Datum Shift

[[Me...]]

The cookbook has True Position instructions for MMC/Datum shift per ISO for Bores but nothing for Shafts.

I've noticed when applying MMC for OD's/Shafts, Calypso automatically changes the evaluation settings to Outer Tangential. I know from other parts of the cookbook, other GD&T evaluation settings recommend Minimum Circumscribed on an OD/Shaft...

Is there any sort of difference between Minimum Circumscribed & Outer Tangential for OD/Shaft's?

Also, what evaluation settings do you fine folks use for MMC/Datum Shift for True Positions? The cookbook settings have me a little frustrated at this point.

[[De...]]

Outer Tangential and Minimum Circumscribed are effectively the same thing on an external shaft.

Using Outer Tangential is also correct on an internal bore. While Minimum Circumscribed would need to be changed to Maximum Inscribed.

In my opinion, Outer Tangential is the better choice since it is the proper choice regardless of whether the feature is internal or external, so as a programmer you don't have to alter your evaluation setting from one to the other if you copy a diameter characteristic and use it for multiple internal or external diameters - the evaluation method of outer tangential will still get you the correct evaluation.

[[Ri...]]

There is no difference between Outer Tangential and Max Inscribed/Min Circumscribed. As Derek suggested, you can make Outer Tangential a default evaluation and not have to worry about inside outside features. An interesting application not often trained is the use of Inner Tangential. If I was measuring a bore prior to a finishing operation, I might use the Min Circumscribed or Inner Tangential evaluation to make sure I had sufficient material for 100% cleanup. For a shaft I might use Max Inscribed or Inner Tangential.

[[Me...]]

Wait, wouldn't we use Inner tangential for a Shaft when MMC is applied?

[[De...]]

Per the standard you need to find the actual size of the smallest diameter hole that the shaft would fit in.

The smallest hole the shaft would fit in would be the same as minimum circumscribed or outer tangential for a shaft.

Inner Tangential on a shaft would be the same as the maximum inscribed. Maximum inscribed would be the largest circle that fits inside of all the points collected - or as Richard explained - it would be essentially tell you at what size the shaft would 'clean up' when turned.