T-shaped microprobe and gap measurement standard for inner microstructures

•New T-shaped microprobe for inner micro structures was developed.•Novel measurement standard for measurements in double flank contact was developed.•New mathematical approach for laminar description of helical flank surfaces is described. To an ever-increasing extent, the constantly progressing miniaturization of components requires a constantly decreasing size of complex internal microstructures such as internal microthreads. These are used in large numbers, among other things in the watch industry and in medical engineering. At the same time, these workpieces are allowed to show only very small tolerances. Thus, it is indispensable to carry out measurements in parallel to the manufacturing of the parts. However, reliable quality assurance for these internal microstructures has been impossible so far. To overcome this situation, the Physikalisch-Technische Bundesanstalt (PTB) has developed – in cooperation with partners from industry – a complete chain of procedures for the calibration of internal microthreads down to a nominal diameter of 0.7mm and a pitch of 0.175mm. To determine the most important measurement parameter – the pitch diameter – on microthreads, probe tips with a diameter significantly smaller than 300μm down to 120μm are required. For this purpose, a new tactile T-shaped microprobe was designed which has been realized and adapted to a coordinate measurement machine (CMM). An important characteristic of the new design is that the spherical probe tips can be exchanged in the case of wear while the expensive body of the stylus shaft can be re-used. In addition, a novel measurement standard has been developed to check the gap probing. Whereas common measuring routines in the field of coordinate metrology characterize the spatial probing behavior of a probe under the condition of single-point touching, the new standard allows measurements to be performed in double flank contact. The measurement thus comprises the complete measurement strategy, including self-centering probing. The standard consists of ball pairs which represent one gap, for example, of a screw thread. The ball diameters and the ball distances vary as a function of the thread geometry (diameter, profile angle and pitch). The gap distances which are essential for the determination of the probing behavior were calibrated on a micro-CMM by applying the triangulation method.