A CMM customer survey rated accuracy as the seventh most important feature of a CMM. Number one was compatibility with CAD computers followed with graphically drawing a picture of the part while inspecting it. While these features are nice they have little real impact on the manufacturing process. The new generation of academia engineers have spent too much time looking into the halo of computer screens and not enough time with the fundamentals and physics of shaping material.

Tool makers used to measure twice and cut once. The golden rule of 10:1 was followed: If a process is required to manufacture something to an accuracy of A, then the inspection uncertainty must be less than A/10.

In the past decade the price/performance ratio of inspection equipment has not kept pace with machine tools and manufacturing requirements. The user has been taught that a 3:1 or less ratio is acceptable because that was all he could afford. (The bean counters prevailed.)

Suppose you have a final product tolerance of 0.006". Proper practice and Federal Law dictates that you must "GUARD BAND" your inspection data. Inspection readings are the reading ± the inspection uncertainty. To manufacture a good part, it must now lie within a new manufacturing tolerance zone. If you follow a 3:1 ratio philosophy then the your inspection is capable of 0.002".