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.