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Numerical control
Numerical control of machine tools may be defined as a method of automation in which various functions of machine tools are controlled by letters, numbers and symbols. Basically a NC machine runs on a program fed to it. The program consists of precise instructions about the manufacturing methodology as well as the movements. Fox example, what tool is to be used, at what speed, at what feed and to move from which point to which point in what path, all these instructions are given. Since the program is the controlling point for product manufacture, the machine becomes versatile and can be used for any part. All the functions of an NC machine tool are therefore controlled electronically, hydraulically or pneumatically.
In NC machine tools one or more of the following functions may be automatic:
(i) Starting and stopping of the machine tool spindle.
(ii) Controlling the spindle speed.
(iii) Positioning the tool tip at desired locations and guiding it along. Desired paths by automatic control of the motion of slides.
(iv) Controlling the rate of movement of tool tip.
(v) Changing of tools in the spindle.
Initially the need of NC machines was felt for machining complex shaped small batch components as those belonging to an aircraft. However, this spectrum currently encompasses practically all activities of manufacturing, in particular capital goods and white goods. Thus the range covered is very wide. Besides machining with which we are concerned, NC has been used in a variety of manufacturing situations. The majority of applications of NC are in metal cutting machine tools such as milling machines, lathes, drilling machines, grinding machines and gear generating machines. Besides a number of metal forming machine tools such as presses, flame cutting machines, pipe bending and forming machines, folding and shearing machines also use NC for their program control. The inspection machines called Co-ordinate Measuring (CMM) are also based on NC. Lastly the robots basically may be material handling units, but their control principles are very close to the NC. Besides these applications listed for manufacturing, other applications such as filament winding or assembly machines based on the NC principles can also be seen in the industry.
NC machines have been found suitable for the following:
(i) Parts having complex contours, that cannot be manufactured by conventional machine tools.
(ii) Small lot production, often for even single (one off) job production, such as for prototyping, tool manufacturing, etc.
(iii) Jobs requiring very high accuracy and repeatability.
(iv) Jobs requiring many set-ups and/or when the set-ups are expensive.
(v) Parts that are subjected to frequent design changes and consequently require more expensive manufacturing methods.
(vi) The inspection cost, which is a significant portion of the total manufacturing cost.
One or more of the above considerations would justify the processing of a part by an NC machine tool.
Numerical Control is superior to conventional manufacturing in a number of ways. The superiority comes because of the programmability. These are as follows:
(i) Parts can be produced in less time and therefore are likely to be less expensive. The idle (non-cutting) time is reduced to minimum. This of course depends on the way the part program for the part is written. The endeavour of the machine tool builder is to provide a facility whereby the non-cutting time can be brought to the minimum. It is possible to reduce the non-productive time in NC machine tools in the following ways:
(a) by reducing the number of set-ups
(b) by reducing set-up time
(c) by reducing workpiece-handling time
(d) by reducing tool-changing time.
These make machines highly productive.
(ii) Parts can be produced more accurately even for smaller batches. In conventional machine tools, precision is largely determined by human skill, NC machines, because of automation and the absence of interrelated human factors, provide much higher precision and thereby promise a product of consistent quality for the entire batch.
(iii) The operator involvement in part manufacture is reduced to a minimum and as a result less scrap is generated due to operator errors. No operator skill is needed, except in setting up of the tools and the work. Even here, the set-up has been simplified to a great extent.
(iv) Since the part program takes care of the geometry generated, the need for expensive jigs and fixtures is reduced or eliminated, depending upon the part geometry. Even when a fixture is to be used, it is very simple compared to a conventional machine tool. It is far easier to make and store part program (tapes).
(v) Inspection time is reduced, since all the parts in a batch are identical, provided proper care is taken about tool compensations and tool wear in part program preparation and operation. With the use of inspection probes in the case of some advanced CNC controllers, the measurement function also becomes part of the program.
(vi) The need for certain types of form tools is completely eliminated in NC machines. This is because the profile generated can be programmed, even if it involves three dimensions.
(vii) Lead times needed before the job can be put on the machine tool are reduced to a great extent, depending upon the complexity of the job. More complex jobs may require fixtures or templates if they are to be machined in conventional machine tools, which can be reduced to a large extent.
(viii) CNC machining centers can perform a variety of machining operations that have to be carried out on several conventional machine tools, thus reducing the number of the machine tools on the shop floor. This would save floor space and result in less lead-time in manufacture. This would also result in an overall reduction in production costs.
(ix) The set-up times are reduced, since the set-up involves simple location of the datum surface and position. Further, the number of the set-ups needed can also be reduced. All this translates into lower processing times. A component can be fully machined in a single machining center or turning center, each of which having wider machining capabilities. In conventional manufacture if the part has to be processed through a number of machine tools which are located in different departments, the time involved in completion and the resultant in process inventory would be large. This would be greatly eliminated by the use of NC machine tools.