Modelling of pretravel for touch trigger probes on indexable probe heads on coordinate measuring machines

This paper presents a pretravel model for touch trigger probes mounted on indexable probe heads, which can rotate and tilt the probe into a number of orientations for coordinate measurements on coordinate measuring machines (CMMs). Pretravel accounts for the majority of touch trigger probe errors and is caused by bending deflection of the stylus shaft. A trigger force model is derived and used to model bending deflection of the stylus shaft at the trigger instant. Only one model parameter needs to be calculated using the probe calibration data. Experimental data associated with thirteen probe orientations were used to validate the model. It is shown that the model can effectively predict pretravel distances associated with various probe approach directions. The standard deviations of prediction errors are less than 0.71 µm, indicating that the proposed model can be used to compensate for pretravels occurring in touch trigger probe applications.