Simultaneous requirement with NO datum

[gabimo]

Here is one more for you: is the simultaneous requirement in effect if the positional callout is with No datum?

Per Jim Meadows:
“No simultaneous requirement exists if no datum references are used.”

http://www.geotolmeadows.com/newsletters/2013/sep2013.htm

Per Mark Foster:

Not everyone believes, and the standard is silent, on whether or not simultaneous requirements should apply to all things positioned to NO datum reference -- i.e. Is "no" DRF the "same" DRF?
Linkedin Thread: A-B-C or What?

http://www.linkedin.com/groups/ABC-What

One GD and T Expert says it is not in effect, but the other says, wait a minute: the standard does not say that!!

Isn’t it the same issue like: “Kurlikovski” composite on a single feature versus pattern of features or Kurlikovski versus Neumann on the perpendicularity thread ?

http://www.eng-tips.com/viewthread.cfm?qid=341776

http://www.eng-tips.com/viewthread.cfm?qid=251955

 

[axym]

This is an interesting question. I definitely agree that the different GD&T consultants and committee members would not agree on this.

There is a certain amount of evidence to support either side, but I would say that the reasoning for "no" outweighs the reasoning for "yes".

Section 4.19 in Y14.5-2009 states the following:

"A simultaneous requirement is where two or more geometric tolerances apply as a single pattern or part requirement. A simultaneous requirement applies to position and profile tolerances that are located by basic dimensions, related to common datum features referenced in the same order of precedence at the same boundary conditions."

Position or Profile zones could be located by basic dimensions, even if no datum features were referenced. But it seems difficult to argue that the absence of datum features qualifies as "related to common datum features". To me this is too much of a stretch, so I say that the standard disallows it. Others may interpret this wording differently, however.

Where functionality is concerned, I would say that in most cases we would not want sim reqts to apply when there are no datum features referenced. Typically, FCF's with no datum feature references are refinements of other FCF's that do (for example, controlling the coaxiality of two holes). If sim reqts was the default for these cases, we would have to specify a lot of SEP REQT annotations to override it.



Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

To muddy water a little bit more, here is what Mark said in one of his answers given in following discussion:

http://www.linkedin.com/groups/How-...roups_most_recent-0-b-ttl&goback=.gmr_1781397

Dated 16 days ago from today: "In fact, I do not believe that this part even requires any datum features. If you simply defined all the dimensions on this part as basic and then controlled all the surfaces simultaneously with profile (with no datum references), I think you get a part that functions as you desire."

My solution would be following:
Since the standard does not explicitly state whether this falls under simultaneous requirement concept or not (in other words, different interpretations may happen), use SIM REQT notation under each FCF without datum feature references that simultaneous requirement is intended to. If all FCFs on a drawing work this way, general note should work fine.

 

[CheckerHater]

CAD DATA IS MASTER.
ALL DIMENSIONS ARE BASIC.
ALL FEATURES ARE PROFILE 0.010 SIM REQ.
No drawing.
No GD&T.

Be careful what you wish for ;-)

 

[pmarc]

No GD&T? Whole GD&T is in the second and the third sentence of your note

 

[axym]

To be fair, I'm not sure that Mark was proposing that all of the features on the part be controlled with the same Profile tolerance. The standard does allows this, and there is even a specific tool for it in 2009 - the ALL OVER annotation and accompanying symbol. This allows all features on the part to be controlled with a single Profile tolerance that covers everything. This is a useful tool, but it can definitely be abused if used as a catch-all to avoid thinking about tolerances. A best practice would be to use the ALL OVER tolerance for general control of less important features, in conjunction with specific datum features and tolerances on the more important functional features.

Here's another detail regarding the original question, which relates to the concept of patterns and "grouping". In the definition of Pattern in Section 1.3.42 on page 6, a pattern is defined in the following way:

"pattern: two or more features or features of size to which a locational tolerance is applied and are grouped by one of the following methods: nX, n COAXIAL HOLES, ALL OVER, A<->B, n SURFACES, simultaneous requirements, or INDICATED."

So this is part of the puzzle of determining whether or not multiple FCF's with no datum features referenced need to be evaluated simultaneously. If the features are grouped by one of the above methods, one could make the case that a pattern is defined and that the tolerances should be evaluated simultaneously. If they are not, I would say that the tolerances do not need to be evaluated simultaneously. Unfortunately this doesn't tell us whether the actual rule of simultaneous requirements applies (from Section 4.19).

Just to complicate the matter further, there are other grouping methods that are not mentioned in 1.3.42. One of them is ALL AROUND, which is very commonly used. Another is describing the grouping in a general note.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[fsincox]

I would think it should be noted just to be safe since it is not specifically covered.
Frank