ASME Y14.5 Position question

[JacobCheverie]

I believe that this is a bad design, but I am looking for some input. I have a thick slab whose width is datum A. I put two spherical I.D.'s in the slab at a distance X apart from one another. I put a ball inside of each sphere. Each ball has a cylindrical I.D. thru bore. Geometrically, I have a plane with two cylinders that are free to rotate throughout space with the centroid being on the plane at a distance X from one another. Call the first thru bore datum B.

Does it make sense to call out a Position tolerance on the second thru bore to [A|B]? The Position is controlling the distance X. Datum A constrains two degrees of rotation, and one of translation. Datum B constrains the remaining two degrees of translation and is constrained perpendicular to datum A. Now the direct centerline between the two thru bores has to be basic distance X, but the feature is free to rotate.

Do I constrain it to A? If so, why? If not, why not?

I don't understand GD&T as applied to an assembly permitting relative motion between features/datums, and I don't believe that it makes much sense.


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[Kp61dude!]

I believe that this is a bad design, but I am looking for some input. I have a thick slab whose width is datum A. I put two spherical I.D.'s in the slab at a distance X apart from one another. I put a ball inside of each sphere. Each ball has a cylindrical I.D. thru bore. Geometrically, I have a plane with two cylinders that are free to rotate throughout space with the centroid being on the plane at a distance X from one another. Call the first thru bore datum B.

Does it make sense to call out a Position tolerance on the second thru bore to [A|B]? The Position is controlling the distance X. Datum A constrains two degrees of rotation, and one of translation. Datum B constrains the remaining two degrees of translation and is constrained perpendicular to datum A. Now the direct centerline between the two thru bores has to be basic distance X, but the feature is free to rotate.

Do I constrain it to A? If so, why? If not, why not?

I don't understand GD&T as applied to an assembly permitting relative motion between features/datums, and I don't believe that it makes much sense.


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Does it make sense to call out a Position tolerance on the second thru bore to [A|B]? Yes it does.

Do I constrain it to A? If so, why? If not, why not? Yes. Because the print calls it perp and not an angle. Thru bores might be free to rotate now but once mated to it's part will it, maybe yes/no. Also, imagine the part rotating about perpendicular to current view (provided attachment) and the bores remaining perp to view X axis, distance X can go down to 0 and all the way back to it's maximum again when it rotates back to datum A being parallel with current view's X axis. Hope that makes sense and if it doesn't I'm not surprised as I'm terrible at explaining things without waving my arms and making sound effects...

Prints/Drawings should be designed in a way to not have to consider it's mating part, though it does help knowing what it mates to as it helps you better understand what the design engineer was trying to convey (or failed to...).

[JacobCheverie]

I believe that I understand what you're saying. Ultimately, I know that they are trying to control the basic distance. I think this part does require you to consider it's mating situation to really understand that kind of control.

That being said, if the design should be done so that the mating part does not have to be considered, then what kind of control(s) would make sense when describing parts exhibiting relative motion? I do not believe ASME Y14.5 captures this situation at all. I also believe that if the appropriate tolerance stack-up analysis was performed throughout component levels, this assembly condition would be satisfied without the use of an ambiguous control.

[Rich P]

but the feature is free to rotate.

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The pattern of 2 cylinders are free not the feature it self
The positioned cylinder axis must be perpendicular to A and // B cylinder at the B relationship Basic to meet the positional requirement shown

[JacobCheverie]

How do we address the fact that if I simulate my datums with a contracting pair of parallel planes and an expanding pin, the feature is free to spin? Do we just align it in such a way that gives us a passing result? Is the drawing meant to visually imply that the measurement, and really thus the control, only applies when the feature is parallel to B?

[A|B] establishes a fixed coordinate system (one rotational DOF is left unconstrained, but that doesn't much matter here) and the feature is not stationary, so how can we talk meaningfully about it's location?

What is THE position of a fly in the room? Do we say "well, when I put it at X it's position is X"? That doesn't really tell anybody anything.

[DungT]

You have to constrain it to A. Very high chance that you would get cosine error if you don't and the result would be smaller that it actually is

[JEFMAN]

This case should be exactly the same with an unitary part with 2 diameters (for example a part with a plane as A, a hole as B and a pin as C), which is a classical case...

[JacobCheverie]

JEFMAN If this was a part without relative mobility allowed, then if the feature was out of perpendicular to A, that would negatively effect the Position result. In the case of relative motion, the error in perpendicularity can be compensated for until nonexistent, similar to a situation involving MMB/LMB.

This situation is like asking for the perpendicularity of each side of a hinge assembly - As the hinge opens/closes, the perpendicularity changes.