UNIT-5 COMPUTER NUMERICAL CONTROL MACHINES

1. Presented By
Prof.S.Sathishkumar
Assistant Professor
Department of Mechanical Engineering
Vel Tech (Engineering College)
Avadi-Chennai-62
MANUFACTURING TECHNOLOGY-II
UNIT –V
CNC MACHINES

2. INTRODUCTION
Numerical Control (NC) machine tools – CNC types,
constructional details, special features, machining centre, part
programming fundamentals CNC – manual part programming
–micromachining – wafer machining

3. Comparison of conventional m/c Vs CNC m/c

4. Numerical control (NC)
 Numerical control (NC) is a form of programmable automation in which
the mechanical actions of a machine tool are precisely controlled by means
of an programme of instructions.
 NC is applied to a wide variety of manufacturing processes such as
machine tools (e.g. drilling, milling, turning, bending, punching, etc.), and
non-machine tool applications such as assembly, adhesive bonding and
inspection.

5. NC machining can be divided in to three types:
1. Traditional numerical control (NC) where mechanical
actions of a machine tool are precisely controlled by means of an
programme of instructions.
2. Computer numerical control (CNC) where each machine has
it own computer.
3. Direct numerical control (DNC) where a number of machines
are served by one computer

6. Components of an NC System
A Numerical control system consists of three basic
components:
1. Part Program
2. Machine Control Unit
3. Processing Equipment

7. ComponentsofanNCSystem

8. Components of an NC System

9.  The program of instructions or part program consists
of a detailed set of commands to be followed by the
processing equipment through actuators and sensors.
 Each command specifies a position or motion that is to be
accomplished by the work head relative to the workpiece.
 A position is defined by its x-y-z coordinates.
 Typical instructions include spindle speed, spindle
direction, feed rate, tool selection, and other commands
related to the operation.
1. Part Program

10.  Part programs are typically stored on
computers and sent directly to the machine
controller over a communications cable.
 The part program is prepared by a part
programmer, a person who is familiar with the
details of the programming language and also
understands the technology of the processing
equipment.

11. 2.Machine Control Unit (MCU)
 The machine control unit (MCU) converts the part
program into a usable format and executes it in the form of
the mechanical actions of the processing equipment.
 The MCU consists of both hardware and software.
 The hardware includes the microcomputer, components to
interface with the processing equipment, and certain
feedback control elements.

12.  The software in the MCU includes control system
software, calculation algorithms, and translation
software to convert the NC part program into a usable
format for the MCU.
 The MCU also permits the part program to be edited in
case the program contains errors, or changes in cutting
conditions are required.

13. 3.Processing Equipment
 The processing equipment accomplishes the sequence
of processing steps to transform the starting work
piece/part into a completed part.
 It operates under the control of the MCU according to
the instructions in the part program

14. Advantages
 Increased productivity
 Greater manufacturing
flexibility
 Improved accuracy
 Reliable and Safe operation
 Shorter cycle times
 lower manufacturing lead
times,
 Reduced nonproductive time
 Reduced human error
 Machining of complex 3-D
shapes
 Multi-operational machining

15. Disadvantages
 High initial investment
 High maintenance cost
 Need skilled Part programmer
 Not cost effective for low production cost
Applications
 Machine tools applications (e.g. drilling, milling,
turning, bending, punching, etc.)
 Non-machine tool applications (e.g. assembly, adhesive
bonding and inspection.

16. 2. Computer numerical control (CNC)

18. 3. Direct/ Distributed numerical control (DNC)

19. Classifications of NC Machine
1. Motion control : Point to point (PTP), or
Positioning ,Continuous or contouring Interpolation
2. Control loops : Open loop and
Closed loop
3. Positioning systems : Incremental and
Absolute positioning
4. Power drives : Hydraulic,
Electric,or Pneumatic

20. Types of coordinate/Positioning system
There are two types of coordinate system they are:
1. Absolute coordinate system
2. Incremental coordinate system
1. Absolute coordinate system
 Absolute program locations are always given from a single
fixed zero or origin point.
 The zero or origin point may be a position on the machine
table, such as the corner of the worktable or at any specific
point on the work piece.
 In absolute dimensioning/programming, each point or
location on the work piece is given as a certain distance from
the zero or reference point.

21. 1.Absolute coordinate system

22. 2. Incremental coordinate system
 In incremental system, the work coordinates change because
each location is the zero point for the move to the next
location.
 Last Point is considered as origin point for next
point/location.
 Incremental program locations are always given as the
distance and direction from the immediately preceding
point.

23. Types of positioning system
There are two types of positioning system. they are:
1. Point to point/ Positioning
2. Continuous/ Contouring
3. Interpolation
1. Point to point
Point-to-point positioning is used when it is necessary to
accurately locate the spindle, or the work piece mounted on
the machine table, at one or more specific Locations to
perform such operations as drilling, reaming, boring, tapping,
and punching.

24. 1. Pointto Point
• Moving at maximum rate from point to point.
• Accuracy of the destination is important but
not the path.
• Drilling is a good application.

25.  Point-to-point positioning is the process of positioning from
one coordinate (XY) position or location to another,
performing the machining operation, and continuing this
pattern until all the operations have been completed at all
programmed locations.
 In the above fig. point 1 to point 2 is a straight line, and the
machine moves only along the X axis; but points 2 and 3
require that motion along both the X and Y axes takes place.
As the distance in the X direction is greater than in the Y
direction, Y will reach its position first, leaving X to travel in
a straight line for the remaining distance. A similar motion
takes place between points 3 and 4

26. 2. Continuous Path
• Controls both the
displacement and the
velocity.
• Machining profiles.
• Precise control.

27.  Contouring, or continuous path machining, involves
work that produced on a lathe or milling machine,
where the cutting tool is in contact with the workpiece as
it travels from one point to the next.
 Continuous path positioning is the ability to control
motions on two or more machine axes simultaneously to
keep a constant cutter-workpiece relationship.
 All axes of motion might move simultaneously, each at a
different velocity.
 The programmed information in the CNC program must
accurately position the cutting tool from one point to the
next and follow a predefined accurate path at a
programmed feed rate in order to produce the form or
contour required

28. 3. Interpolation
 The method by which contouring machine tools move
from one programmed point to the next is called
interpolation.
 This ability to merge individual axis points into a
predefined tool path is built into most of today’s MCUs.
 There are five methods of interpolation: linear, circular,
helical, parabolic, and cubic.
 All contouring controls provide linear interpolation, and
most controls are capable of both linear and circular
interpolation.
 Helical, parabolic, and cubic interpolation are used by
industries that manufacture parts which have complex
shapes, such as aerospace parts and dies for carbodies

29. a) LinearInterpolation
 Linear Interpolation consists of any programmed points
linked together by straight lines, whether the points are
close together or far apart.
 Curves can be produced with linear interpolation by
breaking them into short, straight-line segments.
 This method has limitations, because a very large
number of points would have to be programmed to
describe the curve in order to produce a contour shape.
 A contour programmed in linear interpolation requires
the coordinate positions (XY positions in two-axis work)
for the start and finish of each line segment.

31. b) Circular interpolation
 Circular interpolation has greatly simplified the process
of programming arcs and circles.
 To program an arc, the MCU requires only the
coordinate positions (the XY axes) of the circle center,
the radius of the circle, the start point and end point of
the arc being cut, and the direction in which the arc is to
be cut (clockwise or counterclockwise)

33. Types of Motion control system
There are two types of control systems. They are:
1. Open Loop Systems
2. Closed Loop systems

34. Open Loop Systems & Closed Loop systems

35. 1. Open Loop Systems
 The term open-loop means that there is no feedback.
 In open loop systems the motion controller produces
outputs depending only on its set points, without
feedback information about the effect that the output
produces on the motion axes.
 In open loop systems, the set points are computed from
the instructions in the Part program and fed to the
controller, which may reside in a different
microprocessor, through an interface.
 These motion commands may be in the form of electrical
pulses (typical for step motor drives) or analog or digital
signals, and converted to speed or current set points by
the controller.

36.  These set points, in turn, are sent to the power electronic
drive system that applies the necessary voltage/current to
the motors.
 The primary drawback of open-loop system is that there
is no feedback system to check whether the commanded
position and velocity has been achieved.
 If the system performance is affected by load,
temperature or friction then the actual output could
deviate from the desired out.

37. 2. Closed Loop systems
Closed-loop control, continuously senses the actual position
and velocity of the axis, using digital sensors such as
encoders or analog sensors such resolvers and tacho
generators and compares them with the set points.
The difference between the actual value of the variable and
its set point is the error. The control law takes the error as
the input and drives the actuator, in this case the servo
motor and its drive system, to achieve motion variables that
are close to the set points

38. Closed loop systems can achieve much closer tracking of
set points even with disturbances and parameter variations
in the system with with temperature.
Closed-looped systems, on the other hand, require more
complex control as well as feedback devices and circuitry in
order for them to implement both position and velocity
control.
Most modern closed-loop CNC systems are able to provide
very close resolution of 0.0001 of an inch.

39. Coordinates/axis on a Lathe
Most lathes are programmed on two axes.
 The X axis controls the cross motion of the cutting
tool.
 Negative X (X-) moves the tool towards the spindle
centerline; positive X moves the tool away from the
spindle centerline.
 The Z axis controls the carriage travel toward or
away from the headstock

40.  The directions of movement in a machine tool are based
on a co-ordinate system allocated to the axes of motion
of the slides.
 As in the conventional lathe there is movement of slides
in two directions along two axes,
 one axis being parallel to the spindle axis and the other
axis perpendicular to the spindle axis.
 The axis parallel to the spindle axis is called Z-axis.
 The axis perpendicular to the spindle axis is called X-
axis.
 Since the machine co-ordinate system has two axes, X
and Z, it is referred to as the X-Z co-ordinate system.

41. Coordinates/axis system on a Lathe

42. Coordinates/axis system on a Lathe

44. Coordinates/axis system on a Milling

45. Coordinates/axis system on a Milling

46. The milling machine can be programmed on three axes:
 The X axis controls the table movement left or right.
 The Y axis controls the table movement toward or
away from the column.
 The Z axis controls the vertical (up or down)
movement of the knee or spindle.

48. Reference/Origin Points
Three reference points are either set by manufacturer or
user.
1) Machine Origin
2) Program Origin
3) Part Origin
1) Machine Origin
The machine origin is a fixed point set by the machine tool
builder. Usually it cannot be changed. Any tool movement
is measured from this point. The controller always
remembers tool distance from the machine origin.

49. Reference points and axis on a lathe

50. Reference points and axis on a Milling

51. 2) Program Origin
It is also called home position of the tool. Program origin is
point from where the tool starts for its motion while executing a
program and returns back at the end of the cycle.
This can be any point within the workspace of the tool which is
sufficiently away from the part. In case of CNC lathe it is a
point where tool change is carried out.
3) Part Origin
The part origin can be set at any point inside the machine's
electronic grid system. Establishing the part origin is also
known as zero shift, work shift, floating zero or datum. Usually
part origin needs to be defined for each new setup.
Zero shifting allows the relocation of the part. Sometimes the
part accuracy is affected by the location of the part origin.

52. Part Program
 A part program is a set of instructions given to a
computerized numerical control (CNC) machine to
control its operation.
 Numerical control (NC) information is generally
programmed in blocks of codes/words.
 Each word conforms to the EIA standards and they are
written on a horizontal line

53. Block of Information

54.  A program is made up a number of blocks. Similarly a
block is made up a number of words.
 A word consists of an address character and a string of
digits (alphanumeric character).
 Each code conforms to EIA (Electronic Industries
Association) standards, are in a logical sequence called a
block of information.
 Each block should contain enough information to
perform one machining operation.
 Each block is separated by a semicolon(;) or End of the
block (EOB).

55. Types of NC codes
1. G-codes/Preparatory codes
 Preparatory functions are used to specify the control
mode of the operation.
 G-codes are always programmed at the start of the block.
 The term "preparatory" in NC means that it "prepares"
the control system to be ready for implementing the
information that follows in the next block of
instructions.
 G-codes describe the type of machine movement, type of
interpolation, type of dimensioning, time related
functions and activate certain operating conditions
within the control.

56. The action of G codes is either modal or block by block.
 G codes once programmed, remain active until another
G code of the same group is programmed, after which
the previous one gets cancelled, are said to be MODAL.
 G code which remains active only in the block in which
it is programmed, is said to be BLOCKWISE ACTIVE
or ONE SHOT G CODE.

59. 2. M-codes/Miscellaneous codes
Miscellaneous functions use to perform a group of
instructions such as coolant on/off, spindle on/off, tool
change, program stop, or program end.
M-codes are often referred to as machine functions or M-
functions.