Introduction to Computer
Numerical control
Types of CNC
• Milling Machines can divide in to three categories:
By number of axes - Two , three or more
By the orientation of axes- Vertical or Horizontal
By presence or absence of tool changer
Types of CNC
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Plasma
Spring forming
Laser Cutting machine
Flame cutting machine
Electric Discharge Machine
Water Jet
Welding
CNC Punch Press
Types of Control System
• Point to point Tool Movement
• Point to point control system causes the tool to move to a point on
the part and execute an operation at the point only
• Continues path tool Movement
• Continues path controller are named because they cause the tool to
maintain continues contact with the part as the tool cuts a contour
shape
Machine tool Axes
• Numerical machines can also be classified on the number of axes, or
direction of motion.
• Machine centers usually have two-and – half, three, four, or five
axis with three or four axis are more common
• Lathes and turning centers generally have between two and four
axes
Loop System
• Loop system send electrical signal to drive motor controllers and receives
feedback. One of the important factor that determine the tolerances to which a
part can be cut is the loop system type.
 Open Loop System
• An open loop system utilizes stepping motors to create machine movement by
electrical signals. the motor connected to Acme lead screw
• The lead screws used in this system generates friction and backlash. Backlash can
cause positioning Error
Close Loop System
• Special motors called servos are used for executing machine
movements in closed loop system.
• Motor type can be
• AC servos,
• DC servos,
• Hydraulic servos.
• The speed depend on amount of current or hydraulic fluid
passing through it.
• Servos are connected to the spindle and they are connected to
the machine table through the ball lead screw
Ball Lead Screw
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Ball lead screw is the hart of the drive system
Advantages of ball lead screw over Acme lead screw are:
1. Precise position and repeatability
2. High Speed capability,
3. Less Wear
4. longer life
Position Loop
• Position loop is closed by a device called an ENCODER.
• An encoder is a disk with three rings around it
• 1000 slits in the first ring,
• 1000 slits in the second ring,
• Only one slit in the third
• There are three light receiver on the other side of the disk
• Encoder passes the light to the receiver when the slits line up
with the light and receiver. This create pulses
• The CNC control knows there are 1000 pulses per revolution so
by counting the pulses it knows the encoder’s position exactly
Homing (ZRN)
• Homing moves axes to a known position where
each axes is initialized
Cartesian coordinate system
• Any point on a machine part, such as a hole to be drilled, can be described in
terms of its position.
• the system that allows us to do this is Cartesian coordinate
Machine axes
• Machine axes are established according to the industry
standard report EIA RS - 267A
Axes configuration
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X axes moves from right to left as you face the machine
Y axes move toward and away from you
The Z axes is the spindle movement up and down spindle .
A move toward work is Z(-Z)
A move away from work is Z(+ Z)
Types of tool positioning
• Incremental Positioning
• Absolute Positioning
End of presentation