RMB for width as tertiary

[Viper555]

I'm having issues with a concept in particular that I was hoping someone could shed some light on (Y14.5-2009).

Part in question: Slender bar with hole in center of larger face (through thickness) and one hole on each side mirrored about the center hole (on same face). Larger face is primary (FS or NS not important), center hole is secondary (perpendicular to primary), and center plane of width is tertiary (position to primary and secondary). All datum reference callouts are RMB.

The example I'm looking at explains how by not using a translation modifier on tertiary, it is possible that the collapsing simulator on tertiary can cause the simulator to not grab on to both sides of the part at the same time when calling out a positional tolerance to primary, secondary, and tertiary of the mirrored holes. I believe the reasoning is that since the secondary datum takes precedence, the converging simulator only does so to the extent that the center plane it derives is coplanar with the secondary, even if only one side makes contact.

It further explains that only by using the translation modifier would you permit the total collapse of the simulator, but would result in a center plane not coplanar with datum axis B.

I can't find anything in the Y14.5-2009 standard that supports the idea that RMB does anything except collapse entirely on the part, but I'm sure it's in there somewhere. Could someone point me in the right direction? Hopefully my example is clear enough.

Thanks!

 

[3DDave]

One should start at what the rest of the universe would call the virtual condition, but Y14.5 says that the virtual condition can only be defined in exactly one way for exactly one purpose, so all other cases that are also virtual conditions for the same feature that are different and depend on the underlying references are waiting under the surface like a snag in a lake. This textbook is an example of one of those snags.

 

[Burunduk]

pmarc,
I would expect the datum features to be inspected first. Then the Regardless Material Boundary adjustable datum feature simulator to be operated to set the reference for inspecting the features that depend on the datum features. I realize under some circumstances the order of inspection could be reversed, but I doubt that that this type of considerations should set the rules in Y14.5.

3DDave,
The MMB can be calculated, as I detailed, so what's the big snag in the lake? Is it the semantics of your preferred "Virtual Condition" Vs. other context-related terms pertaining to worst case boundaries?

 

[pmarc]

pmarc,
I would expect the datum features to be inspected first. Then the Regardless Material Boundary adjustable datum feature simulator to be operated to set the reference for inspecting the features that depend on the datum features. I realize under some circumstances the order of inspection could be reversed, but I doubt that that this type of considerations should set the rules in Y14.5.

I wasn't thinking about reversing the order of inspection. I was thinking about whether the inspection needs to be stopped if a datum feature turns out to be non-conforming, or it can be continued.

 

[3DDave]

That's the problem. In one frame of reference the geometry selected as the datum feature conforms to its requirements, but Y14.5 has nothing to say about what is considered conforming in another frame of reference. Outside of the horse-blinder definition of virtual condition, this could be evaluated, but Y14.5 makes certain that cannot be done. About one much discussed figure, Y14.5 specifically demands not to evaluate the allowable/conforming geometry as a basis and to instead substitute a fake result.

 

[Burunduk]

I wasn't thinking about reversing the order of inspection. I was thinking about whether the inspection needs to be stopped if a datum feature turns out to be non-conforming, or it can be continued.

What I was saying is that if you inspect the datum features first, you reject the part before the RMB datum simulator is applied, so there is little use for a rule that helps you notice some problem with the datum feature in the next step set-up.

 

[greenimi]

I wasn't thinking about reversing the order of inspection. I was thinking about whether the inspection needs to be stopped if a datum feature turns out to be non-conforming, or it can be continued.

I remember you had the very same question few years ago. However I don't know what is the correct answer.