Comparison between CNC and NC Machines

Production equipment with computer numerical control is a major component of computer integrated manufacturing (CIM). CNC machine tools paved the way for introducing flexible automation on the shop floor. The technology of computer numerical control is today applied to a large spectrum of material processing equipment. The developments in CNC have been one of the major factors that have contributed to the evolution of CAD/CAM and subsequently CIM. For manufacturing a component, the CAD/CAM process generates a NC program which can run on the CNC machine.  The  fundamentals  of  numerical  control,  their types  and  the programming  techniques are  presented  in  this  chapter.  The features of CAM software and CNC systems are discussed in detail.  The integration of CNC machines in the CIM environment is also dealt with.

Comparison between CNC and NC Machines

NC Machines. The control systems and machine tools in numerically controlled machine tools have varying complexities and capabilities. In the initial stages, the NC machine tools had NC systems added to the machine but only to control the position of the work piece relative to the cutting tool. The operator had to select the cutting tools, speed and feeds etc. In the next stage, the capabilities of the machine  tools  improved relationship,  the  material and  in  addition  to  maintaining  cutter removal  was  also  controlled  by  the and  work  piece numerical  control system. The mechanical design of the machine tool was also improved with the development of recirculating ball screw and better slide ways. These machines are referred to as NC machines. The instructions to the NC machines are fed through an external medium, i.e. paper tape or magnetic tape. The information read from the tape is stored into the memory of the control system called 'buffer storage' and is processed by the machine step by step. So when the machine is working on one instruction block, the next block is read from the tape and stored in the memory of the machine control system. Since the part cannot be produced without a tape being run through the machine, these machines are also called tape con-trolled machines. The tape has to be run repeatedly depending on the number of components to be produced. Also if there is even a minor change in the design of the component, the tape has to be discarded and new tape with changed program has to be produced.

CNC Machines. In case of computer numerical control machine tools, a dedicated computer is used to perform all the basic NC functions. The complete part program to produce a component is input and stored in the computer memory and the information for each operation is fed to the machine tools i.e. motors, etc. The part programs can be stored in the memory of the computer and used in future. The conventional NC machine tools are not much in use these days. CNC machine tools are widely used due to many new control features available on these machines. Some of the additional features available in CNC machine tools are as follows:

(a) The part program can be input to the controller unit through key-board or the paper tape can be read by the tape reader in the control unit.

(b) The part program once entered into the computer memory can be used again and again.

(c) The part program can be edited and optimized at the machine tool itself. If there is any change in the design of the component, the part program can be changed according to the requirements.

(d) The input information can be reduced to a great extent with the use of special sub-programs developed for repetitive machining sequences. For common operations such as drilling holes on a pitch circle, special cycle programs can be built and stored in the computer memory. These sub- programs or sub-routines can be retrieved and used any number of times within a part program; only certain parameters have to be specified and the computer control carries out the necessary calculations and subsequent actions.

(e) The CNC machines have the facility for proving the part program without actually running it on the machine tool. The control system processes the part program and the movement of the cutting tool in each operation is shown on the monitor screen (video display unit). The shape of the component which will be produced after machining is also shown on the screen without actual machining taking place.

(f) CNC control unit allows compensation for any changes in the dimensions of the cutting tool. When a part program is written, part programmer has a particular type and size of cutting tool in mind. But while actually using the part program on the machine tool that particular cutter may not be available. CNC control system allows the compensations to be made for difference between the programmed cutter and the actual cutter used.

(g) With the CNC control systems, it is possible to obtain information on machine utilization which is useful to the management. The control system can provide the information such as number of components produced, time per component, time for setting up a job, time for which a particular tool has been in use, time for which machine has not been working and fault diagnosis, etc.

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