Step 7: Fabrication - Frame Welding
At this point, you are ready to weld the bikes together. I went over to my neighbor's work shop for this step. His knowledge of welding and metal fabrication really helped me do things right the first time.
We set the two bikes up in the jig and to be completely sure everything was square we took a long straight piece of scrap steel and clamped it to each bottom bracket. Doing this ensured that once the cranks and chains were on, they would be parallel and inline with each other.
Then we welded the rear forks to the front seat stays. In between was a thin steel piece that I cut to size to obtain more welding surface. The location of this union was almost exactly where the old rear brake mount was. Only after this first weld did we proceed to cut the rear end off of the front bike. If we would have cut the bike up before welding, it might have warped due to the excessive heat introduced by the welding.
Next we measured and cut a sturdy piece of schedule 80 pipe to span the bottom brackets. It took a fair amount of grinding and refitting to get it just right. Once we were happy with its fit, we welded it into place. My neighbor actually came up with the brilliant design to use the chain stays and their natural bends to join the seat stays and the bottom bracket support. The two chain stays were welded together resembling a "Y" and were large enough to slide right over the seat stays which added to the strength and allowed us to adjust the height before we welded it to the bottom bracket span.
Once we finished welding the frames together, I ground down the welds and used some metal filler paste to smooth it all out. Reminder: it's a good idea to ensure everything is square before each weld to avoid a misalignment.