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Tertiary Datums and Bolt Circles
4

Tertiary Datums and Bolt Circles

Tertiary Datums and Bolt Circles

(OP)
Hello all,

I'm a CMM programmer, and at my current place of employment and my last job, I have struggled to find someone who knows the answer to this question. On a circular part with a bolthole circle that has a true position tolerance applied to it, how many datums are required?  All the engineers I've spoken to seem to think that one (governing X/Y) is enough, two (one for X/Y and one for Z) is generous, and three (one for X/Y, one for Z, and one for rotation) is overkill to be avoided at all costs.  My problem is this: Because of the nature of CMMs, I am forced (as far as I know) to arbitrarily choose one of these holes to set as angularly "perfect" - in essence, make it a tertiary datum - and dimension the rest of the holes off of that hole.  Even worse, in parts with more than one bolt-circle, I am faced with the prospect of taking the position of holes from one bolt circle relative to a single hole in another bolt-circle, or doomed to specify several holes as band-aid rotational datums - and try to dimension the entire part this way.  It just feels wrong, I'm almost certain that's not the way it's supposed to be, but every other programmer I've met does the exact same thing and never thinks twice about it 'til I bring it up - and the head of our drafting department continually beat around the bush trying not to give me an answer without me realizing he was doing it.

See the attached example drawing for clarification, please ignore missing dimensions, the fact that the diameters of the boltcircles aren't specified, etc... what EXACTLY does this mean?  Can those bolt-circle rotate relative to each other?  The head of drafting muttered something about "being on centerlines so they can't," but what reference do I use to determine if they have or not if the tabs on the sides aren't datums or aren't present?  

 

RE: Tertiary Datums and Bolt Circles

In your example there does not appear to be any control over the rotation of the bolt circles.

If rotary control - clocking - is required I believe it should be explicitly stated on the drawing.

See ASME Y14.5M-1994 2.7.3, 4.4.3 & figure 4-6.

Not sure where this "three (one for X/Y, one for Z, and one for rotation) is overkill to be avoided at all costs" mentality comes from.  If angular orientation - clocking - is functionally important then it should be explicitly stated.

Showing features in line means nothing - think about it, how inline would they have to be, what is the tolerance on it?

However, I'm answering from a drawing point of view, not inspection so maybe I'm missing something in your question.

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RE: Tertiary Datums and Bolt Circles

(OP)
No, there's nothing different about inspection, only limitations placed on our capability by software.  The standards are the same, thank goodness.  Thank you for the references to the drawing standard, that told me exactly what I needed to know - that essentially, if there is no clocking datum, the bolt-circle can spin every-which way as long as the holes are in position relative to each other.  Well, that's one problem solved - most of our drawings are wrong.  And I think the problem created by the CMM software is just that... an unacceptable limitation build into software that was written far after the 1994 standard was adopted.  Oh well, I guess I'll complain that we need the new version!

RE: Tertiary Datums and Bolt Circles

Surprisingly, those two patterns are not allowed to rotate relative to each other (beyond the small position tolerance that each hole is given, of course).

In the 1994 ASME standard, this is spelled out in paragraph 5.3.6.1 (or 7.5.4.1 of the 2009 edition):  "Where multiple patterns of features are located relative to common datum features not subject to size tolerances, or to common datum features of size specified on an RFS basis [that's you], they are considered to be a single pattern."

In other words, the 13 holes are to be gaged simultaneously. Well, on a CMM they might not literally be measured simultaneously, but we can't measure the 9 holes and then jiggle the part around differently to measure the 4 holes; I think the clocking of the patterns is established once you hit any 2 of the 13 holes; everything else is rotationally locked from that point on.

To avoid this situation, the designer would have to add some sort of note saying that they are to be gaged separately, although the standard makes it sound like that option is only available if datum A is specified with the "M" modifier (shrug).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Tertiary Datums and Bolt Circles

(OP)
Yes, I had noticed that caveat about separate gaging only permissible with a datum at MMC when I was reading the standard.  I do have a question for you though: you said, "I think the clocking of the patterns in established once you hit any 2 of the 13 holes; everything else is rotationally locked from that point on."  May I ask why two are required instead of one?  The reason I ask is, I was taught to take hits on one hole, construct a line between the center of the datum diameter and that hole, and rotate my X/Y axes to this line.  Is there another way to do it, even if you have an odd number of holes?

RE: Tertiary Datums and Bolt Circles

Yeah, I guess one hole is enough -- I had forgotten that the center of the part was already established.  That's what happens when I try to think late at night...

But I'll stand by the rest of my post  smile

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Tertiary Datums and Bolt Circles

Daekar,

Answering to your original question:
"Can those bolt-circle rotate relative to each other?" - in the way as it is specified on your sketch they can't. Because they are referenced to the same datums in the same precedence, they have to be considered as a single pattern of holes. And as John-Paul said the allowable rotation is only within the small position tolerance that each hole is given. I also agree with Kenat that showing features in line does not mean anything and is irrelevant as long as tolerance betweeen these features is not specified.
If you want to have two patterns as separate ones you would have to type SEP REQT beneath both position feature control frames as it is shown for example on fig. 7-54 in Y14.5M-2009 standard.

In terms of constraining rotation of the pattern(s) of holes:
If we assume that outer tabs are not there or are not specified as datum features, picking one of the holes as tertiary datum seems to be the only solution.
The question is why shouldn't we consider one of outer tabs as tertiary datum feature? In my opinion this would be very reasonable choice.

John-Paul also said that if one wants to specify separate requirements:

Quote:

the designer would have to add some sort of note saying that they are to be gaged separately, although the standard makes it sound like that option is only available if datum A is specified with the "M" modifier (shrug).
Please take a look at figs. 4-40 and 4-41 in 2009 standard. Datum feature A is specified on RMB basis there, so it looks like separate requirements concept doesn't require datum feature specified on MMB basis.  

RE: Tertiary Datums and Bolt Circles

I am curious how the outer (3) lugs are dimensioned and is there any functional requirement of orientation for them relative to what we have?
Frank

RE: Tertiary Datums and Bolt Circles

Good points, pmarc.   It illustrates a minor inconsistency in the new standard.  SEP REQT can be used on RMB datum references per Fig 4-41, yet in paragraph 7.5.4.2 it says that when MMB is invoked, SEP REQT is an option.  You'd think they would also mention that option in 7.5.4.1 (RMB datums)!
 

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Tertiary Datums and Bolt Circles

We would run on CMM as follows:
Align using specified datums -A- & -B-, then clock via one of the outbd holes (@12:00)as -C-.
(This is not so much driven by CMM software. You have to have a complete axis in order to check anything to model/dwg. You have to start somewhere.)
Run all 13 holes per model/dwg.
Run Best fit with a rotation axle @ C/L of -A- & perpendicular to -B-.
This would meet dwg requirements and give the product all the tolerance.

RE: Tertiary Datums and Bolt Circles

I agree, John-Paul.
Maybe I am missing something, but my understanding of para. 7.5.4.1 is that when all datums in FCF are specified on RMB, separate requirements are should not be considered at all. But fig. 4-41 shows exactly opposite situation.   

RE: Tertiary Datums and Bolt Circles

Daekar,
I work with CMM people quite a bit and they all have similar concerns as yours. This issue is covered in the standard because of "the simultaneous requirement" the ISO has chosen to take more of an understanding like you would, namely, if it is not specified it is not implied "the principle of independency". As far as the number of datums issue you mention this is most likely a result of the evolving nature of the standard. It was acceptable, and still is to some here, to reference only one datum for patterns, most of the guys we all work with "prefer the old ways". The biggest issue with your drawing is how the other features are called out and is there a requirement left implied that should be stated. Plus and minus dimensions are not covered by the security blanket of the "simultaneous requirement".
Frank
 

RE: Tertiary Datums and Bolt Circles

Daekar,

The group has pretty much covered everything, but here are a couple of additional comments.

All 13 holes must be measured in the same coordinate system, and that coordinate system must be leveled to B and centered on A.  There is no tertiary datum feature specified, so the clocking of the coordinate system is not constrained.  But the CMM software requires you to constrain the clocking anyway - that's one of the difficulties with inspecting GD&T using CMM's.  As HGMorgan described, pick something convenient as a clocking datum.  Then have the CMM software float the rotational degree of freedom and "best fit" the pattern, to calculate the optimal clocking and smallest actual value.

Different CMM software has different best fitting and optimization functionality.  A simple rotational optimization like the one in this example should be well within the reach of most software.  Some are able to properly handle optimization for datum features referenced at MMC, but a lot aren't.  Only specialized post-processing software will handle more complex types of datum reference frame optimization correctly.

This is familiar territory - I've been through it before with various different CMM softwares.  Get in touch with me if you would like to discuss more details.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Tertiary Datums and Bolt Circles

(OP)

Quote:

The question is why shouldn't we consider one of outer tabs as tertiary datum feature? In my opinion this would be very reasonable choice.

Yes, it would seem so - but the issue is that the drafters where I work have been creating drawings without tertiary datums even when they're technically needed for 40 years, and they're not about to go back and fix them just because I point out they're wrong.  

Quote:

I am curious how the outer (3) lugs are dimensioned and is there any functional requirement of orientation for them relative to what we have?

Well, actually, I did this drawing in about 5 minutes from my imagination - it is not a real part.  What I'm gathering is, if there is, as you asked, a functional requirement that the lugs and bolt patterns align, then a tertiary datum and reference to it in the true position feature control frame is necessary.  If there is no functional requirement that the pattern clocking correspond to the lugs, then no third datum is required.

Quote:

We would run on CMM as follows:Align using specified datums -A- & -B-, then clock via one of the outbd holes (@12:00)as -C-.(This is not so much driven by CMM software. You have to have a complete axis in order to check anything to model/dwg. You have to start somewhere.)Run all 13 holes per model/dwg.Run Best fit with a rotation axle @ C/L of -A- & perpendicular to -B-.This would meet dwg requirements and give the product all the tolerance.

Quote:

All 13 holes must be measured in the same coordinate system, and that coordinate system must be leveled to B and centered on A.  There is no tertiary datum feature specified, so the clocking of the coordinate system is not constrained.  But the CMM software requires you to constrain the clocking anyway - that's one of the difficulties with inspecting GD&T using CMM's.  As HGMorgan described, pick something convenient as a clocking datum.  Then have the CMM software float the rotational degree of freedom and "best fit" the pattern, to calculate the optimal clocking and smallest actual value.Different CMM software has different best fitting and optimization functionality.  A simple rotational optimization like the one in this example should be well within the reach of most software.  Some are able to properly handle optimization for datum features referenced at MMC, but a lot aren't.  Only specialized post-processing software will handle more complex types of datum reference frame optimization correctly.This is familiar territory - I've been through it before with various different CMM softwares.  Get in touch with me if you would like to discuss more details.  

Well these two posts seem to be good confirmation that we have been doing things ALMOST right - the optimization you mention is the final step that we haven't taken.  We're using PC-DMIS 3.7 CAD++ (which is a bit old, now) so I'm not sure what kind of options there are - I'll find out tonight.  If I have trouble, Evan, I might take you up on your kind offer of assistance. smile

I can't thank you all enough for your help - hopefully this will be helpful for more people than myself.  I'm so glad I found this forum...

RE: Tertiary Datums and Bolt Circles

Daekar,

I would only like to add a comment to consider:
From theoretical point of view, if we imagine that not only 13 holes but also 3 outer lugs are all positionally toleranced in reference to the same datums, in the same precedence (e.g. like you have |B|A|), then this group of 16 features should be considered as single pattern unless otherwise stated. In my opinion tertiary datum for constraining rotation would not be needed in this case, because the configuration/relationship between the features would have to be exactly as it is shown on a drawing.
  

RE: Tertiary Datums and Bolt Circles

(OP)
pmarc, that is an excellent point!  It didn't occur to me that features other than holes could be part of the pattern, but of course - as long as they had true position tolerances with the same datum references, that would make sense.  Good heavens... I don't think our CMMs can do that.  I found out that it is possible to create a "feature set" of circles or cylinders and apply true position to a hole pattern that way, but I don't think it will work with linear distances, boxes, etc.  The unfortunate thing is, even with just holes, it will only give you values for the pattern with datums at RFS.  I'm all about educating people, but I don't think I'll tell the drafters about that idea (they'd start using it out of spite), because right now they would never put a true position on something like those lugs.  Not unless a customer like Lockheed-Martin required it, anyway.

RE: Tertiary Datums and Bolt Circles

Daekar, here is a link where you can find a little bit more info about the topic. It also mentions that simultaneous requirements can be very difficult to inspect with CMM's.

http://www.tec-ease.com/gdt-tips-view.php?q=126

As I remember this was discussed on the forum some time ago.  

RE: Tertiary Datums and Bolt Circles

(OP)
Well, I've been learning and refining my programming for the last few days, and I have run into a little snag, not unrelated to this thread topic, so I thought I'd post it - see the example image with this post.  The issue is with the curved slots, for the most part: to which bolt-pattern are they dimensioned?  Or are they dimensioned to any bolt pattern at all?  Particularly in light of the fact that the two bolt circles are not required to clock together (different datum references), this seems to be unclear.  The problem I'm struggling with is, I don't really know how this information could be conveyed.  Ironically, this is dimensioned identically to a real part drawing we use, where the actual design function requires that the two bolt circles clock together, and that the slots be within the title-block tolerance (0.5 degrees) of the boltholes they are (supposedly? assumed to be?) dimensioned to.

RE: Tertiary Datums and Bolt Circles

This now has developed into the kind of issue that I was getting at earlier, we really need to look at a parts functional feature relationship as a whole, by over simplifying the problem you are over simplifying the result and may miss critical relationships.
In your drawing you are calling out the outside pattern of holes to the OD and the inside to the ID so you have lost all possible benefit of simultaneous relations for clocking, now, if there is a relationship required you will need to state it. Does the function really need one pattern to the inside and one to the outside?
As for the slots, the lower slot would almost skirt by if you have a default title block angularity tolerance (and it is functionally acceptable) by changing the radii to diameters and adding position tolerances. You would be mixing +/- and GD&T which leaves "implied datum" discontinuities. By choosing to dimensioning the upper slot to its center the issue gets more complicated even though the other slot could also be considered as dimensioned from the center too (another discontinuity in the old system) so you are really crying out for using a profile call out on the slots which will require definition with basic dimensions and allow you to specifically state a relationship. I, myself, am not opposed to position on slots, but, by defining as an angle and radius you are pretty well cutting yourself out of traditional "in the box" position territory,  I am assuming most will disagree with using position in this case, conceptually I do not.
The important thing is this: if there is a relationship required, using a GD&T method allows you to specifically state what it is.
Frank
 

RE: Tertiary Datums and Bolt Circles

Sorry,
I am in to big of a hurry, the width of the slots would need to be specified directly by removing one of the radius dimensions if using a positional call out.
Frank

RE: Tertiary Datums and Bolt Circles

Position is perfectly valid with slots, though there are some that prefer to use surface profile.

ASME Y14.5M-1994 shows some examples as I recall, though you need to invoke the boundary concept to get full benefit.  Take a look at section 5.10.1 for instance, especially 'C'.

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RE: Tertiary Datums and Bolt Circles

Ken,
Thanks, I read, to my dismay, in "The Dimensioning and Tolerancing Handbook" that position tolerances can't be applied to angles, in this case we have angles used for definition of the extent of the radial slots. I mentioned above that I "conceptually" do not see why you can't position angles and benifit from MMC/LMC, that is why I wanted comments.
I am assuming it will be the ease of measurement issue that comes up with tapers and concentricity vs. runout too. To me ooncerns for ease of measurment are fine but when push comes to shove it seems to me it is not the most important issue, determining functional requirements are. I, of course, am not am inspector.
Frank

RE: Tertiary Datums and Bolt Circles

Who wrote and published "The Dimensioning and Tolerancing Handbook" that you are referring to?  Could it be a case of differing interpretations of the actual standard, such as those that occur with other DRMs for example?
If you can find where the actual standard states that positional tolerances can't be applied to angles, then I would accept that as written; otherwise you have to consider that is only someone elses interpretation, and may have resulted from inspection issues as you mention.  I agree that issues of this type, while important, should not trump funtional definition unless specifically addressed in the standard.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Tertiary Datums and Bolt Circles

Is that the big hardcover book by Paul Drake?
 

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Tertiary Datums and Bolt Circles

(OP)
Hello all,

I worked up a slightly more complete drawing that more correctly conveys the design intent of our actual part as I understand it.  Hopefully, I grasped how to properly dimension for surface profiles, I really didn't like the idea of trying to use position on the slots.  I eliminated a cylindrical datum and elected to give the former datum feature a positional tolerance relative to the datum which remains.  Does this look like a more "by the standard" way to dimension this type of part?  Please try to overlook the poor dimension placement, the CAD software I'm working with seems to be rather limited in that area.

RE: Tertiary Datums and Bolt Circles

Sorry , off topic...

Just curious, is that Moog the suspension company, or Moog the servovalve company?

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Tertiary Datums and Bolt Circles

(OP)
Actually, we're called "MOOG Components Group," a subsidiary company of the international MOOG company which makes a large variety of parts for military and aerospace customers.  Our particular branch specializes in electrical, hydraulic, and fiber-optic slip rings for ground vehicles, aircraft, and space applications.

RE: Tertiary Datums and Bolt Circles

Ah, different animal.  I was fortunate to be employed at Moog Engine Controls Division designing servovalves for jet engines some years back... one of the best companies I've had the pleasure to work for.
(I guess the brainwashing really took hold winky smile)

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Tertiary Datums and Bolt Circles

(OP)
Ah, was that located in the UK?  We have a sister company there that I believe makes some engine parts that go in either the Eurofighter or the Mirage 2000, something like that.

RE: Tertiary Datums and Bolt Circles

No, this division was located in Clearwater, Florida.  The division was shut down about 18 years ago, but the parent location is still going strong near Buffalo, NY.  There were also European divisions, but I don't remember exactly where.
I haven't kept up with the company, and it may very well be related to your Components Group.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Tertiary Datums and Bolt Circles

Daekar,

If the most important issue for you was to assure simultaneous requirements for location of hole patterns and slots, I think you did it correctly. However I still have some minor comments (more about the conformance of the drawing to the standards):

1. You did not state on the drawing which version of ASME Y14.5 you are following (1982?, 1994?, 2009?). The symbols you chose for datums are valid only in 1982 standard.

2. I would put basic dims. dia. 1.014 and dia. 0.613 on the left view instead of typing EQ. SP. on |1.014| and |0.613|. I think your method is not covered anywhere in 1994 and 2009 standards. BTW the abbreviation for equally spaced is EQLSP (see fig. 4-38 in 2009 std.).

3. I would remove basic dim. 2x |.060| for "width" of the slots and put 2x |R.363| for their inner radii. This will precisely define tolerance zone for profile of surface control as well.

4. Use 4x |R| instead of 4x FULL RADIUS.

5. Add "all around" symbol to the leader of profile of surface FCF.

6. Move a little bit datum A symbol. It can confuse that center plane of 1.576 should be taken as a datum instead of bottom flat surface.  

RE: Tertiary Datums and Bolt Circles

(OP)
pmarc,

Thank you so much for your specific critique of the drawing, that type of detailed evaluation is very very hard to find! smile  I made the changes you suggested to the best of my understanding.  Yes, you are correct, my goal was to determine the best way to convey simultaneous requirements for the slots and holes, I'm glad it came across the way I intended.  I am surprised that the "EQLSP ON" with the basic diameter of the bolt circle isn't in the standard, it seemed to me to be a good way to reduce drawing clutter.

RE: Tertiary Datums and Bolt Circles

j-p,
that is the one.
Frank

RE: Tertiary Datums and Bolt Circles

Daekar,

Just 3 more things and that would be all:

1. Important one - attach datum B symbol to the perpendicularity callout of cylindrical dia. .292 feature, not to its surface. You had it on previous sketch. This will clearly say that datum B is an axis of this cylinder.

2. Also important, but only from standard conformance point of view - remove .030 value from R dimension of slots. Simply leave 4X |R|. Take a look at fig. 1-29 from 2009 std. as a reference.

3. Small one - abbreviation EQLSP can be placed next to dia .080 and dia .029 dims., instead of below. This would allow you to save some space on the drawing.  

RE: Tertiary Datums and Bolt Circles

pmarc,
why do you not like the width of the slots specified?
Frank

RE: Tertiary Datums and Bolt Circles

Daekar, and one more thing that I have overlooked before:
- add diameter symbol before .002 in perpendicularity FCF of dia .292 cylinder.  

RE: Tertiary Datums and Bolt Circles

Frank,
I do not think I am able to give you reasonable answersmile. The only problem I see in specifying the width of the slots is how this can be achieved on the drawing with the use of a CAD system. On the sketch where it was presented, this was shown as a typical distance between two parallel lines, while in reality this is a distance between curved surfaces. This explanation might look trivial, but the CAD system I use is also not able to do it in different manner and that's why I thought that specifying two radii would be better and easier to apply. Of course if somebody is able to put such 'fancy' dimension with the use of his CAD system, I do not see nothing wrong in specifying width of the slots.  

RE: Tertiary Datums and Bolt Circles

For shapes with fully rounded ends the preferred dimensioning method is overall dimensions (ASME Y14.5M-19941.8.4).  So Daekers original scheme was in some ways better, though the dia of the centerline of the slots should have been given, not the outer diameter.

However, given it's being used with profile it doesn't matter much.

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RE: Tertiary Datums and Bolt Circles

pmarc,
Cool, thank.
Frank

RE: Tertiary Datums and Bolt Circles

pmarc,
Sorry, thanks.

Obviously from the new sketches the ID I referred to earlier was not an ID at all.
Frank
 

RE: Tertiary Datums and Bolt Circles

(OP)
I'm having trouble convincing my supervisor that the title-block tolerance on angles doesn't apply to the clocking relationship between holes defined with basics/TP tolerance and other features specified with standard dimensions.  I have a few examples but I don't know how to post more than one picture at a time, so I'll start with the kind of thing we usually get.

When we get a drawing like the one attached to this post, it is typically the intention of the engineer that the tabs and holes clock together.  I'm looking for a way to explain why/how this isn't so. I also need to be able to offer an alternative method, so I created the drawing in the next post.

RE: Tertiary Datums and Bolt Circles

(OP)
In the drawing attached to this post, I was attempting to use the fact that features given the same datum features in the same order in their true position FCFs are required to be gaged together.  However, I'm not sure that I did it correctly.  Can you offer any insight?  How would you guys convey this information, and how is it specified in the standard?  I can't find a thing...

Oh yes, please ignore the extra bolthole circle that doesn't actually go through anything.  I had a tertiary datum hole there but decided not to put one in this example, just forgot to remove the circle. Don't want to overload any brains when I have to explain this.

RE: Tertiary Datums and Bolt Circles

Daekar,

Drawing #1:
The way it is specified does not assure that the centerline of a hole is alligned with the centerplane of corresponding tab. These are only someone's intentions that the features clock together. If there is no geometric tolerance between the features specified on the drawing, the features are not related to each other at all.

Drawing #2:
This is exactly the alternative method of specifying clocking between the features. The way you did is correct. I would only delete 9x40deg basic dimension and put EQLSP after 9x .200 dim. for tab width (just as you did for holes) or remove EQLSP from the holes and leave 9x40deg dimension. Take a look at para. 4-19 in Y14.5-2009 std. and figs. 4-40 and 4-41 for confirmation of simultaneous and separate requirements concept.

Hope this helps.

RE: Tertiary Datums and Bolt Circles

I vote for number 2 also, run thew centerline down to the hole and remove the equally spaced and let the inline 9X cover both. Do you care about the OD and ID, I would have to assume the answer is: "no"?
Frank

RE: Tertiary Datums and Bolt Circles

I suspect your supervisor is thinking of a rule about "implied zero" dimensions, meaning that things drawn on a common centerline (like the tab and the hole at the 12 o'clock position) are assumed to be centered on each other. That may be true from a drafting perspective, but our real question is about the tolerance on this implied relationship.
I'm not so quick to dismiss what he's saying -- after all, a 90 degree implied angle takes on the title block's angular tolerance, so why not a zero degree implied angle?

But like the other posters, I say that your second picture is much better because now the tabs are called out with GD&T, using the same datum references as the holes. The standard is clear that those are gaged simultaneously (see post #4 above for the the exact rule).
So now your tabs and holes are rotationally "locked" together, with only their respective position tolerances as the allowable clocking error.
 

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Tertiary Datums and Bolt Circles

(OP)
Thanks for the confirmation, guys, I really appreciate it.

Belanger, as far as the "implied zero" rule applying in this case, the problem I have with that is that the exact nature of the relationship is unclear.  Does this mean that only the hole and tab directly dimensioned have to be within the title-block angle tolerance?  Or that each tab has to be within the title-block anglular tolerance of its corresponding hole regardless of how far out of position the hole is?  Do they also have to conform to the given angular dimension between the tabs as well, then?  What if there are not the same number of holes and tabs, but four of them happen to be on the same centerlines at the four cardinal angles, how do we determine things then?  Do you apply the title-block angle tolerance only to those four?  There are too many variables for it to be a reliable method of communicating functional design intentions, as I see it.  

Now, if there were the same number of holes and tabs and they were dimensioned with standard linear and angular dimensions with a "typical" notation, then I would say that yes, that they would clock together and the "implied zero" would indeed apply.  They would be sacrificing a decent amount of their tolerance zones because they would no longer be round nor potentially subject to MMC bonuses, but I believe this would convey the design intent.

Thoughts on those scenarios?
 

RE: Tertiary Datums and Bolt Circles

The 8 X 40º carries the same logic around to all the features, but you're right: We would still have to decide if the tab's angular tolerance should be looking to the tab next to it, or to the hole beneath it?

Ugh -- tell 'em that your proposed solution is the way to go!
 

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

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