TEMPERATURE PERTURBATION EFFECTS IN A HIGH-PRECISION CMM

Two definitive temperature perturbations were introduced into the environment of a high precision Leitz PMM-866 coordinate measuring machine (with scale temperature compensation) using heaters arranged to simulate aspects of horizontal and vertical temperature gradients in the Y-Z plane. A 250 mm ULE and Zerodur ball-bar was repeatedly measured by the CMM. The CMM itself was used to probe the specimen table as a reference plane during the course of the temperature change. A vertical temperature gradient of up to 9-degrees-C across the granite lintel comprising the probe-carriage slideway produced an approximately linear change of apparent ball-bar length with time at a rate averaging about 1-mu-m deg-1 C. Changes of slideway straightness and apparent specimen table height were also measured. However, a similar temperature rise of one steel bridge supporting column relative to the other produced a variable small effect, without change of apparent ball-bar length. It is shown that the effect of introducing the vertical gradient can be predicted assuming a curvature of the portal beam giving rise to a gradient of pitch along the slideway and resulting in probe tip position errors which, to a first approximation, vary linearly with duration of the temperature gradient. A finite element thermal stress analysis technique is employed to explore the measured distortion of the slideway beam.