The American Society of Mechanical Engineers (ASME) has published a new standard that identifies elements that will impact measurement uncertainty from thread calibration processes. Standard B1.25 is titled, ‘Measurement Uncertainty Factors in the Calibration of Screw Thread Gages’ and in the interests of full disclosure, be advised that I chaired the committee that prepared it.
At the outset, the committee agreed that it would focus on the metrology involved in thread gage calibration and leave acceptance criteria and other quality considerations to other standards available from ASME.
There are a number of situations that brought this document into being many of which have been overlooked by or unknown by some facilities calibrating thread gages. This situation has led to measurement disputes and unrealistic uncertainty claims leaving end users of the calibration data confused and wondering as to who is ‘right’ when a dispute arises. Gage manufacturers and specialty calibration facilities were well represented on the committee being the most affected when their work is rejected by calibration facilities that are not up to standard from a technical point of view.
Our committee included most of the people who write the standards for thread gages and make or calibrate them for a living. The advantage of this comes up when discussions of when a particular feature of a thread gage should be included in uncertainty budgets and when it doesn’t have to be while remaining in compliance with the original standards. This knowledge and experience is hard to find these days and a further byproduct of it are the handy hints and formulae that appear throughout the document.
A general introduction to measurement uncertainty is provided for those not familiar with it along with some definitions related specifically to thread gages. It is not a ‘how to’ guide in the preparation of an uncertainty budget, it’s a listing of the various calibration processes and depending on the hardware used, what elements of each measurement that has to be included in an uncertainty budget. Processing that information into a budget is the subject of other standards that are noted in the reference section of the document.
Since there are several types of equipment used to calibrate thread gages, I feel one of the most significant sections covers different ways the work is done. Having so many specialists in the field on the committee meant that the pros and cons of different methods and hardware were explored with cautionary notes where applicable. This section alone can protect companies from buying equipment for the work that will not comply with contemporary standards or that is not precise enough for gages to North American standards despite fancy brochures and off the wall claims by equipment sellers.
Readers looking for ‘typical’ uncertainty values in thread gage calibration will be disappointed as none are included. The reason for this is that a previous document – B1 Technical Report on measurement uncertainty did provide such data that was ‘borrowed’ by many and used to reflect their capabilities when the processes and/or hardware did not support the data. The result was ongoing disputes with some making claims to uncertainties better than NIST!
A series of graphics is used to show how measurement uncertainty is applied in typical situations, usually where a measurement is made by two different laboratories with differing results and what it all means at the end of the day.
Many people using calibration services get into lengthy discussions regarding the ratio of measurement uncertainty and, in this case, gage tolerances. Invariably, rules of thumb such as 10:1 come up as being required for their work meaning they want the calibrating lab’s uncertainty to be about one tenth or 10% of the gage tolerance. Unfortunately, there is a limit to such rules called ‘state of the art’ and this becomes evident with thread gages where round-robin tests have shown the industry’s uncertainties can range from thirty to fifty percent or more of the tolerance involved. Section 9 of the document explains this and gives some guidance for the situation.
While only nine pages long, this ASME standard contains a wealth of information including equipment criteria, correction factors, hints for better measurements and reasons why variations arise between different calibration methods, equipment and standards. Anyone calibrating thread gages should have a copy on hand to back up their day-to-day operations and sort out disputes that arise from time to time.
