A Robust Pretravel Model for Touch Trigger Probes in Coordinate Metrology

This paper describes a pretravel model for vertically-oriented touch trigger probes with straight styli widely used on coordinate measuring machines (CMMs). Advances in machine accuracy and tighter part tolerances leave probe error as one of the major error sources in dimensional measurement processes. Pretravel accounts for the majority of touch trigger probe errors and is caused by bending deflection of the probe stylus shaft. A trigger force model was first derived and used to model the probing force at the trigger instant. Experimental probing force data were used to validate the trigger force model. The trigger force model was then used to model the bending deflection of the stylus shaft at the trigger instant. Experimental x-y-z coordinate data were used to validate the pretravel model. It is shown that 24 points of equatorial data measured in the probe calibration process can be used to accurately identify the only model parameter. Also, 12 points of equatorial data provides reasonably good results. Model predicted pretravels associated with 1561 probe approach directions were compared with experimental pretravels (range: 16 μm, when using a 50 mm stylus) and the standard deviation of prediction errors can be reduced to around 0.6 μm.