First step is to give the new frame an identity. And not just some cold number, but a name to go with it.
Here are the materials all laid out. Tubing from left are seat stays, chain stays, down tube, seat tube, top tube, head tube. Note the markings on the main tubes. Those indicate the butt transitions where the tubes go from thick on the ends to thin in the middle. The rear dropouts and seat stay bridge are shown on the left. The lugs and BB are on the right.
The chain stays are then set up in our tube mitering fixture.
The next tube to be worked is the seat tube. This tube has a double cut. The first cut gives the shape of the inside of the BB shell.
The next cut makes room for where the down tube and seat tube overlap. This cut is at 90 degrees to the first cut and approximately 60 degrees to the down tube. The rotory table sets the ST/DT angle and the upright peg sets the tube perpendicular to the first cut.
The next tube is the top tube. The first cut is made at approximately 73 degrees.
Next, the cut length is set on the fixture. You can see here that the tube needs to be rough cut to length.
This precision turned dowel is set on the fixture and the tube is butted against it. This dowel is parallel to the cutter. This keeps the miters in perfect plane with each other and helps create a straight frame.
Next is the down tube. There are 3 cuts on the down tube. One where the tube butts against the head tube, one to give it the shape of the inside of the BB shell, and a third where the seat tube and down tube overlap. What's that you say? Didn't we take care of that with the second cut on the seat tube? Yes, but I hate looking into the shell and seeing a piece of tubing just hanging there not doing anything. So I cut it off. And then when you look into the BB shell, all you see is a nice, clean interior.
The down tube cut length is set and the peg sets the tube perpendicular to the first cut.
Next is the third cut. A pivoting dowel is again used that is parallel to the cutter.
And then the third cut is made.
Here is the completed main triangle and chain stays. The rest of the braze-ons are also laid out.