chapter motion and control system

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CHPTER 4

Numerical control machine tool (NC)

Chapter contents:

1- Basic components of the system

2- NC coordinate systems3- Motion Control systems

4- Interpolation methods

5- Problems with Conventional NC

6- CNC

7- DNC

8- Applications of NC

9- Advantages and disadvantages of NC10- NC Part Programming

Manual part programming

Computer assisted part programming (APTAPT, Automatically Programmed Tool)


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Numerical control:

It is a form of programmable automation in which the mechanicalactions of a machine tool or other equipment are controlled by a

program containing coded alphanumeric data. The data represent

relative positions between the workpiece and work head (e.g., cutting

tool) as well as other instructions needed to operate the machine.

1- Basic components of the system

1) Program of instructions.

2) Machine control unit.

3) Processing equipment.

ProgramMachine

control unit

Process equipment


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1- Program of instructions

Is the detailed step-by-step set of directions, which tell the machinetool what to do.

- In machine tool applications, the program of instructions is called a

part program, and the person who prepares the program is called a

part programmer.

- In these applications, the individual commands refer topositions of

a cutting tool relative to the worktable on which the workpart is

fixtured. Additional instructions are usually included, such as

spindle speed, feed rate, cutting tool selection, and other functions.

- The program is stored formerly into punched cards, punched tapes.

Now, these technologies have been replaced by magnetic tape,

diskettes, and electronic transfer of part programs from a computer.


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2- NC coordinate systems

It is a standard axis system by which the position of the workhead relative to the work part can be specified

In order for the programmer to make the sequence of positions

and movements of the cutting tool relative to the workpiece, it is

necessary to establish a standard axis system by which the

relative positions can be specified.

NC programming is based on the Cartesian coordinate system

(sometimes on the polar coordinate system as well).

2-D coordinate system (zero point, positive and negative

directions for the two axes).

3-D coordinate system (zero point, right hand rule, the positive

and negative directions, and rotary motions a, b and c).

Zero point types (fixed zero, full shift zero, and full floating

zero).


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NC types with respect to the number

of axes; Two axes NC, Three axes NC,

Four axes NC, and Five axes NC

Milling Turning

The coordinate systems for the drilling and milling and turning

machine tools.

Position determination;

- Absolute positioning (x=6 and y=8) and

- Incremental positioning (x=4 and y=5).

y


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3-Motion Control systems

1) Point-to-point system (PTP), also called positioning system.

2) Straight cut system

3) Contouring system (continuous-path NC system).


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4- Interpolation methods

a) Linear interpolation

Y = tan (/2) cutter

radius.

X = tan (/2) cutter

radius.


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b) Circular interpolation

1) Inside tolerance (chord).

2) Outside tolerance (tangent).3) Inside and outside tolerance.


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c) Cutter size compensation.

Length compensation.

Wear compensation. Radius compensation.

Offset compensation.

5- Problems with Conventional NC

1) Part programming mistakes

The mistakes can be either: syntax errors or numerical errors.

It cannot achieve the best sequence of processing steps.

2) No optimal speeds and feeds

The conventional NC cannot provide the chance to make

changes in the feeds and speeds.

The programmer must set the speeds and feeds at the worst-

case conditions.


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3) Punched tape

4) Tape reader

5) Controller The NC controller is a hard-wire, which can not easy

improves, while the soft-wire is easy to improve with out the

change in hard-wire.6) Management information

The NC system cannot provide timely information on the

operational performance, such as piece counts, machine

breakdowns and tool change.


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6- CNC

6-1 Features;

1. Storage of more than one part programs2. Various forms of program input

3. Program editing at the machine tool

4. Fixed cycle and programming subroutines

5. Interpolation

6. Positioning features for setup

7. Cutter length and size compensation

8. Acceleration and deceleration calculations

9. Communication interface.

10. Diagnostics

a) Control start-up diagnostics.

b) Malfunction and failure analysis.c) Extended diagnostics for individual components.

d) Tool life monitoring.

e) Preventive maintenance notices.

f) Programming diagnostics.

chapter motion and control system

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