Boundary on slotted features

[dezignr]

I am reviewing a welded assembly drawing that brings together two flanges and one tube. Both flanges contain slotted hole patterns that are defined on their respective detail drawings. They are being related/clocked one to the other on the assembly drawing as follows.

One flange is utlized as the origin in the dimensiong scheme and its mounting surface is identified as primary datum A. Its slotted pattern is identified as secondary datum B with BOUNDARY under the datum callout.

The slots in the opposing flange carry the following callout:

"6X Slot
Position tol .010 RFS relative to datum A & datum B RFS
BOUNDARY"

I could use some help with interpretation. To determine the boundary condition it is necessary to refer to the detail drawings that contain the size limits for the slots. So I'm a little confused with regard to the boundary condition of the datum B slots since there is no associated geometric tolerance on the assembly drawing. The boundary condition for the slots on the other flange must also be determined by reviewing the slot size limits from its detail drawing, but then it seems I would need to consider the .010 position tol on the assembly drawing. Any help would be appreciated.

 

[CheckerHater]

As your datum B is called out RFS (actually it's RMB now) you don't have to know size limits of the slots (as in "regardless of size").

Same thing with your controlled features specifying FRS as well.

In real world you'll have to use some magical slot simulators able to change their several dimensions simultaneously, which doesn't seem practical; but in theory the call-out is OK.

I guess it's a functional requirement.

 

[pmarc]

The thing is that position with BOUNDARY is legal (and interpretable) only if tolerance is specified at MMC, not RFS.

As for datum feature pattern B referenced RMB, it is legal, but looks rather non-functional (unless something like pattern of expandable elongated tabs fit into the pattern of slots B in a real application OR there is a press fit between tabs and slots).

Do size limits of the slots have to be known? I would say yes, because in theory, when internal datum feature(s) is/are referenced RMB, datum feature simulator(s) expand(s) from MMB toward LMB until maximum possible contact with datum feature(s) is achieved. Without knowing the MMB limit, one is not able to define "starting point" of the expantion.

Since geometric tolerance for datum feature B is missing on the assembly drawing, it looks like the drawing is incomplete. It is worth to remember that per fundamental rule 1.4(o) in Y14.5-2009 and similar rule 1.4 in Y14.5M-1994: "Dimensions and tolerances apply only at the drawing level where they are specified. A dimension specified for a given feature on one level of drawing (e.g., a detail drawing) is not mandatory for that feature at any other level (e.g., an assembly drawing).".

 

[CheckerHater]

@pmarc:

According to OP drawing has both BOUNDARY anf RFS specified together. Could you please elaborate where it is forbidden to use them together?

I agree, the functional requirements are kind of murky.

The magical expandable elongated tabs can operate many different ways, so I am not sure we need "starting point". We assume parts passed QC before being sent to be welded, right?

Geometric requirement for datum B is "nice to have", not mandatory. We don't know what was specified on the part level in the first place.

@dezignr:

At least I finally brought some attention to your post

 

[dezignr]

Thanks for the responses! First, my company still works to the 1994 standard. Second, we have since changed the modifiers to MMC for both the position of the slot pattern and the datum feature B slot pattern. So to recap:

Flange 1 establishes datum surface A on its face, and its slotted pattern is datum feature B with BOUNDARY condition invoked.

Flange 2 slots are clocked to flange 1 slots with the appropriate basic dims and by imposing to its slotted pattern a position tol of .020 MMC relative to datum A and datum B MMC at boundary condition. There is also a profile tolerance applied to the face of Flange 2 relative to datums A & B.

So my thinking is the drawing should reference the slot limits of size from the detail drawings. Then tooling would determine the boundary condition for the datum B slots in Flange 1 as simply their MMC size. The boundary condition for slots in Flange 2 would be their MMC size minus the stated .020 pos tol. As for the tooling, I envision some (expensive) bean shaped locaters, or two pins located at each of the slot radii to (sort of) establish the boundaries. Or take it to our CMM guys and let them figure out how to verify it

Thoughts?

 

[pmarc]

dezingr,
Yes, you are correct. If the secondary datum feature is referenced at MMC/MMB on the assembly drawing, the drawing should reference slot limits of size as well as geometric tolerance applied to datum feature B on the detail drawing, unless it is clearly stated (in a note perhaps) that the MMC/MMB of datum feature B is to be taken directly from the detail drawing.

Two pins located at each of the slot radii is a decent workaround, I would say. It just has to be remembered that this method will be ignoring these portions of the slot falling in between the gage pins, so there will be a chance that some actual slots violating the theoretical boundary defined by the positional callout will be accepted by that gage. And the longer the slot in comparision to its height is, the more uncertainty will be introduced to the measurement system.

CH,
Answering to your: "Could you please elaborate where it is forbidden to use them together?"...
The shortest answer is - as it is stated for example in para. 7.3.2 of Y14.5-2009, when positional tolerance is applied RFS, the axis/center plane interpretation, not boundary interpretation, is used to decode the callout.
However, I admit - my initial statement that this kind of callout is not interpretable was too definitive. Even with RFS callout it is possible to imagine the boundary concept working - for each combination of actual height and width of the slot different inner boundary would have to be considered as a volume that couldn't be violated by slot surfaces. It is just that the standard does not provide any instructions on how to treat this kind of animal.

As to your "Geometric requirement for datum B is "nice to have", not mandatory.", I naturally disagree, but I would prefer not to open this can of worms yet again. We have not reached any sort of consensus on this topic so far, and I don't believe we would reach it this time ;-)

 

[CheckerHater]

I agree on both:

It is not always 100% clear when axis or boundary interpretation shall be used, and the term "regardless material boundary" implies that some sort of boundary is still there, we just don't care about it.

And the argument about meaning of Para. 4.9 can go on forever. Llet's not do that.

 

[dezignr]

Thanks all. I am still a little confused about using the position tol from the Flange 1 detail drawing in computing the boundary condition.

My interpretation is boundary for datum B slots = MMC size of slots since no geo tol invoked on that end at the assembly.

Boundary for slots in Flange 2 = MMC size of slots minus assembly level position tol.

Does this make sense?

 

[pmarc]

Because B is referenced secondary in the positional callout for slots in flange 2, not primary, the boundary for datum feature B is not just MMC size of slots in flange 1. It is the MMC size minus a tolerance defining geometrical relationship between datum features B and A. If that tolerance is missing on the drawing, there is no chance to simulate datum feature B at its true Maximum Material Condition/Boundary.

 

[dezignr]

Thanks pmarc - very helpful.