24-03-2012, 04:12 PM
computer numerically controlled (CNC) machines
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INTRODUCTION
Development of computer numerically controlled (CNC) machines is an outstanding contribution to the manufacturing industries. It has made possible the automation of the machining process with flexibility to handle small to medium batch of quantities in part production.
Initially, the CNC technology was applied on basic metal cutting machine like lathes, milling machines, etc. Later, to increase the flexibility of the machines in handling a variety of components and to finish them in a single setup on the same machine, CNC machines capable of performing multiple operations were developed. To start with, this concept was applied to develop a CNC machining centre for machining prismatic components combining operations like milling, drilling, boring and taping. Further, the concept of multi-operations was also extended for machining cylindrical components, which led to the development of turning centers.
ADVANTAGE OF CNC MACHINES
Higher flexibility
Increased productivity
Consistent quality
Reduced scrap rate
Reliable operation
Reduced non productive time
Reduced manpower
Shorter cycle time
High accuracy
Reduced lead time
Just in time (JIT) manufacture
Automatic material handling
Lesser floor space
Increased operation safety
Machining of advanced material
Central Processing Unit (CPU)
The CPU is the heart and brain of a CNC system. It accepts the information stored in the memory as part program. This data is decoded and transformed into specific position control and velocity control signals. It also oversees the movement of the control axis or spindle whenever this does not match the programmed values, a corrective action is taken.
All the compensations required for machine accuracy (like lead screw pitch error, tool wear out, backlash, etc.) are calculated by the CPU depending upon the corresponding inputs made available to the system. The same will be taken care of during the generation of control signals for the axis movement. Also, some safety checks are built into the system through this unit and the CPU unit will provide continuous necessary corrective actions. Whenever the situation goes beyond control of the CPU, it takes the final action of shutting down the system in turn the machine.
Speed Control Unit
This unit acts in unison with the CPU for the movement of the machine axes. The CPU sends the control signals generated for the movement of the axis to the servo control unit and the servo control unit convert these signals into the suitable digital or analog signal to be fed to the machine tool axis movement. This also checks whether machine tool axis movement is at the same speed as directed by the CPU. In case any safety conditions related to the axis are overruled during movement or otherwise they are reported to the CPU for corrective action.
Servo-Control Unit
The decoded position and velocity control signals, generated by the CPU for the axis movement forms the input to the servo-control unit. This unit in turn generates suitable signals as command values. The servo-drive unit converts the command values, which are interfaced with the axis and the spindle motors (Fig.1).
The servo-control unit receives the position feedback signals for actual movement of the machine tool axes from the feedback devices (like linear scales, rotary encoders, resolves, etc.). The velocity feedback is generally obtained through tacho generators. The feedback signals are passed on to the CPU for further processing. Thus the servo-control unit performs the data communication between the machine tool and the CPU.
As explained earlier, the actual movements of the slides on the machine tool is achieved through servo drives. The amount of movement and the rate of movement are controlled by the CNC system depending upon the type of feedback system used, i.e. closed-loop or open-loop system