Fillet Weld

Are you sure you are using the definition of "root gap" correctly? Or is it a case of the drill bit being ground incorrectly, producing an out of round or oversize hole?

Check www.aws.org/ for more hints......

Your root gap should not exceed your welding rod core diameter, or with tig the gap should be slightly smaller than the filler rod diameter. With mig welding you should have tig welded first to give a correct root bead.

"(Australian/New Zealand Standard) AS/NZS 1554.6:2012 Part 6: Welding stainless steels for structural purposes" states the following:

"5.7.3 Alignment of fillet welds and incomplete penetration butt welds

Except for full contact joints, parts to be joined by fillet welds or by incomplete penetration butt welds parallel to the length of the member shall be brought into as close contact as practicable.

The gap between parts shall normally not exceed 5 mm except in cases involving rolled shapes and plates 75 mm or greater in thickness if, after straightening and on assembly, the gap cannot be closed sufficiently to comply with the requirement. In such cases, a maximum gap of 8 mm is acceptable provided that a sealing weld or suitable backing material is used to prevent burn through.

If the separation is 1.5 mm or greater, the size of the fillet weld shall be increased by the amount of the separation or the fabricator shall demonstrate that the required design throat thickness has been obtained."

Therefore, on the basis of this, the rule of thumb is to bring into as close contact as practicable.

"AS 1101.3-2005 Graphical symbols for general engineering Part 3: Welding and non-destructive Examination" shows a standard welding symbol (I believe it is similar to US symbols I've seen) which allows one to specify the design throat thickness (DTT) (along with the weld size).

It looks like you simply have to specify the tolerances fairly tightly so you minimise the gap to get the DTT you want.

Cheers

Malcolm

Thanks for the responses. The wall thickness of my tube to be welded is less than 1 mm, so my weld leg on a 45 degree fillet would also be less than 1 mm, this led me to determine that a V or W letter drill would accomodate the tolerance of the tube OD as well as the tolerance of of the drilled hole itself based on general tolerances for drill bits of that size. A V drill would give me about a 10% probabillity that the 2 parts would not mate due to tolerance overlap (that is to say, smallest holes vs largest tubes), but I was worried that a W drill might give me too much of a gap for largest hole/smallest tube, especially when keeping the parts concentric to eachother becomes more of an issue. Of course not knowing what an acceptable amount of penetration relative to the leg length is makes it difficult to determine the probabillity of "failure" when using the larger drill size.