Datum Reference Frame for Position Control on Axisymmetric Part

[Gwad94]

This is a fairly basic question but I've struggled to find a reference discussing it.

So I've got a stepped cylindrical part that looks like a big tophat bushing. The primary datum is the "flange" face of the tophat that seats against a plate. The secondary datum is the hole through the center of the bushing that will have a shaft go through it. The tertiary datum is two holes that will have dowels press fit into them. Those dowels will go into mating holes on the other part, constraining clocking. After all that, there is a 6-hole bolt pattern that clamps it in place.

So my question is, for the axisymmetric features of this part, such as the flange outer diameter, it makes sense to me to control their location using a position tolerance because I'm not particularly worried about their surface forms. When I create these position controls, it feels like I only need to reference datums A and B to fully define their locations because they are axisymmetric and do not have a "clocking." However, the bolt hole pattern will be defined with position tolerances referencing the A,B,C datum reference frame which is the fully constrained assembly reference frame.

As circled in my drawing below, is there any actual difference between the two position control options I gave for the outer diameter? Does adding datum C somehow change the tolerance zone even though the feature is axisymmetric?

Thanks!

 

[chez311]

4-26 only with the pattern of features referenced RMB instead of MMB - this is not shown in the standard. Regardless, I accept that the paragraph itself can be understood as applying to both cases. I would prefer to specify it in a way that can be interpreted by the standard more easily (represented in a figure). However, if the pattern of RMB datum features of size consists of more than 2 features this may be problematic.

There are many situations which are not represented in the standard by figures, one cannot possibly expect every possible arrangement/situation to be included in a figure. It would certainly help, but in this case I think "Patterns of Features of Size" is pretty much inclusive. What do you believe would be the alternate interpretation of |A|B| that would be different from |A|B-C| ? The |B-C| notation in 4-26 is necessary only because B and C are controlled by separate size tolerances. What also would you believe is problematic with a pattern of more than 2 features? Besides the fact that its technically overconstrained and location/orientation will be likely dictated by the first two features to make contact with their simulators, I don't see any technical limitation.

I agree pylfrm's solution is the more elegant and representative one, I have no issue there.

 

[chez311]

If one pin cannot expand any further, I wouldn't call continued expansion of another pin "simultaneous".

As I've mentioned in other threads, I think the bottom line is that the meaning isn't fully defined. This ambiguity is probably tolerable in many cases, but avoiding it is another reason I often prefer to use the simultaneous requirement approach.

I agree avoiding it is probably the better idea.

That said, the "means this" of Y14.5-2009 fig 4-25 states "datum features simulator is the smallest pair of circumscribed cylinders". While not explicit, it seems to me that suggests that both should be minimized.

 

[Burunduk]

What do you believe would be the alternate interpretation of |A|B| that would be different from |A|B-C|?

I don't know. Perhaps they are the same. A figure with a "means this" portion would help, even if as you said not mandatory for interpretation. It just happens that the only figure referred to by that paragraph shows the version with the hyphen.

What also would you believe is problematic with a pattern of more than 2 features?

For the type of specification I was referring to the problematic thing would be defining something like |A|B-C-D| or |A|B-C-D-E| that doesn't look right to me.

 

[Burunduk]

If one pin cannot expand any further, I wouldn't call continued expansion of another pin "simultaneous".

I agree. This is what I meant also.

 

[pmarc]

Is it the single largest distance between either filtered feature and corresponding associated feature which is to be minimized? The fact that "maximum distances" is plural in the description for items 7 and 8 makes me unsure of the intended meaning. Unfortunately I don't have ISO 5459:2011.

See the attached file for more details:

https://files.engineering.com/getfi...b18b-5ebd4f140a31&file=minmax_association.pdf

 

[pylfrm]

pmarc,

Thank you for the additional information.

For the objective function in A.2.3.2, d(A_i,F_i) is described as "the distance between the features A_i and F_i". It seems like this is probably intended to mean "the maximum distance (normal to A_i) between the features A_i and F_i". Do you agree?


pylfrm

 

[pmarc]

pylfrm,

Is this what you are asking for?

 

[pylfrm]

pmarc,

It seems like the sentence you've circled is saying something different from the formula and definition of variables that follows it.

If I'm reading the circled sentence correctly, each associated feature will have its own maximum normal distance to its corresponding filtered feature, and these maximum distances are to be simultaneously minimized. The issue here is that the idea of simultaneously minimizing multiple interdependent things does not have a single well-defined meaning. The sentence would make sense to me if it had said that the largest of those maximum distances is to be minimized, but that's not the case.

The formula seems to solve that problem by defining a single thing to be minimized: the largest of all the d(A_i,F_i) values. The issue here is that d(A_i,F_i) is implied to be a single value for each feature, but "the distance between the features A_i and F_i" is something that depends on where and in what direction it's measured. The formula would make sense to me if d(A_i,F_i) had been defined as "the maximum distance between the feature A_i and the feature F_i, normal to A_i", but that's not the case.


pylfrm

 

[pmarc]

pylfrm,

I agree that the standard could be much clearer here.

This is how I understand the formula. Because the d(Ai,Fi) is defined as "the distance between the features Ai and Fi", the role of the 'max' preceding the d(Ai,Fi) in the formula is to say that the maximum "distance between the features Ai and Fi" is subject of interest. "i=1,..., N" index under the 'max' means that there are N maximum values to be simultaneously minimized.

As to what the "simultaneously minimized" means, I guess I would understand it the same way as it (the "minmax" algorithm) is understood in the case of establishing a datum plane from a single convex feature, but with the additional constraint that all N maximum values are to be optimized with N associated features perfectly oriented and located (coaxial) relative to each other. At least this is my understanding of the description of items 7 and 8 in the key for figure A.8.

 

[pylfrm]

pmarc,

Thanks for explaining. Despite different justifications, I think we're on the same page about how the "association method for common datums" is supposed to work.


pylfrm