Yamaguchi Framebuilding School - Day 8
Another day of weld prep and brazing! The first task was finalizing the welds securing the rear triangle. Since the joints at the rear wheel are lugged, no fillet brazing will be performed there. The fillet brazing at the top of the seat stays is also quite minimal - just a few extra drops of material. The main welding event of the day was fillet brazing the bottom bracket area. Four tubes meet the BB shell in this area, leading to a lot of compound curves and contours. The BB shell is also a thick section, so a great deal of heat is needed. Fortunately, Koichi’s instruction really came together for me here, and the brazing was completed cleanly and easily (for once!)
All of that welding was done using techniques described on previous days. The afternoon task of assembling, setting up, tacking, and final brazing the fork was quite different! First, the lower, narrow end of the fork blades were trimmed to fit neatly into the wheel-mounting lugs. Then these lugs were tacked on and brazed in place. The fork blades were then both dry-fit into the fork crown, and a temporary axle was clamped in place. The fork blades were then trimmed to length, again using multiple attempts to get the fit just right. After this, all of the joints were cleaned, scoured, and fluxed in preparation for welding. The parts were all placed in a specialty fork setup jig, which was also used to dial in the fork fork offset. A clever trick is the use of two bubble levels to eliminate any twist in the assembly.
After setting up the fork jig in a horizontal orientation, it is moved to an assembly stand for welding. The fork crown is tacked to the blades in the usual fashion. Then the crown is brazed to the blades using a great deal of heat to “wick” the molten bronze down in to the joint. The normal quitting time came and went, but we kept working until the brazing was finished!
Fork welding jig, with bubble levels used to eliminate twist. The ruler in the foreground is used to dial in the correct offset between the steering tube axis and front wheel axis.
The front fork, after brazing. The bottom bracket welds are cooling in the background.
At this point, the structural welding for the bike is done! I’m really surprised by how much my skills have improved in such a short time. Tomorrow we will add a few braze-on bits like waterbottle bosses, and then begin filing and finishing the welds.
