Yesterday, I spent about two hours filing the fork crown. Talbot recommends doing some rough finishing on the crown before brazing the steerer to it in order to reduce the amount of mass that needs to be heated, making it easier to heat the whole joint with a simple hardware store torch. I removed the rough casting finish off the crown, leaving the bulk of it shiny and relatively polished. In retrospect, I would have liked to measure the weight of the crown before and after the filing operation; honestly, I'm not sure I removed more than a few grams.
After work today, I assembled my equipment on our apartment balcony:
Using mineral spirits and a rag, I cleaned the steerer tube and the fork thoroughly. This is an important step to ensure the braze is not contaminated; metal is generally treated with a preservative before you, the user, receive it (I believe it's mineral oil-based), and must be removed. Cleaning the inside of the tube was difficult, but I was able to get the first inch or so by stuffing the rag into the tube with my finger.
Here's what the parts looked like after cleaning:
The cleaning really highlighted the minor marking and scratching leftover from using the emory paper on the parts. It also revealed some imperfections that I missed when I was doing the finishing work earlier (a couple of nicks, likely from the hacksaw). Smoothness of the parts is important to ensure a strong joint, and I think that the two parts were finished enough to braze them together.
Here are the parts and the brazing materials:
The roll of silver solder is worth about $125. the flux cost about $10.
I clamped the steerer tube into the workbench and applied flux to the joint end:
White flux is used for standard ferrous materials; there is also a black flux that is used for stainless steel. I coated the inside and out with a generous amount of flux. I've seen some framebuilders' websites that show significantly more flux than what I used, but I honestly didn't see the point - my impression was that it was just going to make a mess in places that weren't being brazed.
Applying flux to the inside and outside of the crown, and fitting the two pieces together:
As I mentioned before, the crown has small tabs to keep the steerer in place. If yours doesn't, and the crown slides on the steerer, you can pin the two together by drilling a hole through and through (notice the small round dimple) and holding everything together with a steel nail. You can then drill it out later when you make the brake hole. The whole assembly is clamped in this orientation to allow gravity to help the solder flow through the joint.
Now - deep breath, fire up the torch, and get heating. Some observations:
The flux started running literally as soon as the fire hit it. It dripped down the steerer and on to the workbench, which I did not expect at all. It didn't seem to be a problem, and it was pretty easy to remove from the workbench.
I expected a bit of difficulty getting the parts heated, but within a few minutes I noticed a dull red glow coming from inside the steerer tube. Encouraging! As the joint continued to heat, the flux started to bubble and brown, almost looking like a marshmallow over a fire.
I tried to keep the torch moving around the whole circumference of the tube during the initial heating; as things began to glow, I started concentrating on one half of the tube. Every so often I would test the joint with the silver solder, and all of a sudden it started to melt into the joint. Pushing the solder slowly in to the joint, I kept an eye on the bottom of the joint to watch for the solder to come out the other end.
After the first half of the joint was done, I had noticed the other half got a little cold; it took another couple of minutes to get it back up to temperature. Also, I had seen some minor blacking on the steerer tube below the joint. It was difficult to tell if it was the metal discoloring or the flux overheating.
There was one small section that was very difficult to tell if solder had penetrated all the way through, but I kept pushing more in and didn't notice anything further coming out the bottom (I think it was spilling into the inside of the tube). Just like that, it was over.
The finished joint, still hot:
The dripped flux is on the opposite side of the tube. I let it cool in the bench for a few minutes while I put stuff away and filled up a bucket with warm water. Using a stiff wire brush and the water, I cleaned the remaining flux off the assembly:
And here is what the assembly looks like with all the flux and soot finally removed:
You can see the point of least penetration in the center of the photograph; luckily, it managed to come all the way through! I think the joint should be pretty strong.
Another interesting point - the crown blackened where there was no flux present. The steerer remained relatively unchanged, except for a minor discoloration in one spot that I removed with the brush. The crown also started oxidizing fairly quickly after I dried it off; After the blades are attached, I'll have to go over the whole assembly with more emory paper to remove the rust before painting.
The next step before this is 100% complete is to file the joint down again to remove the excess solder. After that, on to the fork blades. I still haven't settled on a method to bend the blades; a visit to an industrial supply store disabused me of the idea of using a conduit bender. I'm in the process of calling local frame builders to see if any of them have the capability to bend tubing and who can lend me a hand.
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