So the more I consider it, the more I don't trust my bridge moldings just glued to the plywood. Chances of de-lamination (ripping loose) seem too high. My monochord has survived for weeks with this arrangement, one of the things I built it to test. However, I don't think I want to operate so close to the edge: even if the blocks do hold the tension, they are extremely fragile. The tall nails sticking out of the top provide plenty of leverage, so that bumping into the nails sideways is likely to tear the block off of its footing. I know this from experience...
The clincher was examining the trigonometry of the forces, particularly the sideways force. Give any kind of side-angle to the string hitching position, and the forces mount rapidly (as double the sine). And I will need a lot of side-angle, to avoid excessive length on some of the string tails, due to my straight-line bentside. Angles of 20 degrees or more quickly get above 25% of the string tension, applied as sideways force.
So I'm thinking I should probably have designed these blocks to have screws (or dowels) going all the way through the plywood. I don't really like putting so many holes in the soundboard, either; especially in the treble, it amounts to a pretty severe perforation, a "dotted line" along which I hope it won't tear or buckle. If I had planned ahead, the holes would have helped in positioning the blocks for mounting, and the screws would have obviated the need for my complex and awkward clamping rigs. Instead, I now have the delicate task of locating the right place to drill through the plywood from the back side, and into the bottom of each bridge-block, just to the right depth for the screws I will use, not penetrating the upper surface. Tricky, but doable I think.
I made a little helper item to mark the positions of the blocks for drilling: a pair of matching 1x2s, glued together at one end in a sandwich with a small piece of the soundboard plywood, so that they form kind of a two-pronged fork, with a space in between equal to the thickness of the soundboard. Thus, when I slide this "fork" over the edge of the soundboard and bring the end of the upper 1x2 up against the corner of one of the bridge-blocks, the end of the lower 1x2 shadows its position on the back side of the soundboard, and I can mark the spot with Sharpie. Taking the time to glue-up this little tool was much preferable to struggling to mark the positions with straightedges and my T-square, the same struggle repeated 49 times.
Next I need to make a little block with a straight hole drilled through it, on the drill press; this will let me drill straight-ish holes to a specific depth using the handheld electric drill.
My diced-up moldings for a bridge idea seems to have acoustic merit, and with a conventional solid-wood soundboard, they'd probably work fine glued on the surface. It's when I combined that cheapskate-ordinary-materials idea with another cheapskate-ordinary-materials idea, the plywood soundboard, that unforeseen (but arguably foreseeable) problems arose. The cheaper and easier-to-obtain materials end up costing quite a bit of labour and design complexity, to get acceptable performance out of them. Oh well, that's pretty much my whole strategy here, to devise a way to build a good instrument using almost no money, but almost unlimited amounts of time and tedium. For some of us, it's a good bargain.
In other news, I have noticed that my oak nut molding has split, all along the top along the line of the nails. So far, the bridge moldings still remain un-split. So basically every example where I've tried this nails-into-oak thing, has split (also the monochord shows splitting), except those cases (the bridge moldings) where I used copious glue-sizing to pre-empt the splitting. I shoulda used glue on the nut, as well! Oh well, now I can see if it makes a difference. None of the nails feel noticeably loose, which would clearly be a problem.
I'll make this beast work somehow; but in the future, no more oak for instrument parts! There's gotta be something more appropriate, and I'm thinking it might be maple.
"It's oak, so it broke"?
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