Rotational orientation on circular parts with cylindrical Datum

[ligo]

Here is a simplified drawing of a circular plate with two hole patterns and a single hole. Case A, B show the same hole locations, and case C has the single hole, and one hole pattern, coincident to the 2nd & 3rd datum planes.

Is my interpretation of Y14.5 section 4.4.2, 4.4.3 correct?

There was a similar discussion in thread 169440

http://www.eng-tips.com/viewthread.cfm?qid=169440

But I'm not sure if my understanding from that is correct?

Thoughts?

Thanks

 

[KENAT]

thread1103-267816 is probably quite relevant especially:

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."

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[fsincox]

ligo,
Ken is right on this, your hole patterns can not rotate. You must abandon all logic when you think of this and just accept and follow the rules of the game as given. ASME Y14.5M-1994 (and 2009 too, I assume) as noted by Ken above defines what is called "the Simultaneous Requirement" it is just simply a rule of the game. ISO does not happen to play that way, so be it. If you are playing by our rules this is how the game is played. I do not happen to agree with this particular rule and feel it is a hold over from a earlier simpler time time when the desire was to minimise GD&T on prints and shows a bias to serve manufacturing (all can be made with one jig), but, if we do not all agree to play by the rules what is the point of a standard?
Frank

 

[ligo]

Ah yes of course I see now that the two patterns are locked together in all 3 cases. But how about in relation to the single hole present in all three cases and used as a datum in case B? Can the two patterns together rotate relative to the single hole in Case A, but not in cases B & C?

 

[pmarc]

ligo,

I would say that in all 3 cases the two patterns can rotate relative to the single hole only within specified position tolerance and that's all. In each example these patterns are tied to the single hole - only the way it was done is different. Cases A & C are classical examples of simultaneous requirements and there is no difference in interpretation of these two. Case B is introducing third datum feature but this changes nothing in terms of location relationship of patterns and the hole.

I think it would be good to discuss about this on a specific case that you are probably dealing with, because the topic of simultaneous requirements is so vast that we could probably talk about it for hours. I mean it would be good to know more about how your component functions to decide if you need third datum feature and if you need simultaneous requirements at all.

 

[pmarc]

Frank,

What did you mean by saying:
"ISO does not happen to play that way, so be it."

 

[ligo]

pmarc

So if both patterns and the hole can only move within positional tolerance in all 3 cases. When does one need a Datum-C for clocking? Examples in Y14.5 (fig. 4-6, 4-7) show a keyway as Dateum-C, but I'm failing to see the difference between the keyway in their example and my single hole.

 

[KENAT]

"or to common datum features of size specified on an RFS basis"

So if your datum B was specified at MMC, as is often the case with diameter datum features, then you'd explicitly need to add the clocking.

I might argue that when 'clocking' is important it's maybe best to explicitly specify it since while valid, relying on 'simultaneous' requirements means relying on your suppliers and everyone else to understand it's implications.

I only realized its implications in this application recently, so I wonder how many other folks realize it.

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[fsincox]

pmarc,
All I mean is the ISO has chosen to go another way on this, is all. Simultaneous requirements are not automatically applied and my latest ISO reference materials use "CZ" added into the individual tolerance reference frameworks next to the tolerance specification and material modifier to invoke a "simultaneous requirement" (common zone). I am sorry if I confused anyone by speaking of it like a game, but, I do think it is really an arbitrary issue, only made important by historical differences. Therefore I am trying to express that this is more of a “we all choose to play a game with a generally understood set of rules” and applying logic here is not really a factor. Philosophically, this is another area where I would agree with the ISO, in that, to me normal engineering logic (assume the worst) would default to their choice. I suspect it is a hold over from the same era as our old default practice of implied MMC on tolerances and datums (1973).
Frank

 

[pmarc]

ligo,

- What I said is that the two patterns can move relative to the single hole within positional tolerance, not all of them together. I was only focusing on this relationship. All your 3 examples have the same meaning in terms of it.

- Tertiary datum is needed for clocking exactly in situations like the one shown in example 4-6 (1994 std.). But this is one way of dealing with the problem. Another would be to omit tertiary datum and specify positional tolerance for keyway and holes with relation to the same datum reference frame |A|B M|. By doing this you would have typical simultaneous requirement situation and then a configuration of toleranced features would have to be exactly the same as it is specified on a drawing. For sure this second option is much less popular than the first, but the geometrical interpretation is identical.

So in fact your examples presented both possibilities of obtaining the same result.

To read more about the second option (applied to a slightly different example) you can take a look at:

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

This not only describes the idea of the concept but also gives some advantage/disadvantage comparison of both methods.

 

[pmarc]

Frank,

Do you have access to ISO 5458 std? I have 1998 version in front of me and in paragraph 4.4 you'll find two situations described when the group of features should be considered as a single pattern. There is nothing about CZ there. Common zone concept is most often used in situations like flatness control to specify coplanarity of two or more surfaces. As you know in Y14.5 std. profile of surface does this.

 

[ligo]

pmarc,

- I understood "relative to the single hole within positional tolerance" but in my effort to be brief in my reply I over simplified, thank you for noticing and reiterating, I made it look like I misunderstood.

At this point I think I'm having a hard time envisioning a circumstance where a datum for clocking would be required.

Since you asked about the specific circumstances that bring up my "clocking datum" questions, here are 3 drawings: an assembly drawing, and 2 fabricated parts.

This is a vacuum chamber in which a directionally sensitive seismometer will be placed. Orientation of the seismometer is critical, so there are features to align it to the base plate, and the base plate to next assembly up. There are also orientation marks on the edge of the plate and cover for visual alignment. These were made Datum-C originally because we wanted to make our alignment (clocking) intentions obvious. Our fabricator says that Datum C is unnecessary, and if they tried to use the related datum feature (v-grove) to actually align to, it would be a very difficult setup. Unfortunately our knowledge the details of how these items will be fabricated is very limited.

 

[ligo]

apparently I can only attach one drawing at a time

 

[ligo]

ditto

 

[KENAT]

ligo, take a look at the last sentance of the excerpt from 14.5.

If your datum B was invoked at MMC then it looks to me like the clocking datum would be required.

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[fsincox]

ligo,
By the way, the "simultaneous requirement" does not automatically apply to the lower level of composite tolerances.
Ken,
The simultaneous requirement applies "at the same material condition, as applicable" ASME Y14.5M-1994, 4.5.12, pg 70.
Not sure what "as applicable" means. The referenced figure in the paragraph is shown on a datum framework referenced at MMC.
Frank

 

[pmarc]

ligo,

In my opininion a datum scheme you have chosen for POD BASE (#1) should be revised. Basing on how the part functions my proposal would be as follows:
- Use 3 flat surfaces as you have now for primary datum. (According to Y14.5 standard you should specify profile of surface without any datum reference as a form control instead of flatness).
- And the tricky part - use 3 holes for a screw (#25) as secondary/tertiary datums. How it should be technically done? Again take a look at a hint:

http://www.tec-ease.com/gdt-tips-view.php?q=187

I think this will tell you a lot.
- Then specify positional tolerance for other holes with reference to these datums. I think projected tolerance zone concept (P) would work here.

By using similar datum scheme also for POD TOPHAT (#2) I think you will achieve what is needed - proper seismometer orientation with relation to the mounting surface.

 

[fsincox]

pmarc,
I do not have access to that version currently. I do have an older version of that standard. What two options do they show? I agree in general that the standard is the final "bible" ,however, all of these standards are also works in progress. The book I have was released in 2006.
Frank

 

[pmarc]

Frank,

Here is a picture of the cited paragraph:

 

[fsincox]

pmarc,
Thank you, judging from the page you have provided I would imply no real difference between ISO and ASME when identical datums and modifiers are stated and shown on a common axis.
Frank