Hi All,
Here are some thoughts. The first one is that this topic is a rabbit hole, and it's going to get weird! The features on "thin" parts can confound some of the traditional GD&T approaches and give us headaches in CMM inspection as well. The features are technically 3D surfaces but function as if they are 2D lines.
There are several things going on here, but let's start with Feature C. Let's say that the Perpendicularity tolerance for feature C is 0.1 mm to A and B. The requirement is that all of the points on the surface C must lie within a zone 0.1 in thickness, with the requirement that the zone is exactly perpendicular to both Datum A and Datum B. There is no requirement to measure it in a certain way, but most CMM software imposes certain restrictions. Feature B is technically a planar surface (there will be a vertical surface in the CAD model, however thin). In order to apply a Perpendicularity tolerance, most CMM software requires that the surface be measured as a Plane. This becomes inconvenient with thin parts, if the diameter of the CMM stylus is comparable to the part thickness. We might only be able to touch points at one depth, and we need points at at least two different depths to define a Plane feature. I would say that this particular issue represents limitations in the CMM hardware and software. If we had a CMM that collected points differently (smaller stylus or perhaps non-contact scanning), then we could measure points at different depths. The part might be thin, but it's not infinitely thin (it must have a finite thickness).
If we think about the Perpendicularity tolerance on feature C, what does it really control? I agree that there is a "psychological" aspect that applies here, because of the term "perpendicularity". What does it mean for feature C to be perpendicular to A and B? All points on feature C must be within a zone 0.1 in thickness, with that zone oriented exactly perpendicular to A and B. We can say that this controls the feature's orientation or "tilt" relative to A, but we really don't have to say that. We only say it because Perpendicularity is described as an orientation tolerance. When feature C is very thin and has essentially zero depth, the "orientation" aspect relative to A effectively drops away.
Feature B defeats some of the usual GD&T approaches as well. Feature B is technically a feature of size (outer width) made up of 2 parallel planar surfaces. The traditional GD&T approach would be to control this feature with a Size tolerance and a Perpendicularity tolerance. On a very thin part, even measuring the size can be ambiguous and require assumptions, but I won't go into that just now. The Perpendicularity tolerance definitely causes major issues when the part is very thin. What does the Perpendicularity tolerance actually control? The center plane of the feature's UAME (Unrelated Actual Mating Envelope) must be within a tolerance zone that is exactly perpendicular to Datum A. So to measure this we must establish the UAME, and that's the hard part! On a thin feature with slightly tilted or irregular surfaces, the UAME can be extremely difficult and error-prone to establish (especially the orientation of the UAME, which is the only thing we care about in this case). If we're using a CMM and can only measure points at one depth, it's impossible. With physical equipment, the UAME would be the "minimum circumscribed slot" of feature B. Imagine putting vise jaws around an outer width feature on a sheet metal part, to establish its center plane. I would go as far as to say that width features on thin parts defeat Y14.5's definition of the UAME and the resulting center plane.
So where does that leave us? Feature B still functions somehow, even if we can't measure the perpendicularity of its center plane. So what could have been specified instead of Size and Perpendicularity? As others have suggested, Surface Profile is always an option. There is another more obscure option that I would like to bring up as well, that is probably much closer to capturing the real requirement on feature B. If the Perpendicularity tolerance was specified at MMC instead of RFS, then we could apply the Surface Method. This is described in the text of Y14.5-2018 (Section 9.3.5), but there is no example illustrating it. In a similar way to Position at MMC, an orientation tolerance at MMC can be interpreted as creating a VC boundary for the surface of the feature. In the case of outer width feature B on the OP's thin part, the specified Perpendicularity tolerance would be added to the MMC size of the feature. Inspecting the Perpendicularity would amount to verifying that the feature fits between two parallel planes that are spaced apart by the VC size, and oriented exactly perpendicular to Datum A. This could be done easily with a hard gage, or calculated from the CMM points even if they were all at the same depth. There is no requirement to establish the UAME and center plane.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
