Presentation on theme: "C ONTROL IN M ACHINE T OOLS Chapter 7. I NTRODUCTION Any manufacture on a machine tool is achieved through the change of the shape and the size of a blank."— Presentation transcript:
I NTRODUCTION 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.
R EQUIREMENT OF M ACHINE T OOL C ONTROL 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
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. M ECHANICAL C ONTROL E LEMENTS - C AM C ONTROL
M ECHANICAL C ONTROL E LEMENTS - F EEDING A BAR STOCK
M ECHANICAL C ONTROL E LEMENTS - S AFETY C ONTROL To avoid erroneous simultaneous engagement of mechanical drives, Safety control devices are used.
H YDRAULIC AND H YDRO - MECHANICAL CONTROL ELEMENTS - D IRECTIONAL V ALVE
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 H YDRAULIC AND H YDRO - MECHANICAL CONTROL ELEMENTS - D IRECTIONAL V ALVE
Used in automatic lathe to copy the profile of a template. For a neutral position of the tracer lever, H YDRAULIC T RACER C ONTROL U NIT
Push Button Control Electrical Limit Switch Electric tracer Control E LECTRICAL 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. E LECTRICAL L IMIT S WITCH
N UMERICAL C ONTROL (NC) OF M ACHINE T OOLS 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 a) An Input unit b) Reading and Parity Check circuits c) Decoder d) Interpolator and comparator e) Position Control Circuit f) Velocity Control Circuit g) Acceleration Circuit h) 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. N UMERICAL C ONTROL (NC) OF M ACHINE T OOLS
CNC M ACHINE T OOL 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 M ACHINE T OOL
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.
E LECTRICAL 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
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