CNC versus conventional machining

CNC versus traditional machining traditional machining relies on a expert operator to govern the desktop instrument handwheels to supply a required element. The operator has to exa­mine the drawing routinely, during the operation, to assess the dimen­sions that follow; and ought to decide (manually calculate) by means of how a lot each handwheel must be became to supply the favored result.It is a time­ingesting pastime in itself.

Very often, metal removal requires a series of cuts before the final result is achieved. Measurement of the part must be carried out in between these cuts. It is almost impossible to predict the final condition of the part, during multiple cuts, even though the handwheels have calibrated scales.

Because of limitations involved in the design of conventional machine tools, much tool changing, tool setting and workpiece re-setting is often involved during the machining cycle. It is apparent that the time required to machine a part, and hence the time during which the machine and the operator are engaged on the job, is much greater than the actual cutting time. These disad­vantages are compounded when the operator has to make a number of similar parts from the same drawing. If their nature does not permit loading and clamping into jigs or fixtures, then inevitable errors of varying size, position and form will result.

In addition, many conventional machine tools have speeds and feeds governed by mechanical design features such as fixed-speed gearboxes. Thus, the choice of a feed or speed is a compromise depending on the gear ratios built into the machine tool. Optimum cutting conditions are rarely realised.

Many “automatic” machine tools have evolved over the years in an attempt to overcome some of the above limitations. Copy lathes, capstan lathes and turret lathes were early examples. Sequence control based on cams and later plug-board-operated pneumatic systems also made important contributions.


These approaches are characterised by extremely long set-up times by spe­cialist setters. This meant that, once set up, the machines had to run for long periods and produce many thousands of parts to justify the long set-up times. It was common to over-produce whilst the machine was tooled up. This meant increased work in progress and working capital tied up in stock. Very often, production bottlenecks due to jobs queuing for certain machines disrupted production schedules.

By contrast, CNC machines offer complete control of all axes, under opti­mum cutting conditions. Extremely short set-up times are possible since stan­dard tooling is all that is required. The need for jigs and fixtures is almost eliminated. Indeed, their presence can be an encumbrance to the flexible contouring facilities of CNC machines. Simple clamping arrangements are often all that is required.


Part programming is often carried out by specialist part programmers, away from the machine. The facility to prepare new jobs away from the machine means that the machine tool spends a greater proportion of its working time actually cutting metal.

Extremely good accuracy and repeatability of the components produced enables a greater uniformity of production. There are also attendant reduc­tions in fitting costs, assembly costs, inspection costs and the elimination of scrap and re-work items. Moreover, once a job has been machined, the data that produced it (the part program) can be retained, saved and loaded back to produce identical parts at a later date. Figure 1/3 illustrates the comparison of machining components by CNC and conventional means.

The quality of the finished job is no longer under the control of the operator but under the control of a computer-run part program. This ultimately trans­lates into lower costs per part and much-reduced lead times.