An Introduction to Computer Numerical Control
Introduction
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 motor rotates.
Introduction
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 commands.
Numerical Control (NC) & Computer
Numerical Control (CNC) mean the
same.
Introduction
A major manufacturing development in
past 60 years.
Resulted in:
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new manufacturing techniques
higher production levels
higher quality
stabilization of manufacturing costs
Evolution of CNC
Single items produced by craftspeople
Interchangeable Parts
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Eli Whitney (Cotton Gin)
Manual labor was still the most cost
effective method.
WW II manufacturers could not
maintain quantity & quality parts.
Evolution of CNC
Machinists could produce superior
quality but not at high volume that was
required.
As quantity increased, quality decreased
due to human factors
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.
ENIAC
ENIAC
CNC & WW II
Need to manufacture large amount of
products for the war.
Need for quantity and quality.
U.S. Air Force set up companies to develop
and produce NC systems to handle volume
and repeatability.
Repeatability: the ability to perform the same
operation over & over within specified
parameters.
Specific Goals
Increase production
Improve quality & accuracy of machined
parts.
Stabilize manufacturing costs.
Speed up production & assembly
operations.
NC Timeline
1949 - First contract awarded for NC
machine.
1951 - servo system for machines
developed.
1952 - tape-fed machine was created.
History
Development of G codes - Punch tape
input (Cartesian Coordinate System)
1970’s Development of computer chips
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Cheaper processing power
Smaller computers
More reliable
Paper Tape Control
Paper Tape Control
Paper Tape Control
Strip of paper tape with holes in it.
Machine read pattern of holes and
performed the required operation.
Paper Tape Control
Disadvantages
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Difficult to identify parts of program.
Programs could be quite large.
Stored on large bulky reels.
Fragile, could rip easily.
CNC
Further developments in the computer
allowed it to be used to control the
machine instead of the paper tape.
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/filesharing w/ other
computers.
Advantages Of CNC
Increased productivity after
programming is completed
Reliability - reduces human error
Often eliminates need for special jigs
and fixtures
Reduces location of part features
Makes possible the machining of
complex shapes requiring simultaneous
3 axis motion
Advantages
Single part and production runs can be
programmed and machined with
minimum effort and cost.
Programs can readily be altered and rerun
Reduced inspection costs (more reliable)
Once programming, setup and verified
the equipment can be operated by a less
skilled operator.
Disadvantages
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.
Definitions
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.
Definition
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.
Resources
Fitzpatrick, Mike, Computer Numerical Control Simplified, 1989 Glenco
Publishing Company. ISBN 0-02-676410-5
Powers, John Jr., Computer Automated Manufacturing, 1987 McGraw-Hill Book
Company. ISBN 0-07-050601-9
Groover, Mikell P. and Zimmers, Emory W., CAD/CAM, Computer Aided Design
and Manufacturing, 1984 Prentice -Hall Inc. ISBN 0-13-110130-7
Luggen, William W., Fundamentals of Computer Numeric Control, Third Edition,
1988 Delmar Publishers. ISBN 0-8273-6496-2
CNC Workshop (CD-Rom), Torcomp Publisher. Cost $39.95 – Contact TorComp
at 888-561-7521