What Determines Root Gap for TIG Butt Welds?

[houlibar]

What factor determines the root gap for making complete joint penetration TIG welds? Is it the tungsten size, filler rod size, thickness of material or what?

I'm starting on a small project and I thought I'd use it to practice CJP butt welds. Will I need to bevel the joints to make it do-able or can I get away without it?

Material: 10 ga. mild steel sheet
Tungsten: 3/32"
Filler rod on hand: 1/16", 3/32" (1/8", 5/32")
FWIW, 300 amp welding maching w/air cooled torch.

I'm thinking it should be gapped 3/32" and use 1/16" or 3/32" filler.

Appreciate your input or any related experiences.

[Diverbill45]

Scott, ............. Most of the time, when I'm doing small gauge material I don't even gap it. Just bevel it (37-1/2 degree +/-) to a knife edge, butt it together and check for any gap. If there's a gap anywhere, remove it. Make a fusion pass, then fill and cap with 1/8" rod. The base material will consume itself, on the backside, on the fusion pass. You can control the suckback of the base material with the speed and heat, on the fill and cap pass.

If you are using a gap, only make the gap as wide as the filler rod you are going to use. On small gauge, I prefer a 3/32" gap and a bevel with no land. After the tackup, the gap will close up a little. Use 3/32" filler wire on the root pass and 1/8' for fill and cap.

I would use only 3/32" tungston for the light gauge material.

Now, this is the way I was taught years ago to TIG weld pipe and it has worked for me and seems to be pretty fast.

Hope this helps.

[houlibar]

Thank you for the explaination, Bill! While reading your post I realized too, that they will not all be butt welds but Tee welds too. The reason I wanted to do pen welds on the Tee's is because it will be harder to get to the inside to make fillets. I suppose I could fit the sheet together like the figure below with the corners touching and work it that way too?

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[Diverbill45]

If you bevel it, you shouldn't have a problem with penatration. If you do, just put a little gap on it and that should work.

[pioneer]

I know this is a "cheap" trick, but on thin metal tee's i sometimes take and
and clip a piece of rod the length of the metal i'm working on and lay it in
where the pieces meet. Then i crank up the tig a little hotter and work it
fast (almost like a solder fill joint). Most of the time it makes for an strong
attractive joint. (we won't talk about the other times)

brad

[JT Metalworks]

Nothing wrong with putting that filler rod in there. Some of the books I've read recommend that practice when doing heavy plates to allow for the contraction. The rod gets squished in the joint as it cools and there's no warpage on the alignment.

[texrednek]

Bill,
I have been doing just exactly this stuff at school as my final class for my degree. I was told to run a 37 1/2degree bevel, for an 1/8" rod use a gap one rod size bigger, so a 5/32" gap and to keep the rod in the puddle, but only the very forward part. I will try your suggestion tommorrow see how I like it. Thanks