Universal measuring machine

A Universal Measuring Machine (UMM) is a measuring device for objects where geometric relationships are the most critical element, where dimensions are specified by geometric positions (see GD&T) rather than absolute coordinates.The original use of these machines was to inspect gages and parts produced by jig grinding.While there are some similarities to coordinate measuring machines (CMMs), their purpose and accuracy range are quite different.CMMs typically move in three dimensions and use a tactile probe for measurement, while UMMs use a continuous scanning probe to align the main axis (4th axis) with the part geometry.

Initially, general-purpose measuring machines were created to meet the need for continuous measurement of geometric features with absolute and comparative capabilities, rather than point-based coordinate measurement systems.CMMs provide a quick way to detect absolute points, but geometric relationships such as runout, parallelism, perpendicularity, etc. must be calculated rather than measured directly.Universal measuring machines can meet this need by aligning precise spindles with electronic test indicators with geometric features of interest, rather than using non-scanning Cartesian probes to estimate alignment.The pointer can be precisely controlled and moved over the part along a linear axis or radially around the main axis to continuously record the profile and determine the geometry.This gives general purpose machines a very powerful advantage over non-scanning measurement methods when profiling planes, radii, profiles and holes, as the details of features can be down to the resolution of the probe. More modern CMMs do have scanning probes so the geometry can be determined similarly.

In practice, the universal measuring machine of the 1970s was a very slow machine that required a highly skilled and patient operator to use, and the precision built into these machines far exceeded the needs of most industries.As a result, universal measuring machines are uncommon today, found only as specialized machines in metrology laboratories.Because the machine can make comparative length measurements without moving the linear axis, it is an invaluable tool for comparing standard gauges and length standards.While universal measuring machines were never a mass-produced product, they are no longer offered on a production basis, but are made to order, tailored to the needs of metrology laboratories purchasing it.Manufacturers who perform work that must be measured on such machines often choose to subcontract the measurements to laboratories that specialize in such work.

A universal measuring machine placed under corrective interference control and using a non-contact measuring head can measure features to the millionth of an inch within the entire machine envelope, while other types of machines are limited in the number of axes or measurement accuracy.The accuracy of the machine itself is negligible because the environment in which the machine is placed is the limiting factor for effective accuracy.Early mechanical machines were built to maintain an accuracy of 10 to 20 millionths of an inch across the entire range of the machine, and thanks to incredible machine design and forethought, the same accuracy is maintained today without computer compensation.