Unit6 Computer Numerical Control

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COMPUTER NUMERICAL CONTROL
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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.
.

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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.

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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

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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.

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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

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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

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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).

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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.

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spindle

rotation
table

z
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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.

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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.

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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.

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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

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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

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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 ;

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- 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.

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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

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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.

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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.

Unit6 Computer Numerical Control

Unit6 Computer Numerical Control

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