4.1 Where and when to measure

Using the surface texture as a check on the process is well established but there are a number of issues to consider. These are: where to measure, what to measure and when to measure.

Ideally, the measurement should take place as the workpiece is being made, so as to avoid scrapping more than one part. This in-process possibility is very difficult to achieve because of the hostile environment in which the part is made. Often, it is the presence of metal chips and/or coolant spray that makes measurement difficult but there is a trend towards dry cutting, which may help alleviate this problem. The number of parameters that could be measured and displayed in-process is limited although robust surface-measuring equipment cannot be expected to perform well.

Another possibility is to have the surface tested when the part has been made but not moved. The surface instrument, which is hand-held, has somehow to be perched on the part when the machining has stopped and then the measurement recorded.

Alternatively, the part could be removed from the machine (the lathe, say) and measured with an instrument located near to the machine tool. This is usually called in situ measurement and the availability of instrumentation with more parameters is much higher than in the in-process case.

It would normally be the case that the in situ tests are carried out during every shift. The tests in the cases where the measurement is on or near the machine...