Unit6 Computer Numerical Control

F T ra n sf o F T ra n sf o
PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/1
he

he
k

k
lic

lic
COMPUTER NUMERICAL CONTROL
C

C
w om w om
w

w
w. w.
A B B Y Y.c A B B Y Y.c

UNIT 6


OBJECTIVES

General Objective :To understand the concept and principles of computer
numerical control (CNC) system.

Specific Objectives : At the end of the unit you will be able to :

Ø Understand the main components of the CNC system,

Ø Understand the point-to-point system (positioning),

Ø Understand the contouring system (continuous
system), and

Ø Write a simple CNC milling program.
.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/2
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

INPUT

6.0 INTRODUCTION

Computer numerical control is a system in which a control microcomputer
is an integral part of a machine or a piece of equipment (onboard computer). The
part programmes can be prepared at a remote site by programmer, and it may
incorporate information obtained from drafting software packages and from
machining simulations, in order to ensure that the part programme is bug free.
The machine operator can, however, easily and manually programme onboard
computers. The operator can be modify the programs directly, prepare
programme for different parts, and store the programmes.
Because of the availability of small computers having a large memory,
microprocessor(s), and programme-editing capabilities, CNC systems are widely
used today. The availability of low-cost programmable controllers also played a
major role in the successful implementation of CNC in manufacturing plants.
Numerical Control is a system where machine action is created from the
insertion of Numeric Data. The Numeric Data is, in the beginning, written
words in an easily understood code of letters and numbers (alphanumeric
characters) known as a programme, which in turn is converted by the machine
control unit (MCU) into the electrical signals used to control the machine
movements.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/3
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

The relationship between the words "Numerical" and "Control" is shown
below.
NUMERICAL CONTROL

An instructional expression, To control such machine
in a language of numbers, actions as:

which represents a series of Directing Altering
commands for specific Commanding Timing
Prescribing Ceasing
machine tool movements Sequencing Guiding
Initiating

Two important points should be made about N.C. First, the actual N.C.
machine tool can do nothing more than it was capable of doing before a control
unit was joined to it. There are now new metal removing principles involved.
N.C. machines position and drive the cutting tools, but the same milling cutters,
drills, taps, feeds, and other tools still perform the cutting operations. Cutting
speeds, feeds, and tooling principles must still be adhered to. Given this
knowledge, what is the real advantage of numerical control?
Primarily, the idle time or time to move into position for new cuts is
limited only by the machine's capacity to respond. Because the machine receives
commands from the machine control unit (MCU), it responds without hesitation.
The actual utilisation rate or chip making rate is therefore much higher than on
a manually operated machine.
The second point is that numerical control machines can initiate nothing
on their own. The machine accepts and responds to commands from the control
unit. Even the control unit cannot think, judge, or reason. Without some input
medium, e.g., punched tape or direct computer link, the machine and control

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/4
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

unit will do nothing. The N.C. Machine will perform only when the N.C. tape is
prepared and loaded and cycle start is initiated.

6.1. NC OPERATION

CNC stands for Computer Numerical Control. An N.C. system in which a
dedicated stored program computer is used to perform basic control functions.
The functions of a CNC Controller are:
1. To read and store programme information.
2. To interpret the information in a logical command sequence.
3. To control the motion of the machines mechanical members.
4. To monitor the status of the machine.
The interpretation of programme commands by a machine control unit
and its conversion of those commands into machine motion is complex. The
basic elements and operation of a typical NC machine are shown in Fig. 6.1. The
functional elements in numerical control and the components involved follow:
a. Data input: The numerical information is read and stored in the
tape reader or in computer memory
b. Data processing: The programmes are read into the machine
control unit for processing.
c. Data output: This is information is translated into commands
(typically pulsed commands) to the servomotor (Fig. 6.2 and 6.3).
The servomotor then moves the table (on which the work piece is
mounted) to specific positions, through linear or rotary
movements, by means of stepping motors, leadscrews, and other
similar devices.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/5
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Computer:
Input command,
Processing,
Output command

Limit switches
Position feedback

Drive signal

Figure 6.1. A schematic illustration of the major component
of a computer numerical control machine tool

Work table

Pulse train Stepping
motor Gear

Lead screw

Figure 6.2. An open-loop control system for a numerical-control machine

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/6
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Work table

Input Dc
Comparator DAC servomotor Gear

Feedback signal Lead screw Position sensor

Figure 6.3. A closed-loop control system for a numerical-control machine

6.2. INDUSTRIAL APPLICATION

6.2.1. Metal Machining
Lathes of all types
Milling Machines of all types
Drilling Machines
Jig borers
Electric Discharge Machining (including wire cut machines)
Laser cutting machines
Machining centres
Turning centres
All types of grinding machines
Gear cutting machines

6.2.2. Metal Forming
Punching and nibbling
Guillotines
Flame cut and profiling
Folding

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/7
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Pipe bending
Metal spinning

6.2.3. Finishing
Plating
Painting

6.2.4. Assembly Joining -
Pick and place robots, spot and seam welding machines and robots,
riveting, looming of wires and assembly of components into printed circuit
boards.

6.3. CNC AXIS CONVENTIONS

CNC axis classification follows the three-dimensional Cartesian
coordinate system and is established in BS 3635: 1972: Part 1. Fig. 5.3 shows the
tree primary axes and the associated rotational axes.
Most machines have two or three slide ways placed at right angles to one
another. On CNC machines each slide is fitted with a control system, and is
identified with either the letter X, Y or Z.
Conventions have been adopted as to the naming of each axis. The axis of
the main spindle, whether it is the axis of the tool spindle or the axis about
which the work piece rotates is called the Z axis.
The X axis is the motion of the largest travel of the primary movement (in
case there is more than one).

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/8
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

The Y axis then makes the third motion and is the shorter primary
movement.
In addition to these primary linear axes, there is provision for Rotary
axes. They are designated A, B and C, with A rotary about the X axis, B rotary
about the Y axis, and C rotary about the Z axis.
It is often required to command a motion parallel to X, Y or Z axes within
the realm of a secondary motion, or a tertiary motion within special automatic
cycles such as describing the amount of finish allowance on a turned part, or to
describe the distance of advancement of a drill during a drilling cycle etc. etc.

Table 6.1. NC axes

Linear Axes X Y Z
Rotary Axes A B C
Secondary Linear U V W
Interpolation I J K

Tertiary motion codes differ considerably, but the address characters
variously used are P, Q, R, D, L, E, and H.
The z-axis is parallel to the main spindle of the machine. It will be
horizontal on a lathe or horizontal machining centre and vertical on a vertical
machining centre.

The x-axis is always horizontal and at 90o to z.
The y-axis is at right angles to both the x and z axes.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/9
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

spindle

rotation
table

z
x

y

Figure 6.3. CNC axes

6.4. NC MACHINE SUB-UNIT

We have already seen the many and varied applications of numerical
control to the manufacturing and other industries, now we will look at the
methods of controlling machines. There are three sub units to study:

The machine tool itself.
The control unit.
The control system.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/10
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

6.4.1. The Machine Tool

A machine tool is a device designed to cut away surplus material
and leave a component of the required shape and size. To do this a
machine tool must be capable of:
- Holding the work piece securely
- Holding the cutting tool securely and driving it with suitable power.
- Moving the tool and work piece relative to one another precisely enough
to achieve accuracy of size and surface finish.
In addition, provision must be made for altering the spindle speed
and feed rates, tool changing, supply of coolant etc. On a conventional
machine an operator controls these functions and sets or alters them
when he considers it necessary, the decision resulting from his training,
skill and experience. Obviously, the machine settings may differ between
operators as will the time taken to read scales, set positions, change tools,
alter speeds and feeds, engage drives and set up the work piece etc. CNC
Automatic Control can be applied to these functions and so result in
consistent and reduced machining times through optimised cutting data,
fast accurate positioning between cuts and fast automatic tool changing.

6.4.2. The Control Unit

The CNC Machine Control Unit (MCU) has to read and decode the
part programme, and to provide the decoded instructions to the control
loops of the machine axes of motion, and to control the machine tool
operations.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/11
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

The main grouping of parts of a control could be considered to be:
The Control Panel.
The Tape Reader,
The Processors
The first part of the control panel is the human interface that
allows various modes of machine or control operation to be initiated, from
switching on and homing, to programme loading and editing, to setting
work positions and tool offsets, manually controlled movements and
commencing the automatic cycling of a programme. Information about
machine status and condition is available to the operator via VDU
screens, gauges, meters, indicator lights and readouts.
The tape reader is the device used to transfer the programme
information contained on a programme tape into the control unit. Most
tape readers are of the photo-electric type which offers high speed reading
with reliability and accuracy providing the tape is in good condition and
the reader is kept clean and free of paper dust particles.
The processors within a control are the electronic circuits that
permit conversion of part programme data into machine motions and they
may be classified into two main sections. The data processing unit and
the axis control processor. The function of the data processor is to decode
the commands of the part program, process it and provide data to the axis
control processor which then operates the slide drives and receives
feedback signals on the actual position and velocity of each axis.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/12
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

The Data Processing Unit includes the following functions:
i. The input device, such a tape reader.
ii. Reading circuits and parity checking logic.
iii. Decoding circuits for distributing data to the controlled axes
iv. An interpolator to supply velocity commands to the axes, either
singly or in combination.
The axis control processor consists of the following circuits:
i. Position control loops for each and all axes.
ii. Velocity control loops.
iii. Deceleration and backlash take up circuits.

An MCU is adaptable to virtually any machine, the differing control
motions and codes being a result of the way the control has been
programmed. This permanent resident program is known as an executive
programme and resides in the read only memory (ROM) of the control,
whereas the N.C. programme resides in the Random Access Memory
(RAM). RAM allows external access and alteration if necessary, while
ROM is programmed by the manufacturer and cannot be accessed through
the control keyboard.

6.4.3. Control System

There are two types of control systems used on NC machines. The
point-to-point system and the continuous-path system. Point-to-point
systems are not so common these days, but they operate only in straight
lines, which are suitable for positioning moves on a drilling machine or
limited use on a lathe or milling machine, where at best 45% cuts are
possible with two axes running continuous path controls allow angular

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/13
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

path and radius motion because the control interpolator has the ability to
move the axis drive motors at varying velocities.
The point-to-point controls were NC controls, while the continuous
path controls could be NC or CNC controls.

NOTE: NC is a general term used for Numerical Control and is also a term
used to describe controls that run directly off tape. CNC is a
specific term for Computer Numerical Control. CNC Machines are
all NC machines, but NC controlled machines are not CNC
machines.

6.5. PROGRAM INPUT

Programmes can be produced and entered (loaded) by any of the following
methods where available.
a. Punched Tape
b. Computer
c. Direct Input

6.5.1. .Punched Tape

Punched tapes may be made of paper or plastic (Mylar) and have a
standard width of one inch (25.4mm) for the eight track (8 bit) format
used in numerical control.
A tape punch unit is connected to a teletype or similar keyboard
and produces a punched tape during typing of the program. Alternatively,
a punch unit can be connected to a personal computer (P.C.) and the
completed programme punched. The holes punched in the tape form

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/14
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

certain patterns and the pattern represents a value when read by the tape
reader on the MCU.
The pattern is a code in itself, and complies to a standard, either
E.I.A. ( Electronics Industries Association ) which uses an odd number of
holes or I.S.O. ( International Standards Organization ) which uses an
even number of holes for each character. The I.S.O. code is most
commonly used in Australia. The E.I.A. code is known as an odd parity
system, and the I.S.O. code as an even parity system.
One of the tracks of each is assigned as a parity track and a hole is
sometimes punched there automatically to maintain the parity. The
purpose of parity is to check during tape reading for errors caused by
unpunched holes, dirt or oil spots etc.

6.5.2. Computers

Personal computers can be used to type the programme in its
entirety while being visible on the screen, so mistakes can easily be
spotted and corrected before the programme is loaded into the machine.
The programme can be down loaded to the machine via a
connecting link (interface cable) or via punched tape if a punch unit is
connected to a computer.

6.5.3. Direct Input

Programming at the machine may eliminate the need for tape
punching equipment and computers, but the machine is usually non-
productive during this time.

Programming can be done by several methods, such as ;

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/15
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

- by typing the program directly into the memory of the M.C.U. through
the edit function.
- programmes produced in this way can include all functions available, are
stored in the machine ready for use at any time and can be output to a
punch unit or computer for external storage.

6.5.4. By Manual Data Input (M.D.I.)

M.D.I. may not allow all programming functions to operate, the
programme can only be used once, and as it is not stored in the main
memory cannot be output to an external device. Additionally, some
controls allow only one line (block) of programme to be entered and
executed at a time.

6.5.5. Interactive Programming.

Some controls allow programming by a method that may be
simpler and speedier than conventional N.C. program language. These
methods usually take up considerable memory space and so fewer
programmes could be stored in the M.C.U. than if they were prepared by
N.C. language. Most of the controls allow
for external storage of these programs providing the necessary devices are
at hand. Interactive programming can also be known as "conversational",
"symbolic", "blueprint","IGF" etc.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/16
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

6.5.6. . External Devices.

A programme can be loaded directly from a P.C. as described, or from
other specialised units that have been designed as a portable device for
loading, transferring or storing N.C. programs. The devices may store the
information on floppy discs, hard discs, magnetic tape, or through solid
state circuitry. Those with discs or magnetic tape would be no larger than
a briefcase or perhaps a tissue box, down to pocket calculator size for solid
state devices.

NOTE: Where punched tape was once the only practical way for most
programmers to transfer (or load) their programs into the controls, it is
presently being overtaken by personal computers connected directly via
interface cable. Other popular and convenient methods are simply
through the edit mode or by interactive (conversational) programming.
Companies specialising in complex die work probably use DNC more often
now. Inputting through M.D.I. mode is common providing the restrictions
noted are adhered to.

6.6. NC PROGRAMMING

6.6.1. Job Planning
1. Sketch the part. Add incremental or absolute dimensions.
2. Ascertain fixturing. Select fixtures which have minimal projections
above the part.
3. Identify a set-up point. Locate the set-up point near:
1. A corner of the part
2. A spot above the fixture

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/17
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Consider space requirements for:
1. Part loading and unloading
2. Tool change.
4. Plan operation sequence Mark sequence pattern of sketch.
Test program data for accuracy.
5. Record necessary data for
each movement of the table
and tool on the program
sheet.
6. Record instructions for Identify, specific:
the machine operator. 1. Tools needed.
2. Speed and feed data
3. Tool change points
4. Console switch setting

6.6.2. Incremental

The word "incremental" may be defined as a dimension or a
movement with respect to the preceding point in a prescribed sequence of
points. Each positioning move is described quantitatively in distance and
in direction from a previous point rather than from a fixed zero reference
point.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/18
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

In incremental mode all moves are with respect to the last position
reached.

N10 G91
Y N15 G01 X10.Y10.F300.
N20 Y10.
N25 X20.
40 N30 X10.Y20.
30 N35 X20-Y-30.
20 N40 X-10.Y-10.
10 N45 X-50.
X N50 M02
10 20 30 40 50 60

6.6.3. Absolute

The data in the absolute system describes the next location always
in terms of its relationship to the fixed zero point. The zero point when
used as a programme datum is known as the programme origin.
The G90 code sets the control up in absolute mode. All moves are
performed with respect to the axes zero.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/19
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

N10 G90
Y
N15 G01X10.Y10.F300.
N20 Y20.
N25 X30.
40
N30 X40.Y40.
30
20
N35 X60.Y10.
N40 X50.Y0.
10
N45 X0.
10 20 30 40 50 60 N50 M02

6.6.4. Linear Interpolation

Under this command the machine tool will move in a straight line
at a defined feed rate. Combined axis motions (angled moves) will be
executed at the programming feed rate as the control will reduce the
velocity of both axes accordingly.

E.g. G01 X200. F250.
G01 Move in a straight line
X200. A distance of 20O.mm
F250. At a feed rate of 250.mm/min.

NOTE: If a new line with G01 is listed again somewhere below, the F250 does
not have to be written again. This is called modal.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/20
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Example:

A block as shown below is to be machined, write a program in absolute mode.

%
10 70 10
10
G90 -
G01 X100 F300
Y70
X90. Y80
90

X20.
80

80
X10. Y90.

70
XO. Y80
Y0.
100
M02

6.6.5. Circular Interpolation

In circular interpolation mode G02 will cause the path to be
transcribed in a clockwise direction and G03 will cause counter-clockwise
motion.

G02 - Clockwise
G03 - Counterclockwise

In circular interpolation there are a number of points to be remembered:
The end point of the arc is defined as X and Y coordinates exactly the
same as if commanding linear motion.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/21
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

The centre of the arc is defined with respect to the start point in the I and
J words as an "increment" from this point.
For G02 and G03 to function the feed rate "F." must be specified.

Example:

N5%
N10 G90
N15 G01 Y110. F200.
N20 G02 X20. I10.
N25 G03 X30. Y100. I10. 110

N30 G01 X90. All radius – R10

N35 G02 X100. Y90. J-10.
N40 G01 Y10,
N45 G02 X90. Y0. I-10.
N50 G01 X0.
100
N55 M02

6.7. PROGRAM DEFINITION

To enable the machine to operate automatically it is necessary to put into
its memory a programme or set of instructions to carry out the required
operation.
a) Programme.
A programme is a series of instructions to the machine, set out in
sequence to -produce a complete machining operation. A programme is
made up of a series of blocks.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/22
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

b) Block.-
A block or programme line is a set of instructions to the machine
that are carried out simultaneously. A block is made up of one or more
Words and is terminated by an End of Block which is the Line Feed
Character.

c) Word.
A word is a specific instruction to the machine that will affect a
particular machine function. Every word consists of a Letter Code and a
Numerical value.

Examples of Dimensional Words: X100. Y2.345 F0.25
Examples of Non-Dimensional Words: N25 G90 M03 S1200

Dimension words can be written in various ways, depending on the
control. Let's take the examples X100. Y2.345 some older controls cannot accept
decimal points, so both dimensional words would be written X100000 Y2345,
with Y showing all decimal places. With these controls, if the X word was written
as X100, it would be interpreted as one-tenth of a millimeter, not one hundred
millimeters.
If a control accepts decimal points, then ALL dimensional words should
have a decimal point. On any control, non-dimensional must NOT have a
decimal point. The method of writing words beginning with a letter is known as
word address format and is now almost universally used.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/23
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

6.7.1. Program

(Start of Program)
(Material 25.mm. dia.)
(Grip 120.m.m. from Front of Jaws)
N01 G71G90G95
N02 G50X100.Z130.
N03 S2000M03

N04 G00X26.Z119.T0101 BLOCK

(Select Turning & Facing Tool)
N05 GO1X2.F.O4
N06 GOOZ120.
N07 X24. WORDS

N08 G01Z20. N10 GOOX100. Z130. T0100

N09 X26.
N10 G00X100.Z130.T0100
N11 M02

WORD ADDRESS The letter at the beginning of each word is called the
address character.

e.g. XYZ for Axis designating word
F for Feed rates
G for Preparatory functions
M for miscellaneous function
N for Sequence numbers

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/24
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

CNC mills, drills and machining centers are all equipped with cycles to
perform drilling, reaming, counter boring, boring and tapping operations. Some
others have pocketing cycles, slot cutting cycles, hole pattern cycles etc, all of
which are designed to save programming time and effort.
CNC lathes usually have cycles to cover drilling, grooving/parting, screw
cutting, repetitive cut (automatic roughing) operations and others. Each cycle
has its own G code to control the sequence of motions and an accompanying set
of words to define the parameters of those motions. These words have addresses
such as: R,P,Q,D,E,I,K,H,B etc.

6.7.2. Program Preparation

CNC programmes can be prepared manually, where the programmer
usually roughs the programme out on paper, then produces it via a keyboard
device of the type detailed below, or by assisted preparation in which a computer
plays a predominant role -such as when CAD/CAM packages have been installed
for design and programming.
The programmer must posses knowledge and skills in planning machining
sequences, fixturing, cutting data, cutting tools, calculations, as well as being
familiar with the machines he is programming. To implement these skills to best
effect a programmer should be prepared to observe critically his programs in use
and modify them as necessary in order to gain maximum machine utilization.

6.7.3. Operation of program

Before a machine can set into automatic motion a program must be
checked for errors. A simple typing mistake - an incorrect code, a minus sign
instead of a zero, the exclusion of a decimal etc, could cause and expensive

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/25
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

machine crash. Anyone who considers their programmes to be without error and
not in need of careful and conscientious trialing has an attitude problem and is
placing expensive machinery and operators safety at risk.
There may be many ways in which a programme can be checked for
errors, but a programme can only be proved 100% by running the machine and
producing a part.
Error checking can be performed in a variety of ways:
Verification: Read through the print-out (NOT the handwritten
manuscript) carefully - sometimes mistakes can be seen
easily.
Trialing: This involves the execution of the programme without
actually cutting the part and may be carried out in several
ways depending on the type of machine, or control, or even
the philosophy of the person in charge. Adhere to the later unless
you can put up good reasons for alteration.
Trialing usually consists of running the machinewith the single
block switch active, that is, each block will only be executed by
pressing cycle start, in conjunction with the programme
being displayed on the screen.
Quite often the dry run mode is switched on to hasten
Proceedings. 'Dry Run' results in all machine motion being
executed at a preset rate, usually in the region of 50% to 80%
of the rapid traverse capability of the machine. The actual axis
velocity can be overridden from 0% to 100%. The
disadvantage of dry running a programme is that feed rates will
be masked, and attention must be paid to determining the
actual programmed feed rate for each block. This may be
displayed on the screen.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/26
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Every movement the machine makes during programme
trialing should be expected and accountable to the
programmer, if not, those motions should be checked for
viability, and if necessary, a more thorough understanding of
the machine operation should be sought.

Editing: Wherever errors are found, they should be corrected and
rechecked, be it on the machine or at the programming
station. Whenever a programme is edited on the machine, a
note should be made on the print-out so the master or
original programme can also be corrected. A better method is
to punch out a programme from the control after successfully
producing a component.

6.8. TYPES OF CONTROL SYSTEM

There are two basic types of control systems in numerical control: point-
to-point and contouring.
a. In a point-to-point system, also called positioning, each axis of the
machine is driven separately by lead screws and, depending on the
type of operation, at different velocities. The machine moves initially
at maximum velocity in order to reduce non-productive time, but
decelerates as the tool approaches its numerically defined position.
Thus, in an operation such as drilling (or punching a hole), the
positioning and cutting take place sequentially (Fig. 5.4).

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/27
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

After the hole is drilled or punched, the tool retracts upward and
moves rapidly to another position, and the operation is repeated. The
path followed from one position to another is important in only one
respect. It must be chosen to minimize the time of travel, for better
efficiency. Point-to-point systems are used mainly in drilling,
punching, and straight milling operations.

15
10 45
C.P
15
(0,0)

10
1 2

4 3 45

Incremental (G90) Absolute (G91)

Position Coordinate Coordinate Position Coordinate Coordinate
(X) (Y) (X) (Y)
C.P. -15 15 C.P. -15 15
Point 1 10 -10 Point 1 25 -25
Point 2 55 -10 Point 2 45 0
Point 3 55 -55 Point 3 0 -45
Point 4 10 -55 Point 4 -45 0

Figure 5.4. Point-to point system

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/28
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

b. In a contouring system (also known as a continuous path system), the
positioning and the operations are both performed along controlled paths
but at different velocities. Because the tool acts as it travels along a
prescribed path (Fig. 5.5), accurate control and synchronization of
velocities and movements are important. The contouring system is
typically used on lathes, milling machines, grinders, welding machinery,
and machining centres.

Cutter Machined
radius surface

Cutter
path

Work piece

Figure 5.5. Continuous path by a milling cutter

6.9. PROGRAMMING CODES

A number of standard codes are used to reduce the amount of
programming effort needed to command commonly used machining operations,
instructions and conditions. These are commonly known as:
G codes – call up machining commands
M codes – call up machine control activities
T codes – call up tool selection
F codes – call up feed rates
S codes – call

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/29
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

- modal codes remain active after being entered, unless they are cancelled
by another G code; and
- non-modal codes are only active in the programme block in which they
appear.

6.9.1. G codes (preparatory codes)

The majority of manufacturers follow the same practice in designation of
codes, but their detailed implementation mav differ.

Sample G codes
GOO Rapid movement for position
GOI Linear interpolation used for straight-line feed
G02 Circular interpolation, clockwise
G03 Circular interpolartion, counterclockwise
G04 Dwell, a programmed stop to the tool movement
G17 Circular interpolation xy plane
G18 Circular interpolation xz plane
G19 Circular interpolation yz plane
G20 Inch units
G21 Millimetre units
G28 Return to home position
G29 Return from home position
G31 Reverses programmed direction of x axis
G32 Reverses programmed direction of y axis
G41 Tool radius compensation left
G42 Tool radius compensation right
G43 Tool length compensation-positive direction
G44 Tool length compensation-negative direction

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/30
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

G70 Imperial unit
G71 Metric units
G80 Cancel canned cycle
G81 Drilling cycle
G82 Drilling cycle with dwell
G83 Deep hole drilling
G84 Tapping cycle
G85 89-boring cycles
G90 Absolute mode
G91 Incremental mode

6.9.2. M codes

These control the auxiliary functions of the machine.
MOO Program stop
M02 End of program
M03 Spindle on, clockwise
M04 Spindle on, counter clockwise
M05 Spindle off
M06 Tool change
M07 Oil mist coolant on
M08 Flood coolant on
M09 Coolant off
M30 End of tape

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/31
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

6.10. WRITING A PROGRAM

Figure 5.6. To cut a ‘S’-slot/groove with a point-to-point
method and a continuous path/contouring system

Table 5. Reference points and X and Y coordinates to cut a ‘S’-slot/groove
with a point-to-point method and a continuous path/contouring system

Position Coordinate (X) Coordinate (Y)
C.P. 0 0
P. 1 45.0 -25.0
P. 2 70.0 -25.0
P. 3 60.0 -65.0
P. 4 45.0 -50.0
P. 5 60.0 -50.0
P. 6 49.393 -75.607
P. 7 38.787 -65.0
P. 8 15.0 -65.0

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/32
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

To machine the above component (as in Fig.5.6), below is the programme
that can be followed;

N10 G71 G90 S1500 T1 N130 G00 M00
N20 G00 X0 Y0 N140 G00 X0 Y0
N30 G00 X70.0 Y-25.0 Z10.0
N40 G01 Z-5.0 F250
N50 G03 I-25.0 J0
N60 X45.0 Y-50.0
N70 G01 X60.0 Y -50.0
N80 G02 I0 J-15.0
N90 X49.393 Y-75.607
N100 G01 X38.787 Y-65.0
N110 X15.0 Y-65.0
N120 Z10.0

Description of The Above Programme

NXX – block number
Block No. 10 – set machine to use metric unit, incremental coordinate,
spindle speed 1500 rpm, choose tool no. 1.
Block No. 20 – rapid movement to centre point (C.P).
Block No. 30 - rapid movement to point 1 (P. 1), cutting tool distance is
5.0 mm from the surface of the work piece.
Block No. 40 – cutting tool cuts 10.00 mm deep, feed 250 mm/min
Block No. 50 – circular interpolation, counter clockwise, radius 25.0 mm

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/33
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Block No. 60 – tool ends interpolation cutting at P. 4
Block No. 70 – linear interpolation until P. 5
Block No. 80 - circular interpolation, clockwise, radius 15.0 mm
Block No. 90 - tool ends interpolation cutting at P. 6
Block No. 100 - linear interpolation until P. 7
Block No. 110 - linear interpolation until P. 8
Block No. 120 – tool rises up 10.0 mm
Block No. 130 – program stops
Block No. 140 - rapid return to centre point (C.P).

6.11. ADVANTAGES OF COMPUTER NUMERICAL CONTROL

i. The component programming tape and the tape reader are used
once only when the programme is copied into the computer
memory, not only this practice wills same time but it will also
reduce errors.
ii. The programming tape can be edited on the shop floor, when the
machine is placed/located. Editing, correction and optimising; such
as machine tool operations, spindle speeds and speeds; are usually
done in the test run of the tape.
iii. Computer numerical control can easily changes into metric system
if the programme is in the imperial units.
iv. It is widely used in industry. It is easily adaptable in a
computerised industry system.
v. Increased flexibility – the machine can produce a specific part,
followed by other parts with different shapes, and at reduces cost.
vi. Greater accuracy – computers have a higher sampling rate and
faster operation.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/34
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

vii. More versatility – editing and debugging programmes,
reprogramming, and plotting and printing part shape are simpler.
viii. Programmes are stored on the machine ready for use.
ix. Programmes and data can be modified on the machine.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/35
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

ACTIVITY 6

6.1. Briefly state four (4) advantages of numerical control system.

6.2. You are given a drawing of a component. List down the steps you would take to
operate a NC machine in order produce the component.

6.3. Write a short paragraph on three (3) basic components of a numerical control system.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/36
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

FEEDBACK ON ACTIVITY 6

6.1. The advantages of numerical control system are:
i. The component programming tape and the tape reader are used
once only when the programme is copied into the computer
memory, not only this practice wills same time but it will also
reduce errors.
ii. The programming tape can be edited on the shop floor, when the
machine is placed/located. Editing, correction and optimising; such
as machine tool operations, spindle speeds and speeds; are usually
done in the test run of the tape.
iii. Computer numerical control can easily changes into metric system
if the program is in the imperial units.
iv. It is widely used in industry. It is easily adaptable in a
computerised industry system.
v. Increased flexibility – the machine can produce a specific part,
followed by other parts with different shapes, and at reduces cost.
vi. Greater accuracy – computers have a higher sampling rate and
faster operation.
vii. More versatility – editing and debugging programs,
reprogramming, and plotting and printing part shape are simpler.
viii. Programs are stored on the machine ready for use.
ix. Programs and data can be modified on the machine.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/37
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

6.2. Job Planning
1. Sketch the part. Add incremental or absolute dimensions.
2. Ascertain fixturing. Select fixtures which have minimal projections
above the part.
3. Identify a set-up point. Locate the set-up point near:
1. A corner of the part
2. A spot above the fixture

Consider space requirements for:
1. Part loading and unloading
2. Tool change.
4. Plan operation sequence Mark sequence pattern of sketch.
Test program data for accuracy.
5. Record necessary data for
each movement of the table
and tool on the program
sheet.
6. Record instructions for Identify, specific:
the machine operator. 1. Tools needed.
2. Speed and feed data
3. Tool change points
4. Console switch setting

6.3.(a) Machine Tool - a device designed to cut away surplus material and leave
a component of the required shape and size. It holds the work piece, cutting tool
and moves the tool and work piece relative to one another precisely enough to
achieve accuracy of size and surface finish. It can also alter the spindle speed
and feed rates, tool changing, supply of coolant etc.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/38
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

(b) The Control Unit - reads, decodes the part programme and provides the
decoded instructions to the control loops of the machine axes of motion,
and to control the machine tool operations. There are three main parts of
the control unit namely, the Control Panel, the Tape Reader and the
Processors

(c) Control system - there are two types of control systems used on NC
machines - the point-to-point system and the continuous-path system.
The point-to-point systems operates only in straight lines, which are
suitable for positioning moves on a drilling machine or limited use on a
lathe or milling machine, where at best 45% cuts are possible with two
axes running continuous path controls allow angular path and radius
motion because the control interpolator has the ability to move the axis
drive motors at varying velocities. The point-to-point controls were NC
controls, while the continuous path controls could be NC or CNC controls.

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/39
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

SELF-ASSESSMENT 6

1. Numerical control machine can be done in absolute coordinate (G90) and incremental
coordinates (G91). What is the difference between the two coordinates.

2. By using G90 and G 91 coordinates write a program to cut a component in is the .below
figure.

30

J20
70

35

20
60

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
J3103/6/40
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

FEEDBACK OF SELF-ASSESSMENT 6

1. (a) G90 Absolute mode - The data in the absolute system describes the next
location always in terms of its relationship to the fixed zero point. The
zero point when used as a programme datum is known as the programme
origin. All moves are performed with respect to the axes zero.

1. (b) G91 Incremental mode - dimension or a movement with respect to the
preceding point in a prescribed sequence of points. Each positioning move
is described quantitatively in distance and in direction from a previous
points rather than from a fixed zero reference point.

2.
G 90 G 91
Point
X Y X Y
Origin Point 0 0 0 0
1 30 -70 30 -70
2 30 -40 0 30
3 70 -15 40 25
4 90 -35 20 -20
5 90 -70 0 -35

PD rm PD rm
Y Y
Y

Y
er

er
ABB

ABB
y

y
bu

bu
2.0

2.0
to

to
re

re
he

he
k

k
lic

lic
C

C
w om w om
w

w
w. w.

Unit6 Computer Numerical Control

More Related Content

What's hot (14)

Viewers also liked (16)

Similar to Unit6 Computer Numerical Control (20)

More from mokhtar (20)

Recently uploaded (20)

Unit6 Computer Numerical Control