reader, and computer or USB
communication.
Machine Control Unit (MCU)
The machine control unit (MCU) is the heart of a CNC system. It is used to perform the
following functions
a) To read the coded instructions.
b) To decode the coded instructions.
c) To implement interpolations (linear, circular, and
helical) to generate axis motion commands.
d) To feed the axis motion commands to the amplifier
circuits for driving the axis mechanisms.
e) To receive the feedback signals of position and
speed for each drive axis.
f) To implement auxiliary control functions such as
coolant or spindle on/off and tool change.
Drive System
A drive system consists of amplifier circuits,
drive motors, and ball lead-screws.
The MCU feeds the control signals (position and
speed) of each axis to the amplifier circuits.
The control signals are augmented to actuate
drive motors which in turn rotate the ball lead-
screws to position the machine table.
Feedback System
The feedback system is also referred to as the
measuring system.
It uses position and speed transducers to
continuously monitor the position at which
the cutting tool is located at any particular
instant.
The MCU uses the difference between
reference signals and feedback signals to
generate the control signals for correcting
position and speed errors.
Machine Axis Designation
Machine axes are designated according to the "right-
hand rule“.
When the thumb of right hand points in the direction
of the positive X axis, the index finger points toward
the positive Y axis, and the middle finger toward the
positive Z axis. The main axes to be designated are
the linear axes and the rotary axes.
First axis to be identified is the Z-axis. This is then followed by
the X and Y axes respectively.
Offline programming linked to CAD programs.
MDI ~ Manual Data Input.
Manual Control using jog buttons or `electronic
handwheel'.
Word-Address Coding using standard G-codes and
M-codes.
The position of the tool is described by
using a Cartesian coordinate system. If
(0,0,0) position can be described by the
operator, then it is called floating zero.
In defining the motion of the tool from
one point to another,
either absolute positioning mode or
incremental positioning mode
can be used.
In this mode, the desired
target position of the tool for a particular move is
given relative to the origin point of the program.
In this mode, the next
target position for the tool is given relative to the
current tool position.
INFORMATION NEEDED by a CNC
1. Preparatory Information: units, incremental or absolute positioning
2. Coordinates: X,Y,Z, RX,RY,RZ
3. Machining Parameters: Feed rate and spindle speed
4. Coolant Control: On/Off, Flood, Mist
5. Tool Control: Tool and tool parameters
6. Cycle Functions: Type of action required
7. Miscellaneous Control: Spindle on/off, direction of rotation, stops for part movement
This information is conveyed to the machine through a set
of instructions arranged in a desired sequence – Program.
WORD-ADDRESS CODING
Example CNC Program
N5 G90 G20 Each instruction to the machine
N10 M06 T3 consists of a letter followed by a
N15 M03 S1250
N20 G00 X1 Y1 number.
N25 Z0.1
N30 G01 Z-0.125 F5
N35 X3 Y2 F10 Each letter is associated with a
N40 G00 Z1 specific type of action or piece of
N45 X0 Y0
N50 M05 information needed by the machine.
N55 M30
G Codes
G00 Rapid traverse G40 Cutter compensation –
G01 Linear interpolation cancel
G02 Circular interpolation,CW G41 Cutter compensation – left
G03 Circular interpolation, CCW G42 Cutter compensation-
G04 Dwell right
G08 Acceleration G70 Inch format
G09 Deceleration G71 Metric format
G17 X-Y Plane G74 Full-circle programming off
G18 Z-X Plane G75 Full-circle programming on
G19 Y-Z Plane G80 Fixed-cycle cancel
G20 Inch Units (G70) G81-G89 Fixed cycles
G21 Metric Units (G71) G90 Absolute dimensions
G91 Incremental dimensions
M Codes
M00 Program stop
M01 Optional program stop
M02 Program end
M03 Spindle on clockwise
M04 Spindle on counterclockwise
M05 Spindle stop
M06 Tool change
M08 Coolant on
M09 Coolant off
M10 Clamps on
M11 Clamps off
M30 Program stop, reset to start
Common types of CNC machines
CNC Lathe machine
CNC Milling machine
CNC Drilling machine
CNC Grinding machine
CNC Laser cutting machine
Water jet cutting machine
Electro discharge machine
DIFFERENCE BETWEEN NC, CNC AND DNC
1. Numerical Control Machine
The program is fed to the machine through magnetic tapes or other such media.
The original NC machines were essentially basic machine tools which were modified to have motors for
movement along the axes.
An NC machine is numerically controlled but has no memory storage and is run off of the "tape" each
time the machine cycles
2. Computer Numerical Controlled machine
The machines are interfaced with computers.
This makes them more versatile in the sense that, suppose a change in dimension of a part is required.
A CNC machine has memory storage and the program can be stored in its control.
3. DNC
Number of machines are controlled by a central computer.
Applications
Parts needed in a hurry
Parts with complicated contours
Parts requiring expensive jigs and fixtures
Parts those have several engineering changes
Cases where human errors can be extremely costly
Parts requiring close tolerance or good repeatability
ADVANTAGES OF CNC MACHINE
Productivity
Machine utilization is increased because more time is spent cutting and less time
is taken by positioning. Reduced setup time increases utilization too.
PROFIT increases as COST decreases and as PRODUCTIVITY increases.
PRODUCTIVITY through AUTOMATION
helping the workers to
perform their tasks more
efficiently
AUTOMATION
Transfer of the skill of the
operator to the machine
Cont…
Quality Higher accuracy
1. Parts are more accurate.
2. Parts are more repeatable. Reduce lead time
3. Less waste due to scrap. Higher flexibility
Reduce scrap rate
Machining Complex shapes
1. Slide movements under computer control. Reliable operation
2. Computer controller can calculate steps. Consistent quality
Reduced manpower
Management Control
1. CNC leads to CAD Increased productivity
2. Process planning Reduced non productive time
3. Production planning
Increased dimension precision of the work part through high basic precision of the
machine tool (1/1000mm).
DISADVANTAGE OF CNC MACHINE
1. Machine Cost is Higher than Conventional Machine.
2. Higher Maintenance Costs .
3. Higher Labor Costs .
4. Higher Machine Attachment Costs.
5. Higher Tooling Costs.
6. High maintenance requirements
"A robot is an automatic device that performs functions normally
ascribed to humans or a machine in the form of a human."
OR
"A robot is a software-controllable mechanical device that uses
sensors to guide one or more end-effectors through programmed
motions in a workpiece in order to manipulate physical objects.
OR
"An industrial robot is a reprogrammable multifunctional
manipulator designed to move material, parts, tools or specialized
devices through various programmed motions for the performance
of a variety of tasks"
The functions of a robot can be classified into thre areas :
• Sensing the environment by external sensors.
Example : Vision, voice, touch, proximity and so on.
• Decision making" based on the information received
from the sensors.
• "Performing" the task decided.
Difference between a Robot and an Automated Machine
• In automation,the machine produces a job following a set of operational
sequence, while a robot can be made to do different jobs at different
times and in different sequences.This can be achieved by programming.
A robot can be programmed to change the sequence of task while a fixed
machine set to perform certain tasks in sequence can not be
programmed.An automated machine does not have sensory feedback to
reprogramme the predetermined path.
Objectives of using Industrial Robots
• To reduce production time.
• To minimise the labour requirement.
• To raise the quality level of products.
• To increase productivity.
• To enhance the life of production machines.
• To minimise the loss of man-hours on account of accidents and diseases
The various components of a robots are enumerated and discussed below:
1. Base.
2. Manipulator arm.
3 End-effector.
4. Actuators and transmissions.
5. Controller.
6. Sensors.
Cont....
Base:
The base may be fixed or mobile
Manipulator arm:
The most obvious mechanical configuration of the robot is the manipulator arm.A robotic manipulator arm consists
of several separate links making a chain. The arm is located relative to the ground on either a fixed base or a movable
base. It has a free-end where an end-deflector or gripper or some times a specialised tool holder (for holding, say, a
welding gun) or any powered device (say, a drill) is attached.
End-effector:
Robot end-effector is the gripper or end of arm tooling mounted on the wrist of the robot manipulator arm.
The wide range of gripping methods include :
(i) Mechanical clamping.
(ii) Magnetic gripping
(tii) Vacuum (suction) gripping
Actuators and Transmissions:
Actuators :
The robot arm can be put to a desired motion with its payload if actuator modules are fitted in to provide power
drives to the systems.i.e. Numatic drive,hydraulic drive,Electric drives.
Transmissions:
"Transmissions" are elements between the actuators and the joints of the mechanical linkage
Cont...
Controller:
The "controller" provides the intelligence that is necessary to control the manipulator system.
Sensor:
The sensors perform the following functions :
• To act as feedback devices to direct further actions of the manipulator arm and the end effector (gripper), and
• To interact with the robot's working environment.
• Two types of sensor
Tactile sensor (Contact sensor)
Force sensor,Torque sensor,Touch sensor,position sensor
Non-tactile sensor(contactless sensor)
Proximity sensor, electro-optical sensor,Range imaging sensor
• The six basic motions or degrees of freedom (DOFs) are as follows:
• Vertical motion: The entire manipulator arm can be moved up and down vertically
either by means of the shoulder swivel, i.e., turning it about a horizontal axis, or by
sliding it in a vertical slide.
• Radial motion: Radial movement, i.e., in and out movemernts, to the manipulator
arm is provided by Elbow extension by extending it and drawing back
• Rotational motion: Clock wise or anticlock wise rotation about vertical axis of
manupulator arm.
• Pitch motion: Up and down movement of wrist
• Roll motion: Enables of rotation of wrist
• Yaw: Enables rightward and leftward swiveling movement of the wrist
The majority of commercially available robots can be
grouped into four basic configurations :
(i) Cartesian coordinate configuration.
(ii) Cylindrical configuration.
(ii) Spherical configuration.
(iv) Jointed-arm configuration (Revolute)
Flexible Manufacturing Systems (FMS)
• An FMS is a “reprogrammable” manufacturing system capable of
producing a variety of products automatically. Conventional
manufacturing systems have been marked by one of two distinct
features:
• The capability of producing a variety of different product types, but
at a high cost (e.g., job shops).
• The capability of producing large volumes of a product at a lower
cost, but very inflexible in terms of the product types which can be
produced (e.g., transfer lines).
• An FMS is designed to provide both of these features.
FMS Components
• Numerical Control (NC) machine tools
• Automated material handling system (AMHS)
• Automated guided vehicles (AGV)
• Conveyors
• Automated storage and retrieval systems (AS/RS)
• Industrial Robots
• Control Software
Flexible Manufacturing System
Computer
control
room
Tools
Conveyor
Machine Machine
Pallet
Load Unload
Terminal Finished
Parts
goods