Hello forum

I am trying to maximize the potential of our thread micrometers. The official nomenclature is probably "interchangable anvil screw thread pitch diameter measurement micrometer" or something, you know the type.

I have a question regarding thread distortion of form. Let's say I have a thread plug with a pitch diameter of 1.000" and 8 threads per inch. If I set my thread mic to read accurately on this thread plug and then measure a different thread plug with a pitch diameter of 1.000" and 32 threads per inch, will the thread mic read accurately?

If the distortion of form error is very small, then I could set all our pitch mics using a 1" pitch diameter thread. If the distortion of form error is significant, I would imagine the real deal would be having master thread plugs at 1" pitch diameter in all the thread leads that we use, 8, 10, 11, 12, 13, 14, 16, 18, 20, 24, 28, ad infinitum.

Does anyone see where I'm going with this? I want thread micrometers to have very low MU factors and be very accurate. As it stands, I'm finding discrepancies all the time. Zero setting with a thread mic standard and measuring a thread plug yields .0005" discrepancy, same discrepancy going from thread plug to thread plug of different pitch.

Is distortion of thread form with varying pitch a factor here?

My experience has been that each time that you change the anvil and spindle you need to recalibrate the thread mic. I have never been a big fan of pitch mics as the "touch" used can make the difference of pass or fail. It is a quick way of getting close and then check with wires.

Have a great day.

You are correct, the feel factor of pitch diameter micrometers is substantial. I suppose I should look into this as the source of .0005" error more critically. We do class the pitch diameter mics as +/- .001" instruments, and that's tricky because most class 2 english threads have a tolerance of around .005".

Is the distortion of form error a significant source of inaccuracy? In the hypothetical situation posted above, how would you figure the error created by the distortion of form as measured by a pitch micrometer?

I believe to check the distortion of form you would have to use an optical comparator. We have actually had hardware come from a vendor with the wrong included angle in the threads. I hope this helps.