My Simple Note Chapter 3 : Numerical Control Systems

 
CNC COMPONENT

a) The machine tool
b) Controller - Machine control unit (MCU)

CNC CONTROLLER MANUFACTURES:

• General Numeric
• Fanuc
• Genearal Electic
• Bendix
• Cincinati Milacron
• Haidenhain
• Allen Bradley
• G & L Electronic

2 TYPES OF CONTROL SYSTEMS

a) POINT TO POINT SYSTEM

• Move straight line
•  limited hole operation such as

                  - reaming

                   - boring

                   - drilling

• straight milling cut parallel to a machine axis
• cutting 45 degree possible but not angle or arc
• speed is same

b) CONTINUOS PATH SYSTEM

• Has ability to move it drive motor
• Cutting of arcs and segment is easily

CNC SYSTEM

3 SUB-SYSTEMS IS:

1. Control

2 Drive

3. Feedback

1) CONTROL SYSTEM

• The function of interface is to store and read the program and tell other conponent what to do

• The control act as the user interface for operator to set up and run the machine.This also called MCU ( machine control unit) which have two basic control

                    a) Proprietary- expensive and high realibility

                    b) PC- Based - low realibility

                    c) Motion Control

2) DRIVE SYSTEM ( SERVOMECHANISMS)

A) STEPPER MOTOR

• Move a set amount of rotation ( a step) every time motor receive electronic pulse

B) DC AND AC SERVOS

• Widely used variable- speed motor on small & medium continuos path machine
• A servo does not move a set distance
• Ac motor have high power than DC
• The current removed, motor stop turning

C) HYDRAULIC SERVOS

• Variable speed motor
• High power than A) and B)
• Large machined is used

3) FEEDBACK SYSTEM

Purpose:  provide control and information about the status of motion control systemI

• It will compare from desired condition to actual condition & make correction
• Most obuious information to be feedback to the controll on CNC machine

LOOP SYSTEM

a) Open Loop 

• Use stepper motor
• The machine receive information from reader & store in storage device
• If want to use it send to motor
• After the motor is finished, a signal is sent back to motor
• Other next instruction is requested
• Have no feed back

b) Close Loop

  

• Other than stepper motor
• Information is sent to drive motor , the position of motor is monitored by system
• Compare what have been sent to
• If error is detected, machine stop execuitng program until the inaccurancy is corrected
• The machine receive information from reading device and store in storage device

INTERPOLATION

• A device is called interpolator because the drive have to move simultaneously from start to end of the command
• An interpolation provide two function

      a) Calculate individual axis velocities to drive the tool the programmed path at a given speed rate

      b) Generate thousand of intermediate coordinate point along the programmed part between star point to end point

LINEAR INTERPOLATION

• Calculate point to point
• GOO - for rapid movement
• G01- for cutting curve..need speed rate

CIRCULAR INTERPPOLATION

• To overcome difficulty in programming arcs & circles
• G02- Cutting clockwise
• G03- cutting antiClockwise

REFERENCES POINTS

• Fixed referances point
• On workpiece declared by programmer
• Tooling

   MACHINE REFERENCE

 The machine zero or home position is the origin of the machine’s  coordinate system and is built in to the machine tool  and it is a fixed position

- All the machine movements are measured from this reference or origin point where the MCU continually calculates the machine 

movements from the origin point

- On vertical machining centers, the machine origin for the X and Y axes is usually at the top right corner of the table. The Z axis is usually set at the tool change position 

 -The machine origin on the lathe is usually set at the top right side of the machine

POSITIONING SYSTEM

a) Incremental- calculate direct from point - G91

b) Absolute- calculate from origin- G90