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?
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
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tertiary Datums and Bolt Circles
RE: Tertiary Datums and Bolt Circles
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
RE: Tertiary Datums and Bolt Circles
But I'll stand by the rest of my post
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tertiary Datums and Bolt Circles
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:
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
Frank
RE: Tertiary Datums and Bolt Circles
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tertiary Datums and Bolt Circles
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
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
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tertiary Datums and Bolt Circles
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
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
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.
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.
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.
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
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
RE: Tertiary Datums and Bolt Circles
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
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tertiary Datums and Bolt Circles
RE: Tertiary Datums and Bolt Circles
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
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
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'.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tertiary Datums and Bolt Circles
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
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
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tertiary Datums and Bolt Circles
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
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
RE: Tertiary Datums and Bolt Circles
(I guess the brainwashing really took hold
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Tertiary Datums and Bolt Circles
RE: Tertiary Datums and Bolt Circles
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
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
Thank you so much for your specific critique of the drawing, that type of detailed evaluation is very very hard to find!
RE: Tertiary Datums and Bolt Circles
that is the one.
Frank
RE: Tertiary Datums and Bolt Circles
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
why do you not like the width of the slots specified?
Frank
RE: Tertiary Datums and Bolt Circles
- add diameter symbol before .002 in perpendicularity FCF of dia .292 cylinder.
RE: Tertiary Datums and Bolt Circles
I do not think I am able to give you reasonable answer
RE: Tertiary Datums and Bolt Circles
However, given it's being used with profile it doesn't matter much.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tertiary Datums and Bolt Circles
Cool, thank.
Frank
RE: Tertiary Datums and Bolt Circles
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
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
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
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RE: Tertiary Datums and Bolt Circles
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
Frank
RE: Tertiary Datums and Bolt Circles
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
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
Ugh -- tell 'em that your proposed solution is the way to go!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems