Contents show

MAJOR COMPONENTS RELATED TO CNC MACHINE
TOOLS

Any CNC machine tool essentially consists of the following parts:

Part program:

• A series of coded instructions required to produce a part.

• Controls the movement of the machine tool and on/off
control of auxiliary functions such as spindle rotation
and coolant.

• The coded instructions are composed of letters, numbers
and symbols.

Program input device

• The program input device is the means for part
program to be entered into the CNC control.

• Three commonly used program input devices are
punch tape reader, magnetic tape reader, and
computer via RS-232-C communication.

Machine Control Unit

The machine control unit (MCU) is the heart of a CNC
system. It is used to perform the following functions:

• To read the coded instructions.

• To decode the coded instructions.

• To implement interpolations (linear, circular, and
helical) to generate axis motion commands.

• To feed the axis motion commands to the amplifier
circuits for driving the axis mechanisms.

• To receive the feedback signals of position and speed
for each drive axis.

• To implement auxiliary control functions such as
coolant or spindle on/off and tool change.

Machine Tool

• CNC controls are used to control various types of
machine tools.

• Regardless of which type of machine tool is
controlled, it always has a slide table and a spindle
to control position and speed.

• The machine table is controlled in the X and Y
axes, while the spindle runs along the Z axis

Feed Back System

• The feedback system is also referred to as the
measuring system.

• It uses position and speed transducers to continuously
monitor the position at which the cutting tool is located
at any particular instant.

• The MCU uses the difference between reference signals
and feedback signals to generate the control signals for
correcting position and speed errors.

Drive System

• Drives are used to provide controlled motion to
CNC elements

• A drive system consists of amplifier circuits, drive
motors, and ball lead-screws.

• The MCU feeds the control signals (position and
speed) of each axis to the amplifier circuits.

• The control signals are augmented to actuate drive
motors which in turn rotate the ball lead-screws to
position the machine table.

POWER DRIVES

In machine tools, power is generally required for driving the main
spindle, saddles and carriages and to some auxiliary units.

• The motors used for CNC system are of two kinds

1. Electrical – AC , DC or Stepper motors

2. Fluid – Hydraulic or Pneumatic

• In CNC, usually stepper and servo electrical drives are used

• They exhibit favourable torque-speed characteristics and are
relatively inexpensive.

STEPPER MOTOR

A stepper motor is a pulse-driven motor that
changes the angular position of the rotor in steps.
Due to this nature of a stepper motor, it is
widely used in low cost, open loop position control
systems.

Types of stepper motors: Permanent Magnet
Employ permanent magnet
Low speed, relatively high torque
o Variable Reluctance
Does not have permanent magnet
Low torque

MAJOR COMPONENTS RELATED TO CNC MACHINE TOOLS