Introduction, History, Advantages, Disadvantages

History, Advantages,
Disadvantages, Applications,
Elements of Machine Control
Introduction to the concept
of computer numerical
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
• 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
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
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
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
• Metal, wood, plastic, etc. cutting
• Flame cutting, waterjet cutting, laser
• Sheet metal forming and punching
• Electrical discharge machining
• 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
Elements of Machine
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
Feed rate (how fast)
Rotary Movements
• A about X
• B about Y
• C about Z
2. Machine spindles
• Electric motors
• Hydraulic motors
Parameters of control for
• 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.