Universal measuring machines (UMM) are

measurement device Instrumentation a collective term for measuring instruments that are used for indicating, measuring and recording physical quantities. The term has its origins in the art and science of scientific instrument-making. Instrumentation can refer to ...

s used for objects in which geometric relationships are the most critical element, with dimensions specified from geometric locations (see GD&T) rather than absolute coordinates. The very first uses for these machines was the

inspection An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activit ...

of gauges and parts produced by

jig grinding A jig grinder is a machine tool used for grinding complex shapes and holes where the highest degrees of accuracy and finish are required. The jig grinder is very similar to a jig borer, in that the table positioning and spindles are very accur ...

. While bearing some resemblance to a

coordinate-measuring machine A coordinate measuring machine (CMM) is a device that measures the geometry of physical objects by sensing discrete points on the surface of the object with a probe. Various types of probes are used in CMMs, the most common being mechanical and l ...

(CMM) its usage and accuracy envelope differs significantly. While CMMs typically move in three dimensions and measure with a touch probe, a UMM aligns a spindle (4th axis) with a part geometry using a continuous scanning probe. 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, such as runout, parallelism, perpendicularity, etc., must be calculated rather than measured directly. By aligning an accurate spindle with an electronic test indicator with a geometric feature of interest, rather than using non-scanning cartesian probe to estimate an alignment, 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 determine geometry. This gives the universal machine a very strong advantage over non-scanning measuring methods when profiling flats, radii, contours, and holes, as the detail of the feature can be of at the resolution of the probe. More modern CMMs do have scanning probes and thus can determine geometry similarly. In practice, the 1970s-era 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 uncommon, only found as a special-purpose machine in

metrology Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fra ...

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 Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...

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

{{reflist *American Society for Precision Engineering, ''Achieving Accuracy in the Modern Machine Shop'' * Wayne R. Moore, ''Foundations of Mechanical Accuracy'' Dimensional instruments Metalworking measuring instruments