'Introduction to CNC

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UNIT - III
COMPUTER AIDED MANUFACTURING
What is NC ? 
• Numerical control (NC) refer to control of a
machine or a process using symbolic codes
consisting of characters and numerals.
• The concept of NC was proposed in the late
1940s by John Parsons who recommended a
method of automatic machine control that
would guide a milling cutter to produce a
curvilinear motion in order to generate smooth
profiles on the work-pieces.
Components of traditional NC systems
Advantages of NC systems
• Better control of the tool motion under optimum cutting conditions.
• Improved part quality and repeatability.
• Reduced tooling costs, tool wear, and job setup time.
• Reduced time to manufacture parts.
• Reduced scrap.
• Better production planning and placement of machining operations 
in the hand of engineering
Computer Numerical Control (CNC)
• Computer numerical control (CNC) is the
numerical control system in which a
dedicated computer is built into the control
to perform basic and advanced NC functions.
• CNC is a computer assisted process to control
general purpose machines from instructions
generated by a processor and stored in a
memory system.
Components of modern CNC systems
Advantages of CNC systems
• CNC machines can be used continuously and only need to be switched off for
occasional maintenance.
• It requires less skilled people to operate unlike manual lathes/milling
machines etc.
• CNC machines can be updated by improving the software used to drive the
machines.
• The manufacturing process can be simulated virtually and no need to make a
prototype or a model. This saves time and money.
• Once programmed, these machines can be left and do not require any human
intervention, except for work loading and unloading.
• These machines can manufacture several components to the required
accuracy without any fatigue as in the case of manually operated machines.
Types of control circuits in CNC
• There are two types of control circuits which is used to operate a 
CNC
Open Loop system
Semi-closed Loop system
Closed loop system
Open Loop Control system
• Programmed instructions are fed into the controller through an input device.
• These instructions are then converted to electrical pulses (signals) by the
controller and sent to the servo amplifier to energize the servo motors.
• The cumulative number of electrical pulses determines the distance each servo
drive will move, and the pulse frequency determines the velocity.
Open Loop Control system
• The primary drawback of the open-loop system is that there is
no feedback system to check whether the program position
and velocity has been achieved.
• The open-loop system is generally used in point-to-point
systems where the accuracy requirements are not critical.
• Example Simple drilling operations.
Semi - Closed Loop Control system
• In an Open loop system, if any one
feedback is achieved by attaching
an encoder. The system is called as
Semi-closed loop system.
• The figure shows, only one
encoder is used
• It serves as both position and
velocity feedback unit.
• This encoder is mounted at the
back of the servo motor.
• This system may cause error due
to ball screw, coupling, etc.
Closed loop system
• The closed-loop system has a feedback subsystem to
monitor the actual output and correct any discrepancy
from the programmed input.
• The feedback system could be either analog or digital.
• Analog systems measure the variation of physical
variables such as position and velocity in terms of
voltage levels.
• Digital systems monitor output variations by means
of electrical pulses
• Closed-loop systems are very powerful and accurate
because they are capable of monitoring operating
conditions through feedback subsystems and
automatically compensating for any variations in real-
time.
Closed loop system
Point to Point (PTP) CNC Control
• The primary function of the PTP control system is to move a
tool from one point location to another specified point.
• It is used to do hole operations such as drilling, boring,
reaming, tapping and punching.
• The path the tool follows may be one of three paths,
depending on the control being utilized:
• Axial Path :
• The tool moves in a right-angle path parallel to the two
positioning axes, X andY.
• This is the slowest method because the axes are moved
one at a time.
Point to Point CNC Control
• 45 Degree Path
• The tool moves at a 45 degree angle until one of
the axes is lined up with the new position.
• Initially, both the X and Y axis move
simultaneously until one of the specified
coordinates is reached, and the tool then travels
in a linear mode to the specified coordinate
position.
• Linear Path
• In this mode, the controller has the capability to
synchronize the motion in both axes, X and Y, to
generate a linear path.
• This method provides the quickest path from one
point to another, but sophisticated equipment is
required to coordinate the speed of each axis to
maintain a straight line
Continuous path system
• It is used in CNC machine tools such as
milling machines.
• These machines require simultaneous
control of axes.
• Its function is to synchronize the axes of
motion to generate a predetermined path,
generally a line or a circular arc.
• The contouring system has the capability
of controlling its drive motors
independently at various speeds as the
tool moves towards the specified position.
Classification based on the number of axes
• Movement of table in X direction is called as X axis.
• Movement of table in Y direction is called as Y axis.
• Movement of spindle in Z direction is called as Z axis.
• Rotation of table in X axis is called as A axis.
• Rotation of table in Y axis is called as B axis.
• Rotation of spindle in Z axis is called as C axis.
The machine is classified as 
2 Axis 3 Axis 4 Axis
5 Axis 6 Axis
2 Axis - Machine
• During Operation or Machining, two axis can be used 
simultaneously.
• It has One rotation (fixed) and Two Linear movement (movable)
• Example CNC Lathe.
3 Axis Machine
• During Operation or Machining, three axis can be used simultaneously.
• It has One rotation (fixed) and Three Linear movement (movable)
• Example CNC Milling.
Horizontal Milling machine Vertical Milling machine
z z
4 Axis Machine
• During Operation or Machining, four axis can be used simultaneously.
• It has One rotation (fixed), One rotation (movable) and three Linear movement 
(movable)
5 Axis Machine
• During Operation or Machining, five axis can be used simultaneously.
• It has One rotation (fixed), Two rotation (movable) and three Linear movement 
(movable)
6 Axis Machine
• During Operation or Machining, all the six axis can be used
simultaneously.
• It has Three rotation (movable) and three Linear movement
(movable)
• Refer the video for the movement of six axes in a 6 axis machine.
Direct Numerical Control (DNC)
• Here a mainframe computer is used to
coordinate the simultaneous operations of a
number NC machines.
• The main tasks performed by the computer
are to program and edit part programs as
well as download part programs to NC
machines.
• Machine tool controllers have limited
memory and a part program may contain
few thousands of blocks. So the program is
stored in a separate computer and sent
directly to the machine, one block at a time.
Applications of CNC
Applications of CNC
Applications of CNC
Applications of CNC
Applications of CNC
Applications of CNC
Applications of CNC
THANK YOU
NEXT TOPIC:
CNC PROGRAMMING (To be continued..)