Surface Profile and Size 2

[AMontembeault]

I'm looking at a series of prints which detail a round feature of size, controlled by a datum-less surface profile. My question is, should the diameter be basic, or limit toleranced?

My inclination is that it should not be basic - that a datum-less surface profile is just a form control (in this case, no different than circularity or cylindricity), and profile must be used as a refinement of an independent size tolerance, as described in ASME Y14.5-2018 section 11.2.

That said, I have doubts, because I often read, even in this forum, that size is controlled, and I look at Figure 11-10 in Y14.5-2018, and all of the size dimensions are basic and the all over surface profile has no datums.

What nuance am I missing?

 

[3DDave]

You are right - the intersection doesn't define the true profile - it locates it. You must by now know the difference between defining a shape and defining an offset from a point on that shape.

One can just as easily create the true profile, pick a point on it, create a chord meeting the given chord depth, then a horizontal at a given offset angular offset from that chord at a given distance from the original point. Any CAD software will find the perimeter completely defined and not over-constrained.

In CH's original all three elements are coincident by definition. The width is poorly controlled, but that's not the subject at hand, is it? Your substitute suffers the same problem.

 

[Burunduk]

The location of a true profile/tolerance zone is part of its definition. When I say "defines" I mean definition in all aspects: location, orientation, form.

As part of not defining it, a directly toleranced dimension does not locate a true profile (the theoretical geometry that the profile tolerance zone is disposed about), and therefore it doesn't locate any specific intersection point within that true profile.

You keep describing how to draw the geometry. Refer again to my "hole in a rectangular prism example" from 30 Jun 20 03:29. Anyone can easily draw a square with a hole in the middle of it. Will the position tolerance properly control the hole to be normal to one face and centered to the height and width within the specified tolerance if the appropriate datum features are not referenced?

 

[3DDave]

I'd rather not follow yet another distraction. Your example is still flawed. Concentrate on that instead. The profile is over-constrained and the width is still mis-constrained.

 

[Burunduk]

Nothing I said is a distraction as I was addressing the points you were bringing up. I suggest you pay attention to this "distraction", you may eventually find it informative.

If by "over-constrained" you mean over-dimensioned by the X dimension in this sketch, it is not meant to be one of the basic dimensions to be specified on a drawing. The context is within the post from 29 Jun 20 19:46.
Or do you still find datum feature B to be unnecessary after all those explanations?

The width has nothing to do with what is being discussed (the profile tolerance). The sketch is not intended as a fully defined drawing.