Tertiary Datums and Bolt Circles 5

[Daekar]

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?

 

[fsincox]

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

 

[fsincox]

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

 

[KENAT]

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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[fsincox]

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

 

[ewh]

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." - [small]Robert Hunter[/small]

 

[Belanger]

Is that the big hardcover book by Paul Drake?


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

 

[Daekar]

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.

 

[ewh]

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." - [small]Robert Hunter[/small]

 

[Daekar]

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.

 

[ewh]

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 ;-))

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

 

[Daekar]

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.

 

[ewh]

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." - [small]Robert Hunter[/small]

 

[pmarc]

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.

 

[Daekar]

pmarc,

Thank you so much for your specific critique of the drawing, that type of detailed evaluation is very very hard to find! 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.

 

[fsincox]

j-p,
that is the one.
Frank

 

[pmarc]

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.

 

[fsincox]

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

 

[pmarc]

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

 

[pmarc]

Frank,
I do not think I am able to give you reasonable answer. 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.

 

[KENAT]

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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