huangyhg

referencing a datum feature of size multiple times
i have a drawing with multiple pages. datum a is defined as a feature of size on page 1. on page 2 we dimension another feature to the center plane of datum a. the center plane of datum a is represented by a centerline in this view, but because the centerline can be tied to multiple features shown in the view, it is necessary to define it as the center plane of datum a. how should this be accomplished?
i understand that asme 14.5 forbids a datum flag to be attached to a centerline. this leaves only the use of a note or changing the dimension scheme. if a note can be used, what is the proper syntax?
take a look at asme y14.5m-1994 fig. 3-4.
that being said, there really isn't any reason to reference a datum twice on the same drawing. as long as it is clearly defined on a veiw it should be enough. there is no reason that you would need to be concerned about someone misinterpreting which feature to dimension to as long as you reference datum a in your fcf.
if i had to label atum a on your page 2, i would use a leader pointing to the centerline (centerplane is meaningless in this situation) with the text reading "centerline datum a", or "axis datum a" depending on the view orientation.
aardvarkdw, the key point is that the second view contains multiple features that an centerline could be associated with.
ewh, i originally considered a "centerline", but although the line type used would be a centerline, the actual feature represented is a plane. it appears as a line because itexists in a single dimension in the view.
i'm inclined to go with the note, but it would be nice to know if there is some precident for this situation in the official asme universe.
thanks for the help so far.
aardvarkdw: i too would like to assume that there is no need to repeat the datum, but like ewh has said, i have had complicated machine parts with a width datum, since the part center is machined out and their are enough concentric features, multiple sheets, and detail views to cause confusion as to what indeed is the center line relating to. therefore i also use a leader to the centerline, stating "centerline of datum a"
when in doubt, spell it out.
sorry, i had assumed that datum a was a cylinder. bad habit, that assuming.
instead of "centerline datum a" why don't you just use "centerplane datum a"? it is not ambiguous, and a centerline font would be appropriate.
it occurs to me now that a plane has no center, therefore a note referencing a centerline or centerplane of datum a would also be incorrect.
how about just "datum plane a"?
yes, but it should make no difference. you have already specified datum a. if you are dimensioning from datum a and it is critical to function that this is so, make the dimension basic and reference datum a in the fcf. there is no chance of ambiguity this way. if you mark the datum twice, especially if there are multiple features that you could be refering to, this does nothing but open the door to misinterpretation. i have no problem with putting a clarifying note on the centerline ( i personally wouldn't do it, but i am not looking at your drawing) but i don't think it is nessesary.
surely you should be able to add/create a view or section that would illustrate the datum feature a on the second sheet. i assume from your statement that you are concerned with repeating the datum identifier. is tthat the case? if so, i do not see a problem in repeating it.
i agree that it often simplifies a drawing to repeat datum callouts, and as ringman notes, a section is a very good way to do this. it is not a good idea to add a datum callout to a hidden feature.
one solution i've used in the past was to add an isometric view on each sheet which graphically shows the datum features again, and label that view as reference. there's no precedence in the standard for this problem or for my proposal, but i've never had anyone come back pleading confusion.
jim sykes, p.eng, gdtp-s
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