gd&t on weldments

[TWJR]

Got a fairly simple requirement, but not sure the best way to express it on the drawing. Fairly large weldment, on the order of 6 feet long. Most things not critical, but there are 4 tabs on each side, which have holes that need to line up, as in a hinge. Don't want to put datums all over the place. It's a bit oddly shaped in the first place. Just want to state that each set of 4 holes must be in line with each other within a particular amount. Easiest way that still meets ASME?

 

[drawoh]

TWJR,

I claim that any idiot can make a drawing of a machined part. In most cases, the as-fabricated features are way more accurate than you require. The process undoes all sorts of crappy drafting. You need GD&T on weldments. You need to clearly show the welder which of your features matter, and which ones don't.

It is hard to make suggestions without knowing what your part looks like and what it does. In general, just about everything in GD&T comes from datums. The fabricator and inspector should fixture your part using your datums. The ideal datums are your mounting points. Consider using your tabs as datums. Don't forget to apply sloppy tolerances on all the stuff that is not critical.

--
JHG

 

[CheckerHater]

You could probably get away with what's known as "implied self-datum", but like drawoh mentioned, this won't give you control over the rest of the part.

"For every expert there is an equal and opposite expert"
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[3DDave]

Call it an interrupted feature of size. It will be checked with a suitably sized rod, just like the actual hinge pin.

 

[ewh]

Or you could use Continuous Feature.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV