Yamaguchi Framebuilding School - Day 3
Day three of the framebuilding school begins with an hour or so of brazing practice. This takes the form of building little welded “towers” on a piece of tube. The goal is to build a series of smooth, uniform pillars without surface contamination or inclusions. This requires really precise control of both the torch and filler material. Koichi makes it look easy, but I’m still pretty bad at it! Viewed up close, the results look a bit like rock formations in Moab.
Once a few pillars have been completed, they are allowed to cool for a moment, and struck sideways with a small hammer. If the welding is sound, they produce a musical tone. If it’s not, they break off and fly across the workbench. My results are about 50/50 today.
After welding practice, we move on to being assembly of the frameset. The bottom bracket shell is clamped securely to the assembly table, and the seat tube which I mitered yesterday is positioned in contact. After verifying that the miter joint geometry is correct, both components are removed from the jig and prepared for tack welding. The preparation consists of roughing the surfaces which will accept brazing material using 100-grit sandpaper, followed by a fastidious cleaning with acetone. Koichi repeatedly emphasizes the emphasis of cleaning everything thoroughly - even the rods of brazing filler material are wiped down with acetone.
After cleaning, a compound known as ‘flux’ is applied to the weld area. We use Gasflux compound ‘B,’ which the manufacturer states is composed of “Boric Acid, Sodium Tetraborate Decahydrate and Water.” Translated to english: it’s Borax paste, the same stuff that’s in laundry detergent. After the flux is brushed on to the weld area, the torch flame is used - very lightly - to evaporate the water and produce a white crust. When the full heat of the welding torch is applied, the white flux coating melts and forms a clear ceramic glaze that protects the steel from atmospheric oxygen. But care must be used - if the flux is overheated it turns black and ceases to offer oxidation protection.
After fluxing the parts and aligning them in the jig, small ‘tack’ welds are used to join the seat tube and BB shell. These little welds are strong enough to rigidly hold the frame components in correct relation to one another during assembly. Later, once all the joints have been tacked, full welds will be applied over them. The borax flux coating can be easily removed using hot water.
After the seat tube is tacked to the BB, the top tube and head tube are brought over to the assembly jig. Both ends of the top tube must be mitered. These joints must be precisely the correct distance from one another, and they must not be twisted about the axis of the tube. I mitered one end of the top tube yesterday, now the other end is cut. Then the tube is returned to the jig and the critical measurements between the seat tube and head tube are repeated. The top tube is pulled back out and the miter is adjusted, using a file, accordingly. This process was repeated three times to get the top tube just right. Now, with the BB shell, seat tube, top tube, and head tube in the jig I can almost see the shape of a bicycle emerging… Tomorrow, the downtube will be cut and installed, and the whole ‘front triangle’ of the frame will be tacked in place.
