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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.

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Control in Machine Tools

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  1. Control in Machine Tools Chapter 7

  2. 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.

  3. Requirement of Machine Tool Control • 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

  4. Different Control Elements

  5. Mechanical Control Elements-Dropping Worm

  6. Mechanical Control Elements-Cam Control

  7. 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.

  8. Mechanical Control Elements-Feeding a bar stock

  9. Mechanical Control Elements- Safety Control To avoid erroneous simultaneous engagement of mechanical drives, Safety control devices are used.

  10. Hydraulic and Hydro-mechanical control elements- Directional Valve

  11. 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

  12. Hydraulic Tracer Control Unit

  13. Hydraulic Tracer Control Unit • Used in automatic lathe to copy the profile of a template. • For a neutral position of the tracer lever,

  14. Electrical and electro-mechanical control elements • Push Button Control • Electrical Limit Switch • Electric tracer Control

  15. 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.

  16. Electrical Limit Switch

  17. Electrical Limit Switch

  18. 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 • Reading and Parity Check circuits • Decoder • Interpolator and comparator • Position Control Circuit • Velocity Control Circuit • Acceleration Circuit • Control for other functions DPU (Data Processing Unit) CLU (Control Loop Unit)

  19. 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.

  20. 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

  21. 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

  22. Digital Displacement Measurement Device

  23. 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.

  24. Digital absolute measurement of position

  25. 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

  26. Recirculating Ball Nut

  27. 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

  28. That’s all for today

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