Of all the measurements taken on metal parts, from conveyorized production-line items to specialized, one-of-a kind parts, a company's greatest concern stems from a surface roughness measurement that has varied by a millionth of an inch.
To be sure, concern can be justified for fuel injectors and other ultra tight-tolerance parts. If the scale and proportion of the part is such that the total surface roughness allowance is 10 parts of a million, then 1 part-of-a-million variance is significant. At the same time, it is easy to forget that 10 parts of a million is still microscopic, a fact that is somewhat obscured by using measurement terms such as micro, Ra or roughness.