Estimate of uncertainty for thread-measurement standards

Measuring threads poses very specific problems for metrologists. While the conditions for obtaining measurement traceability remain fairly simple and similar to those for measuring smooth rings and plug gauges, assessing uncertainty requires particular attention when defining the measurand and the mathematical expression modeling the measurement process.

The very nature of thread geometry led the standardizer to define a number of dimensional characteristics, more or less functional, taking into account all or part of the geometric parameters defining the thread and their manufacturing defects.

This specificity requires the metrologist to define the measurand in accordance with current standards. However, they have a certain amount of latitude in terms of the operating mode and the means to be implemented, and are responsible for conducting a relevant and exhaustive analysis of their measurement process and establishing the appropriate mathematical model.

This last point is particularly tricky due to the complex geometry of a thread. The XP E 03-110 standard , in its November 2003 edition, highlighted this complexity, which requires solving a system of nonlinear equations to obtain the value of the flank diameter.

The purpose of this report is therefore to present, for the most common scenarios, the approach to analyzing the measurement process and handling the propagation and composition of the various uncertainty components, based on the exact formulas for calculating the diameter on the flanks.

It should be noted that the developments presented below relate to measurement principles that enable a good level of uncertainty to be achieved. This study therefore excludes all measurements taken using calipers, dial gauges, thread micrometers, etc.