# Diameters measurement with 3D touch machine like a CMM

#### Marco Bernardi

##### Mark_Navigator
Dear all,

I'd like to solve and clear some doubts about diameters measurements like in the thread title:

when you measure with a CMM e.g. a diameter of a piece, usually is used the best-fit algorithm to get the best approximated diameter value and the roundness error to evaluate the circle form.
Then, but could not be useful to get the minimum circumscribed and the maximum inscribed circles, referred to the roundness error and not only the roundness error?
And more, for a coupling analysis with another part, should not be more indicated the minimum and maximum diameters got from the circle form construction? (that is different from roundness error that considers the circle center, instead of singles and separates diameters, that has the barycentre translating)

#### Marco Bernardi

##### Mark_Navigator
Up, up, thank you to everyone wants to go on with the thread.

#### John Predmore

Trusted Information Resource
The best-fit circle is not always the best choice, the answer depends on what information you want and what you will do with the information. If you measure the outside diameter of a shaft which will be inserted into a hole, the minimum circumscribed circle could be better to predict how the shaft will fit in the hole. I think you know this. What is your question?

#### Ron Rompen

Trusted Information Resource
Most CMM software that I have used in the past few years has the option of reporting Min circumscribed/Max inscribed/Average (least squares) diameters. If your CMM doesn't give you these options, then perhaps you should investigate a software upgrade.

#### Marco Bernardi

##### Mark_Navigator
The best-fit circle is not always the best choice, the answer depends on what information you want and what you will do with the information. If you measure the outside diameter of a shaft which will be inserted into a hole, the minimum circumscribed circle could be better to predict how the shaft will fit in the hole. I think you know this. What is your question?
You're right John, it depends which application is intended for that diameter.
However, what I want to clarify then is that max inscribed and minimum circumscribed diameters, could and usually have different center than the minimum and maximum diameters got to evaluate the roundness value with the least-squares actual diameter calculation, isn't it?
Another question: why is not considered with GD&T to give out the information of those min. and max. diameters got with least-squares calculation, instead of only roundness value, that could be positioned in different ways compared to the actual diameter got?

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