Chapter 7
CONTROL IN
MACHINE TOOLS
INTRODUCTION
•
Any manufacture on a machine tool is achieved through
the change of the shape and the size of a blank (or raw
material) usually by removal, addition and/or plastic
deformation of the material of the blank.
•
This demands a pre-determined relative motion between
the tool and the blank.
•
The characteristics objective of the control elements
of a machine tool is to attain this pre-determined
relative motion.
REQUIREMENT OF MACHINE TOOL CONTROL
1.
2.
3.
4.
5.
6.
7.
Accuracy
Rapid action
Mnemonic feature: reduced number of control
lelements
Control against excessive force, torque etc.: overload
safety devices.
Interlocking feature
Additional requirements for manual control
Reliability
DIFFERENT CONTROL ELEMENTS
MECHANICAL CONTROL ELEMENTSDROPPING WORM
MECHANICAL CONTROL ELEMENTSCAM CONTROL
MECHANICAL CONTROL ELEMENTSCAM CONTROL



Plate control- When movement is short and
perpendicular to the cam axis. Feed of parting
tool.
Face type-movement is short and perpendicular
to cam axis at an offset.
Drum type-movement is parallel to the cam axis.
MECHANICAL CONTROL ELEMENTSFEEDING A BAR STOCK
MECHANICAL CONTROL ELEMENTSSAFETY CONTROL
To avoid erroneous simultaneous engagement of mechanical drives,
Safety control devices are used.
HYDRAULIC AND HYDRO-MECHANICAL
CONTROL ELEMENTS- DIRECTIONAL VALVE
HYDRAULIC AND HYDRO-MECHANICAL
CONTROL ELEMENTS- DIRECTIONAL VALVE



Designed to change the direction or to open and
close the passage of the oil in two or more lines.
They can reverse the motion or stop them.
Directional valve can be hand operated,
mechanical, electrical etc
HYDRAULIC TRACER CONTROL UNIT
HYDRAULIC TRACER CONTROL UNIT
Used in automatic lathe to copy the profile of a
template.
 For a neutral position of the tracer lever,

m  mo
p1 A1  p 2 A2
p 2  p1
A1
A2
,but , A1  2 A2
So , p 2  0 . 5 p1
ELECTRICAL AND ELECTRO-MECHANICAL
CONTROL ELEMENTS

Push Button Control

Electrical Limit Switch

Electric tracer Control
ELECTRICAL LIMIT SWITCH
The limit switch supplies a limit signal when an
adjusted limit is exceeded in either direction,
especially when a control valve has reached a final
position
This signal is suitable for reversing control signals,
initiating visual or audible alarms and for connection
to central control or alarm systems.
ELECTRICAL LIMIT SWITCH
ELECTRICAL LIMIT SWITCH
NUMERICAL CONTROL (NC) OF MACHINE TOOLS
Numerical control is a method to control the motion of
the machine tool components by means of coded
instructions.
NC machine tool consists of
Machine tool
 MCU(Machine tool Control unit)

MCU Consists of
An Input unit
DPU
b) Reading and Parity Check circuits
(Data Processing
c) Decoder
Unit)
d) Interpolator and comparator
e) Position Control Circuit
CLU
(Control Loop
f) Velocity Control Circuit
Unit)
g) Acceleration Circuit
h) Control for other functions
a)
NUMERICAL CONTROL (NC) OF MACHINE TOOLS
Control Loop Unit (CLU) uses the Data
generated by DPU to make control signals.
 DPU and CLU are hardwired into the MCU of
NC Machine tool.


Drawbacks of NC machine tool
Much time needed to correct any mistake in program.
 Data input tapes damages easily.
 Cutting and feed speed cannot be changed during
cutting.
 Mechanical moving parts for tape reading is not
suitable for high speed operation.

CNC MACHINE TOOL
Due to technological advancement, a dedicated
computer is used as DPU in NC machine tool.
 Fairley complex program can be written with key
board and stored in memory.
 This system is commonly known as Computer
Numeric Control (CNC).
 Advantages of CNC machine Tools:






Enhanced product quality
Greater manufacturing flexibility
Reduced production lead time
Reduced inspection
Complex machining
CNC MACHINE TOOL







Higher effective machining time
Effect of change of design of a part
Machine tool damage reduced
Less material handling
Reduce skill of operator
Significant reduction of cost
Disadvantages of CNC machine Tool



High Investment
High skilled programmer and electrician required
High rigidity and better dynamic characteristics are
necessary to incorporate when building
DIGITAL DISPLACEMENT MEASUREMENT
DEVICE
DIGITAL DISPLACEMENT MEASUREMENT
DEVICE
A feedback transducer monitors the position of
the moving element of a machine tool and
converts a minimum displacement into discrete
numerical values.
 The photo diodes of the scanning element send a
signal every time a mark on the scale is
encountered.
 Signals are added in electronic counter, which
provide an accurate measure of distance
travelled.

DIGITAL ABSOLUTE MEASUREMENT OF
POSITION
ELECTRICAL STEPPER MOTOR
It is a synchronous motor specially designed to
rotate a specific number of mechanical degrees
for every electrical pulse received by its
controller.
 The relation between degree of spindle rotation
and electrical degree

m 


Speed,
Nm 
1
NP
2
p
e
n pulse
Typical angle of rotation are, 1.5, 3, 7.5, 15
degrees
RECIRCULATING BALL NUT
RECIRCULATING BALL NUT
The motion of tool or job in NC machine tool faces
the problem of friction an backlash.
 To eliminate and compensate these problem, ballnut screw assembly is used.
 The balls roll in the helical channel of the thread
from.
 Advantages of ball-nut assembly

That’s all for today