NC and CNC machines and Control Programming

Introduction to NC and CNC machines

CNC controls and RS274 programming

History of CNC

1949
US Air Force asks MIT to develop a "numerically controlled"
machine.

1952
Prototype NC machine demonstrated (punched tape input)

1980-
CNC machines (computer used to link directly to controller)

1990-
DNC: external computer “drip feeds” control programmer
to machine tool controller
Motivation and uses

To manufacture complex curved geometries in 2D or 3D

was extremely expensive by mechanical means (which
usually would require complex jigs to control the cutter
motions)

Machining components with repeatable accuracy

Unmanned machining operations

Advantages of CNC

- Easier to program;
- Easy storage of existing programs;
- Easy to change a program
- Avoids human errors
- NC machines are safer to operate
- Complex geometry is produced as cheaply as simple ones
- Usually generates closer tolerances than manual machines
Conventional milling machines

Vertical milling machine

Conventional milling machines

Vertical Milling machine architecture

Conventional milling machines

Horizontal Milling machine architecture

How does the table move along X- Y- and Z- axes ?

NC machines

Motion control is done by: servo-controlled motors

CNC terminology

BLU: basic length unit 

smallest programmable move of each axis.

Controller: (Machine Control Unit, MCU) 

Electronic and computerized interface between operator and m/c

Controller components:

1. Data Processing Unit (DPU)

2. Control-Loops Unit (CLU)
Controller components

Data Processing Unit:

Input device [RS-232 port/ Tape Reader/ Punched Tape Reader]

Data Reading Circuits and Parity Checking Circuits

Decoders to distribute data to the axes controllers.

Control Loops Unit:

Interpolator to supply machine-motion commands between data points

Position control loop hardware for each axis of motion
Types of CNC machines

Based on Motion Type:

Point-to-Point
or
Continuous path

Based

Open on Control Loops:
loop
or
Closed loop

Based on Power Supply:

Electric

or
Hydraulic
or
Pneumatic

Based on Positioning System

Incremental
or
Absolute
Open Loop vs. Closed Loop controls
Open loop control of a Point-to-Point NC drilling machine

NOTE: this machine uses stepper motor control

Components of Servo-motor controlled CNC

Motor lead screw rotation table moves

Motor speed control feedback position sensed by encoder

Two types of encoder configurations

Motion Control and feedback

Encoder outputs: electrical pulses (e.g. 500 pulses per revolution)

Rotation of the motor  linear motion of the table: by the leadscrew

The pitch of the leadscrew: horizontal distance between successive threads

One thread in a screw  single start screw: Dist moved in 1 rev = pitch

Two threads in screw  double start screw: Dist moved in 1 rev = 2* pitch
Example 1

A Stepping motor of 20 steps per revolution moves a machine table

through a leadscrew of 0.2 mm pitch.

(a) What is the BLU of the system ?

(b) If the motor receives 2000 pulses per minute, what is the
linear velocity in inch/min ?
Manual NC programming

Part program: A computer program to specify

- Which tool should be loaded on the machine spindle;

- What are the cutting conditions (speed, feed, coolant ON/OFF etc)
- The start point and end point of a motion segment
- how to move the tool with respect to the machine.

Standard Part programming language: RS 274-D (Gerber, GN-code)

 History of CNC

The RS274-D is a word address format

Each line of program == 1 block

Each block is composed of several instructions, or (words)

Sequence and format of words:

N3 G2 X+1.4 Y+1.4 Z+1.4 I1.4 J1.4 K1.4 F3.2 S4

sequence no destination coordinates dist to center of circle tool

feed rate spindle speed

preparatory function
miscellaneous function
Manual Part Programming Example

Tool size = 0.25 inch,

Feed rate = 6 inch per minute,
Cutting speed = 300 rpm,
Tool start position: 2.0, 2.0
Programming in inches

Motion of tool:
p0  p1  p2  p3  p4  p5  p1  p0
1. Set up the programming parameters

Programming in inches

Use absolute coordinates

Feed in ipm

N010 G70 G90 G94 G97 M04

Spindle speed in rpm

Spindle CCW
2. Set up the machining conditions

Machine moves in XY-plane

Use full-circle interpolation

Feed rate

Spindle speed

N020 G17 G75 F6.0 S300 T1001 M08

Tool no.

Flood coolant ON
3. Move tool from p0 to p1 in straight line

Linear interpolation

target coordinates

N030 G01 X3.875 Y3.698

4. Cut profile from p1 to p2

Linear interpolation

target coordinates

N040 G01 X3.875 Y9.125

or

N040 G01 Y9.125

X-coordinate does not change  no need to program it

5. Cut profile from p2 to p3

Linear interpolation

target coordinates

N050 G01 X5.634 Y9.125

6. Cut along circle from p3 to p4

circular interpolation, CCW motion

target coordinates

N060 G03 X7.366 Y9.125 I6.5 J9.0

coordinates of center of circle

7. Cut from p4 to p5

Linear interpolation

target coordinates (Y is unchanged)

N070 G01 X9.302

8. Cut from p5 to p1

Linear interpolation

target coordinates (see step 3)

N080 G01 X3.875 Y3.698

9. Return to home position, stop program

Linear interpolation

target coordinates (see step 3)

N090 G01 X2.0 Y2.0 M30

end of data

N100 M00

program stop
Summary

CNC machines allow precise and repeatable control in machining

CNC lathes, Milling machines, etc. are all controlled by NC programs

NC programs can be generated manually, automatically

Additional references: RS274D code descriptions