We didn't weld the joints, they are brazed, in lugs. Welding is waay too complicated to do on something like this, unless you have a lot of experience, and the ability to do proper controlled heat pre- and post-treatment. Frames by Klien and Cannondale are all robo-welded for consistency.
The fittings you see on bike frames at the joints of the tubes are called "lugs". Lugs on bike frames are there to only hold the tubes into position, they are too soft and flexible to really take much stress. Good bike frame tubes (like the Reynolds that we used) are called "double-butted", which means they are thicker at the ends, they are made by piercing a solid rod with a mandrel, no seams in the tube. The change in thickness happens inside the tube, the outside diameter is constant. Think about how 2 tubes intersect at an odd angle, say 72deg (the angle of my head and seat tubes). If you're building a roll cage, you only need to get it close, then the MIG wire fills in the gaps, and it works because usually the wire filler material is stronger than the base metal (the tubes). Well, with bike frames, with a brazed joint, that mitered cut of the tube must be very tight, no gaps, as brazing can't span a large gap.
So, what we did was carefully draw out each joint (again, no CAD at the time), then we created templates in paper to wrap around the tube, to scribe a cut line. We cut it with a hand hacksaw and a fine blade, then spent a lot of time filing the profile of the cut so it was a very tight fit against the joining tube. Lots of trial and error work, very tedious. But it makes a much stronger joint.
Folks that do this a lot will have a lathe setup with cutters that are the same precise size as the joining tube, they use the cutters to slice the mitered tube off, with a perfect miter. Much faster, obviously. We did it all with cheap hand tools, we didn't even have a drill press.
It's also important to lightly sand off the mill scale from the tubes, so the flux can stick to the tube.
Our first brazed joints were a mess, we were so obsessed with making sure to fill the lug with brazing rod material that we got flux and brazing globs all over the place. Think of the first time you ever soldered a copper pipe joint, you'll know what I mean. We soon learned that the flux was a huge PITA to file off, it was like glass. We got better with each joint, by the end of the 4th bike there was very little filing to do. Kinda like mudding drywall, the more you do it, the less sanding at the end to get it to look nice.
The kit we bought only provided the tubes and lugs, dropouts (the place where the wheels bolt on), the fork crown, nothing else. So we had to hand-make fittings for terminating cables, which we did with plain Grade 5 hardware store 1/4" bolts. We made water bottle mounts, and mounting points for racks and lights, all by hand, from stuff we scrounged.
Dropouts were chromed with a car battery and a little tub of chromium fluid (try buying that these days), if was really interesting.
We painted the frames in the garage, hung from the ceiling, with rattle cans, then slowly cured the paint in the kitchen oven (my Mom wasn't happy). We developed a method of making decals by using Chart-Pak rub-on letters (remember, no computers, no laser printers, nothing like that was available at the time), on gummed mailing tape. Once the decals were applied, we sprayed clear over the entire frame.
Good times. I should try to scan the contest entry photos, they came out great.
