Control in Machine Tools
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Transcript of Control in Machine Tools
CONTROL IN MACHINE TOOLS
Chapter 7
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. Accuracy2. Rapid action3. Mnemonic feature: reduced number of control
lelements4. Control against excessive force, torque etc.: overload
safety devices.5. Interlocking feature6. Additional requirements for manual control7. Reliability
DIFFERENT CONTROL ELEMENTS
MECHANICAL CONTROL ELEMENTS-DROPPING WORM
MECHANICAL CONTROL ELEMENTS-CAM 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 ELEMENTS-CAM CONTROL
MECHANICAL CONTROL ELEMENTS-FEEDING A BAR STOCK
MECHANICAL CONTROL ELEMENTS- SAFETY CONTROL
To avoid erroneous simultaneous engagement of mechanical drives,Safety control devices are used.
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 AND HYDRO-MECHANICAL CONTROL ELEMENTS- DIRECTIONAL VALVE
HYDRAULIC TRACER CONTROL UNIT
Used in automatic lathe to copy the profile of a template.
For a neutral position of the tracer lever,
HYDRAULIC TRACER CONTROL UNIT
1 1 2 2
12 1 1 2
2
2 1
2
0 5
om mp A p A
Ap p ,but ,A A
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Push Button Control
Electrical Limit Switch
Electric tracer Control
ELECTRICAL AND ELECTRO-MECHANICAL CONTROL ELEMENTS
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
ELECTRICAL LIMIT SWITCH
NUMERICAL CONTROL (NC) OF MACHINE TOOLSNumerical 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 ofa) An Input unitb) Reading and Parity Check circuitsc) Decoderd) Interpolator and comparatore) Position Control Circuitf) Velocity Control Circuitg) Acceleration Circuith) Control for other functions
DPU(Data Processing Unit)
CLU(Control Loop Unit)
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.
NUMERICAL CONTROL (NC) OF MACHINE TOOLS
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
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
CNC MACHINE TOOL
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
Speed,
Typical angle of rotation are, 1.5, 3, 7.5, 15 degrees
2m ep
1m pulseN n
NP
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, ball-nut 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