Geometrical instruments – Gauge – With support for gauged article
Patent
1995-07-08
1996-11-12
Bennett, G. Bradley
Geometrical instruments
Gauge
With support for gauged article
33 1M, G01B 500
Patent
active
055727980
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to metrological apparatus. Typically, such apparatus carries a stylus and is used for measuring the surface of a workpiece contacted by the stylus, for example to determine the form or shape of the workpiece.
In a known form of metrological apparatus, an arm is movably mounted on a support structure, and carries the stylus. The stylus is typically provided as part of a probe which may be movable along the arm. In use, the arm is supported over a work table on which the workpiece is placed. The support structure may be made movable to carry the arm over the work table. Alternatively, the work table may comprise a turntable, in which case the support structure is typically not movable relative to the turntable, but the arm is movable towards or away from the turntable.
In such a prior art structure, the support structure includes a bearing member, which acts as a datum. The arm bears against the bearing member as it moves relative to the support structure, and the bearing member defines its position. A measuring means, for example an optical grating, will typically be mounted close to the bearing member, for providing a measurement of the position of the arm along the bearing member. In this configuration, a number of factors act to limit the accuracy with which the position of the stylus can be measured, thereby limiting the accuracy with which the surface of the workpiece can be measured.
The bearing member, acting as a datum, theoretically constrains the arm to move precisely in a predetermined direction, which is typically a straight line. However, any inaccuracies in the manufacture or mounting of the bearing member will cause corresponding inaccuracies in the movement of the arm, so that the positioning of the arm cannot be more accurate than the positioning and shape of the bearing member.
Any deviations of the position of the arm from its correct intended path will tend to increase with distance along the arm away from the bearing member. If the arm is intended to move in a straight line, the part of the arm contacting the bearing member will normally have the straightest path. In order to ensure comparability between different measurements of the workpiece surface, made by contacting the workpiece with the stylus at different positions of the arm relative to the support structure, the workpiece should be placed in the line where the arm follows most closely its perfect intended path. However, this is the place where the arm contacts the bearing member which acts as a datum, and consequently the presence of the bearing member obstructs the workpiece and it cannot be positioned there. Accordingly, when the stylus is contacting a workpiece, movement of it in the direction of movement of the arm will follow a path which tends to be less accurate (typically less straight) than the datum path defined by the bearing member.
The position of the arm relative to the support structure is typically measured by use of a measuring means provided close to the bearing member acting as a datum. This is inevitably spaced from the position of the stylus and the workpiece. Consequently, the precise position of the stylus in the direction of movement of the arm cannot be determined owing to errors known as Abbe errors. These errors can arise from twisting the arm or from inaccuracies in its movement due to inaccuracies in the datum, and mean that the precise position of the arm at the point where the stylus is located cannot be determined exactly from measuring the position of the arm at the location of the measuring means.
According to an aspect of the present invention, there is provided a metrological apparatus having a support structure with first and second spaced apart parallel bearing members, both acting as datums, and an arm carrying a stylus which bears against both bearing members. The bearings between the arm and the bearing members are of a form in which the clearance between the arm and each of the bearing members adjusts automatically. Typically, the bearings are arranged so
REFERENCES:
patent: 4483079 (1984-11-01), Band et al.
patent: 4887360 (1989-12-01), Hemmelgarn et al.
patent: 4947557 (1990-08-01), Gapshis et al.
patent: 5148600 (1922-09-01), Chen et al.
patent: 5333386 (1994-08-01), Breyer et al.
Koordinatenmessgerate, Fertigungsmesstechnik, Handbuch fur Industrie und Wissenschaft, 1984, pp. 271 to 276.
Bennett G. Bradley
Rank Taylor Hobson Limited
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