Bronze Welding for Lugless Bicycle Frames

By Mark Stonich

Several years ago, I wrote an article about flux and lugless fillet construction for The Framebuilders Gazette. I mention “Brass” filler metal in the article, which has caused confusion for at least 2 of you. I understand that most manufacturers have since added a small amount of Tin to what had been known as “Naval Brass”, so it is now known as “Naval Bronze”

Since the article was written for more experienced builders, I’ll start with a few;

Suggestions For Beginners

1. Flux fumes are nasty, I ventilate and use a 3M welding mask (Abt. $7)

2. Before brazing a joint on your frame, try to do a practice joint as similar as possible. Especially try to use the same wall thicknesses. 0.035″ wall tubing will overheat much faster than 0.049″. 0.035″ or even 0.024″ aren’t that tough, if you’ve practiced with these thicknesses. Aircraft grade chrome-moly is designed to be welded, so a little overheating isn’t that dangerous, but warping is increased, and if you char the flux it won’t work. For brass, bronze or silver, it doesn’t matter if you practice on mild steel, just so the wall thickness is the same.

3. The flame I use is nothing like a welding flame, There is a slight excess of acetylene, and a large tip is used to reduce gas velocities. The idea here is to get a gentle, quiet but large flame to warm up a large area evenly without localized hot spots. The tip is usually held 2-4″ (50-100mm) from the joint. I flux coat the outside of joints to be bronze welded. I use a Smith’s #205 tip, 0.048″ (1.2mm) i.d., on tubing down to 0.028″ (0.7mm) wall thickness. I use the same tip to silver braze. Try it!

4. The large flame, plus pre-heating the joint, leads to less concentration of heat and reduces pitting, which is caused when the filler is locally over heated. I’ve also had good luck moving the torch from side to side to put more of the heat on the tubes and less on the filler. Pits in fillets occur when zink in the filler alloy is boiled off by overheating. I’ve seen pits in old French tandems about 5mm diameter.

I believe 99% of all pits occur when you are re-melting solidified filler. This is most true when you get all the way around to the start of the joint, where the filler has had time to cool. To reduce pitting, take plenty of time to bring the joint back up to temperature before trying to restart a puddle in the filler.

5. Crashed bikes are a good source of tubing for these practice joints. I’ve been known to use tubes from wrecked bikes in even my nicest frames. My stoker is the neighborhood recycling czar, so it seems logical. Lots of energy is spent producing steel, why not reuse it. Seat and chain stays especially.

6. Building forks is really a pain, if your design allows it, just add rake to an existing fork for an LWB recumbent, or shorten one for an SWB.

7. Always use silver for braze ons. Cadmium free Harris 1200 is a good choice.

8. Illuminate well. The brighter your work area, the less your vision will be obscurred by the flame/flux haze. I use a lighter #3 lens in my goggles when brazing, and sunglasses when silver soldering

9. Cosmetics aren’t usually that important on HPVs but a good looking joint is usually a strong one.

10. The very best steels are only about 6 times as strong as brazing alloys, so the root depth of a fillet only needs to be 6 times the wall thickness of the tube.

11. Nickel silver is a bit stronger than the other brazing alloys. So, with thinwall tubing, a skillful welder, like Mark Zeh, can get away with fillets only slightly larger than TIG welds.

bicyle frame bronze welding

The following is exerpted and updated from the article. The flux I mention is now more commonly used by framebuilders. The article assumes that you know how to use a torch, which you can learn at Vo-Tech school night classes. Many years ago 12 members of our club signed up for a welding class that had only 12 openings. They showed the teacher the original article so he would know what they wanted to do. He showed them how to run the torches, and 12 weeks later 14 bikes had been built. Our Exalted President, Dave “weld first and take measurements later” Krafft built 3 of them. The class was on Tuesday, so every Monday night, everyone was cutting and filing like crazy.

Flux and lugless frame construction.

By Mark Stonich 1984
I usually build lugless as most of my frames are a little odd, (Mixtes, Tandems or HPVs). I have been using a flux designed for use with Nickle-Silver brazing rod. I use Welco #17, but Alstate #11 and Unibraze #111 are supposedly equivalent. You nay have to have your welding supplier order it, as it’s not too common. Your supplier may recommend trying Harris Hi-Temp Silver soldering flux. It can be used in an emergency, but is definitely “Plan B”.

Nickel-Silver flux has a couple advantages for the framebuilder over powder brazing flux.

1. It is a water soluble paste, similar to silver solder flux. As such, it can be removed by soaking in water, a little help from a small stainless steel brush speeds things up. As this flux requires no mechanical removal, the joint can be left as brazed.

2. You paint a thin coat of it on your rod, after removing oxides from the rod with scotchbrite. Therefore, you can work continuously, instead of dipping into powder flux, so your puddle doesn’t cool down and have to be reheated as often.

The flux that comes on flux coated rods is harder to remove, and is too thick. You get so much flux buildup when you are working that you can’t see the shape of your fillet. If you use pre-coated flux, scrape 1/3 to 1/2 of it off.

I use only Low Fuming Bronze filler rod (usually 3/32″) for fillets. I know some builders have such good temperature control that they never get pitting when using Brass, but I’m not one of them. (BTW I always buy LFB in 3/32″ rod and Naval Brass (or Naval Bronze) in 1/16″ so I never mix them up.)

Other use Nickle-Silver rod to get the maximum strength from smaller fillets. I find it a little tricky to get a nice looking joint with Nickle-Silver, probably because I only build about one frame a year and don’t get that much practice. The bronze puddle seems to be more “syrupy” and easier to control.

bronze welding bike frame

Regarding Nickle Silver, Mark Zeh wrote me;
” When I used to do a lot of fillet bikes, I developed a nasty skin rash. The filler rod I was using was a nickel silver rod. Cutting to the chase, it turns out that nickel is a contact toxicant and you can become sensitized to it over time. Now I always wear a long-sleeved shirt and long pants when I’m Dyna-filing fillets to keep the dust off of my skin as much as possible. I wear the disposable 3M weld fume masks during welding and brazing, and the 3M nuisance dustmasks during finishing. ”

To use this flux, I clean the joint, with sandpaper, scotchbrite or SnoBowl, and paint on a thin coat of flux.

Since writing the above, I’ve been hearing great things about fillet brazing using two Gasflux products. C-04 rod, which melts at the same temperature (about 1620 F) as other brasses, but once molten it forms a very liquid puddle with a high surface tension. And, their Type “B” Blue Paste Flux. You can get these from; Hank Folson of Henry James Bicycles, Inc. Hank’s prices seem to be no more than you would pay in a welding shop for LFB and Nickel Silver flux.

At highly stressed locations, like head tubes, I try to draw some bronze into the interior of the joint for a little extra strength. Especially on the sides of joints, where it is often impossible to build a thick fillet.

I always work somewhat uphill when building bronze fillets. The cooling backside of the puddle acts as a shelf to support the more liquid section under the torch.

I (usually) get a nice looking concave fillet with a smooth transition from tube to fillet. I think 3 things help this happen.

1. With the flux on the tubes before any heat is applied, fewer oxides form on the tubes during pre-heating.

2. I apply the rod at the sides of the puddle instead of the center which promotes wetting of the tubes.

3. Remember to concentrate on heating the tubes, not the bronze. Ideally you want the puddle just above the melting point and the surrounding area just below, so you need to heat a large area to prevent a steep temperature gradient.

I’m certainly no pro at this, but at least I’ve had no joint failures. Yet!

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