Originally Posted by
rgppgr
So how then are the chain stay, down and seat tube openings attached to the shell? I can see no seam that's been welded. Although it looks like the inside has been machined. So I guess the thread material was put in after the shell was machined then threaded.
They're not attached separately but rather formed when the piece starts as a flat sheet. A mandrel presses bulges into the sheet for the seat tube, down tube, and chain stays, then the sheet is formed into a cylinder and welded along the cylinder seam. Then the ends of the bulges are machined off to create openings for the tubes, and threads are cut into the shell before brazing.
