Checking Clearance of diameter

[mattfurtaw]

I am having trouble coming up with a good way of measuring the clearance on 2 different parts. This is a valve and the diameter must be within .0002 of clearance. I would like to make a report showing the overlay of each but I have trouble getting there. Right now I just measure points around the diameter take the difference of the readings at each hit. Is there an easier way ?

[Mark Farren]

I personally would not use a CMM to check that close of a diameter tolerance, too much error involved. I know this doesn't answer your question, but that's probably going to be the typical response.

IMNSFHO

[#2#]

I'm not exactly sure what you are trying to do but measuring a diameter with a tolerance of .0002 - I wouldn't use a CMM unless there is absolutely no other way to measure it.

Edit - WMFS

[Jan d.]

I would be careful to measure the size.

What you can do though is to mount a ring gauge of similar size on the machine, close to your part. Calibrate the probe your normal way. Then measure the ring gauge and measure the diameter you need to measure with the SAME speed, approach and retract and probe orientation. If you measure the ring gauge 0.000050" undersized, I found that you pretty much assured that you will measure your feature also 0.000050" undersized. So correct for the size by adding 0.000050" to the measured diameter.

I do this many times, verifying size on a known artifact of similar size. Works good for me.


Jan.

[VADER]

I think it all depends on how intimate you are with your CMM. I feel very comfotable measuring a tolerance that invovles this little of clearence. I do it everyday and get good correlation with hard gages as well as customers. Its all about how well you know your machine.

[\v/inston]

how large of a diameter are you checking? if its not that big you may want to try one of these bore gages they are extremely accurate and i think you can rent them.

http://sunnen.net/gageit/pg-700e.jpg

http://sunnen.net/gageit/form.jsp

[John Kingston]

I have to agree with some of the others. The CMM is not a good tool for such a tight tolerance. Pins, air gages, bore gages are the way to go.

[mattfurtaw]

The 2 diameters are small. We are looking at anywhere from .250" to .750"
We have been unsuccessful trying to verify the clearance. We use a DEA Mistral.
Thanks for all your input.

[Jan d.]

You are suffering from tunneling. No doubt. See my reponse here:

http://www.pcdmisforum.com/showthread.php?t=2289

Suggestions:
- run the machine quite slow (make sure you calibrate at the same speed as you measure) because that usually improves the tunneling slightly (up to a point where you get bad effects from not going fast enough).
- as I suggested before, try to master on a ring gauge.


Jan.