ISO position tolerance for pattern when datum is the pattern hole axes 2

[Mhrob]

Hello all. First time post here. Soon-to-be graduated engineer currently in an internship.

So I am working on preparing drawings for some brackets I’ve designed, and I have a request to change something from management that I am not sure how to go about.

For a composite position tolerance (in ASME), I am used to something similar to the image shown below. 3 datums in the upper segment with TEDs tied to the datums that controls movement of the pattern AS A UNIT, and then a lower segment that further refines the tolerance….in this case, spacing between the individual holes and the perpendicularity to datum A (I think….)

However, I have been told to make my two 7 mm hole pattern as datum B. I realize in ISO that if you have a pattern set as a datum, you put B-B in the tolerance box when referencing other feature tolerances to that…..but what about the pattern itself? How will my upper segment change now? I want to control the holes as a unit within a 2 mm zone, and then control individual spacing by using a 0.5 mm zone. But what datum now goes into the upper segment? I can’t put B-B in that can I? Because the pattern itself is datum B…..

 

[greenimi]

I have been told to make my two 7 mm hole pattern as datum B. I realize in ISO that if you have a pattern set as a datum, you put B-B in the tolerance box when referencing other feature tolerances to that…..but what about the pattern itself? How will my upper segment change now? I want to control the holes as a unit within a 2 mm zone, and then control individual spacing by using a 0.5 mm zone. But what datum now goes into the upper segment? I can’t put B-B in that can I? Because the pattern itself is datum B…..

If my understanding is correct, I don't think you need datum feature B in the definition of those two 7mm holes. In the tolerance indicator of the 7mm holes you would have |A|Y|Z| (for example) in the upper segment, then in the lower segment only A (and maybe some CZ's after the tolerance zone value). Then this group of two tolerance indicators, upper and lower, would have a datum feature indicator attached to it and named "B"
So, other features in your bracket part would have B-B in their tolerance indicator definition.

Again, this is how I understood your question, but I might not be on a right track here.....

 

[Mhrob]

Hello,

Thanks for the response. I think I should have just supplied the actual drawing to explain.

So this is what I had in mind, but the issue I'm having is whether or not the upper segment here is correct. The problem is that I really only have 2 datums on this drawing: the flat surface that contains the 7mm hole pattern as datum A, and then the axes of the pattern of 7 mm holes as B.....so really, the only datum I can reference is datum A, which to me, would only control perpendicularity WRT to A; with the lower segment that doesn't have a datum controlling the spacing of the holes relative to each other.

Also, wouldn't the 4 dimensions circled need to be basic? Management told me the only basic dimension would need to be the spacing between the 7mm holes....but that just seems wrong to me. The tolerance on the 11 mm hole I am confident is correct, because the horizontal and vertical dimensions of the hole will be controlled WRT B-B, and the perpendicularity is controlled WRT A.....its just the datums for the pattern that is throwing me off.

 

[greenimi]

Also, wouldn't the 4 dimensions circled need to be basic?

Yes. They need to be basic.

The problem is that I really only have 2 datums on this drawing:

Make more datum features then.
For example, if 10 (circled) is basic then why this dimension is not coming from a datum? Why you cannot make the upper flat surface -horizontal surface- a datum feature? Lets say datum feature Y.
And then, consequently, the vertical left surface datum feature Z. So, as I said before the upper tolerance indicator will be A|Y|Z|
You should locate those 2 holes on the bracket somehow, don't you?
And I think you forgot some CZ after 0.5 and before the M in the lower tolerance indicator and datum feature A as a primary datum feature.
CZ= combined zone.

 

[Mhrob]

I agree with what you are saying 100%. See the drawing below. What you just described is exactly what I put on my initial drawing. However, I received mark ups and was told that "the only datums needed should be Datum B as the axes of the two 7mm holes, and the flat surface that those holes are drilled into as datum A. They will drop two pins in those holes and but it up against a flat surface on a fixture and the part will be fully constrained". That's why I'm unsure now as to how I can build that tolerance box when my only datum outside of the pattern of the holes is datum A...


In terms of the lower tolerance zone, I thought if you leave a datum feature off, it's indicating that the reference for the tolerance is the features within the pattern. So for example, the spacing between each hole is relative to the other hole. I've seen it done before, but maybe I'm not understanding something correctly..

 

[pmarc]

Mhrob,

A single position tolerance callout with just a single datum feature reference (which is nominally perpendicular to the axes of the holes) applied to a pattern of features - |POS|dia. 2(M)|A| in your case - not only controls spacing within the pattern, but also perpendicularity of the features to the specified datum. If the pattern is going to be a secondary datum feature for the part, there is no other datum feature reference needed in the position tolerance for that pattern, because it is the pattern that all other features of the part will be located from.

The second position tolerance callout with no datum feature reference - |POS|dia. 0.5(M)| in you case - is added to refine/tighten the hole-hole relationship.

Some other thoughts/comments:
-- In the latest ISO standards, composite tolerance frames are not used, as greenimi pointed out. Single segment callouts with an additional modifier releasing translational degrees of freedom are used instead.
-- The reference to the datum pattern B in the geometric callouts on the drawing should be (B-B)(M), not B-B(M).
-- Per the latest ISO standard on geometrical tolerancing at MMC and LMC, ISO 2692:2021, the tolerance values in both positional callouts applied to the pattern of holes shall be followed by the CZ (combined zone) modifier, again, as greenimim pointed out.
-- Regarding the list of comments you attached (I assume it is the feedback you received from your management), do you know how you are going to control the perimeter of the part (item #3 on the list)? Since the most appropriate way would most likely be profile tolerance, then all dimensions in that view, except the size callouts for the holes, should be theoretically exact/basic/boxed. But this goes against item #5 on the list, which, in my opinion, is simply wrong anyway, because the 40 and 19.2 dimensions between the pattern of dia. 7 holes and the dia. 11 hole should be theoretically exact too, if positional tolerancing is to be used to control location of the dia. 11 hole.

 

[greenimi]

Pmarc,
Should I understand that in your proposal CZ is needed on both segments (upper and lower position) and in mine case the CZ is needed in the lower position only?
Am I correct?
Just for clarification:
Your proposal is
Pos | xxxx CZ |A|
Pos | yyy CZ|
Mine is
Pos | xxx |A|Y|Z|
Pos | yyy CZ|A|
(I ignored for a second diametric symbols and MMC's)
I was trying to follow posted page 57 from the OP initial inquiry. I know some perpendicularities are needed for the upper horizontal feature to A and for the left vertical surface to A and Y.
Thanks.
Good to see you here again.

 

[pmarc]

greenimi,

Yes, you are right. My proposal requires CZ in both tolerance frames. Your proposal would require CZ in the lower frame only.

Unfortunately, the answer may be different depending on the version of the ISO 2692 standard used. Per the 2006 and 2014 versions, the presence of the (M) modifier after the tolerance value means that CZ is not needed, but if the position tolerance was Regardless of Feature Size, RFS, the CZ would be needed. This was an unncessary complication, therefore in the 2021 version they unified the practice and now CZ is needed regardless if the tolerance has been specified at RFS or MMC/LMC basis.

 

[greenimi]

pmarc,

upper[/b] frame only.]

Is this a typo? You meant lower frame only? (or I guess upper or lower frames in ISO could be used interchangeably, as both frames will have its own position symbol)

 

[Mhrob]

Pmarc,

Thank you for a very detailed response.

So when you were describing the upper-segment tolerance, you said it "not only controls spacing WITHIN the pattern...". Can you clarify what you mean here? I am assuming you mean like, the spacing of the pattern as a unit relative to the basic dimensions. For example, a fuel pump is being mounted to this bracket using the two 7mm holes. There are 2 flanges that are part of the pump body that have slots in them where the bolts are to go through. Therefore, the pump can move around on the bracket to a degree....I'm not really concerned about that. What needs to be tightly controlled is the spacing between the holes to ensure the pattern will align with the slots on the pump, which is why I refined that in the lower segment. So as long as the upper segment is allowing the pattern as a whole to shift within a 2mm tolerance zone, I am ok with that (i.e, if the 10.8 mm dimension becomes 9.8, that is ok.....as long as the spacing between the holes vertically and horizontally is no more than 0.5 mm)

About the perimeter control, yes, I planned on a profile of a surface. I was thinking the same thing about the dimensions all needing to be basic.....I didn't think that was right either, but I saw an image from an ISO guide book that said otherwise (see first image below).

I did have another question about the profile of a surface as well. The area around the anti rotation tab on the right needs to be tighter than the rest of the perimeter. I know you can define two points and then use the double arrow symbol to say "profile of the surface between points A and B must be......". Would I be able to define the rest of the perimeter with the all around symbol still? (see second image below).

 

[pmarc]

greenimi,

Yes, this was a typo. I meant the lower frame, of course. I made the necessary correction in my previous reply.

 

[greenimi]

Mhrob,
I am not pmarc (and I am sure he will come to "slap" my hand if I say something wrong), but I think you could use profile in between points C and D on the other contour in the same way as you did for the anti-rotation tab.
In other words 2 profiles between C and D and the leader lines will show two different contours. One profile between C and D within 1mm and the other profile also between C and D within 2mm (so, remove the circle symbol and add between C and D symbol on the 2mm profile).
In my opinion is clear enough.

 

[pmarc]

So when you were describing the upper-segment tolerance, you said it "not only controls spacing WITHIN the pattern...". Can you clarify what you mean here? I am assuming you mean like, the spacing of the pattern as a unit relative to the basic dimensions. For example, a fuel pump is being mounted to this bracket using the two 7mm holes. There are 2 flanges that are part of the pump body that have slots in them where the bolts are to go through. Therefore, the pump can move around on the bracket to a degree....I'm not really concerned about that. What needs to be tightly controlled is the spacing between the holes to ensure the pattern will align with the slots on the pump, which is why I refined that in the lower segment. So as long as the upper segment is allowing the pattern as a whole to shift within a 2mm tolerance zone, I am ok with that (i.e, if the 10.8 mm dimension becomes 9.8, that is ok.....as long as the spacing between the holes vertically and horizontally is no more than 0.5 mm)

What I meant is that the upper frame will effectively control perpendicularity of the axes of the holes relative to datum plane A, while the spacing between the holes will be controlled by the lower frame. If I were to do a tolerance stack-up to calculate the variation in the distance between the axis of either of the dia. 7 hole and the side wall, the contributors that would affect the calculation would not only be the hole size and the upper position tolerance specifications, but also the profile tolerance value applied to the side wall.

I did have another question about the profile of a surface as well. The area around the anti rotation tab on the right needs to be tighter than the rest of the perimeter. I know you can define two points and then use the double arrow symbol to say "profile of the surface between points A and B must be......". Would I be able to define the rest of the perimeter with the all around symbol still? (see second image below).

There is no need to use the all around symbol in this case. As greenimi explained, C<-->D notation below the second tolerance frame will work just fine.

Unfortunately, there are two more things to add to both profile tolerances:
1. The CZ modifier after both tolerance values. Without it, the default understanding (due to the default independency principle in ISO) is that the profile requirement applies to each feature between the points C and D individually. And since the |A|(B-B)(M)| specification does not fully constrain all degrees of freedom of the tolerance zones (because of the presence of the (M) modifier after B-B that allows for so called datum feature shift), the drawing should clearly specify that the individual profile tolerance zones for the features between points C and D need to be considered as a single combined zone.
2. SIM notation after both tolerance frames to indicate that both sets of combined profile tolerance zones are to be considered a single pattern.

I know this all sounds weird and scarry, especially for a person used to ASME Y14.5 standard.

 

[greenimi]

And here it is an interesting aspect: if the datum system for the profiles is A|Y|Z| then I think CZ and SIM is not needed at all.
But I would argue that for the non anti-rotation contour (2mm profile) the datum system A|Y|Z| seems more feasible/maybe read functional, than for the anti-rotation one 1mm profile (which I think would functionally use A|(B-B)(M)|).