Data processing: measuring – calibrating – or testing – Measurement system – Orientation or position
Patent
1997-05-28
1999-07-20
Barlow, John
Data processing: measuring, calibrating, or testing
Measurement system
Orientation or position
702150, 702167, 702168, 702169, 33503, 33504, 33505, 33554, 33589, 33644, G01B 528, G01B 703
Patent
active
059267810
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to measuring the roundness of a component using a roundness measuring machine.
Typically, a roundness measuring machine will have a motorised turntable and a holder for a measuring probe. It is normally used to measure the roundness of a component such as a crankshaft which is intended to have a circular cross-section. In use, the component is placed on the turntable with its axis co-axial with the axis of rotation of the turntable (known as "the spindle axis"), and the tip of a stylus of a measuring probe or gauge which is mounted on the holder is brought into contact with the surface of the component. The turntable is rotated and the angular position of the turntable together with the output from the measuring gauge are repeatedly logged. The logged data provides detailed information about the precise shape of the cross-section of the component at the level where the stylus tip contacted it. Such data can be processed, for example to provide a measure of the extent to which the shape of the cross-section has waveform undulations rather than being truly circular.
As is well known in the art, such roundness machines are normally provided with a centring and levelling mechanism, by which the turntable can be moved radially with respect to the spindle axis and also can be tilted. When a component is placed on the turntable, the axis of the component will normally be slightly offset from the spindle axis for turntable rotation, and additionally the axis of the component may not be precisely parallel to the spindle axis. By operation of the centring and levelling mechanism, the turntable is moved and tilted so as to bring the component axis to be parallel to the spindle axis (levelling) and to bring the component axis into line with the spindle axis (centring).
Normally, a centring and levelling operation comprises rotating the turntable and the workpiece twice, with the stylus tip contacting the workpiece at different heights, and analysing the data thus obtained to provide instructions for the amount of sideways shift of the turntable in each of two directions and the amount of tilting of the turntable about each of two axes. U.S. Pat. No. 4,731,934 (Barnaby & Mills) and EP-A-0240150 (which are incorporated herein by reference) disclose an automated arrangement in which the centring and levelling mechanism is driven by four motors which are controlled by computerised processing means so as to perform the centring and levelling movements as calculated by the processing means from the data obtained during the measurements of the workpiece taken at two heights as discussed above. As mentioned in U.S. Pat. No. 4,731,934 and EP-A-0240150, the centring and levelling mechanism may alternatively be manually operable, and processing means can be arranged to output to an operator information about the adjustments required. Various designs of manually operable centring and levelling mechanisms are known.
After the centring and levelling operation has been completed, the precise shape of the cross-section of the component can be measured at various heights as desired, by rotating the turntable about the spindle axis while the stylus tip contacts the workpiece, and logging the consequent data. In the analysis of this data, it is possible to detect and measure slight eccentricity of the measured cross-section with respect to the spindle axis, resulting from a residual off-centredness or tilt of the workpiece. It is known in the art, and has been discussed in various publications by D. G. Chetwynd, to determine the degree of eccentricity by fitting the data for the shape of the cross-section to the equation for a limacon, and the resulting information about the degree of eccentricity can be used to re-centre the data relating to the positions of the stylus tip during rotation of the workpiece so that the measurements are centred on the centre of the measured cross-section rather than on the spindle axis. For accurate work of the type for which roundness machines are normally used, such a techn
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Barlow John
Taylor Hobson Limited
Vo Hien
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