【转帖】ordinate 0,0 not at a datum

yang686526

ordinate 0,0 not at a datum?
this is not how i would prefer to detail a part, but there is a specific motivation i won't go into.
what i have is a large part with many features.  i have 3 datums selected (a plane, and two bores).  the part is round and neither of the bore datums has axis at the center.  now, for a few reasons, people would like to see ordinate dimensions with 0,0 at the center of the part even though the positional tolerances reference a, b, and c which are unrelated (except by basic dims) to the center.  is this ok?  i'd rather put 0,0 at my datum b, but that causes other logistical problems.  any help is appreciated
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is there an actual feature at the center to dimension from, or is it the theoretical center?
it's a round part with a round bore in the middle (approx 24" bore).  the issue is that the round bore is just clearance and is not critical to part function and so i'd like to not have it be datum.  the other (smaller ~ .5") bores are not at center but are critical to part function and serve as alignment features.  people like 0,0 at center to easily be able to confirm location of critical features relative to other parts in design reviews (among other reasons).
take it to inspection an the machine shop and ask them if they can locate and measure 0,0 at center, with dims located basic to the center.
chris
solidworks/pdmworks 08 2.0
autocad 06
as long as the gd&t places the features w.r.t. the correct references, the 0,0 location shouldn't really matter.  i do get bugged when 0,0 is "in space", in the center but not on a feature.
i agree with thetick. an (0,0 basic linear dimensional reference) ... does not a datum make. the coordinate system that is constructed using datum features a|b|c would put a 0,0 reference point where the axis of b's ame (actual mating envelope) intersects with a... oriented for rotation relative to c. the 0,0 reference must then be translated to what appears to be the center of the part using datum feature b's basic dimensions and all diameters that are shown coaxial with the (0,0 basic linear dimensional reference) will then have x and y displacement error from 0,0.
editorial comment...
it does seem a bit senseless, however, to not have 0,0 correspond w/ primary datums.
pinsnpads,
i was curious what your inspection and machine shops would say.
chris
solidworks/pdmworks 08 2.0
autocad 06
this is always hard when i don't know the whole story and i may be wrong, but here is something to chew on.  
it seems to me that the wrong functional primary drf is being specified here since there is such a strong desire to have an origin separate from the current specified drf.  additionally, maintaining an assembly clearance implies to me a functional requirement.  
so i ask a question and suggest a possible solution.  the question is whether you have considered adding an axial datum to the main bore feature defining the center of the part and utilize multiple drf's or not?  if not please see, para 4.5.11 and 4.5.11.1 in the standard.
last but not least, combining methods will surely lead to problems.  i suggest pick one or the other, never both.  my preference is gd&t.
i agree with thetick's first response, and with pauljackson, that the xyz coordinate system origin in the model or drawing can be put anywhere.  the origin doesn't have to correspond directly with the datum features, and it often can't.  with many datum feature configurations, patterns in particular, there is no unique "logical" location for the (0,0) origin.  it's arbitrary.
the reasons for requesting the xyz origin at a certain place often have nothing to do with part function - they are usually something to do with convenience for nc programming, fixture reference, or something like that.  but that's not a problem - put the xyz origin wherever they want it.  that's the beauty of gd&t and basic dimensions - you can lay them out in whatever coordinate system you want, and it's all equivalent.
having said that, all of this only works if the distinction between datum features and features at the (0,0,0) origin is understood properly by all concerned.  if the inspection report shows a hole that is nominally (0,0) with actuals of(0.003, -0.005), this can be counter-intuitive if you're expecting datum features to always be at the origin (as some machine shops might).
evan janeshewski
axymetrix quality engineering inc.
thanks for the responses.  i think this confirms my thought which was that i could have 0,0 wherever it "needs" to be even if that is not a datum which will be used to locate critical features.  i think this introduces the possibility for confusion by vendors, but my alternatives are worse in this case!