Brazing the Bottom Bracket

To assemble the front triangle (actually a quadrangle, composed of top tube, seat tube, head tube, and down tube) you need to first braze the bottom bracket.

We did the bottom bracket first because it's easier to adjust the angles of the BB. Should we need more angle, all we had to do is file down a little on the inside of the shell, close to the center of the BB. It's also easier to fit the other lugs and tubes together with the bottom bracket prepared as a constant.

We measured the tube so as not to cut off too much.

Here we are preparing the angles on the Bridgeport machine

First cut

Another view. Note all the cutting fluid. Mitering takes time. The miter doesn't so much cut as it knocks metal out of its path. This creates a lot of friction, so a miter cut is a several minute long process. My arm grew tired controlling the machine. 

Here's the final seat tube being cleaned up and prepared for brazing to the bottom bracket. 

Before brazing, the bottom bracket needs to be cleans and reamed. 

This is the finished BB. Its spikes are sharper and the insides are clean. Also, the edges are nice and square. The tube fit a little loosely, especially at the pointy parts, so I knocked the spikes closer to the tube with a brass hammer. You can see how some of the points look to be pointing in a little bit. 

With plenty of flux, we begin the brazing process.

My classmate is showing here that it's important to heat the BB more than the seat tube because it has much more mass. Using heat, my classmate draws the silver down into the bottom bracket, guaranteeing total penetration of silver in the joint. 

Another angle. As you can see, the flux is evaporating away. Luckily, he's almost done, just cleaning up.  It's important to note that all this heat deforms the bottom bracket a bit. It will need to be re-faced and the threads will need to be chased out with machining tools (Campagnolo tools in a wooden case, very nice).

With the seat tube solidly brazed in, we stick the down tube in place and mark where it sticks out too far. We'll cut this off with the Bridgeport machine. 

The down tube is just tacked in place, not permanently brazed in, just in case it needs to be modified later. 

This, obviously, is just a mock up, but you can see how we measure and check alignment. The BB is locked into the work table, then we use a surface gauge to check that the distance next to the BB is the same as that down at the end of the tube. Any time I apply heat to any joint, alignment must be checked. If needed, the tubes are bent into shape with hand power.
The little block with the holes in it is called a 123 block. Its dimensions are one by two by three inches. It's a useful tool for quickly measuring things, and also works as a level. On top of it is a slide-level, which is useful for moving an un-brazed tube up and down by little bits. 

Next, we braze together the rest of the big tubes.