Abstract: A metrological apparatus has a movable arm 15 which moves up and down between two bearing members 13, both of which define datum positions for the arm 15. The arm 15 has air bearings on the bearing members 13, so that it adopts a position equidistant between the bearing members 13. In this way, the position of the arm 15 contains the average of the errors in the two datum positions defined by the bearing members 13. Where the bearing members 13 have symmetric errors, these will be cancelled. Since the path of maximum straightness of movement of the arm 15 is spaced from the bearing members 13, it is possible to put a workpiece 3 in this path and a turntable 5 for supporting the workpiece 3 is arranged to have its rotational axis in line with the path of maximum straightness.

Abstract: In a metrological or other sensitive apparatus, the transducer or other operative member (22) is mounted on a support structure (14 to 18) which is carried on a chassis (2) by a first set of dampers or isolating means (10) and the chassis (2) is mounted on the floor by a second set of dampers or isolating means (6). The two sets (10, 6) have different natural frequencies and the first set (10) is arranged so that any vibration of the support structure tends to be rotational centered in the zone of operation (21) of the transducer or other member. This is achieved by arranging that the stiffness of the first set of dampers (10) is higher in the direction (24) passing through the zone of operation (21) than in the transverse direction.

Abstract: A bearing structure, preferably for supporting a rotary table (4) in metrological apparatus, includes both a journal bearing (34,36,38) and a thrust bearing (56,66). The journal bearing is made up of three dry pads (34,36,38), one (38) of which is radially adjustable, and the thrust bearing (56,66) comprises first and second air bearings. The axis of rotation of the bearing is vertical, and the lower (66) of the air bearings is less stiff than the upper one (56).

Abstract: Metrological apparatus is provided with a turntable (4) which is movable horizontally and pivotally for centering and levelling. The turntable (4) is supported at three points (A,B,P), one (P) of which is fixed and the other two (A,B) of which are vertically movable by motorized jacks. Centering and levelling is achieved automatically by means of a computer (72) which calculates the required horizontal and pivotal movements of the turntable (4) from data obtained by sensing the workpiece surface with a transducer (12).