2/3 Inspection and measurement
Machine tools can be made to produce complex components under CNC control. The next logical step is the utilization of a machine that can check and inspect the same elements under CNC control. The basis of dimensional measurement or inspection has already been provided in the part program used to manufacture the components.
Three-dimensional coordinate measuring machines (CMMs) are the result. Such devices are not restricted to measuring individual components. Assemblies and sub-assemblies may be regulated by probing and thus inspected by comparing the actual dimensions with the required dimensions. The ability to “remember” positions and sizes, together with the ability to “re-state” datums, make these machines extremely versatile. Control software within the tools makes comprehensive information available to the operator via visual display units and printed hardcopy. Desired dimensions, actual dimensions, actual errors and their locations can be provided. Computational facilities can also take into account, and compensate for, any specified tolerances relating to component features. In specific circumstances, results from inspecting a first-off component may be fed back automatically to the machining process, allowing automatic adjustments of cutter path movements.
Most CMMs operate in a stand-alone mode since various problems exist in trying to incorporate them into a flexible machining system. Such issues include:
a) The machines are susceptible to vibration.
b) Strict environmental control has to be observed to minimise the effects of heat and humidity on dimensional and volumetric changes.Components have to be allowed to reach thermal stability after the heating effects of the machining process.