# Concentricity of two ID's (Inside Diameters)

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#### awall

Hello, I am trying to measure concentricity between two ID's. I've attached a snippet of the print so you can see exactly how it is called out.

I have a gage with 4 probes. Two probes measure the 96 ID and two probes measure the 134 ID. You place the part on the gage and spin the part 360?. It gives you the average measurement of the two diameters. The gage is ran using gage metrics software.

Using this gage is it possible to measure concentricity as called out on the print? If so, how?

This is measured on a CMM once per shift but the customer is requiring 100% inspection on concentricity between these two ID's. I would like the data from the gage to correlate to our CMM.

#### Attachments

• Concentricity.pdf
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#### Michael_M

Trusted Information Resource
Can not really see how the functional gage you are using can be used to measure the concentrically unless the probes are on the same center-line as datum -A-. In general, concentricity cannot be checked with a functional gage.

Can you get with the customer and use process control to set concentricity. If you are using a lathe to turn both diameters and you turn them both at the same time (same operation), it would be very difficult to fail on the .05 concentricity. You may be able to allow the CMM check once per day to ful-fill the requirements.

#### Proud Liberal

##### Quite Involved in Discussions
A lot depends on how you are holding the part and picking up the data from the probes. Since concentricity requires an analysis of centerlines, it doesn't sound like your setup can account for establishing datum -A- RFS (two probe points on a diameter wouldn't work).

I'd check the Cpk of the concentricity and approach the customer with the statistical argument to reduce the sample frequency to less than 100%. Assuming this is a machining operation and both diameters are turned without un-chucking the part, the relationship between the two should be a function of the condition of the turning center (and for all practical purposes constant).

Has this dimension been a problem in the past?

A

#### awall

Thank you both for your quick replies.

I've attached a picture of the gage we have so you can get a better idea of what I have to work with. I thought since the gage would be measuring both diameters from the same centerline I could use it somehow to measure concentricity.

This part is machined on a CNC, both dimensions are cut without being removed from the chuck.

This dimension hasn't been an issue for us historically, but we haven't been able to get the customer to budge on letting us reduce the frequency of the check, the language barrier makes this extremely difficult. Currently the gage and CMM concentricity data do not correlate and this is causing a major headache for Quality and Production trying to produce good parts.

A

#### awall

Forgot to attach picture, sorry about that.

#### Attachments

• gage.jpg
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#### Golfman25

Trusted Information Resource
I don't think you'll be able to do it with that gage. Unless you can develop some mathematical formula between the two diameters and the centerline.

Probably easier to just make up a new gage to measure concentricity - and get a direct measure. Or check it while in the CNC.

Good luck.

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#### zancky

imho If the requirement is changed adding two maximum material principle, you can check concentricity with a stepped plug gauge

#### Stijloor

Super Moderator
To all the posters above: What GD&T Standard does apply?

The definition per ASME Y14.5 is quite different from ISO.

Virtually ALL concentricity callouts on ASME-based drawings are wrong/misapplied.