Rotation 1

[aniiben]

ASME Y14.5-2009
Part assembled on A (flat surface) ( flatness considered--not shown in this sketch for simplicity) then
centered on B (OD) (perpendicularity to A considered--not shown in this sketch for simplicity) and
bolted down with 4 screws (Ø.475 holes) (basic dimensions considered--not shown in this sketch for simplicity)

The other 4 holes (not dimensioned for size in my sketch) are non-functional (aid-process only)

Not sure about rotation.

Can I use C(modified at M) to stop the rotation only? Nothing is stopping the rotation except 4 bolts in the same time.
Is A, B(M) and C(M) valid? Legal? Why? Why not?

 

[drawoh]

aniiben,

As shown, your datum[ ]C feature is the four-hole pattern, which is over constraint. You want to apply the datum to one hole only. I don't know how serious you are about your numbers. I would use one of the Ø.005 toleranced holes on the outside as a datum feature.

--
JHG

 

[chez311]

As shown, your datum C feature is the four-hole pattern, which is over constraint. You want to apply the datum to one hole only.

I see nothing wrong with this - isn't this the whole point of utilizing a pattern as a datum? To prevent being ambiguous as to which hole is actually the datum feature on a symmetric part like this - especially since the entire pattern is the functional feature since all 4x are utilized with bolts to mount the part, not just one. As far as I understand it the standard allows this and the pattern will only arrest the final rotational degree of freedom and orient the 2x datum planes created by B - I don't think this results in over constraint.

I don't know how serious you are about your numbers. I would use one of the Ø.005 toleranced holes on the outside as a datum feature.

While I wouldn't disagree that it would result in a tighter tolerance/better orientation of the holes and other features in reference to each other, these are not the functional holes - I would probably recommend leaving the datum as the 4x bolt holes (.475 dia) unless there are issues in assembly, the field, or if maybe misalignment results in aesthetic issues the customer is not satisfied with. Otherwise the current scheme allows manufacturing to take advantage of the looser tolerances.

Edited for grammar and formatting

 

[3DDave]

Since C(M) for all four holes is defined in the A|B(M) DRF and B(M) is defined in the A DRF, there is no conflict from the over constraint. In fact, even using a single hole C(M) is an over constraint as it controls X and Y when only theta_Z is required; one more DOF than needed.

By not using all four holes it is possible for the related features to be forced away from the inspection-acceptance orientation of the DRF.

As it is, there is no need to have a datum feature C reference at all. There is a simultaneous requirement between the position and profile tolerances at the datum feature A DRF level and there is also one at the [A|B(M)] DRF level.

 

[drawoh]

3DDave,

If I build a fixture to mount this thing for inspection and the datum[ ]C feature is a single hole, I will pick it up with an appropriately rotated diamond pin. I agree that the four holes inside look functional, but the four holes on the outside are more accurately positioned for some reason.

Another option is to forget about the big diameter datum feature, and use a hole pattern as the secondary datum feature.

--
JHG

 

[semiond]

I don't see anything wrong.
To remove any doubts about using the holes pattern to stop rotation, check fig. 4-26 (page 69) at ASME Y14.5-2009.

 

[chez311]

drawoh,

The OP already stated the 4x outside holes are for assembly aid only, not functional in the final assembly which is why I suggested leaving the datum structure as it is with the bolt holes as the datum feature however I could see changing these to the datum feature C if for example any of the issues I mentioned arise.

In regards to changing the secondary datum I assume the OD was utilized as a datum feature for a reason I probably wouldn't change that. If you're concerned about over constraint 3DDave put it very well but I'll expand on that - "technically" (in mathematic/geometric terms) utilizing a pattern is inherently over constraint but thats not what happens here, it only controls as many DOF's as needed and as I said allows you to be unambiguous as to which hole is important or should be utilized for fixturing/gauging/measurement - they're all important. At least thats my understanding.

 

[aniiben]

To remove any doubts about using the holes pattern to stop rotation, check fig. 4-26 (page 69) at ASME Y14.5-2009.

Standard example shows B (M) as secondary. In my situation, C(M) is tertiary.

Another option is to forget about the big diameter datum feature, and use a hole pattern as the secondary datum feature.

Well, the part is centered on B. Not sure "forgeting" B is a good (functionally) option....

Again, the concern is the rotation and overconstraing DOF's for the ID shown with Ø.010 L(M) , all around profile for the outher surface. (Maybe less for the aid-holes, shown with Ø.005(M) as those holes are used only in the manufacturing process. These holes are inspected with a functional gage and if they pass the functional gage then sure in the subsequent operation the part will fit their fixture).

Therefore, a complete and unambigouos product defintion is expected for the outer surface and the ID, both of them shown with A, B(M) and C(M)

 

[chez311]

So after looking through the standard, including the 4-26 semiond pointed out, I see that the standard does not explicitly say a patterned datum like this can be utilized as a tertiary datum. I still stand by my previous statement that it makes sense to me that it can be, that it is no more over constraint than inherently specifying a pattern as a datum (regardless of precedence) or as 3DDave pointed out even using a single hole as a tertiary datum.

I'm fully open to interpretations on this however, feel free to pick this apart. I'm here to learn. Does anyone have something that supports or refutes this?

 

[semiond]

In the standard example the psttern of holes constrains more degrees of freedom. In your example - only one rotational DOF. Translational DOF's that the pattern could restrain, are taken care by datum B.
Still nothing wrong with it. In functional assembly, the features will constrain the part similary to your scheme.

 

[drawoh]

aniiben,

Try visualizing a fixture for your four-hole pattern. I provide four undersized pins. As I rotate your part, the first pin to make contact, controls rotation about the big diameter. Rotation can be controlled by one of your straight edges. The big diameter can be positioned from your hole pattern. I do not know what your part does.

You have specified a Ø.020" position for your tertiary datum, and you are locating holes to Ø.005" from this. Again, I do not know how serious you are about your numbers.

--
JHG

 

[chez311]

(Maybe less for the aid-holes, shown with Ø.005(M) as those holes are used only in the manufacturing process. These holes are inspected with a functional gage and if they pass the functional gage then sure in the subsequent operation the part will fit their fixture).

So are you actually fixturing off these 4x tighter tolerance, non-functional holes in your manufacturing process or just checking them? It sounds like you are, but I want to verify.

Try visualizing a fixture for your four-hole pattern. I provide four undersized pins. As I rotate your part, the first pin to make contact, controls rotation about the big diameter.

And? If the tolerances specified are acceptable to OP/OP's customer then is there an issue? No different than if say the center big diameter was positioned from the 4x hole pattern - since it is specified at MMB the part could be easily produced in such a manner that only 2x of the 4x holes make contact with a such a fixture/functional gauge.

EDIT - drawoh sorry just realized you might be indirectly agreeing with me, I read it as a reason why its not viable or allowable. Oops.

 

[semiond]

drawoh,
To stop rotation around datum feature B diameter, it is possible that 2 pins may need to make contact with the holes, not just one (since B is called out at MMB).
The fact that datum feature C is a pattern only means that whatever pin(s) will make contact, will constrain the rotation. This perfectly makes sense, since in real life application, there also won't be preference to one particular screw.

 

[chez311]

semiond - exactly, which is the advantage of specifying a pattern like this. You're not giving priority to one hole in the pattern, which is in reality no more important than the rest, as well as not forcing an inspector/operator to choose or make a decision when measuring/gauging it as to which is the datum feature since the part is symmetric. Even if the pattern didn't have symmetry the first statement still stands.

 

[pmarc]

Here is something to think about:
Since datum features B and C are referenced MMB in two position and one profile callouts, what makes you think that when this part is mounted in the gage containing datum feature simulators A, B and C, the datum feature B will be the one that will always stop translation and always leave the rotation-constrainer job to datum feature C?

 

[semiond]

chez311,
Well said. In fact, other than wondering why the nonfunctional holes that are also controlled by the datum feature pattern are specified with the tightest tolerance (like drawoh noted), i have no doubts about the correctness of the OP's scheme.

 

[aniiben]

pmarc,
To answer your question I have to say ( and I learned it from this forum) that we are trying to reflect the physical reality of this assembly.
The part is sitting "flush" on A, centered on B and the rotation is stopped by C (hopefully)

 

[chez311]

pmarc - considering we don't actually have the callout for the OD in the example provided by the OP its hard to answer that precisely, but my gut tells me that the combined tolerance on the OD is probably much smaller than that shown for the 4x bolt holes. If this is the case then it is possible for the tertiary datum C to control translation, but only when produced at the very extreme of a much larger tolerance zone. Even if this is the case, again what is the concern? Even in this extreme set of circumstances the translation and rotation on a gauge is stopped by tertiary C - it would still have to meet the tolerance set for datum B.

 

[semiond]

pmarc,
I think your question can be answered only after tolerance analysis - the OP will have to check on CAD and see if the numbers he put in the feature control frames correspond with the DRF and the role of each datum feature. My feeling is that only datum feature C may stop rotation, but at a certain condition datum feature C may also constrain translation, while it's not intended to. It is the least material boundary for datum feature B that needs to be limited to avoid that.

* Edit: the worst case scenario: datum feature B (OD) produced at LMB, datum feature C (4 holes) produced at MMB. the OP will check that condition on CAD, assembled, with the fixture (which hopefuly represents the mating part at worst case) and he's good (i think).

 

[drawoh]

aniiben,

When I apply a datum to a Feature Of Size (FOS), I prefer that this be very much more accurate than the tolerances referenced from it. There are specific practical applications where a sloppy FOS can be used at MMB, but the concept gets very messy, very fast, as noted above.

--
JHG