Pretravel compensation for vertically oriented touch-trigger probes with straight styli

Touch-trigger probes are commonly used on coordinate measuring machines (CMMs) and machine tools for dimensional measurements in modern manufacturing environments. Pretravel is a highly repeatable error occurring in touch-trigger probe applications. Advances in machine accuracy and tighter part tolerances have resulted in pretravel being one of the major error sources in dimensional measurement processes on CMMs and machine tools. This paper presents a pretravel model for vertically oriented touch-trigger probes with straight styli. A triggering force model based on principles of mechanics was derived and used as the basis to develop the pretravel model. Experimental probing data were used to show that the model can effectively predict probe pretravel in various probe approach directions. In applying the model to predict probe pretravel, only one model parameter needs to be calculated from the probe calibration data. Furthermore, only 24 points of experimental probe calibration data are needed to identify the model parameter and to perform effectively the prediction. The pretravel model presented in this paper is robust and can be used to reduce probe pretravel in vertical probe applications. The pretravel model can be implemented by means of a software correction module which can be incorporated into machine systems.