Originally Posted by
bulgie
Do you use a gas lens? If no, have you tried one and decided against it? I couldn't imagine going back after using one.
It's a must with Ti, but pretty useful for steel too. I welded more Ti than steel, and only really welded steel frames after I was already doing Ti, so I just used the same torch and the same gas lens for steel, and I liked it. I'd recommend the "large" lens and big cup to match, then you can get away with shocking amounts of stickout.
For those who don't know, a gas lens makes the argon flow
laminar, which means no turbulence. The turbulence you get without a lens sucks in air from the surroundings. This video from Miller shows it pretty well:
https://youtu.be/hGasy7wnALA Notice how the length of the laminar flow region gets shorter as the gas flow (cfh) goes up, which can be counter-intuitive. So if you're getting air coloring your weld, it can help to turn the argon flow
down. Note also how the length of the laminar flow region goes up as the cup size (diameter and length) go up, so try the biggest cup you think you can fit in there..
Even with the lens, there were still places where I'd build a foil dam for Ti, but under the DT/HT joint wasn't one of them. The lens seemed adequate to me for that spot, even with Ti. I also welded with a plug ("heat sink") in the end of the HT, but that was more for back-purging, not really a heat-sink per se. Same as with the gas lens, I started doing that plug and back-purge thing for Ti, and just kept doing it the same way for steel because I liked the results.
Past-tense because I haven't welded a bike in over 20 years, so my advice is old and maybe obsolete.
Mark B in Seattle
I am using a gas lens with a No 8 cup, and about 10lpm which is about 20cfh, but I go to 12lpm if I'm in one of these tricky spots with extra stickout needed. I've tried a much bigger cup with more stickout but I found the big cup gets in the way and you need even more stickout... so kind of a vicious circle. Maybe my big cup is
too big. You also always need some pre-flow with that setup because there's such a big space you're trying to fill up with gas.
I used the foil in three places: bottom of DT/HT, where the ST joins the DT, and CS to BB shell. ST/DT I probably could have got away without it and possibly DT/HT as well but I dunno... it was quite a lot of stickout. Never tried backpurging as not attempted stainless or Ti, but I know some people do use it for CrMo as well.
How did you manage the back of the SS to ST on a road-style frame with a horizontal TT and quite a long ST with is about 30mm diameter at the top? That's like a 57 degree angle and with the two stays very close together. I've found it's basically impossible to reach inside there, but I can TIG braze it. I make a bigger fillet that's sort of bridging across a bit (which means I can reach it) and then I can zap the fillet with the torch and it kind of falls in. Actually seems to work quite well.