Universal measuring machine

Universal measuring machines are measurement devices used for objects in which geometric relationships are the most critical element, with dimensions specified from geometric locations rather than absolute coordinates. The very first uses for these machines was the inspection of gauges and parts produced by jig grinding. While bearing some resemblance to a coordinate-measuring machine (CMM) its usage and accuracy envelope differs significantly.

Originally, universal measuring machines were created to fill a need to continuously measure geometric features in both an absolute and comparative capacity, rather than a point based coordinate measuring system. A CMM provides a rapid method for inspecting absolute points, but geometric relationships must be calculated rather than measured directly. In theory? By using an accurate spindle with an electronic test indicator rather than a probe, a universal measuring machine fills this need. The indicator can be accurately controlled and moved across a part, either along a linear axis or radially around the spindle, to continuously record profile and geometry. This gives the universal machine a very strong advantage over other measuring methods when profiling radii, contours, and holes, as the detail of the feature can be of any arbitrary resolution. The analog record generated by the machine can be fed directly into a compensator on computer numerical control (CNC) machine tools, correcting the machine on a per-job basis.

In practice, the universal measuring machine is a very slow machine that requires a highly skilled and patient operator to use, and the accuracy built into these machines far outstripped the needs of most industries. As a result, the universal measuring machine today is only found as a special-purpose machine in metrology laboratories. Because the machine can make comparative length measurements without moving linear axes, it is a valuable tool in comparing master gauges and length standards. While universal measuring machines were never a mass-produced item, they are no longer available on a production basis, and are produced on a to-order basis tailored to the needs of the metrology lab purchasing it. Manufacturers that perform work that must be measured on such a machine will frequently opt to subcontract the measurement to a laboratory which specializes in such.

Universal measuring machines placed under corrected interferometric control and using non-contact gauge heads can measure features to millionths of an inch across the entire machine's envelope, where other types of machine are limited either in number of axes or accuracy of the measurement. The accuracy of the machine itself is negligible, as the environment the machine is the limiting factor to effective accuracy. The earlier mechanical machines were built to hold 10 to 20 millionths of an inch accuracy across the entire machine envelope, and due to incredible machine design and forethought, remain as accurate today without computer compensation.

References

• American Society for Precision Engineering, Achieving Accuracy in the Modern Machine Shop
• Wayne R. Moore, Foundations of Mechanical Accuracy