Well the quetion say it all.

At the beging of the summer I attened a welding class at evening college. We cover oxy acyetaline welding (prob spelt wrong sorry), brazing and mig welding. From what I under stood from the lecturer brazing is mainly used on light stress joints because you are not getting the same deep penertation and fusing you get with welding. You have a beefed up solder joint. While I can see the point that you can braze together two diffrent materials in an homogonse frame why would you braze over welding?

Also when frames are welded they are alway seem to be tig welded. Why is this? I have spoken to several welders including the lecturer who tell me that once set up correctly you can mig weld steel, alu, stainless just as well as a tig welder and once set up a mig is much quicker. The only possiblty that they hinted at was that was that tig welding tends to be a prittier weld than a mig weld.

I am not trying to say I know better than all of you guys who have been doing this for years (as I after doing the course I realise how much there is to know) I would just like to understand the reaoning behind this. Could it to be down to wall thickness?

T

Bicycle tubing is very thin and easy to melt through or otherwise damage with welding temps. A brazed lug joint is actually very strong. The tube will almost always fail before a lugged joint will when a bicycle frame is overstressed.

Thanks San Rensho super quick reply. Only one thing I don't quite get. When welding we use a netral flame and occationaly you would knock the oxygen and go to a oxidising flame. This resulted in moor heat input and hence resulted in bowling a whole in the metal Iwas welding. We also used a oxidiing flame when brazing. So how come with welding a bike frame a nutalising fram would be too hot but an oxidising flame (which from my limited experience seemed to be hotter) would not be too hot?

After thought: Could this be because when you weld you are pooling the the metal parts you are welding together and dipping the filler rod into the pool but brazing you are only getting them cherry red and and then melting the filler rod into the joint allowing the cappilory action to pull the filler into place?

Any thought on the tig vs mig anyone?

Well, for brazing it seems that most like to use a neutral to carburizing flame for building fillets and a carburizing flame for lug brazing- I've never tried it, but from what I've been told, an oxidizing flame is a big no no for bike brazing because you don't want to add ummm....oxides.
I've done some mig ang tig and I think you are right on one count, tig is used because it's prettier- but it's prettier because it is so much more controllable you need to be able to control the pulse to make those nice welds and to keep from destroying the thin tubes used for bikes. Also, (and I don't know this for sure) I don't think they make mig welders with the inverters you need to do aluminum.

There are steel bicycle tubes manufactured now that can be welded; in fact some of them actually become stronger after welding.

Traditionally frames have always been brazed not because a weld would fail but because the tube would fail right next to the weld due to the tube being very thin. Many bicycle tubes are heat treated to strengthen them. The trick in brazing properly is to apply enough heat to make the lugged joint but not to get the tubes too hot over a large area; retaining as much of the tube's inbuilt strength as possible.

A properly brazed lugged joint is tremendously strong and a lug spreads the stresses over a larger area, not pinpointed in one place as with a weld. This is also the reason lugs are cut into curved or other fancy shapes, and not just cut square like a pipe fitting. A square edge would create a stress point and the tube would likely fail at that point.

To sum up; yes a welded joint may be stronger, but in any structure there is no point in making a joint far stronger than the material you are joining. And if the structure is stressed enough the material will fail before the joint.

I don't mean to sound contrary to Dave, or any others, but my opinion is that the benifits of brazing vs. welding, and/or lugs vs. lug-less has been much over blown.

The way the tubes are joined is not all that important really. Lugs are cool, all my bikes have lugs, but they don't build into a better frame than TIG or fillet brazed. The important thing is to have a good quality joint - pick your own joining method. Even air-hardening alloys, like TT Platinum, can be brazed at low temp with no real concern - check Henry James' web site for evidence on this fact.

One exception to this thought is Oxy-Acc welding; my understanding is that this is not a prefered method on bicycle tubing - too much heat.

I think you're confusing two different things, AC/DC and inverters/transformers. AC is wat you need to TIG weld aluminum. inverter/transformer has to do with how the welding maching steps up (or down) the input power. MIG welding aluminum is very popular, but not in the bicycle industry.

"I don't mean to sound contrary to Dave, or any others, but my opinion is that the benifits of brazing vs. welding, and/or lugs vs. lug-less has been much over blown."

I totaly agree with that coment. There are tons of welded frames including many of the top racing brands where metal is still used. One should separate what people fear will happen (unless it's nearly impossible to avoid) from what happens when proper practices are obsered.

Many bikes use metal tubing about the same thickness at the joints as with aircraft structures. Aircrafts require certification, you can't just hang out a shingle and pass them off to willing cutomers. The approved joinery process is welding either gas or TIG. You will get laughed out of town if you try to pass off lugs or brazed structures. I'm not knocking the latter that obviously work, but any suggestion that welding is a lessor process when properly performed is ridiculous.

Another principle is that the hotter process causes less damage than the low heat process. Obviously you have to shield a welding process or the metal will burn, but as far what occurs in a shielded process with high heat, the result is a smaller haz not a larger one. The fast high heat approach does not allow time for the heat to move out of the haz if the welder is competant. While the time needed to bring a lug up to speed does allow heat to creep out further. It's lower heat, but as far as drawing the hardness is concerned it's more that sufficient. The point is that the chromo tubbing we mostly use is not hardened. The discussion gets a lot more complex if you are dealing with hardened steel, but to get things up to snuff you would have to post heat treat amd that woud melt out your bronze anyway.

I also don't buy the happy talk that air hardeing metals are a good thing. Harder is not necesarilly better and the process may or not be controlled well enough to allow a better result from TIG welding. The strength of bikes comes largely from proper sizing of the structure and the thicker material in the ends of the tubes. It seems to be fairly forgiving judging by the savage extremes people impose on bikes, and the comparitively low failure rate of any type of assembly. So just as tubes survive hardening they also survive softening, whatever happens wherever along the haz.

The thing that I see determining the choice is basically what people start with. If you only had OA torch you could make forks with lugs and you could either lug the frame, or braze it. If you Tig you can weld some fork designs and obviously the frame. It talkes both types of gear to do everything freely. So by the time people get to the point where they might want to add the other for some part of the bike, they seem just as likely to have developed some kind of affiliation to the craft they started with . The melted look of brazing, the pearls of TIG, or the graphic potential of lugs. Then they cut themselves off from other methods or pick them up as they prefer.

O rly?
Back to the google board you go.

Wow lot of info here. thanks for the reply guys. Well when I have a house with a workshop a frame will be on my list of things to build, weled or brazed I shall see. thanks for the info again.

The "Rivendell Reader" published an article comparing the three methods. A skilled builder built three bikes using lugs, and the two "common" lugless methods. It was no surprise to me that the lugged frame bike was the strongest of the three. What surprised me was that the weight differences in the frames was trivial. I had always assumed that using lugs added significant weight to a frame.

Although the tests of the three frames proved that the lugged frame was the strongest of the three, the results also showed that each frame was WAAY strong enough for everyday riding. But, of course, the lugged frame was the "pretty" one.

i think mig will also introduce more impurities into the weld than a properly back purged tig weld