Measuring Pitch Diameter as a Datum

[[Ja...]]

Hi all

Wondering if anyone has ever come across something similar to this; pitch diameter is a datum (A)...I've read threads online where people have stated measuring PD on a CMM isn't reliable. We also have a perpendicularity tolerance face (B), referenced to the PD (A).

What would the recommended methodology be in checking this in calypso?

Thanks
Jack

Perp Dat B.JPGPD Datum A.JPG

[[Da...]]

Measuring the pitch diameter of a thread is next to impossible on a CMM. You'd need a probe small enough to touch the center of the flanks of the thread.

And then this perpendicularity call-out... sigh... I don't know why designers always come up with such BS.

The perpendicularity call-out is for an angle of 90°. A tolerance of 0.001" on a plane with a diameter of more than 7.6" (the drawing doesn't show the OD, but let's say 7.6" for the sake of argument) can be understood as a slope:

0.001" / 7.6" = 0.000131578

If you turn the plane and pitch axis around ( pitch axis to plane instead of plane to pitch axis), you would have to multiply this value by the length of the pitch axis to get a comparable tolerance.

0.000131578 * 0.885" = 0.000116447"

That's around two microns, a precision unachievable for a CMM to measure the orientation of an axis. And you can't even measure the pitch diameter.

All the vital information you could take from the drawing would be that the plane and thread must be turned in ONE setup.

Designers should know never to call-out a large element to a tiny datum.

[[Jo...]]

I would also report flatness of both planes. That will be a component of error in perpendicularity and parallelism. Technically the slope analogy is only possible with perfect form of the plane, being a total wifth zone. The M odifier on Primary Datum is also ridiculous, unless you are able to quantify it. That is a MOBILITY on Datum, and doesn't give you "bonus" tolerance.

[[Je...]]

If you have a RT this is not a difficult task but will require a rather small stylus to hit the flank of the thread in an area near the PD. A self-centering scan of (Rotations*360°) over the length of the axis will give you good results. From there a formula will need to be applied to calculate PD vs actual measured size (c.o.b. over 60 included angle) to report PD actual. The axis of the thread is not going to be sufficient to control a perpendicularity of -B- to -A- however as the length of the axis is roughly 12% the diameter. A minimum of 1:1 is recommended while a 3:1 ratio is strongly preferred for a repeatable measurement.

Performing this measurement with a star probe will be significantly more cumbersome to program and longer to run but not impossible.

[[Ke...]]

I have had good luck with getting custom probe diameters from suppliers. Just contact their sales rep and let them know what the pin diameter specification is. If you have other methods of verifying the pitch diameter, then a standard stylus may be "close enough"

[[Ri...]]

Something I have been toying with for a while now.
Custom Torus Styli for collecting PD of threads to establish as Datums or for True Positions.
I'm not 100% sure and I believe I could accommodate most sizes from 20mm on up.

This drawing and STEP file is based on a 42x2 OD thread.
I still have to complete the Stylus as it is in basic shape (Model).
I've become more curious on how this would work for OD/ID threads Inch or Metric.
Copy right CPC LLC 2023

M42x2 Model views.JPGHelical Thread Disc Step file.zipM42x2 Thread Wire Size.JPGHelical thread Disc_M42x2.pdf

[[Jo...]]

I think that has promise. The thread over wire method is just a pin size that allows protrusions above Major so you can Mic it. I wouldn't be concerned with an exact probe diameter(wire size). It is still an assumption that lead and thread profile is correct, and doesn't ensure root,crest are correct.(Probe would still shank out). I have parts that spec. a runout to thread as well. I helix scan the crest. Those parts are made on lathe in one operation so the PD and major are coaxial. I think the most obvious risk, is not contacting the flank and getting bad data. It looks like your chart has recommended sizes to help your selection.

[[Ri...]]

Please sign in to view this quote.

I think you're right, that opens up the capability.