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
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
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.