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T-shaped microprobe for tactile measurements on internal threads

Especially interesting for
  • precision engineering
  • clockmaking
  • medical engineering

Measuring microstructure components is a particular challenge in precision engineering. Probes adapted to internal microthreads (such as threads with a size of M 0.7 × 0.175) were previously not available. In a cooperation with partners from industry, a microprobe was developed at PTB which, based on its special T-shaped geometry, is optimized for the measurement tasks carried out on internal microthreads. A patent application has been submitted for the T-shaped microprobe.

For this new probe, the probing elements used are commercially available microspheres made of ruby with diameters of down to 110 μm that are fixed onto a T-shaped probe stylus of hard metal. The probe stylus is manufactured by means of microwire discharge machining. Whereas small probing spheres in microprobes have often been glued to the stylus, here, a different concept has been pursued: the spheres are clamped inside the probe body. The design of the probe stylus currently allows the fitting of two probing spheres, which are kept in place by clamping forces only and can be exchanged.

A first validation of the procedure carried out on internal microthreads with a nominal diameter of 0.7 mm and a pitch of 0.175 mm was performed on a commercial 3D coordinate measuring machine, like those that are used to measure macroscopic components. To adapt the machine to the requirements of measurements on microcomponents, it sufficed to modify the scanning parameters such as, e.g., the probing force.

The T-shaped microprobe allows tactile measurements of internal microstructures with gap widths of down to approx. 150 μm, as well as a reliable detection and assessment of high-precision and function- relevant internal structures such as those that are increasingly encountered, e.g., in medical engineering. Together with a novel approach for the laminar evaluation of threads, this opens up the possibility of an extensive quality assessment.

Scientific publication

T. Krah, A. Wedmann, K.Kniel, F. Härtig: Measurement and evaluation processes for inner micro structures. Proceedings 13th International Conference of the EUSPEN 1, 120–123 (2013)