Accuracy and repeatability of thread measurements using replication technique

One of the important problems in verifying the dimensional and geometrical accuracy of products is the measurement in hard-to-reach places. One of the non-destructive measurement methods is the indirect measurement, using the replication technique. When evaluating each measurement method in the context of its legitimacy, one should consider its accuracy and repeatability.

This study aimed to determine whether the error values of measurements with the use of replicates are dependent on the thread parameter being checked. Two types of replica materials were used in the study - one in initial liquid consistency, and the other in paste form. Both materials are used on a semi-flexible impression to measure the cross-section on a profile projector or optical microscope. After the replicas were made, they were cut with a double blade cutter to obtain 1 mm slices. The profiles were measured on an iNEXIV VMA-2520 metrology system. As the measurement of the replicas did not provide information on the position of the thread axis, it was not possible to determine the diameter parameters. The thread parameters measured were: thread angle, thread height, pitch and root radius. To assess the accuracy of the replica measurements, the results obtained were compared with the values from the direct measurement of the thread.

The repeatability of the replicas in the context of measuring a given parameter was examined using the analysis of the means method. Irrespective of the replica material used, the largest errors in comparison with direct measurement were recorded for the thread angle. Measurements of this parameter were also characterized by the lowest repeatability. For the other analyzed parameters, the relative error was usually less than 1%.