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.


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.


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

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