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

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Introduction

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

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Introduction to the concept of computer numerical
control
3
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.

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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

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Direct Numerical Control NC machines are
operated directly from a remote computer
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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.

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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

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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)

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Advantages of CNC

• Increases productivity
• High accuracy and repeatability
• Reduced production costs
• Reduced indirect operating costs
• Complex machining operations
• Greater flexibility

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Adv. (Continued)

• Lower operator shill required
• Facilitates flexible automation
• Reduced rejection rates
• Reduced tooling costs
• Improved production planning and control

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Limitations of CNC

• 1. High initial investment
• 2. High maintenance
• 3. Not cost effective for low production levels

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Applications of CNC

• 1. Chip removal
• 2. Grinding
• 3. Unconventional Machining
• 4. Fabrication

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Specifically

• Metal, wood, plastic, etc. cutting
• Flame cutting, waterjet cutting, laser cutting
• Sheet metal forming and punching
• Electrical discharge machining (EDM)
• Welding

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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
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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

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Elements of Machine Control
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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

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Parameters of control for slide movements

• Starting and stopping
• Direction
• Feed rate (how fast)
• Distance

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Rotary Movements

• A about X
• B about Y
• C about Z

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2. Machine spindles

• Electric motors
• Hydraulic motors

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Parameters of control for spindles

• Stopping and starting
• Direction (CW or CCW)
• Speed of rotation

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3. Tooling

• Turrets
• Magazines (chain)
• Manual intervention

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4. Work holding

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

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5. Supporting functions

• Apply coolant
• Blast of air to clear chips
• etc.