Yamaguchi Framebuilding School - Day 4

Today kicked off with a few more hours of brazing practice. Definite improvement, my towers are all strong enough for the hammer test at least. Still pretty ugly though. The timing and concentration required are a lot greater than other welding techniques that I’ve practiced (MIG, TIG).

Compound miter cut into the downtube.

After welding practice, more tube chamfering and set-up. The BB shell, seat tube, top tube, and stem tube were all finished yesterday, and only the downtube remains. It is left for last, since it doesn’t directly set the lengths and angles required to fit the rider properly. However, if the frame geometry is to match the design, it must be precisely the correct length, with miters at precise angles. An additional difficulty, the seat tube and down tube interfere with one another. Therefore the downtube requires a compound miter joint - very tricky for my novice hands. This compound miter was left for last, and dialed in via several successive test fittings in the jig. I think I pulled it out and removed a hair more material at least ten times to get it right!

Front triangle setup for fitting the downtube and tacking the front triangle.

With the downtube mitered, the ‘front triangle’ of the frame is ready for tack welding. All of the tubes are removed from the jig, and the weld areas are roughened with sandpaper and cleaned with acetone. Vent holes are drilled in the seat tube (where it meets the top tube) and the stem tube (where it meets both the top tube and the down tube). Unless these vent holes are drilled the top/down tube will form a closed volume of air. When the heat of welding is applied, this air will expand and blow out the welds.

Brazing prep on the stem tube. The left side is ready for final cleaning, the right needs sanding.

After prep, flux is applied and the front triangle is tacked together. Tomorrow, we will fully braze the joints of the front triangle, and perhaps move on to the fork.

The front triangle, removed from the setup jig and ready for additional brazing.