How I build a bike, part 3

Yikes, It’s been awhile. We left off in the last installment with all of the tubes coped and mocked up in the fixture. There is still some work to do before we tack the frame though. One thing I try to do before tacking the frame is braze on as many..errr…braze-ons as possible. Why? Well, when you heat things, they expand. Now, picture a nicely made frame. It’s all aligned and happy. Then, you go in with a torch and heat up small, localized sections of the tubes with abandon. Things will get squirely on you. If you add the braze-ons beforehand, the tubes have done their thing and you can move on to tacking and brazing the frame straight without worry of undoing all of your hard work later. That was a run on sentence, wasn’t it?

First up are the brake bosses. The frame is in the jig and my handy Henry James jig has an integrate bridge and boss jig. The bosses have been machined to fit the stay very tightly.

Next, the parts are cleaned and placed back in the jig. Flux is then added (the grey stuff). Flux is an acid that becomes reactive at around 900 degrees F. It gobbles up all of the oxides on the surface of the steel and promotes the silver to “wet out” on the surface of the steel. This is a special flux I get from Fred Parr. It resists burning very well and is very easy to rinse off after the joint has cooled.

The joint is heated, the water in the flux boils off and the flux becomes powdery. Hotter yet and the flux liquifies and begins to work its magic. At about 1150F, the silver filler rod is added to the joint. If all goes well, you will be left with a joint that appears to be covered in ice. Good heat control = no burnt flux.

Next come the two cable guides on the seat stays. This is a cross bike with top tube cable routing, so the rear deraileur cable runs along the right seat stay.

Fluxed…

Brazed…



The front derailleur cable is run down the backside of the seat tube, around a pulley and back up to the derailleur. This is the boss for the pulley.

Fluxed…

Brazed…

Next up are the triple stops on the top tube. A witness line is used to be sure the stops are on the top dead center of the top tube.

Fluxed…

Well, you get the idea…



So, if all has gone well in your brazing and you haven’t gurnt your flux, a quick dunk in hot water is all that is needed to remove the flux.

Notice the areas of clean steel where the flux was and the discolored areas just outside. If the flux had not been there, the silver would not have adheared to the steel.





Next up, final tube prep and tacking…
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