Yamaguchi Framebuilding School - Day 5

Example of brazing the inner and outer surfaces of the tube joint in a single pass.

Day five was entirely devoted to brazing. The main structural brazing, or “root pass” on the front triangle welds was completed. This took quite a lot of time, since it’s necessary to use the torch heat to “chase” the molten filler bronze. This way, both the inside and outside of the tube can be welded in one pass.

Koichi also taught me that it is very important to use the exact same amount of brazing filler on both sides of the frame, in order to prevent a balancing issue. The brazing rods are measured and marked in segments of uniform length using a file. Each half of a normal tube-tube weld gets 100mm of bronze filler on this bike. Each half of the bottom bracket weld gets 225mm.

After the welding of the front triangle, the front fork crown was also brazed to the steering tube. These components required a lot more heat to braze than the thin frame tubes!

The brazed assemblies were allowed to cool to room temperature, and then all flux and scaling were stripped. The flux is removed by immersion in hot water. Any scale or other buildup from the brazing process is removed manually with sandpaper and a file. This is super tedious, but necessary to prevent contamination during the next steps.

Next week, we will go over these welds again with a second pass, in order to produce smooth uniform radii on the outside of the frame. This will be a purely cosmetic step - the welds made today are the structural backbone of the bicycle! But before we get to that, tomorrow is the stem-building class.