Geometrical instruments – Gauge – With support for gauged article
Reexamination Certificate
1999-01-20
2001-12-11
Gibson, Randy W. (Department: 2859)
Geometrical instruments
Gauge
With support for gauged article
C033S543000, C033S550000
Reexamination Certificate
active
06327788
ABSTRACT:
This invention relates to a method of surface form measurement and is particularly applicable to the measurement of the radial or cylindrical form of an object such as a workpiece. Radial form is related to the roundness of an object and gives an indication of its deviation from circularity. Cylindrical form provides a measure of how close in shape the surface is to a right cylinder.
In known apparatus for measuring cylindrical form such as the applicants' Talyrond TR 200 (trade mark) measurement apparatus, a generally cylindrical workpiece to be measured is mounted on a rotatable support or turntable. The rotatable support is centred and levelled, for example in the manner described in the applicants' EP-A-0240150 the contents of which are incorporated herein by reference, so that the cylindrical axis of the workpiece is coincident with the spindle of the turntable which defines a rotational axis datum. An axial straight datum is defined by a support column located offset from but parallel to the rotational axis datum. The support column carries a probe arm which is moveable along and transversely of the support column to allow a stylus carried by the probe arm to contact the surface of the workpiece.
The radial or cylindrical form of the workpiece is measured by rotating the turntable and measuring the displacement of the stylus relative to the axial straight datum. As the axis of the cylinder is aligned with the rotational axis datum, the displacement of the stylus relative to the axial straight datum provides a measurement of the radius at each angular measurement position during rotation of the rotatable support. The radial form or deviation of the cross-section of the workpiece from a circular cross-section can thus be determined. In addition, the cylindrical form of the workpiece may be determined by making measurements at different heights along the workpiece by moving the probe arm along the axial straight datum. Movement of the probe arm transversely and longitudinally of the axial straight datum is measured by appropriate gauges, for example optical or linear variable differential transducer (LVDT) gauges.
The accuracy of measurements made using such apparatus depends on the accuracy of the rotational axis datum and, especially for cylindrical form, the axial straight datum. It is possible to define the spindle or rotational axis datum to, typically, within ±25 nm (nanometers) so, generally, the mechanical performance of the axial straight datum is the limiting source of error for radial and cylindrical form. Mechanically related errors in the axial straight datum may arise from, for example, long term mechanical instability, for example changes with temperature or other environmental factors, inaccuracies in the actual straightness of the axial straight datum or its parallelism to the rotational axis datum and variability of the interface between the support column defining the axial straight datum and the carriage carrying the probe arm on the support column.
As described in the manual for the Talyrond TR 200 at section 16.8, the straightness of the traverse, that is effectively the straightness of the axial straight datum, may be checked by mounting a cylinder slightly offset from the rotational axis datum of the turntable normally of the axial straight datum. The stylus is then caused to move axially along the surface of the workpiece by moving the probe arm longitudinally of the support column and the variation in the displacement of the stylus from the axial straight datum with height is plotted to form a first straightness graph. The above-mentioned procedure is then repeated after the turntable has been rotated through 180° to form a second straightness graph. The line bisecting the first and second straightness graphs should be a straight line and any departure from straightness of the bisector represents an error in the straightness of the column support or axial straight datum. The slope of the bisector is a function of any residual relative tilt of the column support and the cylindrical workpiece and is not related to the straightness of the column support.
The above-described straightness check is intended to be carried out from time to time before making a measurement on a workpiece so as to ensure that the straightness of the axial straight datum lies within acceptable limits and to enable calibration of the apparatus.
An embodiment of the present invention provides a method of measuring the form of a surface which is nominally symmetric about an axis, for example a surface having a cylindrical form, which method enables the effects of any deviations in the measurement direction in a reference datum to be eliminated or at least reduced during the measurement process.
In an embodiment, sensing means are used to obtain at least two sets of data for measurement points P
i
(h
y
) on a surface with the points P
i
(h
y
) being at different angular positions (with respect to an axis about which relative rotation of the surface and the sensing means is effected) for each measurement and the at least two sets of data are combined to compensate for any deviations in, for example, straightness or parallelism in a datum to which the measurements made by the sensing means are referenced. For example, the two sets of data may be combined by taking the mean of two values measured for the same location on the surface measured with two different orientations of the sensing means to the reference datum to give an indication of the radius (that is the distance from the said axis to the location) at that location. The difference of the same two measurements may be taken to give an indication of the value of any deviation or error in the reference datum. This enables measurements of the form at a given height along a surface to be made which are not dependent on errors in the reference datum in the measurement direction. By making such measurements at different heights along the surface of the object, the form of the object can be determined without being affected by any deviation or error in the reference datum in the measurement direction.
In an embodiment, two sensing means are used to obtain data for two diametrically opposed points P
i
(h
y
) and P
i+&pgr;
(h
y
) on a surface so allowing two sets of data to be obtained at the same time. In this example a diameter determined by the separation of the two sensing means may be compared with a diameter obtained by combining the polar data which is obtained individually from the two sensing means and which is referenced to a reference datum. This enables any deviation in the measurement direction in the reference datum to be determined. These measurements may be repeated for other points at the same height and for corresponding points at different heights to determine the overall form of the surface compensated for any deviation in the measurement direction of the reference datum.
In another embodiment, the sensing means comprises a single sensing element biassable in at least two different directions and means for adjusting the biasing of the sensing element to change the direction in which the sensing element is biased.
Relative rotation of the surface and the sensing means may be effected by mounting the surface on a rotatable support. As another possibility, relative rotation of the surface and the sensing means may be effected by rotatably mounting the sensing means to a support and rotating the sensing means about the surface.
In another aspect, the present invention provides apparatus for determining the shape, form, texture or roughness of a surface using a sensing means having adjustable biassing means to enable the sensing means to be biased in different directions.
REFERENCES:
patent: 3866829 (1975-02-01), Egawa et al.
patent: 4080741 (1978-03-01), Siddall et al.
patent: 4084324 (1978-04-01), Whitehouse
patent: 4903413 (1990-02-01), Bellwood
patent: 5309646 (1994-05-01), Randolph, Jr. et al.
patent: 5572798 (1996-11-01), Barnaby
patent: 5694339 (1997-12-01), Ishitoya et al.
patent: 59267
Mills Michael
Scott Paul James
Seddon Peter
White Roy George
Whittle Derek Roger
Gibson Randy W.
Lerner David Littenberg Krumholz & Mentlik LLP
Taylor Hobson Limited
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