Measuring Thread Pitch

[Underspec]

I have a call out of .125 Pitch thread / 3.25 Revolutions. I've never measured this kind of feature on the CMM. From what I've been seeing is that it can be done pretty easily. Could use some info! Thanks.

[cmmguy]

That's what thread gages are for....

[Wes Cisco]

Use an autocylinder. Put the pitch in the pitch window. Your rows should be a multiple of the pitch apart. Cylinder diameter will be either the thread minor or major depending on whether this is ID or OD.

[John Kingston]

I don't think anyone is actually measuring thread pitch with the CMM. What you may be thinking of is using the pitch function to measure the location of a threaded hole. This allows the styli to hit on the same part of the thread for each hit. (IE. Crest, or just off the crest at intervals aroud the hole.)

[craiger_ny]

What part is it you want to measure the 0.125 pitch? The 3.25 revolutions? Or is it the thread form itself? I guess I shouldn't really ask as none would be a good idea on the CMM. If you simply want a rough (I mean rough) location reading then do what Wes said, otherwise do what J said. I say get an animated avatar and your problems will all go away.

Craig

[Mark Farren]

I say get an animated avatar and your problems will all go away.

Craig

I second the motion!

And if all you want to check is the pitch, use a comparator or a pitch gage.

[Cumofo]

The Council has spoken

I say get an animated avatar and your problems will all go away.

Craig

I think we should appoint underspec an approved avatar, or he shall be flammed until he cheers up.

[T Miller]

I know it's cheap. but it will get us off your back

http://i62.photobucket.com/albums/h9.../underspec.gif

Save as.....

[david phillips]

thread Depth Revolution

I agree with CMMGuy.
Use thread gage.
Thread gage till 2 finger stop.
then count # of revolutions to remove gage.

[Underspec]

This is a part that's about 2.8" in diameter. I don't have gage that size

[craiger_ny]

I'm getting the fealing you want to measure the actual thread for conformance to the thread spec with the CMM? You better hope it is a standrad thread that there are typically gages on the shelf for because you need a thread gage now, while you are making parts. I'm outta here man.

[Cumofo]

This is a part that's about 2.8" in diameter. I don't have gage that size

look in the machinery's handbook and see what is required to measure thread pitch, then decide if you have the abilitiy with a CMM to reasonably validate that feature.

The cost of the gage should be included in the job.

[John Kingston]

I'm going with Craig. CMM is not the tool for thread conformance.
Try Johnson Gages. They make thread gages for just about every possible scenario.

[cmmguy]

Alright if you must know how to do this...

First you need to get best-wire sized styli for the probe. Typically this is sized by (w = 0.5p x secant(1/2 thread angle). Thread angles are usually 60deg unless you are using British Threads then sometimes they are 55deg - you better double check which ones you have. If you can't find the correct sized tips then you can vary slightly but you will have to calculate the correction. The next thing to do is calculate the correction factor for the actual pitch diameter and the effective pitch diameter based on the corrected wire size and measured diameter. Once you measure the thread with your self centering scanning CMM, take the uncompensated diameter and add 2(sin(30)x(1/2(bws))). Make sure you scan a speed that will maintain a constant gaging pressure of 2.8 lbs - no wait that is if you are using wires, never mind that part about the pressure.

[george frick]

hey-add to wish list of 5.0. cmm with software that gages threads! lol