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Given that frames are generally brazed, not welded and given that MIG welding will work on simple bike hacks what are the downsides of MIG welding these hacks?

For example, I would like to make a frame mounted front rack (rather than handlebar and/or fork mounted rack). I know for sure the welds will be uglier than a professionally brazed job, but are there other issues to be aware of?

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I'm not sure I'd take either of those as given, frankly. Far more frames are Tig welded than brazed, and that's been true for quite some time. And mig welding is too likely to damage the tubing to be really practical. –  zenbike Oct 3 '12 at 3:12
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2 Answers

up vote 7 down vote accepted

The reason that brazing is traditionally used on steel bikes is so that the temper of the chromoly tubes won't be wrecked. (The high temperatures associated with conventional welding would seriously damage the tubing.)

However, Cannondale, in their push to develop good aluminum bikes, invented the technique of rapid welding (since aluminum can't be brazed, and traditional welding worked poorly). This technique was transferred to the manufacturer of steel bikes, which is why most modern steel bikes are welded (without lugs) rather than lugged and brazed.

The question is, can you do your MIG welding rapidly enough to not destroy the temper of the tubing?

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Your results will depend on how comfortable you are with either method. Brazing tends to be the standby for most hobbyists (including me) because the equipment is easier to setup, it is cheaper, and it is harder to ruin a bicycle (though not impossible). –  WTHarper Oct 3 '12 at 0:14
    
It is worth noting that brazing--using lugs or fillets--is more labor intensive and went out of fashion in the early 90s in favor of TIG welding. You find a few very high quality producers (Waterford, Rivendell) still making brazed frames; more importantly, however, most home builders use a brazing torch. MIG welders are cheap and easy to use, though, and I'm sure many people have modified bicycles with no complications. –  WTHarper Oct 3 '12 at 0:33
    
Thanks Daniel and WTH. Given the problems with MIG I will still experiment with it knowing that I can ruin the frame. @WTHarper (or anyone else) for brazing are the small Mapp Gas + Oxygen canister set-ups workable or do you really need full sized tanks with Acetylene, etc to get something done? –  Arbalest Oct 3 '12 at 13:57
    
The setup I have I bought from a hardware store for ~$100 and uses MAPP and oxygen. For small braze-ons it is best to use silver brazing rods, and for structural modifications use bronze. You'll want some welding glasses, leather gloves, and whatever frame prep tools necessary. Also, buying a small respirator will cut down on breathing any flux fumes. Really that is it! –  WTHarper Oct 3 '12 at 14:21
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As far as I know, nowadays most bikes (not the extra cheap neither the extra fancy) are TIG welded. If I'm not mistaken, MIG is used for lower end aluminum frames.

I have made a lot of hacks and fixes to steel frames (including building a recumbent frame from an old, already cracked, MTB frame), and always used TIG weld, with excelent results, either durability and aesthetics.

The main difference between TIG and MIG is that, with MIG, the electrode and the deposit material are the same (consumable electrode), while with TIG you have a non-consumable tungsten electrode and a consumable, handheld wire rod. Not only the tungsten electrode has a sharper and more precise point, but also it is more probable to the shop to have wire rods from different (and more suitable to your bike) steels, than it is for them to have multiple large rolls of different MIG electrodes.

So, I recommend you to look for TIG welding instead of MIG welding, at least for steel.

Hope this helps!

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Thanks, TIG does sound like the way to go. I asked about MIG because don't have access to welding equipment and the first few places I found that can provide something for me to use all just offer MIG. –  Arbalest Oct 3 '12 at 13:51
    
I usually see the TIG welders I use to go very busy repairing internal-combustion engines (cars, motorcycles), radiators, air conditioning systems, industrial aluminum parts, and the like. In my area they use to advertise like "special welds" as opposed to anything that might be called "regular welds" (garage doors, maybe?). –  heltonbiker Oct 3 '12 at 13:58
    
MIG is very much a diy system because they're super easy to use (basically a welding squirt gun), but beyond that they don't offer the precision and control of a TIG weld. If you don't have access to a MIG machine, it'd be best to find a TIG welder (or learn about brazing and do it yourself! It's fun!) –  WTHarper Oct 3 '12 at 14:16
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