Introduction

History, Advantages, Disadvantages, Applications, Elements of Machine Control

Introduction to the concept of computer numerical

control

Definition of Numerical Control (NC):

• A versatile form of programmable automation in which machine movements and various other functions are controlled by instructions expressed as a series of letters, numbers and symbols initiated via an electronic control system.

History

• 1725; England, punched cards were used to control the cloth pattern of knitting machine operation

• 1863; automatic player piano, used air passing through holes in a perforated paper drum to control keyboard mechanism in a preprogrammed sequence.

• 1952; first NC machine built by MIT

Direct Numerical Control: NC machines are operated

directly from a remote computer

Computerized Numerical Control (CNC): late 60's

early 70's:• When the control system utilizes a

dedicated internal computer positioned on the machine. A dedicated computer is built into the Machine Control Unit (MCU) of one machine tool.

The internal computer allows for the following:

• Storage of additional programs

• Program editing

• Running of programs from memory

• Machine and control diagnostics

• Special routines

• Inch/metric-incremental/absolute switchability

6 Elements of CNC system:

• 1. Part program• 2. Program input device• 3. Machine control unit (MCU)– reads and interprets coded instructions for

machining the particular work piece, then generates electric output signals

• 4. Drive system• 5. machine tool• 6. Feedback system (sometimes)

Advantages of CNC:

• Increases productivity

• High accuracy and repeatability

• Reduced production costs

• Reduced indirect operating costs

• Complex machining operations

• Greater flexibility

Adv. (Continued)

• Lower operator shill required

• Facilitates flexible automation

• Reduced rejection rates

• Reduced tooling costs

• Improved production planning and control

Limitations of CNC:

• 1. High initial investment

• 2. High maintenance

• 3. Not cost effective for low production levels

Applications of CNC:

• 1. Chip removal

• 2. Grinding

• 3. Unconventional Machining

• 4. Fabrication

Specifically:

• Metal, wood, plastic, etc. cutting

• Flame cutting, waterjet cutting, laser cutting

• Sheet metal forming and punching

• Electrical discharge machining (EDM)

• Welding

Fewer employees will be required but those needed

will be technicians with knowledge of metal-cutting

methods, cutting speeds and feeds, work holding,

and tool set-up

Cannot eliminate human involvement totally from the

machining process.

• There is no automatic control system capable of making a decision in the true sense of the word. Its capability is restricted to responding to a manual or computer prepared program

Elements of Machine Control

1. Slide movements• Hydraulically actuated pistons

• Electric servo motors (common)

• Axis of movement are X, Y, & Z (sometimes referred to as U, V, & W)

• Z axis is always related to motion parallel to the spindle axis

• Length of travel of slide movement is called “Coordinate Dimension”

• Slide movement caused by Servo

Parameters of control for slide movements

• Starting and stopping

• Direction

• Feed rate (how fast)

• Distance

Rotary Movements

• A about X

• B about Y

• C about Z

2. Machine spindles

• Electric motors

• Hydraulic motors

Parameters of control for spindles

• Stopping and starting

• Direction (CW or CCW)

• Speed of rotation

3. Tooling

• Turrets

• Magazines (chain)

• Manual intervention

4. Work holding

• Automatic (electric, hydraulic, pneumatic actuated vises, chucks, collets, robots, pallets)

• Manual intervention

5. Supporting functions

• Apply coolant

• Blast of air to clear chips

• etc.