Improved axis determination method for calculation of virtual pitch thread diameter using a point cloud from cmm

Abstract

Thread gauges are widely used to obtain the values of complex parameters of thread surfaces. The virtual pitch thread diameter (VPTD) is the main standard complex thread accuracy figure. The author has previously published the method of VPTD calculation using the data obtained from the coordinate measurement machines (CMM). However, following this procedure, the thread axis position is considered as a-priory known. It is clear that the axis position is not always known in practice. This limitation makes application of CMM for thread accuracy estimation rather difficult. Later, the author has developed a procedure to detect a part thread axis location. However, this procedure is not accurate enough, in particular, for a short thread. The present paper describes the improved procedure to calculate arbitrarily located thread axis. The procedure includes two stages. The first stage is the determination of average coordinates of the points obtained from CMM for end basic segments. At this stage, the corresponding parallel translation of the points forms circular turn. The second stage is the detection of axis inclination angles and axis turn to these angles to its ideal location. The procedure has been checked by computer calculations, which prove sufficient reliability and accuracy of the accepted approach. With the use of this improved procedure of the thread axis detection, the VPTD is further calculated using a previously published method.