1. Unit 5 - CNC MACHINE TOOLS AND PART
G.Ravisankar, Asst Prof , Mechanical, Sri
Eshwar college of Engineering , Coimbatore .
2. UNIT V - CNC MACHINE TOOLS AND PART PROGRAMMING
Numerical Control (NC) machine tools – CNC types,
constructional details, special features, machining
centre, part programming fundamentals CNC – manual
part programming – micromachining – wafer
1. Hajra Choudhury, "Elements of Workshop Technology", Vol.II., Media
2. Rao. P.N “Manufacturing Technology - Metal Cutting and Machine Tools",
Tata McGraw-Hill, New Delhi, 2003.
• CNC: Computer Numerical Control
• Production of machined parts whose production is
controlled by a computer.
• Computer uses a controller to drive each axis of
the machine tool. (X,Y,Z)
• Controls direction, speed, and length of time
• A programmed path is loaded into the computer
and then executed.
• Program consists of numeric point data (X,Y,Z),
along with machine control and function
• Numerical Control (NC) & Computer Numerical
Control (CNC) mean the same.
• A major manufacturing development in past 60
• Resulted in:
▫ new manufacturing techniques
▫ higher production levels
▫ higher quality
▫ stabilization of manufacturing costs
7. Evolution of CNC
• Single items produced by crafts people
• Interchangeable Parts
▫ Eli Whitney (Cotton Gin)
▫ Manual labor was still the most cost effective
• WW II manufacturers could not maintain quantity
& quality parts.
8. Evolution of CNC
• Machinists could produce superior quality but not
at high volume that was required.
• As quantity increased, quality decreased due to
9. Evolution of CNC
• ENIAC – developed by the United States Army
Ballistic Research Lab & University of Pennsylvania
• First digital computer.
• Vacuum tube technology. (30,000)
• Used to calculate artillery tables.
• Programming involved setting hundreds of
switches and cables.
12. CNC & WW II
• Need to manufacture large amount of products for
• Need for quantity and quality.
• U.S. Air Force set up companies to develop and
produce NC systems to handle volume and
• Repeatability: the ability to perform the same
operation over & over within specified
13. Specific Goals
• Increase production
• Improve quality & accuracy of machined parts.
• Stabilize manufacturing costs.
• Speed up production & assembly operations.
14. NC Timeline
• 1949 - First contract awarded for NC machine.
• 1951 - servo system for machines developed.
• 1952 - tape-fed machine was created.
• Development of G codes - Punch tape input
(Cartesian Coordinate System)
• 1970’s Development of computer chips
▫ Cheaper processing power
▫ Smaller computers
▫ More reliable
• NC - A method of accurately controlling the
operation of a machine tool by a series of coded
instructions, consisting of numbers, letters of
the alphabet, and symbols that the machine
control unit can understand
• MCU - Machine Control Unit - decodes NC codes
to drive and monitor servo motor movements.
• CNC - Computer Numeric Control - computer
provides machine codes to the MCU.
• Control Systems
▫ Open loop system - servo motor driven by pulses
without feed back encoders.
▫ Closed loop system - servo motor is driven by
electrical pulses. An encoder provides feedback
to verify machine movements.
23. History of CNC
US Air Force asks MIT to develop a "numerically
Prototype NC machine demonstrated (punched tape input)
CNC machines (computer used to link directly to controller)
DNC: external computer “drip feeds” control
programmer to machine tool controller
24. CNC Advantages vs. NC
• Programs could be stored in computer memory.
• Easier to edit.
• More complex parts could be manufactured.
• Use of 3d geometry.
• Networking/file sharing / other computers.
25. Advantages of CNC
• Increased productivity after programming is
• Reliability - reduces human error
• Often eliminates need for special jigs and
• Reduces location of part features
• Makes possible the machining of complex
shapes requiring simultaneous 3 axis motion
• Single part and production runs can be
programmed and machined with minimum effort
• Programs can readily be altered and re-run
• Reduced inspection costs (more reliable)
• Once programming, setup and verified the
equipment can be operated by a less skilled
• Initial cost of CNC machine tools
• Servicing of equipment
• Larger machines require more space
• Personnel must be trained in the programming
and operation of this equipment.
35. CNC terminology
BLU: basic length unit
smallest programmable move of each axis.
Controller: (Machine Control Unit, MCU)
Electronic and computerized interface between operator
1. Data Processing Unit (DPU)
2. Control-Loops Unit (CLU)
36. Controller components
Data Processing Unit:
Input device [RS-232 port/ Tape Reader/ Punched Tape Reader]
Data Reading Circuits and Parity Checking Circuits
Decoders to distribute data to the axes controllers.
Control Loops Unit:
Interpolator to supply machine-motion commands between
Position control loop hardware for each axis of motion
51. Types of CNC machines
Based on Motion Type:
Point-to-Point or Continuous path
Based on Control Loops:
Open loop or Closed loop
Based on Power Supply:
Electric or Hydraulic or Pneumatic
Based on Positioning System
Incremental or Absolute
53. Open loop control of a Point-to-Point NC drilling machine
NOTE: this machine uses stepper motor control
54. Components of Servo-motor controlled CNC
Motor speed control
Two types of encoder configurations
Motor lead screw rotation table moves
position sensed by encoderfeedback
55. Motion Control and feedback
Encoder outputs: electrical pulses (e.g. 500 pulses per revolution)
Rotation of the motor linear motion of the table: by the leadscrew
The pitch of the leadscrew: horizontal distance between successive threads
One thread in a screw single start screw: Dist moved in 1 rev = pitch
Two threads in screw double start screw: Dist moved in 1 rev = 2* pitch
72. Manual NC programming
Part program: A computer program to specify
- Which tool should be loaded on the machine spindle;
- What are the cutting conditions (speed, feed, coolant
- The start point and end point of a motion segment
- how to move the tool with respect to the machine.
73. Part program
The RS274-D is a word address format
Each line of program == 1 block
Each block is composed of several instructions, or (words)
Sequence and format of words:
N3 G2 X+1.4 Y+1.4 Z+1.4 I1.4 J1.4 K1.4 F3.2 S4 T4 M2
destination coordinates dist to center of circle
feed rate spindle speed
84. PART PROGRAMMING
• Part program is a sequence of instructions, which
describe the work, which has to be done on a part, in the
form required by a computer under the control of a
numerical control computer program
• Programming is where all the machining data are
compiled and where the data are translated into a
language which can be understood by the control system
of the machine tool.
The machining data is as follows :
(a) Machining sequence classification of process, tool start up
point, cutting depth, tool path, etc.
(b) Cutting conditions, spindle speed, feed rate, coolant, etc.
(c) Selection of cutting tools.
85. PART PROGRAMMING
While preparing a part program, need to perform the
following steps :
(a) Determine the startup procedure, which includes the
extraction of dimensional data from part drawings and data
regarding surface quality requirements on the machined
(b) Select the tool and determine the tool offset.
(c) Set up the zero position for the work piece.
(d) Select the speed and rotation of the spindle.
(e) Set up the tool motions according to the profile required.
(f) Return the cutting tool to the reference point after
completion of work.
(g) End the program by stopping the spindle and coolant
86. PART PROGRAMMING
Methods of part programming can be of two types
depending upon the two techniques as below :
(a) Manual part programming,
(b) Computer aided part programming
Manual Part Programming
• The programmer first prepares the program
manuscript in a standard format.
• Manuscripts are typed with a device known as flexo
writer, which is also used to type the program
87. PART PROGRAMMING
Computer Aided Part Programming
• Complex-shaped component requires calculations to
produce the component are done by the
programming software contained in the computer.
• The programmer communicates with this system
through the system language, which is based on
• There are various programming languages
developed in the recent past, such as APT
(Automatically Programmed Tools), ADAPT,
AUTOSPOT, COMPAT-II, 2CL, ROMANCE, SPLIT
88. PART PROGRAMMING
Computer Aided Part Programming
A translator known as compiler program is used to
translate it in a form acceptable to MCU.
The programmer has to do only following things
(a) Define the work part geometry.
(b) Defining the repetition work.
(c) Specifying the operation sequence.
91. Direct Numerical Control (DNC)
• Direct numerical simultaneously control the operations of a
group of NC machine tools using a shared computer.
Programming, editing part programs and downloading part
programs to NC machines are main responsibilities of the
computers in a NC system.
92. G - Code Programming
• G – Code Programming
• Originally called the “Word Address” programming
• Processed one line at a time sequentially.
93. Common Format of a Block
N50 G90 G01 X1.40Y2.25 F10 S1500 T01 M03
94. Word Address 1
• N – Sequence or line number
• A tag that identifies the beginning of a block of code.
It is used by operators to locate specific lines of a
program when entering data or verifying the
• G – Preparatory function
• G words specify the mode in which the milling
machine is to move along its programmed axes.
95. Word Address 2
• Dimension Words
X – Distance or position in X direction
Y – Distance or position in Y direction
Z – Distance or position in Z direction
• M – Miscellaneous functions
• M words specify CNC machine functions not related to
dimensions or axial movements.
96. • F – Feed rate (inches per minute or millimeters
• Rate at which cutting tool moves along an axis.
• S – Spindle speed (rpm – revolutions per minute)
• Controls spindle rotation speed.
• T – Tool number
• Specifies tool to be selected.
Word Address 3
97. • I – Circular cutting reference for x axis
• J – Circular cutting reference for y axis
• K – Circular cutting reference for z axis
Word Address 4
98. G Word
• G words or codes tell the machine to perform
certain functions. Most G words are modal which
means they remain in effect until replaced by
another modal G code.
99. Common G Codes
• G00 – Rapid positioning mode
• Tool is moved along the shortest route to
programmed X,Y,Z position.
• Usually NOT used for cutting.
• G01 – Linear Interpolation mode
• Tool is moved along a straight-line path at
programmed rate of speed.
• G02 – Circular motion clockwise (cw)
• G03 – Circular motion counter clockwise
100. Common G Codes, con.,
• G17 – XY plane
• G18 – XZ plane
• G19 – YZ plane
• G20 – Inch Mode
• G21 – Metric Mode
• G28 – Return to axis machine Zero (Home)
101. G Codes: G90, G91
G90 – Absolute Coordinate Reference
References the next position from an absolute zero
point which is set once for the entire program.
G91 – Incremental Coordinate Reference
References the next position from the previous
102. G Codes: Canned Cycles
• G80 – Cancel canned cycle
• G81 – Drilling cycle
• G83 – Peck drilling cycle
• G84 – Tapping cycle
• G85 – Boring cycle
• G86 – Boring cycle
• NOTE: A canned cycle stays in effect until
cancelled by a G80.
103. Canned Cycles: G81
• G81 – Drilling Cycle
• Feed to depth, rapid return
Example of program code:
• N35 G81 X.500 Y.500 Z-1.000 R.100 F1.50
• N36 X1.000 Y1.500
• N37 X1.500 Y2.000
• N38 G80
104. Canned Cycles: G83, G84
• G83 – Peck Drilling Cycle
• Feed to an intermediate depth, rapid out, rapid back
to just above previous depth, feed to next depth,
rapid out, repeat until reaching full depth.
• G84 – Tapping Cycle
• This cycle creates internal threads in an existing
• NOTE: One cannot over-ride the feed rate.
105. Canned Cycles: G85, G86
• G85 - Boring Cycle
• Feed to depth, feed back out.
• G86 – Boring Cycle
• Feed to depth, rapid out.
107. Table of Important G codes
G00 Rapid Transverse
G01 Linear Interpolation
G02 Circular Interpolation, CW
G03 Circular Interpolation, CCW
G17 XY Plane, G18 XZ Plane, G19 YZ Plane
G20/G70 Inch units
G21/G71 Metric Units
G40 Cutter compensation cancel
G41 Cutter compensation left
G42 Cutter compensation right
G43 Tool length compensation (plus)
G43 Tool length compensation (plus)
G44 Tool length compensation (minus)
G49 Tool length compensation cancel
108. Table of Important G codes
G80 Cancel canned cycles
G81 Drilling cycle
G82 Counter boring cycle
G83 Deep hole drilling cycle
G90 Absolute positioning
G91 Incremental positioning
109. M Word
• M words tell the machine to perform certain
machine related functions, such as: turn spindle
on/off, coolant on/off, or stop/end program.
110. Common M words
• M00 – Programmed pause
• Automatically stops machine until operator pushes a button
to resume program.
• M01 – Optional stop
• A stop acted upon by the machine when operator has
signaled this command by pushing a button.
• M02 – End of program
• Stops program when all lines of code are completed. Must be
last command in program.
111. • M03 – Turn spindle on
• In clockwise direction
• M04 – Turn spindle on
• In counter clockwise direction
• M05 – Stop spindle
• Usually used prior to tool change or at end of program.
• M06 – Tool change
• Stops program and calls for a tool change, either
automatically or manually.
Common M words
112. • M08 – Turns Accessory 1 on.
• M09 – Turns Accessory 1 off.
• M10 – Turns Accessory 2 on.
• M11 – Turns Accessory 2 off.
• M30 – End of program
• Similar to M02 but M30 will also “rewind” the program. Must
be last statement in program. If used, DO NOT use M02.
Common M words
113. Zero Points
• Part Zero
▫ Used for absolute programming mode.
▫ Usually a position on the part that all absolute
coordinates are referenced to.
▫ Changes with different parts and programs.
• Machine Zero or Machine Home Position
▫ Fixed for each machine from the manufacturer.
▫ Not changeable.
114. Cutter Path Generation
• Cutter path is generated by moving the tool from
point to point. The points are previously defined
from the part drawing dimensions.
• Each line of code will show the destination point
of where the tool will go to.
• Method of determining intermediate points along a
• Two methods:
• Linear interpolation – cut a path along a specified
angle at a specified feed rate.
• Circular interpolation – cut a path along an arc or
circle at a specified feed rate.
119. TYPES OF CNC MACHINES
In every aspects of manufacturing CNC machines are
used. It can be mainly classified in eight classes.
▫ Mills and Machining centers
▫ Lathes and Turning centers
▫ EDM Machines
▫ Grinding machines
▫ Cutting Machines
▫ Fabrication Machines
▫ Welding Machines
▫ Coordinate Measuring Machines
• CNC – Turning Center • CNC – Machining Center
122. CNC – Machining Center
• It is a machine tool capable of multiple machining
operations on a work part in one setup under NC
• Machining centres are classified as vertical,
horizontal, or universal.
128. Automatic part programming and DNC
Very complex part shapes very large NC program
NC controller memory may not handle HUGE part program
computer feeds few blocks of
NC program to controller
When almost all blocks executed,
controller requests more blocks
CNC machines allow precise and repeatable control in machining
CNC lathes, Milling machines, etc. are all controlled by NC programs
NC programs can be generated manually, automatically