Assembly of Weldments and Significant Figures

Coordinate or GD&T?

New or existing drawing?

ASME Y14.5M-1994?

Will the finished weldment have the correct F,F ans F?

On weldments, tolerance isn't normally x.xxx but fractional.

Try this.
Geometrical tolerances are specified in Annex D of BS EN 1090-2

That may be sort of a right idea, but I don't think tolerances should be relaxed a whole order of magnitude at each step. Maybe by a factor of say 2, though.

Sometimes F, F & F are in the eye of the beholder. The first F is the question here. Sure it may Fit but if the overall assembly is 1/4" long over 34' is that an F or not? Some would say yes, as I would, but some would say no and justifiably so if the print says 34' +/-1/8" and if this assembly is made up of six or seven weldments bolted together I think the first F should be reasonably relaxed.

It would be nice to find a standard rule stating such.

Thanks,

I suspect the F, F & F question has to be answered separately for each industry. What is OK in shipbuilding might be overkill in a small factory building steel tanks for the oil patch, and so on.

I am not at all sure the standard propagation of errors applies in this assembling of multiple weldments, although it sort of makes sense that there will be a much larger error bar given x number of weldments assembled into one piece, than for y weldments assembled, when X>>y. Perhaps there even has to be built in flexibility in assemblage to allow for this? I dunno.

I don't know specifically about weldments, but in structural steel an example would be column plumbness. You are allowed to be out of plumb at L/500 until a maximum of 1" out or 2" in at the 20th story, then beyond the 20th story you are allowed 1/16" per story..........., and you need to always fall within those lines. This information is located in the AISC Code of Standard Practice. Maybe AWS has something similar.

A good designer will look at individual components for tolerances after the tolerance of the final assembly is established, at least where there are no written specifications or established "standards". Tolerance buildup is often overlooked and can create a problems not only on assemblies but also on individual parts with, say, multiple holes in a line. If not dimensioned properly, the buildup can be substantial.