A universal measuring machine (UMM) is used to measure the geometric features of components. The measurement can be performed in both absolute and comparative modes, thereby providing a convenient means to carry out inspection of length and diameters of both plain and threaded work, tapers, and pitch of screw threads to a high degree of accuracy. Figure1 illustrates the constructional features of a UMM.
UMM consists of a fixed head and a movable head, also called the measuring carriage. The part to be inspected is mounted on the clamping device. The spindle centres, which are fitted with sleeves of hardened steel, ensure accurate centring of various tools such as the locating microscope, feller microscope, goniometer microscope, spotting tool, and locating indicator.
Fig.1 Schematic of Universal Measuring Machine
The spindle has an electronic test indicator rather than a probe. The indicator can be accurately controlled and moved across a part, either along a linear axis or radially around the spindle. It continuously records the profile and geometry of the component being inspected, which proves advantageous for the UMM while profiling radii, contours, and holes. A component can also be tested optically by a short-focus microscope carried in a special tool holder. The UMMs, which are presently available in the market, are interfaced with a computer, which enables easier recording of test results and subsequent analysis. A user-friendly software interface enables the user to select the testing procedure, resolution, and type of test probe.
Fig.2 Universal Measuring Machine
Presently, UMMs are being used as special-purpose machines in metrology laboratories. These are valuable devices for comparing master gauges and length standards. However, since UMMs have largely been replaced by coordinate measuring machines, they have limited usage in companies. Whenever the need to use a UMM arises, manufacturers opt to subcontract the measurement to a laboratory that has the facility.