I was shocked by the progress of the frame when I arrived this morning. I remembered it looking and feeling like a bicycle frame when I packed it away for the weekend, but I couldn't be prepared for how it felt to see something that I had made resembling what I wanted it to be. These feelings would prove fleeting, though, since a great deal of work remains.
I assumed that we would tack the rear end of frame to the front triangle before doing the complete brazing on any of the joints. Instead, we began to work the front triangle to completion today, and I continued honing the different techniques for the lugs and the fillets. The process can be very frustrating at times, and incredibly rewarding when done right. For the lugged areas, we use what can best be described as a small-medium sized flame on the torch. If you use too much heat, you can scorch the flux and make it useless (or even inhibit the flow of your braze) or even permanently damage the tubing. However, too little heat and it becomes nearly impossible to keep all the surfaces warm long enough for the braze to penetrate the depth of the joint. I am using 50N nickel-silver brazing alloy for the lugs, which is a high tensile-strength silver-based alloy with a low melting temperature, good for lug work. With tools and supplies in hand, I went to work on the bottom bracket shell. The premise is the same as that of brazing the steer tube onto the fork crown, but the spaces to fill are a bit more intricate, since you're working around miters and the peaks and valleys of the lug shape. Using your torch, you guide the silver around the inner space of the lug - not too much, not too little - by heating the surface. Once the surfaces are hot, you make a little puddle of silver at the seam, then direct your torch where you want the silver to travel. POOF! The puddle disappears. You do it again, and again, until silver peaks out from the opposite side. When you've overheated things and toasted the lugs to the point of carbonization, you step back, let the frame cool, and tackle it with a file. Knock off the blackened and discolored bits, wipe it down with acetone, cover with appropriate flux, and repeat.
The picture is from about half way through the bottom bracket shell work. About another 45 minutes went into finishing it, and it cleaned up a lot. I'll take a picture of the final product tomorrow, when it comes out of the soak tub and looks shiny.
Next, I went to work on the fillet brazing for the head tube. In my case, the fillet brazing is done with a medium-high torch strength and bronze brazing alloy. Since there's no lug, the brazing becomes a different manner of drawing the alloy into a crevice. For fillet brazing, you have to be a bit more careful, since you lose the extra material of the cast lug. You apply direct heat to the two tubes, then add a bunch of the bronze alloy to the heated spot. Once the puddle forms, you then flip the torch to the inside of the tubing, drawing the alloy into the seam between your tubes. This builds up a bond between the two tubes from within, which provides much greater strength than a solely external fillet. You do the internal brazing all the way around your mitered seam, until no more alloy will enter and the puddle starts staying put. At this point, external filleting becomes much, much easier. But, as before, there's always a lot of clean up to do, and the bronze-appropriate flux dries as hard as glass. It's not a ton of fun to use the big, honkin' 12" file to grind off tiny kernels of char, but it has to get done.
Right now, the frame is soaking in some warm water overnight, which dissolves the flux. From there you can see if you need to do more work on a certain area.
Also, check out the before and after pictures of the fork crown! Hot water baths and wire brushes do wonders. Swanksville!
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