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.
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.
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.
