The seat tube is the priority of the main triangle, as it in important to keep it oriented and in phase to build the rest of the main frame off of. With a ti frame, the work necessary to prep the tube before it enters the fixture is a bit more involved, as there are no "ready to go" butted tubes that have the proper inner diameter for your standard sized seat posts. So, how do you get a tube that is right size for your desired seatpost and still light and responsive for the characteristics you are shooting for in the build? You use two tubes.
Let's cut the first tube, a 3/2.5 1.25x.089 ti tube that will be our seat tube top portion. I'll square up the parting tool so we can get going...
With the top insert in place, let's cut our 12 degree angle cut for the top of the seat tube...gotta be precise :)
Back on the Ti frame in the am..