OBJECTIVES

Fundamentals of NC systems Classification of NC systems Advantages of NC systems

NUMERICAL CONTROL

NC system has been defined by EIA as “ A system in which

actions are controlled by direct insertion of numerical data at

some point. The system must interpret at least some portion of

this data.”

NC milling, drilling & boring machines, NC lathe &turning

centers, NC grinders, NC punches& presses, NC welding machines,

NC inspection machines, NC tube bending machines, NC wirecut

EDM, NC wire wrapping machines, NC gear hobbing machines etc.

PART PROGRAM

The numerical data which is required for producing a part is

maintained in the form of series of instructions on a punched tape

and is called “Part program”.

Example:

– N01G90 G21

– N03 M06 T12

– N05 GOO X0 Y0 Z.1 F10 S2500 M03

– N07 G1Z-.5

– N09 G02 X-10. I0J0F20

– N13 G01 X0Y10

– N17 X10Y0

– N19 X0Y-10

– N21 X-10Y0

– N23 M2

NC VS CONVENTIONAL

In NC machine each axis of motion is controlled by a separate

driving device which replaces the hand wheel of the conventional

machine.

Commonly used driving devices on NC machines are AC & DC

servo motors, Stepped motors, Hydraulic motors.

AXIS OF MOTION

Axis of motion means axis along the cutting tool moves relative to the workpiece.

EIA & ISO recommends right hand co-ordinate system.

ELEMENTS OF NC MACHINE

NC machine tool system consists of MCU & NC machine tool itself.

ELEMENTS OF NC MACHINE

DPU: Decodes the instructions received from the tape, process it and provides

data to the CLU. Such data contains new required positions of each axis, its direction

of motion & velocity, auxillary control signals to relays etc.

CLU: Operates the drives attached to machine lead screws and receives feed

back signal on the actual position and velocity of each axes. It also provides a

signal announcing that the previous segment is completed & the DPU can read

new block of part program.

ELEMENTS OF NC MACHINE

DPU includes the following circuits:

Input device

Reading circuits & Parity checking logic

Decoding circuits

Interpolator circuits

CLU consists of following circuits:

Position control loops

Velocity control loops

Deceleration & Backlash takeup circuits

Auxillary function control

CLASSIFICATION OF NC SYSTEMS

Based on motion type

Point to Point and Contouring systems

Based on structure of the controller

NC and CNC

Based on programming method

Incremental and Absolute systems

Based on type of control loops

Open loop and Closed loop systems

POINT TO POINT SYSTEM

Point to point control system is designed to move the tool to the pre destined location.

No control over the path and velocity while moving from one location to another.

Example: NC drilling machine and Spot welding machines.

STRAIGHT CUT NC SYSTEM

The straight cut NC system can move the cutting tool parallel to one of the major axis

at controlled rate as required for machining operation.

The main limitation with straight cut NC is that it can’t perform angular cut.

Example: Shaping machine.

CONTOURING SYSTEM

The continuous path NC can control the movement of more than one axis

simultaneously to generate the desired geometry of the workpiece.

HARDWARE ASPECTS

PTP system would require only Position counters.

L-type NC requires Position counters and velocity control loops.

C-type NC is provided with position loops & position counters for each axis. The

velocity of each axis is decided by interpolation circuits in DPU based on the feed

rate along the given contour.

MATHEMATICAL PERSPECTIVE

P- type NC can position the tool to any point (x,y).

L- type NC can move the tool along x,y axes and can control the feed rates dx/dt &

dy/dt but only one at a time.

C-type can perform L-type and simultaneous control of dx/dt & dy/dt.

NC Vs CNC

NC system use electronic hardware based on digital circuit technology. CNC

machines employ a computer to control the machine tool and eliminates additional

hardware in control cabinet.

NC is Hardware based logic and CNC is Software based logic.

INCREMENTAL AND ABSOLUTE SYSTEM

In incremental system each position is specified with respect to previous one. In

absolute system all tool positions are specified with respect to one fixed zero point.

RELATIVE MERITS AND DEMERITS

In case of interruption it is easy to bring the tool to the beginning of the segment in

which interruption occurred with absolute system. In incremental system operator has

to restart the program and repeat the entire operation prior to interruption point.

It is easy to change the dimensional data in part program with absolute system,

where as in incremental system the part must be re programmed from the point at

which original part program has been changed.

RELATIVE MERITS AND DEMERITS

Error detection is easy with incremental system.

Mirror image programming is facilitated with incremental system.

OPEN LOOP AND CLOSED LOOP SYSTEMS

In Open loop systems there won’t be any feed back system to check whether the

programmed position and velocity are achieved or not.

In general open loop system uses stepped motor as a driving device.

OPEN LOOP AND CLOSED LOOP SYSTEMS

The closed loop control system measures the actual position and velocity of each

axis and compares with the desired references. The difference between actual and

desired values is error.

OPEN LOOP AND CLOSED LOOP SYSTEMS

Encoder is used as a feed back device in closed loop control systems.

ADVANTAGES AND DISADVANTAGES OF NC

Advantages:

A full flexibility

Accuracy is maintained through the full range of speeds and feed.

Shorter production time

Possiblity of manufacturing complicated contours

Easy adjustment of machine

No need of highly skilled and experienced operator

Operator will have free time

Disadvantages:

Relatively high cost.

More complicated maintenance

A highly skilled and trained part programmer is needed.

FUNDAMENTALS OF MACHINING

Machining is the manufacturing process in which the size, shape or surface

properties of a part are changed by removing the excess material.

Conventional machining processes vs Uncoventional processes.

The cutting conditions in machining with NC machines are : Cutting speed, feed and

depth of cut.

CUTTING CONDITIONS

Cutting speed (V) is defined as the relative velocity between the cutting tool and

the workpiece material and is expressed as m/min. In NC machines spindle speed is

used rather than cutting speed in programming.

Feed (f) is defined as relative lateral movement between the tool and the

workpiece during machining operation. It is expressed as length units/rev. In NC

machines it is also expressed as units of length per min.

Depth of cut (d) is defined as the distance the cutting tool projects below the

original surface of the work and is expressed in mm.

MRR = Vfd

DESIGN CONSIDERATIONS OF NC MACHINE TOOLS

The reason that stimulated the development of NC were the demand for

better accracy and to increase productivity.

However, it should be noted that the combined characteristics of the control

and machine tool determines the final accuracy and productivity of the NC or CNC

system.

RESOLUTION OF NC SYSTEM

Programming resolution : It is the smallest allowable position increment in part

programs and is referred as the BLU.

Control resolution: It is the smallest change in position that the feedback device

can sense.

For better system efficiency Programming resolution = Control resolution

ACCURACY OF NC SYSTEM

Final accuracy of CNC system depends on

a) Computer control algorithms

b) System resolution

c) Machine inaccuracies

System accuracy = ½ BLU + Machine accuracy

Machine designer tries to ensure that the effect of all inaccuracies associated with

machine tool will be under ½ BLU.

REPEATABILITY OF NC SYSTEM

Repeatability is the statistical term associated with accuracy.

The system repeatability is the positional deviation from the average of the

displacements.

Repeatability is always be better than accuracy.

PRODUCTIVITY VS ACCURACY

High Productivity and Accuracy might be contradictory features.

IMPROVING ACCURACY OF NC SYSTEM

Improved machine tool structure

Use of low friction moving components

Avoiding lost motions and isolating thermal sources

High productivity by improving the efficiency of machine using

machining centers

ECONOMICS

Mass production: SPMs arranged in transfer line.

Low volume production : Conventional machines

Medium volume production: NC machines

RETROFITTED NC MACHINE

Adding MCU

Replacing the conventional screw with ball bearing screw

Adding stepped motors/servo motors and feed back devices

INCREASING PRODUCTIVITY WITH NC MACHINES

The total production time comprises of four time components

i. Actual cutting time

ii. Idle or Traverse motion time

iii. Loading & Unloading time

iv. Tool changing time

Actual cutting time can only be slightly reduced with NC machines however

other three components can be reduced substantially.

IDLE OR TRAVERSE MOTION

Idle or Traverse motion time can be reduced by increasing the traverse velocity

thus idle time is reduced and production time is saved.

LOADING & UNLOADING

Loading & Unloading times can be reduced by using two workholding devices

simultaneously on the machine table.

Using Automatic tool change methods

AUTOMATIC TOOL CHANGE (ATC)

In case of machines with ATC large number of tool are stored in the magazines.

Two types of magazines are very common on CNC machines

i. Chain type tool magazine

ii. Carousel type tool magazine

ROTATING ARM MECHANISM

MACHINING CENTERS

Machining centers are more rigid than conventional machines and equipped

with ATCs and rotary tables.

Capable of machining complex parts with high accuracy. May not be economical

for small parts.

Turning centers.

MCU FUNCTIONS

Various functions on the control panel of the MCU are the direct interface between

machine operator and NC system.

MODE SELECTION

Auto mode for continuous execution of part program.

Manual or Dial mode G-code, M-code, V, f, d and co-ordinate information is

entered and executed block by block.

Jogging mode is used for positioning the slides. Jogging mode is used for

coarse positioning and discrete jogging switches are used for fine positionning.

Block by Block mode is used to read and execute one block of information at a time.

COMPENSATIONS AND OVERRIDE

Cutter radius compensation: This feature is used to offset actual tool path from

programmed tool path.

Tool length compensation: This feature is suitable in drilling applications

where varying lengths of drills are used.

Feed rate override: This feature is useful to override the programmed feed rate

in case of adverse cutting conditions.

READOUT DISPLAYS

Sequence number readout: Helps to know the location of programming errors.

Present position readout: Electronically display the present position of one or

more axes.

CNC CONTROLLER

Keyboard to edit part program and CRT screen to display messages.