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A fast measurement and evaluation procedure for the holistic calibration of threads

06.05.2024

In order to support the industry with quality assurance in the area of thread production, PTB has been in close contact with thread gauge manufacturers and calibration laboratories for years. Since then, various research projects have developed measurement and evaluation strategies that set new standards in the holistic calibration of threads. However, these measurement strategies require the use of high-precision coordinate measuring machines (CMM), which are not available to most calibration laboratories due to cost and time constraints.

The measuring methods previously established in thread calibration, e.g. the three-wire and two-ball methods as well as newer stylus methods, each refer to individual axial sections according to VDI/VDE/DGQ 2618 sheets 4.8 and 4.9 [1]. The strategy of these measurement methods excludes a holistic measurement and evaluation of threads, which makes it difficult to meet the increasing demands on manufacturing tolerances.

To facilitate that the holistic characterization of threads can find its way into industrial calibration practice, a measurement and evaluation strategy was developed that can also be implemented with measuring devices that are usually available in calibration laboratories. These are stylus measuring devices for two-dimensional, tactile contour measurements. Such a device was combined with a rotation axis for thread measurement for the first time within a TransMeT project. A new measurement method was developed to characterize the position of the rotation axis. Using the additional rotation axis, thread gauges can for example easily be measured in ten or more axial sections. The measurement data is then transferred to an evaluation software, which adapts the nominal thread geometry to the measured values using the least squares method. With this fitting, the standardized evaluation parameters such as flank diameter, pitch and flank angle can be calculated.


Fig. 1: Tactile contour measurement of a thread gauge M64x6 with a rotation axis on a 2D scanner

The first results of the research project show good agreement between the measured thread parameters both with the established, two-dimensional stylus measurement and with areal thread calibration on a CMM. The time advantage of this method is particularly noteworthy, as it is significantly more efficient than measuring on a CMM, but at the same time only takes a little more time than conventional axial section measurements. In the next few months, the new measurement strategy will be further optimized and the specific measurement uncertainty budgets according to EURAMET cg-10 will be created [2]. A comprehensive comparative measurement with all project partners to validate the quality of the results will conclude the project.


References

[1] VDI/VDE/DGQ 2618 Blatt 4.8 & 4.9; Prüfmittelüberwachung - Prüfanweisung für zylindrische Gewinde-Einstelldorne, Gewinde-Lehrdorne und Gewinde-Prüfdorne; Prüfanweisung für zylindrische Gewinde-Einstellringe, Gewinde-Lehrringe; 04/2006
[2] EURAMET cg-10; Determination of Pitch Diameter of Parallel Thread Gauges by Mechanical Probing; 12/2012.

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