【转帖】simulation of a single daum plane

yang686526

simulation of a single daum plane
i'm looking at 4.5.7.1 of asme y14.5m-1994.  are all the methods in my attatched tiff in compliance with the standard?
the paragraph doesn't say you have to have two datums and reference them a-b but that's all it seems to show in examples.
kenat, probably the least qualified checker you'll ever meet...
i have always used #2 without issues. i would break the line around the first surface similar to a dim.
chris
solidworks 08 0.0/pdmworks 08
autocad 06
figure 4-20 in the 1994 standard with datums a-b has changed in the proposed 2008, figure 4-30 only has one datum with an arrow pointing to the phantom line between the two surfaces. however, when the two surfaces are offset in the vertical direction by a basic dimension, the two surfaces are datums a-b.
whitmiregt,
it would seem technically more appropriate to use datum targets (3) for that example.  there would seem to be a conflict in merging the 2 surfaces to obtain a single datum plane.  if 3 points determine a plane, then that should be all that is required.
am i thinking wrongly?
what i do in a case like this is use a coplanar specification (profile of a surface long with the "2 surfaces" spec) with whatever tolerance value is appropriate for the situation then attach a datum identifier to the fcf. all 4 examples will get your point across but the only two that are directly addressed in the standard are 1 and 2. the only place i know of in the standard where "2 surfaces" is used is with coplanarity. this doesn't make 3 and 4 wrong though, in my opinion.
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thanks all, a tidied up version of 2 is probably what i'll use in this case.
to me 3 is just a clarification of 2, though not in the standard as such.
4 was one concern i had, i've had drawings from a fairly experienced guy that if i recall correctly only had the 2 surfaces and no phantom/leader line.  i didn't think it was really in compliance with the standard and in some cases could be ambiguous because of it (not the example i give though probably).
powerhound, i've done similar with the coplaner callout, i may do it in this case but at present no coplaner tol is called out and it's working but i'm going to look at the function a little closer.
kenat, probably the least qualified checker you'll ever meet...
kenat,
if you are using 2 planar surfaces as a single datum feature, and a plane is still determined by 3 points, how does one know which surface is selected for 2 points and which for 1 point?
would not  3 target points or areas be more appropriate?                                                                                                         
i'd have thought you'd simulate the datum by putting them on a gauge flat surface.  the point is you're treating it as a single surface even though it isn't.
either way, the standard says it's ok (my question was on how to properly indicate it not if it was valid).
you could make the same argument for any datum base on my understanding of your logic.
kenat, probably the least qualified checker you'll ever meet...
per the original post i would say example #1 is per the standard and the only method to use.  examples 2-4 are imho a misinterpretation of the standard and not compliant as is.  the two surfaces features are separate and should not be implied as one.  to do this each surface feature warrants a unique datum identifier and is simulated by their tgc separately.  this is why fig 4-20 shows this.
is there a reason why you would not use this method only, since it is unambiguous and per the example?  
additionally, i would almost always recommend control of any datum, in this case thru the use of profile of surface with a phantom line and the number of surfaces spelled out under the fcf.  to control the co-planarity desired of the two separate surfaces, as described in para. 6.5.6.
also, i caution those of you who have a copy of the "proposed" 200x standard to leave it alone and not cite it until it is released even if it is an improvement.  it only confuses those who are already confused.  after it has been approved and released, have at it.
xplicator 4.5.7.1 penultimate sentence
quote:
where appropriate, an extension line may be used to indicate a continuation of one datum feature across slots or obstructions.
how is 2 a misinterpretation?  are you saying my example isn't 'appropriate'?  if so could you explain why.
somewhat scarily i'm about the most knowledgable gd&t person around here so i try to use the 'simplist' or most easily understood representation of the gd&t so that others will understand.  to me having the extension line, especially if clarified with "2 surfaces" is probably clearest.  however i wanted opinions on if i'd interpreted the standard correctly, especially since the '2 surfaces' isn't mentioned for datums in the standard.
kenat, probably the least qualified checker you'll ever meet...
i personally would leave "surfaces" off. it could be vague to some about which surfaces. if you show the extension line across both surfaces, it is obvious it is those two surfaces. treat it like a dimension.
chris
solidworks/pdmworks 08 1.1
autocad 06