If you’re having problems with thread measurements but the answers you’re getting from your usual sources aren’t solving them, it’s natural that you would try and get some unbiased advice from a specialist. If you take this path, be prepared to answer a lot of questions before any worthwhile answers are forthcoming. This column includes a lot of those questions so you can be prepared and save everyone a lot of time when you talk with a specialist in this area of metrology.
The first question is the obvious one, but the answers are often incomplete: What thread are you concerned about? 1/4-20 UNC might be your answer, but if you neglect to mention that it is a two start or double-lead thread or perhaps a pre-plate thread for example, the recommended solution may not solve your problem. Before I forget, make sure you mention whether it’s an internal or external thread. If you can provide a part drawing showing the thread you can save a lot of discussion time but often even they are not properly specified.