The First Braze

Slightly hungover this morning i headed into my workshop with a coffee to commence the task that i had been dreading ever since i envisioned this project; brazing. After a few weeks of sporadic practicing using a bronze brazing rod i felt that although i was resonably proficient at the skill i was concious that i was always having to heat the tube to a temperature that was difficult to control. I therefore made the decision last week to use silver. Silver has several advantages over bronze, the pricible two are that it flows better, especially into a tightly fitting lug and it has a much lower melting point. The lower melting point is useful in bicycle frame construction becasue the structure of cromoly changes at high temperature making it harder but also more brittle, using silver as a brazing alloy means that the temperature at which this process happens is way above the temperature required to melt the silver. The only disadvantage of using silver is the price, where as a 5kg tube containg around 50 rods of bronze costs £25.00, silver is sold by the rod priced at £14.00 each! Peace of mind does indeed come at a hefty price.

After at least half an hour of searching for my ear protection which i had fastidiously hidden from my self i set about producing my jig. After cutting 6 no. 100mm lengths of box secition i used a plasma cutter to notch out a cradle in each of them, placed them on my brazing table (note modified for extra support realising that if i were to stand inside of the frame whilst brazing it is so small that i would have to direct the 5000 degree C flame at my nuts). This done i dissassembled the the front triangle and prepped it for brazing. The first task was to drill a vent hole in the top of the head tube where the top tube buts against it. The top tube is the only sealed member, if the vent hole was not drilled the expanding gasses within the tube would prevent the flow of silver into the lug due to the pressure. Then the tubes and lugs have to be thoroughly cleaned using a degreaser, coated in a thick paste of flux and then reassembeld before placing in the jig:
Close up of a fluxed lug:
After this is done an angle finder is used to make sure that all the tubes are in the correct alignment:


Once this is done and the jig has been adjusted the brazing can begin, i will spare the details of how this is done (unless anyone requests the info?!) but it is a delicate process involving relitively precise temperature control whilts using a very powerful Oxy-Acetaline flame.

Brazing of the front triangle all done, below is an example of a finished braze, the discoloration is the flux which will be brushed off with a brass wire wheel when i go and buy one tomorrow!
Brazing has gone better than expected and generally pretty pleased with the completion of phase one of the build.