Measuring Cylindricity

What type of CMM hardware are you running? (Probing system? Newer machine?)

Tell us more about your CMMs Model and probing system. It maybe the limitations of your machine.
Cylindricity is the form and straightness of the entire cylinder, in my opinion I don't think the air gage solution is valid.

Beat me to it.

The CMM is a old Xcel 9-15-9. RC1 controller, touch trigger probe. Software is PCDMIS 2015. I used Mutiple probe configurations.

The CMM is a old Xcel 9-15-9. RC1 controller, touch trigger probe. Software is PCDMIS 2015. I used Mutiple probe configurations.

The CMM would be the correct piece of equipment vs the air gage.

The type of machine/probing system setup that you’re working with is not repeatable enough for the tolerance you’re trying to hit

Depending on what kind of touch trigger probe you use, the results can be better or worse. A Renishaw TP200 has a virtually non existent roundness error, whereas a TP20 and comparable probe systems produce a three-lobed error pattern when measuring a perfect round gage. A TP20 in a PH20 however gives better results due to the rotation of the head during probing.

Best choice for form measurements is always a scanning system.

The air gage has physical limits when it comes to form measurement.

We are running a part with a 3" cylinder. The cylinder has a .0002 cylindricity call out. For 10 years we have been using a air Gage to measure this feature, we zero the Gage & spin it throughout the cylinder & the total reading is what we use as our result. Anyway, I was asked to start measuring this on our CMM, I get the cylindricity reading at around .001 every time, I measured a master ring the nominal size of this cylinder & also get the cylindricity around .001. Can I hear everyone's input on measuring cylindricity please.

1.) Which way is more accurate, CMM or air gage (I have tried multiple sized probes & stylus lengths.
2.) Is cylindricity of a cylinder the same as roundness of a circle??

2) Cylindricity is a 3d version of roundness aka circularity. It's like the difference between runout and total runout.

1) Generally speaking, an air gage can be extremely accurate for size. Using one for cylindricity is going to be dependent on how accurate the turntable is and how accurately the cylinder is aligned to the center of rotation. I doubt that your CMM is accurate enough to measure anything with a .0002 tolerance much less cylindricity. The probe accuracy alone is probably greater than the part tolerance. I also assuming that the probe head has to be rotated to reach all the way around the cylinder. Indexing error is probably greater than part tolerance. In addition, with a touch probe, vast amounts of surface won't even be touched unless management is willing to spend a day or so probing. At the tolerance level being discussed, a nearly invisible scratch would throw the part out of tolerance, so a high point density is required.

There are dedicated systems out there for measuring circularity and cylindricity. Think Mitutoyo has one among others. CMM's are great but they can't do it all; sometimes specialized tools are required.