Below we have the beginning of a steel race frame. Perhaps one of the most important pieces of the whole is the seat tube, as it is truly the spine that everything else is built off of. I thought I'd walk you through my process of creating the seat tube...
I begin with a True Temper OX Platinum single tapered tube. The tube is mic'd and marked showing the end of the thick walled section and the end of the taper to the thin wall section of the tube.
The thicker, shorter section of the tube is located at the bottom bracket. The thin section for the top of the tube does not have enough mass to support the heat from welding the top and the seat stay tubes, let alone the dynamic leverage from the rider's weight passing through the seat post. To create a more stable structure, I take a section of 4130 heavy walled 1.25" OD tubing and bore it out to fit my anticipated 27.2 mm seat post.
I then flip the tubing around and turn down a shoulder in the piece that will then be pressed into the seat tube and circumferential welded. This thicker walled piece will resist deforming during welding and offer the physical support necessary for the rider's weight.
The sleeve is cleaned and pressed into the seat tube with a little mechanical force. It is then welding in place using my tubing roller and a steady hand.
Once welded, off to the cold saw we go to cut a 12 degree miter into the top of the post.
A nice clean cut, the piece then goes back to the lathe and is polished up inside and out, double checking the ID to insure there has been no shrinkage during welding.
This seat tube is then aligned parallel to the table and rotated against a 90 degree machine plate to determine the apex of the cut. This apex will be marked and becomes the center line for our binder.
The binder is held in place using my high quality, precision .99 cent spring clamp. Fantastic!
Using his incredible chin controlled welding powers, the binder is tacked in place on both sides. Frankie places a tack on opposite sides of the slot to equalize any rotation/pulling from the application of heat. Notice he's so good, he can do it in his sleep!
Once the binder is tacked in place, the seat post heads over to the mill to have the bottom bracket miter cut . I use the binder as our datum point to properly orient the tube so our miter is 90 degrees around and spot on.
The seat tube is then re-fixtured and the water bottle mount holes placed. Here's a shot of me double checking the proper rotation/positioning of the tube using the bb miter and a spare shell, insuring it's square to the table's surface.
The holes are drilled, cleaned up, and ready for flux and heat to set our bosses in place with some silver and a little patience.
I braze the binder in place at the same time, flowing in a little 45% silver and then ready the binder and tube for slotting. A center drill is used to create our terminal relief hole, then a slitting saw makes a pass, creating our binder slot.
A little filing and clean up later, our seat post is ready to be the spine of the frame.