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R&D project

A fast measuring and evaluation method for the holistic characterisation of threads in the calibration practice

Period of the project

Start: 01/09/2021
End:  29/02/2024


Project description

The state-of-the-art procedure to ensure the functionality of screw-threads comprises the determination of thread parameters like lead, flank angles and pitch diameter only in a few predefined axial cross sections and inspection planes. For example, the calibration of screw-thread gauges in DAkkS laboratories is based on the three-wire method or two-dimensional scanning approaches with tactile styli. The established DIN-standards and calibration guides therefore only allow for an insufficient thread flank evaluation, as the 3D thread shape is not extensively probed.

Therefore, within the scope of two pre-projects a holistic measuring and analysis procedure was developed at the PTB, which allows for the acquisition and evaluation of 3D thread point clouds. By fitting a parameterized thread geometry model into the captured point cloud, potential local form deviations can be detected. Furthermore, thread parameters can be deduced locally, e.g., to realize a more accurate determination of the virtual pitch diameter.

However, the new approach requires a universal 3D coordinate measuring machine (CMM) with rotary table. This precondition is not fulfilled by most of the DAkkS laboratories. Therefore, this project comprises an adaption of the 3D calibration method in order to allow for the usage of standard two-dimensional stylus scanning devices together with 1D length comparators. Hereby, the 3D approach can be transferred to the calibration practice without high financial investments. Unlike the CMM-based approach which renders highly accurate thread calibrations on NMI-level possible, the new concept is meant to address subordinate instances within the calibration hierarchy.

In collaboration with four project partners (a contouring scanner manufacturer, two DAkkS-accredited laboratories, and a screw thread gauge manufacturer) a specification sheet is formulated, e.g., listing the necessary adaption tasks with regard to data acquisition based on a 2D scanner with rotary axis. The new measuring strategy is meant to include an appropriate procedure to calibrate the additional rotation axis, as well as a length comparator for dimensional traceability. The existent analysis algorithm will be extended by a new data class to allow for a stylus radius correction or to share meta data effectively.

 

The determination of the measuring uncertainty according to GUM, and the procedure validation by reference measurements complete the project. In conclusion, accredited calibration laboratories have a fast and reliable 3D measuring procedure at their disposal, which, taking the low investment costs into account, is in a reasonable ratio to the expected income from calibration orders.


Project partners

The project is conducted in collaboration with four partners.


Contact

Dr.-Ing. Rüdiger Beermann und Dipl.-Ing. (FH) Achim Wedmann