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Finding and Verifying Pitch Diameter

acgarcia
Good Afternoon:

How would one calculate pitch diameter for external threads on a CMM and verify that the results are correct. I need to check some pretty large acme threads im not too sure about the nominal dimensions.
  • What's are the threads?
    Did you try to google it?
  • dont use the cmm to verify pitch diameter...unless the threads are huge and flat and you have a way to insure the probe hits the correct spot on the start of the thread every time....almost impossible.
  • Good Afternoon:

    How would one calculate pitch diameter for external threads on a CMM and verify that the results are correct. I need to check some pretty large acme threads im not too sure about the nominal dimensions.

    +1 to what Jimmy said....

    Use 3 wire or 3 pin method to verify the pitch diameter, you will get the nominal, tolerance and pin / wire size from the thread standard.

    They are now checking pitch diameters on master API thread gauges in NIST in the US, using Quindos, PMM's, Scanning probes & styli the same size as the wire diameter.

    Way beyond most mortals, like us....Slight smile HTH
  • What's are the threads?
    Did you try to google it?

    We have thread disk, software that is used to calculate the dimensions of most types of threads, UN, Acme, Stub Acme, Buttress. I know the pitch dia of this thread that I am measuring.

    What I am trying to measure right now is the center of a threaded hole. Not worried about the pitch or the minor diameter, i just need the center for now. Its a small thread .375-16 UN and I'm using a 5mm ruby with a simple circle with 4 hits. Then I construct a center point and I'm getting an angle dimension from the hole center point to the center of my part. My tolerance is +/- .5 degrees and i'm measuring OOT by 1 degrees. There is a pitch option for the auto circle but I can' t tell if it is truly in the threads.
  • We have thread disk, software that is used to calculate the dimensions of most types of threads, UN, Acme, Stub Acme, Buttress. I know the pitch dia of this thread that I am measuring.

    What I am trying to measure right now is the center of a threaded hole. Not worried about the pitch or the minor diameter, i just need the center for now. Its a small thread .375-16 UN and I'm using a 5mm ruby with a simple circle with 4 hits. Then I construct a center point and I'm getting an angle dimension from the hole center point to the center of my part. My tolerance is +/- .5 degrees and i'm measuring OOT by 1 degrees. There is a pitch option for the auto circle but I can' t tell if it is truly in the threads.

    As was posted, you cannot dimension & report pitch diameter with a CMM. It's a Basic Dimension.
    But there is a way to ensure the validity of the measured centerpoint of your data.

    Pitch option simply spaces the hits up & down along the surface vector.
    The idea is that if the hole is drilled correctly perp to surface/datums, the probe will land on a flank and follow that flank all the way down.
    Or, land on hit a peak and keep hitting that peak all the way down.
    Or, land on a valley and keep hitting that valley all the way down.

    Best practice is to throw an extra Circularity (roundness) check in right after the hole, with a generous tolerance to account for threads being rough. If the pitch is wrong, or the hole is crooked, or the thread is wrong, or the threads are filled with chips, then the Circularity will show that there's a problem with a crazy bad measured value. If you get halfway decent Circularity then you've found the correct centerpoint.
  • Thanks Josh, looking at my CAD model, it doesn't even show threads, only a small cylinder. Think i'm going to try to measure it as a cylinder with 3 levels with a tight spread. Maybe it will compensate for the thread form. Or look into a getting shank probe.
  • Or look into a getting shank probe.

    NO NO NO!

    A cylinder (or shank probe) will not work!

    Uae the pitch option as was suggested above.
  • Ok, agreed, I'll add the pitch option.

    Why not a shank probe? I thought it what they were made for.
  • Ok, agreed, I'll add the pitch option.

    Why not a shank probe? I thought it what they were made for.

    Your shank probe and thread would need to be perfectly square to each other and the shank probe would need to be perfectly square to the CMM - that's impossible! Think about how an unsquare shank prob will contact the thread - like this /|/ - thread is indicated as the mid line - not good for finding a decent C/L.
  • Your shank probe and thread would need to be perfectly square to each other and the shank probe would need to be perfectly square to the CMM - that's impossible!

    Ah, that makes sense. and sucks.
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