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. Accuracy

2. Rapid action

3. Mnemonic feature: reduced number of control lelements

4. Control against excessive force, torque etc.: overload safety devices.

5. Interlocking feature

6. Additional requirements for manual control

7. 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 m

p A p A

Ap p ,but ,A A

A

So, p . p

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 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 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