the main triangle jig, complete so far as i know. the angle stock is 1/8", mostly 1.5" on a side, but somebody stole my DT piece so that's 2" on a side. i ground the corners down because i kept hitting them. the bolts are all 1/4-20.
the procedure for getting all this aligned and locked down tight: position the angle stock, with two 1/4" holes pre-drilled per piece, along the proper line on the drawing, using the actual tube as a guide. clamp down the ends, checking while you tighten that everything's still aligned. drill through the holes, through the plywood backboard, with a cordless drill and a 1/4" twist bit. with the clamps still tight, set up the bolts, nuts, and washers. tighten down, again checking alignment with the actual tube. remove clamps.
the hold-down bit. material is 1/2" 4130 hex stock cut from a 6" sample piece from McMaster-Carr. turned down to fit inside the fork blade by chucking into a massive drill and taking it to the bench grinder. finished up by chucking into the drill press and filing. really wish i had a lathe.
the bit essentially replaces e-RITCHIE's sacrificial front dropout. the 4130 was cheap and easier to get and hopefully will work at least as well.
the complete hold-down setup. material for the hold-down is 3/16" x 1.5" (?) flat mild steel stock. MIG welded--i'm still learning so they're kinda sloppy. it's not especially precise, as you can see, but it seems like it'll do. the next step is to drill through the hold-down sides and run bolts through to secure it to the bender.
the nut and bolt were in a box of random hardware. i had to grind off the galvanized surface of the nut. they're 1/2"-13 tpi. i pre-drilled a 1/2" hole in the top horizontal piece and reamed it out a little, but the nut ended up a little off-center so i ran a tap through to put some threads in the mild steel piece.
pictured is my 3rd bender, 8" radius. i plan to re-cut the groove using a V-shaped router bit from Grizzly. if i can, i'll sand flat the part that contacts the hold-down bit so it has a nice stable surface to sit on.
i'm also planning to make a 6" radius bender. the idea is to use the 6" bender to start for a tight bend near the tip, then switch to the 8" for the upper part. i'll try some test bends on the 8" and see how it looks first.