Interior framing complete

As of today the interior framing of the instrument is complete:


The instrument received some additional reinforcement in two forms. Firstly, several braces were attached along the full width of the bottom. The stress from the strings pulling on the bentside liner, if one thinks about it, is transferred from the notch at the top of the knee down to the base of the knee, and might cause the bottom to buckle somewhat. Four braces made of Douglas fir, which is strong and stiff, help keep the bottom flat to resist this.

Secondly, a couple of "flying buttresses", also out of Douglas fir, were wedged in between the bentside liner and the bottom. These required some rather complicated compound cuts on each end, which my father ended up calculating on paper. Two of these go at the bass end of the bentside, where string tension is high, and one goes in the treble near the cheek, which is a problematic area in many harpsichord designs: the soundboard often cracks in the treble and creeps into the register gap. Since the compound cuts were close but not quite perfect, I secured the buttresses with epoxy, which has reasonable strength across gaps (most glues are strong only when the joinery is tight).

Lastly, the joints between bentside liner and knees were reinforced with #8 wood screws as extra insurance, and dowels secured the far end of the spine and bentside liners to the corner blocks at the tail of the instrument.

I've inscribed a dated note marking the completion of this stage on the back face of the lower belly rail.

Bentside liner

This merits its own post, even though it is part of the interior framing mentioned in the previous post.

Wood can be bent by soaking, steaming or dry-heating it and then bending it around a curved form. After clamping for a while, it will mostly hold that curve, although it will try to return to its original shape at least a little: a phenomenon known as "springback". Alternatively, one can cut a number of closely spaced saw kerfs along the inside of the curve, and then bend the wood to the desired shape. Although the kerfs weaken the wood, the act of bending closes them up to a fair degree, which restores some strength. In a harpsichord, the kerfs are largely filled up with glue when the bentside and liner are glued together, and this restores even more strength.

I experimented with a poplar scrap the same dimensions as my liners (3/4" thick by 1.5" wide) by clamping it and cutting a series of kerfs spaced 1/2" apart, and cutting through all but about 1/4" of the total thickness. This fine-toothed hand saw proved to have a suitably narrow kerf:


The kerfing parameters were suggested by reading professional harpsichord makers' advice on the harpsichord mailing list. After trying this scrap against the steepest part of the treble curve, I found it would bend readily enough when clamped, even though bending it by hand seemed to imply it wouldn't bend far enough. Obviously it must only be bent inward: the wood breaks all too easily when bent the wrong way, as I found out by deliberately flexing it in the opposite direction (note the broken end above).

This little experiment seemed promising enough, so I cut my actual bentside liner stock a little over-length and got to work. In the treble, I started out conservatively with kerfs 1/2" apart for a small distance, then switched to 1" apart with the idea of going back and filling this portion in with more kerfs later on if necessary. I repeatedly clamped the liner in place as much as I could and tried to bend the unkerfed portion inward carefully. Further down the liner, I switched to kerfs 2" apart, then 4" apart. The kerfs ended up going almost the full length of the liner, though they were of course widely spaced at the far end.

By trial and error—and not as much as I thought I'd need—I went back and filled in some of the treble areas with additional kerfs. I discovered that I'd need to tweak the notch in the treble wrestplank support block to accommodate the curve that the liner wanted to take, so I did that with a 1" chisel. I mitered the far end of the liner at 55 degrees to match the right-hand corner block angle.

Eventually I decided the fit was good. I checked first by eyeballing, then by running a square along the edge of the baseboard and seeing whether the liner curvature was aligned with the curved edge of the baseboard. For the most part the fit was very good. In one place it might be necessary later on to sand the liner a bit; in another, the baseboard edge might need sanding. Before I decide this for sure, I'll clamp my case material in place and see if the boards can conform to these little discrepancies; I suspect they might do so readily enough: the case wood is 3/16" Alaska yellow cedar. The mismatches are very slight, perhaps 1.5 mm or less, so they might not need any correction at all.

On to the glue-up: polyurethane glue again, and C-clamps to bring the liner in against the knees, with more clamps from above to push them into the notch. I took pictures from several angles:






An arrangement of blocks at the bass end of the liner captures the mitered end and keeps it from slipping out of alignment with the tail liner.

Note that I've left the treble end of the liner over-length: it is far easier to leave a generous margin and cut it flush later then to guess the correct length and pre-cut it.

I'm pleased with the way this turned out; it wasn't as difficult or time-consuming as I thought it would be.

After the glue dried, I cut off the overhanging treble portion of the liner and took everything outside to chamfer the inside edges of all the liners. An extended router baseplate helped keep the tool steady:


Where the router couldn't reach—it was blocked by the tip of each knee and in deep corners—I finished the chamfer with a chisel and sandpaper.