Positional Tolerance of Internal Triangle Tangent Cylinder 1

[jonathan8388]

Hi everybody. Long time lurker on these forums, but I think first time posting here. ASME Y14.5-2009 Senior.

Have an interesting application for a part assembly fit that we are working on the tolerancing for. To break it down in a very simplistic state, the mechanics would be similar to a triangular shaped hole (ignoring corner radii) that would house a cylindrical part. The direct functionality that we would care about is the fit of this cylinder inside of the triangular cavity AND the positional error of the cylinder in the as-assembled state.

On the detailed triangle part, I would highly prefer to somehow control that functionality directly (tangent cylinder position) rather than the 3 walls with some profile and extrapolate out what an effect positional error would be. Obviously, I would need to control the entire profile of the triangle with some control, but for the ultimate part functionality, this tangent cylinder position is what I mostly care about.

One can argue that this could be an Irregular FOS type A, so my first off-the-cuff method here was to explicitly state that the UAME for this part is a largest-fit cylinder in the triangular cutout (so there is no different interpretation)and then position/size that UAME directly. Would have a note to state that the size tolerance requirement actually applies to the UAME contacting all 3 surfaces in lieu of an actual “feature size” which doesn’t explicitly exist here

Thoughts?

link to figure is below

https://files.engineering.com/getfi...b32a-4e29-aefc-477e7c0ed462&file=Triangle.JPG

 

[Burunduk]

greenimi,
Looking through ISO 5459 it seems like the standardized use of the term "contacting feature" is different from my use in the context of the above example (of the "VDI" shank). ISO 5459 invokes a "contacting feature" by requiring the CF modifier wherever the geometry type of the "associated feature" (true geometric counterpart) is different from the nominal geometry type of the datum feature.

For example, had the following Y14.5 figure was implemented per ISO 5459, the datum feature reference for A in the profile and position callouts had to be accommodated by the [CF] ("contacting feature") modifier, because even though the datum feature is two cones, the "associated feature" being used is two spheres of fixed diameter.

In ASME it is implied by the use of datum targets and their dimensioning, and no redundant modifier is required to communicate the idea.