Making the Parts
I was able to make the major parts for two pianos from one 15 by 5 by 1 inch piece of curly maple. (One 5 by 7 by 1 inch piece would be enough for one thumb piano, although keep reading for other problems with a piece this small.) I started by resawing the maple into three pieces. Two of these pieces were planed to 3/16ths for the top and bottom and the last piece was planed to 3/8ths for the sides. (If I had started with a piece shorter than 12 inches or so, it would have been unsafe to use the planer to bring the parts to thickness. I was already pushing the safe limits of thickness for planing anyway.) I then cut the top and back to length creating two 5 by 7 inch pieces for both thumb pianos.
I cut the sides to their rough length on the table saw, then used a disc sander and miter gauge to miter the ends and bring the sides to their final lengths. This was much safer than using a miter saw on such small pieces and left no chance of the piece chipping out. The sides measure 5 and 7 inches on their outer edge respectively once cut to size.
Attaching the Back
I glued the back to sides using a bunch of clamps while making sure to keep everything as square as possible. I used a combination of quick grip style clamps and homemade spool clamps: The quick grip clamps kept the sides pulled tight and perpendicular to the back, while the spool clamps filled in the gaps for even pressure all along the length of the sides. The spool clamps are simple to make using 1.25 inch toy wheels,1/4 inch by 3 inch (or longer) bolts and wing nuts. The spool clamps are great for even clamping pressure over a large perimeter you need to glue.
I sanded the exposed top surface of the sides flat and level once the glue was dry using a belt sander. The sides were very uneven after attaching them, so this step would hopefully ensure a tight, gap-free joint with the top when it’s attached later. You could use a sheet of sandpaper glued to a flat surface for any of the steps using a belt sander. A very coarse grit probably won’t take much longer than the powered option, but will take some more effort.
I started by drilling the sound hole in the top. It measures 1.25 inches in diameter and is spaced 1.625 inches from bottom edge of the box. I also drilled holes to accept the t-nuts that will allow the machine screws to pull the aluminum rod down tightly on the keys. These holes were centered, 2.375 inches apart and 1.375 inches from top edge of the box. I epoxied the t-nuts in place from the inside of the top. The t-nuts I used are made to be secured with brads which stop the nut from spinning while tightening the screw and prevent it from falling out of place. They also have t-nuts with sharp spikes built in. Fastening the t-nuts with built in spikes is possible with softer woods like birch ply, but wouldn’t work with a hardwood since the spikes would tend to split the wood, so I used the epoxy instead.