Bidirectionnal position tolerance, rectangular coordinate

[2JL]

Hi everyone

Is it possible to use the scheme described in figure 7-28 2009 standard without a tertiary datum? Attached is a sketch of what we are planning to do.

The hole is a locating feature and basically we only need to be really tight in one direction.
2JL

 

[CheckerHater]

No

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[ewh]

What purpose would the horizontal FCF serve if it is not referencing a datum that could control horizontal shift?

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[2JL]

Thanks CheckerHater and Ewh! Now that I took another look I see the obvious problem with the horizontal FCF.
Does it change anything if datum B was a cylindrical feature? Sorry, I should have refered to fig 7.29 with is closer to what I am looking for (and also for the crappy sketches)!

 

[3DDave]

Even without the tertiary datum the orientations of the tolerance zones are controlled by the simultaneous requirements established by the given references and their depiction at right angles to each other; those orientations are fixed with respect to the surfaces mentioned.

In this case, the pair is controlling the location and orientation from [A|B] and the perpendicularity WRT to A parallel to B.

Why wouldn't one use perpendicularity to [A|B] instead? The standard says that simultaneous requirements don't apply except to position and profile, so one could reorient a part that either passed the position tolerance or passed the perpendicularity one, but could not pass both at the same time; there is nothing in the standard that allows a designer to record his intent that they happen at the same time under these circumstances unless a note is added to the drawing.

I'm guessing the rest of the part perimeter will have some dependence on [A|B|C] or at least simultaneous with [A|B].

It's an intellectual laziness to not realize that a position tolerance controls the location of one end of a feature relative to the other end and all points in between, which is how it controls orientation. It wouldn't be so bad, but for the express exclusion of simultaneous requirements among other controls besides position and profile.

 

[2JL]

Thanks 3DDave for the very detailled answer. Simultaneous requirement does the trick in this case.

2JL

 

[CheckerHater]

Would somebody please explain me how simultaneous requirement applies to one single feature?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[2JL]

I hear you Checkerhater. I spent too quite a time reading paragraph 4.19, trying to figure if the rule was to be applied only on multiple features. And unless I misunderstand the intent of the first two paragraphs, the focus is not on the features but on the FCF.
2JL

 

[CheckerHater]

@ 2JL:
From Para. 4.19 you mentioned:

“In a simultaneous requirement there is no translation or rotation between the datum reference frames of the included geometric tolerances thus creating a single pattern”

So, in your interpretation, “pattern” means ”pattern of FCFs”?



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[2JL]

Not quite, I was refering to the first two paragraphs and the term that got my attention is "part requirement" at the end of the first one. My interpretation is that in my case, the two FCF combine to form a part requirement which is the location of the hole. Am reading this right?

 

[CheckerHater]

@ 2JL:
I think you are reading what was not written, and "part requirement" applies to use of Profile.
Profile tolerance is versatile and can represent several different "part requirements", so it looks like correct choice of words for Para. 4.19


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[2JL]

My thinking is that "part requirement" in first paragraph is to be understand in the broad sense of the term. The second paragraph is there to clarify the types of FCF (part requirements) to which the simultaneaous requirements rule applied.

 

[CheckerHater]

I have no more questions

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[3DDave]

It's almost as if the Committee spent a lot of time being -very- specific in order to be as useless as possible.

They could have said - All identical DRFs are evaluated based on a single selection of datums; that is, they apply simultaneously.

Whoosh, nothing about what counts as a pattern, and without regard to type of geometric control. Just one sentence. And then a sentence that excepts those with 'SEPT REQT' noted on them.

I'd like to know they had some plan in mind, but it doesn't seem like it. It's not even useful for CMM operators or gauge makers to allow the increased mobility that applies to the standard's concept. More like someone proposed this for 1982 or earlier, and no one thought much about it. Just like TGC was taken out of 2009, but it's still in there. Sigh.

 

[weavedreamer]

Paraphrasing from Alex Krulikowski's course on tolerance stacks, between two features that have identical DRF's the shift does not get considered in the stack.

"SEPT REQT" excepts this.