Originally Posted by PaPa

Against my better judgment, I'm gonna respond. Please bare-in-mind, that this repetitive "welding thinwall with MIG" Q&A is starting to grind on me a little - especially when those asking claim, "no problems welding thin wall", but turn right around and litter forums by asking if they can.

To be crystal clear, it isn't usually the MIG, in-and-of-itself that's the problem - it is the human under the hood that ultimately bares the responsibility of success. There is exceptions of course, but generally speaking, there's nothing wrong with using a quality MIG on bike frames... providing you can consistently prevail the elevated complexities of joining thin wall tubing + compound geometries + all position welding.... all at the same time. So in lieu of hammering a keyboard... why not go try it?

Originally Posted by Nessism

My first custom build frame was made by a guy named John Waite that MIG'd lots of frames together. This was in the early 90's and he typically used SLX which is .9 on the butted ends. As long as you don't go too thin on the tubes I'm sure you can build yourself a perfectly serviceable frame with MIG.

Originally Posted by Peterpan1

One problem you can't escape with MIG is cold starts. With MIG a lot of stuff is happening at the same time and is automated, which is why it is so fast. So the arc forms off the filler metal which immediately is advanced into the forming pool. If one is repairing sheet metal that is largely aesthetic aero in nature, then the fact that there is a tiny portion of the weld that is less that perfect is unlikely to be an issue. With high end structural work, like bikes and airframes, this isn't acceptable. With GAS welding or TIG, the torch heats the puddle to max penetration depth, and full liquid state before the filler is advanced into the puddle. So this problem with MIG does not exist.

The proper sequence with bike welding is likely to involve 4 tacks and 4 starts, that means 8 areas in the tube where the welding heat may not be perfect (and there is little likelihood that it is perfect as the process progresses either, since there is no independent control throughout). Overwelding the tacks is a whole other nightmare...

With MIG robotic welding of bikes, I can imagine that they might program the process to control the heat, and they could theoretically overcome the start-up heat, or minimize the starts and stops required. I don't know how they actually do it but process control is at least theoretically possible.

If you are flux shielding your MIG welds, not gas shielding them, then there are a series of other problems to deal with.

Obviously MIG is not suitable to custom bike making, but whether one can skate by on a home project for the fun of it is a different mater.

Originally Posted by unterhausen

Distortion is a real problem with any process. But the real issue about asking this question in this forum is that nobody that posts here on a regular basis uses MIG on bike frames. So we're all just speculating. My way of thinking is that if there were any merit to using MIG on bike frames, people would be doing it. But I'll be the first to admit joining methods are subject to non-technical prejudices.