Constraints on datums for profiles

[[Ja...]]

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If datum feature C were the width (midplane) the label would be attached to the size dimension. In this case, at least per ASME, the datum feature is the single plane.

[[Ma...]]

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Actualy it is on dimension and not on surface. So symetry can be used

[[Ja...]]

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Take a closer look and refer to Y14.5-2009 Para. 3.3.2(a) and 3.3.2(b). The datum feature symbol is placed on the extension line of the feature surface (3.3.2(a)) but not on the dimension line (3.3.2(b)) and is thus the single surface.

Furthermore, compare the datum feature symbol placement for datum C in Fig. 3-3 and Fig. 3-4(h).

[[Ma...]]

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Well thanks for pointing me. I am still learning these details - not so much sure how it is described in ISO which we use.

[[Ro...]]

Look at figure 3.3. Do you see how one side of the dimension extension line is longer than the other side? That is a feature surface extension line and indicates it's just the one surface. Also, read 3.3.2 (a). It needs to be clearly separated from the dimension line when the datum feature is the surface itself. Neither of those is the case here.

[[Ro...]]

If you want to make the argument that the drafter didn't perfectly align the symbol with the width dimension then that's fine. I would disagree that that is the intent of the drawing and that the datum is attached to the dimension itself, not to the surface. To further that, why isn't the symbol on the surface itself? Somebody took the time to attach it to the dimension.

[[Ja...]]

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You, like the rest of us, don't understand the intent of the drawing unless you are the designer or you actively use the described part in it's desired application.

There is nothing in Y14.5 that says the datum feature symbol must be placed on the extension line of the feature outline beyond the dimension line.

You state that the datum feature symbol is not clearly separated from the dimension line. It most certainly is. I can see a clear difference between the two.

You can't assume what you want if you are measuring to a standard. You can only go as far as the standard takes you unless you receive authorization from a(nother) stakeholder.

Para. 3.3.2 makes it very clear that the datum feature is the single side because the separation from the datum feature symbol and the dimension line is obvious on this drawing.

[[Ro...]]

Thanks for the reply, Jacob. Couple of questions if you have the time...

Why is the example shown in the ASME standard clearly show the symbol beyond the dimension line and have a different length for the extension? I'm assuming they did that for a reason. I posit it's to clearly show that it's separated from the dimension line and a single surface. And it avoids the confusion of my second question.

How well does the datum symbol need to be aligned to the dimension? Does it need to be exact? Can it be slightly out of alignment? What criteria determines if it's clearly separated from the dimension line?

As far as your other comments, I don't see how they further this discussion.

[[Ja...]]

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Robert,

The person that drew the figure decided on that direction. I'm assuming that they did that for cleanliness and because space allowed for it. "Clearly separated" does not need to be unidirectional.

The datum feature symbol should be aligned exactly with the dimension if the drawing is done digitally, as there is really no reason for it not to be. Clearly separated should mean some detection of white space between the two.

In any event, the questions are of direction and "closeness", which are both simply undefined in Y14.5 for this example. We both seem to have interpretations that do not match. This represents some ambiguity in the standard which is not unheard of. We can agree to disagree, and the OP should bring this question directly to his engineering team.

[[Ro...]]

Jacob,
I don't disagree, per se, with anything you've posted. Just offering a different and plausible explanation. For me, I'm interpreting the standard verbiage in conjunction with the image, as that's all we have. I think showing the symbol on an extended line, as shown, or exactly aligned to the dimension is absolutely clear and leaves no room for interpretation. Showing it anywhere else opens up the can of worms of "what is clearly separated". Why they didn't just put it on the surface itself is beyond me.

Thanks again for the replies. I appreciate your posts in the group and the sharing of your knowledge.