huangyhg

datum placement
is it legal to use a symmetrical centerline as a datum?
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i would say "no".
no - a datum is reflected using features on the part that would develop the centre line. how is the c/l developed? was it the outside width? hole? one needs to reflect the feature(s) that developed the c/l from a design perspective.
dave d.
i would think it would be ok. unless i am miss-understanding the situation. looking through the gd&t material i have aquired over the years, i think that would just be a datum plane. in "geometric dimensioning and tolerancing" by david madsen this is shown as an ok practice.
but i could be mistaken...
asme y14.5-1994 para 4.3.2 states: "the datum feature symbol identifies physical features and shall not be applied to center lines, center planes, or axes except as defined in paras. 4.6.6 and 4.6.7.", which refer to datums defined from datum points.
the answer is nofff">. no exceptions, "what if", or "i would think". a datum must be a feature. if you use a centerline, it must be the centerline of a feature.
put the datum flag on the feature. if the feature has a centerline, then that centerline is used for certain types of callouts (position, runout).
honesty may be the best policy, but insanity is a better defense.
this is all very helpful. thanks a bunch!
ahh, but there is an exception, just not one pertinent to the op. "...except as defined in paras. 4.6.6 and 4.6.7." is verbatim from the standard.
the only legitimate way to attach a datum symbol to a centerline is to include "axis of" or similar qualifier with it.
ewh - 4.6.6 defines establishing a datum from datum targets while 4.6.7 again establishes a datum from datum targets when there isn't a solid feature to create, as an example, a plane.
i don't think it applies in this situation.
dave d.
i agree, and stated such, with both "which refer to datums defined from datum points" and "just not one pertinent to the op". i was just being facetious about tick's post. i'll try to be more serious in the future.
i think this is at the heart of the debate as to whether or not symmetry even belongs in the gd&t. sym has been dropped and picked up again, but i'm sure it will be up for reconsideration yet again next time the standard gets updated. instead of using sym, see if their is some other gd&t method to accomplish what you wish.
matt
cad engineer/ecn analyst
silicon valley, ca

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