True position producing smaller diameter

[[Ch...]]

When programming a true position call out at MMC, Calypso automatically produces a diameter for the feature being called out.

Regarding the program I am working on, I have (3) circles that I use to recall into a cylinder.

The MMC produced a cylinder diameter that is smaller than the recalled Cylinder and all (3) circles. Yes we are working with very small amount of deviation but I am still curious because we deal with really tight tolerances frequently, so .000043 difference would be nice to understand moving forward.

Does anyone know where it is getting this number from? 😕

Max material error 3.JPGMax material erro.JPGMax material error.JPG

[[Mi...]]

MMC will create a Diameter using the functional size (Inscribed or Circumscribed) and it will constrain it to your primary datum if that applies, your other reported diameter is likely LSQ unless you changed it.

Using the functional size is correct as per the standard, but won't be as stable.

[[Ma...]]

You have used different evaluation method.
Diameter_Cylinder3 is GG ( Gauss )
Diameter_ForCylinder3_1 is GX ( inside )

[[To...]]

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Chances are that one is using Least Squares Quotient (LSQ) and the other is using Outer Tangential Element (OTE). The differences are due any form error that may be present in the feature. From a functional fit perspective, I would use OTE evaluation.

Not sure where to look? Open the diameter characteristic and click on the feature button. At the bottom, you'll probably see a button with one of the two options listed above.

[[To...]]

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Is there a setting for this? I've never seen Calypso automatically change the evaluation on the feature unless they were features used in a datum reference frame. I've seen a window pop up stating that I was not using the correct evaluation, asking me to create a new characteristic with the proper evaluation or edit the existing characteristic to the proper evaluation.

[[Mi...]]

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It doesn't change it, it creates a new one with the "correct" settings. That's why when you hit MMC it will warn you that the characteristic you picked isn't set right and it askes you to create a new one, or adapt the current one.

If somebody hits adapt, not understanding what is happening they are going to be affecting the size that gets output without understanding why.