CNC PP

CNCUpon completion of this topic, student should be to :

Understand the concept and principles of computer numerical control (CNC) system

INTRODUCTION

Numerical Control (NC) is one in which

the functions and motions of a machine

tool are controlled by means of a

prepared program containing coded

alphanumeric data.

Numerical control (NC) machines are

used in manufacturing tasks, such as

milling, turning, punching,drilling and

welding

NC OPERATION

CNC stands for Computer Numerical

Control

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.

An open-loop control system for a numerical-control machine

A closed-loop control system for a numerical-control machine

INDUSTRIAL APPLICATION

Metal Machining Metal Forming Assembly Joining Finishing

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

Punching and

nibbling

Guillotines

Flame cut and

profiling

Folding

Pipe bending

Metal spinning

Pick and place robots,

spot and seam welding

machines and robots,

riveting, looming of

wires and assembly of

components into

printed circuit boards

Plating

Painting

CNC AXIS

Milling

Turning/latheLinear Axes X Y Z

Rotary Axes A B C

Secondary Linear U V W

Interpolation I J K

NC MACHINE UNIT

designed to cut away surplus material and leave a component of the required shape and size

The function of machine tools can give the results in consistence and reduced the machining time

machine tool

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

The main grouping of parts of a control could be considered to be the Control Panel, the Tape Reader and The Processors

Machine control unit

(MCU)

two types of control systems :The point-to-point system and the continuous-path system

The point-to-point controls were NC controls, while the continuous path controls could be NC or CNC controls

Machine control system

NC PROGRAMMING

- Job PlanningJob Planning

Sketch the part Add incremental or absolute dimensions

Ascertain featuring Select fixtures which have minimal projections above

the part

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.

Plan operation sequence Mark sequence pattern of sketch.

Test program data for accuracy

Record necessary data for each movement of

the table and tool on the program sheet.

Record instructions for the machine operator Identify, specific:

1. Tools needed.

2. Speed and feed data

3. Tool change points

4. Console switch setting

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

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 up speed

G codes

Sample G codes

G00 Rapid movement for position

G0I 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

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

M Codes

M00 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

Coordinate system

Incremental (G91)

movement with respect to the

preceding point in a prescribed sequence

of points

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

Absolute (G90)

the next location always in terms of its relationship to the fixed zero point

All moves are performed with

respect to the axes zero

Incremental (G91)

Absolute (G90)

Linear Interpolation

Under this command the machine tool

will move in a straight line at a defined

feed rate

Ex:

G01 X200 F250Meaning :

G01 Move in a straight line

X200. A distance of 200mm

F250. At a feed rate of 250mm/min

Circular Interpolation

G02 will cause the path to be transcribed in a clockwise direction

and G03 will cause counter-clockwise motion

Program Definition

A programmed is a series of instructions to the machine, set out in sequence to -produce a complete machining operation. A programmed is made up of a series of blocks

Programme

A block or programmed 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

Block

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

Word

Type of control system

point-to-point Point-to-point systems are used

mainly in drilling, punching, and

straight milling operations

Contouring used on lathes, milling machines,

grinders, welding machinery, and

machining centres

WRITING A PROGRAM

programming

N10 G71 G90 S1500 T1

N20 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

N130 G00 M00

N140 G00 X0 Y0

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

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).

Description

ADVANTAGES OF COMPUTER

NUMERICAL CONTROL

can be edited 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.

easily changes into metric system if the programme is in

the imperial units.

It is widely used in industry. It is easily adaptable in a

computerised industry system.

Increased flexibility the machine can produce a

specific part, followed by other parts with different

shapes, and at reduces cost.

Greater accuracy computers have a higher sampling rate

and faster operation.

More versatility editing and debugging programmes,

reprogramming, and plotting and printing part shape are

simpler.

Programmes are stored on the machine ready for use.

Programmes and data can be modified on the machine.

Example 1 (use G90)

Example 2

Quiz (use G91)