introduction history, advantages, disadvantages, applications, elements of machine control
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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.
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