John is the jobbing foreman of a medium sized company making parts for the automobile industry. The company has been asked to make a part for a new range of cars and it is John’s job to work out how they are going to measure it.

It was getting dark now and John was alone in the office with only an anglepoise lamp to illuminate the drawing. John took a long hard look at the drawing. It was a complicated part with many features that needed measuring.

His first conclusion was that this component was ideal for that brand spanking new coordinate measuring machine (CMM) that the company had just bought. The CMM was ideal for measuring lots of components quickly and could easily tackle all the features on this component.

Getting out the specification for the CMM, he looked up the maximum permissible error. It was quoted as 5 + L/100 μm.

‘Hang on,’ he thought. ‘The tolerance for roundness on that 60 mm diameter hole is
0.002 mm – better than the CMM can manage. We will have to measure that on our roundness-measuring instrument if we want a measurement uncertainty of about a tenth of the tolerance. While we are at it, the overall length has a tolerance of 0.3 mm. We could easily check that with a calibrated micrometer before we put the component on the CMM.’ A plan was coming together.

‘The diameter, right,’ he thought to himself. ‘That has been specified with a tolerance of 0.005 mm, that will require the high accuracy CMM in the inspection laboratory and we can check the surface texture quality with our hand held surface texture meter. Flatness, parallelism and perpendicularity - all those features could be measured on my CMM.’ John made some notes that would form the basis of a procedure for checking the new component. In the morning, he would read the relevant specification, ISO 14253, before he finalised anything.