Measuring and testing – Surface and cutting edge testing – Roughness
Patent
1997-05-21
1998-12-08
Williams, Hezrone E.
Measuring and testing
Surface and cutting edge testing
Roughness
738665, G01B 5012, G01B 528
Patent
active
058472703
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a way of attaching a stylus to a metrological instrument for measuring the surface form (shape) or texture (roughness) of a workpiece. The invention has particular-application to instruments of the type in which an arm holding the stylus can be moved generally parallel to the plane of the surface, so as to drive the stylus along the surface to make a measurement traverse. However, it is also applicable to other types of metrological instrument, such as machines for measuring the roundness or surface roughness of a rotating component, in which a support moves the stylus radially towards the surface to be measured until the stylus contacts the surface, and then the radial displacement of the stylus tip is measured while the workpiece rotates.
Metrological instruments are normally made so that the stylus is removable and replaceable. This allows different types of stylus to be fitted to the same machine, and in particular allows the user to change the length of the stylus or the features of the surface-contacting stylus tip. Additionally, it may be desirable to remove the stylus when the instrument is not in use, to avoid damage to the stylus or to the instrument if the stylus is accidentally knocked.
In one known arrangement, a stylus is supported in use for pivotal movement of the tip towards and away from the surface of the workpiece, and the mechanism for moving the stylus over the surface of the workpiece ends with a pivotable stylus support to which the stylus can be fitted. The stylus support is mounted on the remainder of the mechanism through precision bearings to allow the pivoting movement of the stylus. The stylus is connected to the support by pushing its end remote from the stylus tip into a stylus-receiving opening in the stylus support, and the stylus is removed by pulling it out of the opening. These movements are substantially linear movements in the direction along the length of the stylus, generally parallel to the plane of the workpiece surface and generally transverse to the pivot axis. In fitting or removing a stylus, the pushing or pulling force applied to the stylus will be transmitted through the pivot bearings to the remainder of the mechanism, which will be held against movement. Accordingly, it is inherent in this method of attaching a removable stylus that the act of attaching the stylus and the act of removing the stylus will apply substantial stress to the pivot bearing.
In this known arrangement, the stylus is provided with a precision locating surface on the part which is inserted into the stylus support. This surface extends in a plane parallel to the direction of movement for attaching or removing the stylus, and a spring in the stylus support presses this locating surface of the stylus against a kinematic mount in the support to locate the stylus accurately in a known and predetermined orientation. However, since the stylus is normally a narrow cylinder (typically about 3 mm diameter), the locating surface is inevitably small, especially in its direction across the width of the stylus, making it correspondingly difficult to locate the stylus accurately and securely.
With apparatus of this type, damage can result if the stylus is knocked sideways or if an attempt is made to rotate it about the pivot axis beyond the ends of its range of free movement. This may happen through accidentally knocking the stylus. Additionally, it sometimes happens that the instrument is inadvertently operated in a manner such that the stylus is driven over a sloping workpiece surface which lifts the stylus beyond its range of movement. Since such events attempt to move a part of the stylus in a manner which cannot be accommodated by the stylus support, they may lead to snapping or splitting of the stylus. More seriously, such events tend to apply an extreme force to the pivot bearings of the stylus support, and may damage the bearings making the instrument unusable. This is particularly serious because repairing the bearings is more expensive than simply replacing
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Larkin Daniel S.
Taylor Hobson Limited
Williams Hezrone E.
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