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Datum feature B in this example is both holes, which constrain the final 3 degrees of freedom. Datum features A|B fully constrain the part.

Thank you, I missed that datum B was in 2 plcs. IF you had a hole and slot as the B and C datums, could you repeat the A-B-C in the lower portion of the composite? Could it be appropriate? I only see examples repeating A and B and they are always parts that resemble blocks. I work with sheet metal parts or molded plastic parts with complex shapes where the datums aren't always square to the part.

Calculating capability with a position tolerance zone and the added bonus tolerance is tricky because the allowed tolerance changes based on the hole size. There is another technique using the calculated virtual condition (MMC hole minus the position). This is a fixed boundary that the hole must not violate. Then measure how close the hole surface gets to the boundary (calculated RAME). Here is a related video for that: czcams.com/video/HSx7ofsad1k/video.html

Yes, ASME Y14.5 requires the true geometric counterpart for C to be located (centered in this case) to datums A and B.

I don't like that rule. It just depends on the application. Sometimes the size is very important (press fit) but where the feature is on the part is not important (big position tol). On a rotating shaft, sometimes the size does not matter but the coaxiality is very important (for balance).

@@GeoTolPro Thank you for your quick response..

No the Z height doesn't affect the measured value. The tolerance zones must be rotationally aligned to that primary datum A plane. The perpendicularity of the datum feature C axis does affect the measured position tolerance value however.

@@GeoTolPro What if C nominally isn't perpendicular to A?

I agree. A lot in this system is what machinists and inspectors would call natural and common sense. Geometric tolerancing is just giving us the language to communicate that "common sense" in a more explicit way.

I would recommend profile tolerance relative the the datum reference frame. If size didn't matter that much, you could have a larger profile tolerance relative to the DRF and then refine with a dynamic profile control relative to the DRF. Profile is definitely the way to go for irregular features.

They are the same. Although ISO doesn't call it "shift". In ISO 2692, it says the MMVC shall not be violated by the datum feature's surface. This means the datum feature could "shift" to meet the requirements of other features on the part. You can not add the datum feature shift as bonus in either standard.

⁠@@GeoTolProcan I find it this info ISO 2692. We are having quite serious discussion over it. Having a solid example might convince people.

No difference on this example because there is only a single feature. The lower frame of a composite constrains rotational degrees of freedom only. This would control orientation only to the lower datums just like the parallelism. However, composite position would be different if there were multiple holes. The lower frame of composite would also control location between the features in addition to orientation to the datums.

That's awesome. Thanks for the compliment!

You want all features located to the same datum reference frame. Even the datum features A and B are located to the common datum A-B. Datum features A and B do not establish the datum by themselves so they need relationships to the A-B also.

Tanks a lot for your support

Flatness "combined zone" is an ISO-GPS concept. The CZ modifier locates the flatness tolerance zones together to also control location between the surfaces (usually to control coplanarity). ASME would instead switch to a profile tolerance to control location between multiple surfaces.

@@GeoTolPro Understood. Thank you very much!

No difference there. The surface must be flat. Then it is also named "datum feature A". Personally, I like to have the symbols together.

No, you don't have to add the MMC modifier. However, the tolerance in the lower frame is usually (not always) calculated based on the MMC clearance between hole and pin so it makes sense to add the mmc modifier.

Dimensions shall be basic when the features are located with position and profile. A dimension can be made basic with either a box around it or a note. (A note is used in this drawing) See Y14.5-2018 section 5.1.1.2

The theoretical tolerance zones are locked to the basic dimensions. The hole axes may shift within the dia 0.2 tolerance zones.

Datum features are selected based on how the part functions (how it mounts in the assembly). It doesn't matter the sequence of operations from manufacturing. When the part is all finished, what are the functional requirements?

Right, a functional design drawing does not replace the manufacturing plan or work instructions for how to make it. Geometric tolerancing on the design drawing only states the functional requirements for the final product. This may have an influence on how the part is made, but it does not state the method of manufacturing or sequence. However, geometric tolerancing could also exist on the manufacturing level to document "in-production" checks to manage process control.

@@GeoTolPro Thanks for this - perhaps this aspect can have a greater emphasis in the videos. I have noted that it's rarely mentioned - for sure you have talked about work Instructions etc but I REALLY WORRY THAT MACHINISTS are really losing sleep over GD&T and how to interpret / apply it. How to tie it all up to their jig, fixtures and vice setups must be a nightmare very often.

Countersinks often "self-center " the fastener. MMC modifiers may not be appropriate here.