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:
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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:
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Increases productivity
High accuracy and repeatability
Reduced production costs
Reduced indirect operating costs
Complex machining operations
Greater flexibility
Adv. (Continued)
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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:
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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
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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.