...how to calibrate or verify screw pitch gages. Any ideas? My first thought is to slap it on an optical comparator and compare with an overlay- but where do I get do thread pitch overlays (or do they even exist?
I am entering this discussion way late, but here goes anyway.
Screw pitch gages are used by some machinists to verify threads that they have machined. When used in this manor they are referred to as profile gages. They are in common use with American Petroleum Institute (API) threads, and even kept as standard, on-the-shelf, items by two gage makers that I know of. They are available with NIST traceable certifications from the manufacturer.
That said, using a made-in-some-third-world-country, mass-produced, die-stamped, Screw Pitch Gage for cutting threads is a little scarey to me.:mg: I do not see the die stamped tool having much precision, but I still can visualize someone using it to check machined threads. Getting it calibrated may be a good thing for them. It is still cheaper than buying a thread gage. Some days it is anything to save a buck.
As for the optical comparator overlays, if you can define it I can get you an overlay. The design work may take a couple of weeks, or you can supply your own cad files for the overlay company to work from. There are standard screw thread overlays available for about US$130 with a 2 week lead-time. These overlays can be certified. There are four levels of certification and I expect the maximum level has NIST traceability.