Legitimate use of Tangent Plane?

[jonathan8388]

Hi All

I have a tooling application that is a plate with 3 spherically tipped pins inserted into a plate at a basic height shown below. The component that will sit on top of it will be sensitive to any tip/tilt caused by varying heights of the 3 pins it sits on, BUT not sensitive from the height to Datum A.

The first thing that came to mind for controlling this is the tangent plane modifier shown below. Never actually used the modifier in practice but I don't see anything else more appropriate or direct.

Is this a legit use of Tangent Plane? I am on the fence about it

(Note that image is incomplete and am only showing dims/tolerances pertinent to my question)

 

[greenimi]

Some people say YES, some other say NO.
ASME Y14.5 committee is explaining it only in the context of a nominally flat surfaces.

 

[greenimi]

https://www.eng-tips.com/viewthread.cfm?qid=440137

Discussed here before

 

[jonathan8388]

Yeah I read that thread a little. The example there is a little different as I think the part is a 360 degree toroidal shape that is one constant surface (albeit curved). In the 2009 standard I don't see anything that specifically disallows the T modifier on a curved surface like that (Paragraph 6.5) so I can see an argument made for extension of principle there.

If I can make the bottom of that toroidal shape a primary datum feature that contacts at 3 high tangent points (which I should be able to) then I don't see why there isn't a legitimate method to also control that same interface with SOME geometric control for those 3 high points.

The one I have is further muddied in that it is 3 separate surfaces that are used to control the tangent plane. I suppose more people would be in the camp of that not being allowed.

But again....I can reverse this and make the tips of those 3 pins a primary datum feature and then control the tooling surface to those 3 pin tips. So i feel there should be some way to do the reverse situation like I'm showing (somehow).

 

[jonathan8388]

We are using the 2009 standard FYI. Just looked up the 2018 standard and they clarified that it is for planar surfaces only in paragraph 9.4. Bummer.

I'll just reverse the scheme and call it a day