Next is the rack- this sits (vertically) behind the keys and acts as the guide for the keys, keeping them moving straight up and down. Here, I've transferred the center of each key to the rack, and extended the lines.
Now the idea is to cut slots (1.7mm wide) in the rack, centered on the previously drawn center lines, for each of the 56 keys. There's probably a number of ways to do this, but I followed the Barnes book here. Build a little jig with two fingers spaced about 1.9mm apart (you can guess I used an old hacksaw blade here).
Center the line drawn on the rack between the two fingers and use a knife to score a pair of lines. Then use a chisel to chop out the slot.
Well crud, perhaps I should have been a bit pickier about the wood I used for the rack. I can probably glue this crack, but I've got 55 more slots to do. Barnes recommends a chisel with a 12 degree edge.
Anyway, here you can see the scored lines and where I chiseled out one of the slots. To minimize friction between the key and the slot, we want just the front edge to be the guide, so we need to chisel at an angle (i.e., the slot will be wider at the back of the rack than the front.
The slot in the back. Need to use a harder backing board too methinks.