Pretravel variation
Pretravel is the distance the probe travels between the stylus touching the surface and a trigger signal being sent to the CMM.
Pretravel is compensated by the CMM computer by probe qualification. However, due to the design of standard probes and the changes in
force required to trigger from different directions, there is a small variation in this pretravel. This is called pretravel variation.
On most CMMs using standard software and very long styli (i.e. up to 100 mm), pretravel variation can become a large factor in the probe's
total margin of error. It is most evident in inspection routines requiring measurement of form (e.g. following contours or measuring
roundness).
Many standard gauging practices, such as measurement of distance between bore centres, are largely unaffected by probe pretravel variation
errors.
Stylus selection
To obtain the best performance, apply the following considerations when selecting and fitting a stylus:
Use the shortest possible stylus length
Minimise the mass of the stylus by using the types with ceramic or GF stems where possible (refer to the Renishaw stylus catalogue for
further information)
Work with the recommended stylus limits (see graph below in recommended stylus limits)
Ensure that stylus balls, threads and mating faces are kept clean
Tighten styli using only the tools provided
Always qualify the styli at the gauging speed set for the part measurement program - if the speed is changed re-qualify the stylus tip
Use the largest ball diameter to minimise surface finish effect on measurement
Whenever possible, use only styli from the M4 range - it is possible to adapt to the M3 or M2 styli, but this will result in reduced measuring
accuracy and the user is advised to establish the suitability for the application
TP7 user's guide
www.renishaw.com
Issued 10 2019
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