Fish Mouths

One of the things that i had been steeling myself for (over the past 6 years!) was the tremendous effort I was going to have to make in creating the tube joints and interfaces.  With limited facilities at home I was preparing myself for cutting and filing each tube joint by hand.  Actually, I find great pleasure in using hand tools, and I'm not bad at it either.... but man, its a slow job.
Added to this the fact that I was planning to weld this frame with my TIG welder which requires much finer fitting tolerances, I imagined that shaping the frame was going to be a log slog.

However, now down in Jo's forge with space, tools, and skills on tap, I was able to re-evaluate this whole process.
Firstly, I decided to ask Jo to do the welding for me.  He uses MIG welding (and he is good at it) - some custom bike builders scorn MIG welding as not being strong or neat enough for bike frames, but as my tubes are larger than most and my design is heavier than usual, we decided that a well executed MIG weld would be perfectly acceptable for my frame - and with Jo doing it, he would be fast, accurate and get a proper penetration.
Secondly, As MIG welding can be a little more forgiving on the fitting tolerances of the tubes (particularly heavy ones like mine) I didn't need to be so fussy about the shaping of the tube interfaces - the fishmouths.

    

There are many ways to design and make a fishmouth in a tube - including hand filing/fitting, drilling with a hole cutter, and using software to print out a cutting template.  Jo introduced me to a very simple method of 2 single straight cuts on a table saw.  There is a great explanation of this process (and much more tube fabrication information besides) on this site.  With a table saw and a hand grinder we were able to create perfectly acceptable fishmouths quickly and easily

Another important decision we made was to not try and make 2 bends in a single piece of tube - so that we wouldn't be faced with having to write off a perfectly accurate bend because we'd cocked up the second or third.  Instead we simple butt welded the single bend sections together (with the use of a tight fitting box-section stub for alignment).  This made it so much easier in the design/assembly/tacking process as we were able to rotate the bends relative to each other to create the perfect shape - and consequently reproduce the exact mirror assembly for the other side of the bike.  With Jo's expert welding on my thick tubes, we all agreed there would be no worries about the strength and integrity of the frame