today i modified parts of my bending setup, and now it's working well.
the big problem before had been that the tip of the blade kept slipping off the hold-down bit and popping up. kinda scary the first few times it happened, although i always had a hand on the blade and a cheater in it so it never went anywhere. but not good for bending.
see this post for a breakdown of the hold-down design, including the original bit:
the problem, i realized, was that the bit was tapered gradually. the blade had nothing to hold it position securely. i did some filing and hacksawing, ending up with this:
this better emulates the R. Sachs method of using a sacrificial front plug-in drop to secure the blade tip.
also, since the original photo, i flattened off a corner of the hexagonal lower portion to better position it in the V-groove.
i don't think i photographed the bender after i modified it to keep the plywood from splitting. as silly as this looks, it works well (especially for a bender that cost maybe $10, excepting the optional $20 router bit). i can hear the plywood creaking apart a little when i'm cranking down, but i can report no kinks in tweaking .9 mm Deda Tre blades.
this is a 90° V-groove; i'd like to try something with a shallower angle, which would probably keep the plywood intact more easily and still support the blade nicely.
the bolts are 1/4"-20 round heads, holes made with a cordless drill, spaced just far enough to allow a washer on either side. i had them about as tight as i could with a big crescent and screwdriver, i might loosen them up for the next bending sesh.