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tell me why this tolerance is wrong

bigtallanddopey
Hi everyone

I have been having an animated discussion with the design engineers this morning as to why I think there drawings is wrong. The design engineer is sat there with the standard which is ISO 8888 and is adamant that he is correct and he can dimension the feature like this and use whatever tolerances he likes in whatever order.



That is the offending tolerance, A,B & C are the overall datums for the whole part and I have no problems with that segment. The lower segment however to me is wrong in so many ways, maybe I am wrong as I am someone who is actually trained in ASME and not ISO, maybe he is right and it can be done like this in ISO,

He claims the F in the circle means measure in free state however ive never seen it placed where it is now. I also believe from my ASME training that the bottom segment refines the top segment so cannot use different datums, and I think this is the same for ISO.

I would usually argue my case louder but this is the senior design engineer here and he is digging his heels in at the moment and not changing the drawing, plus I don't want to look like an idiot if he is right and can tolerance the feature like this.

Anyone have any thoughts?
  • Thanks for the endorsement Vinni, but to be honest some days I am not even sure I undestand ASME GD&T, much less ISO.

    That said, my first question is WHICH STANDARD does the drawing cite? If the Drawing specifies ASME Y14.5-2009 (Or any ASME version) the FCF is wrong. Not that is likely to cut mustard with Mr. Senior Design Engineer.

    If the Standard is ISO, then you need one or more copy(ies) of the standard and training.

    HTH
  • in reply to Wes Cisco
    "some days I am not even sure I undestand ASME GD&T, much less ISO."

    Say it ain't so. I guess I'll take down my Wes the Wizard statue.

    Disappointed
  • "some days I am not even sure I undestand ASME GD&T, much less ISO."

    Say it ain't so. I guess I'll take down my Wes the Wizard statue.

    Disappointed

    Okay: It ain't so.

    Does that make you feel better about your idolatry now? Neutral face



  • Another point : 8888 is a BS standard, not a ISO Slight smile. Maybe NB can confirm it or not ?

    NinjaBadger! Help requested!
  • Thanks for the endorsement Vinni, but to be honest some days I am not even sure I undestand ASME GD&T, much less ISO.

    That said, my first question is WHICH STANDARD does the drawing cite? If the Drawing specifies ASME Y14.5-2009 (Or any ASME version) the FCF is wrong. Not that is likely to cut mustard with Mr. Senior Design Engineer.

    If the Standard is ISO, then you need one or more copy(ies) of the standard and training.

    HTH


    That is one of the problems I am having, We have a copy of the standard in front of us which is BS EN ISO 8888. The engineer is stating that no where in the standard does it say he cannot tolerance the feature like this, and he is correct, it does not seem to say that this is illegal anywhere. On the flip side to that, it doesn't state anywhere an example showing a situation like this where this FCF can be used.

    Whilst everyone is agreed that under ASME this is wrong and cannot be used, however using an ISO or BS standard there seems some ambiguity as to whether it can be used. Jefman's post certainly threw a spanner in the works and it may well be legal.
  • in reply to Wes Cisco
    My faith is shaken, but I shall not falter, oh Wizardy One. Slight smile
  • in reply to vpt.se
    How do you measure (access) A & B in restrained condition?
  • I would say he's right...


    As vpt shows, that position of the (F) is OK, but there's still the question about the datums. I have found nothing (yet) in the ISO standards that says anything about the datums of a composite dimension, although everyone expect them to be like [A|B|C] and [A]. Contrary to ASME, ISO has a general independence policy, except for a number of cases (common centerline/point, identical datums with material conditions, and so on).

    As this (the original question) is a combination of one restrained dimension and one free state, it would probably mean exactly that - two completely separate dimensions.


  • As this (the original question) is a combination of one restrained dimension and one free state, it would probably mean exactly that - two completely separate dimensions.


    That is the meaning I am taking from all this. It could do with an explanation in the standard outlining it though, if it is indeed a legal callout. I get 100% what the designer means now he has explained it, but for me when looking at the drawing it wasn't immediately obvious, the different datum systems confused things for me.

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