Looks like a variation of the technique I use to cut the slot on my 3-stringer headstocks. You drill holes all around the perimeter of the slot then use a chisel and hammer to cut out the wood in the middle. On my slots (which are narrower than yours), I drill holes about 1/4" apart down the center line for the slot, starting with small drill bits, and increasing the diameter of the drill bits with each subsequent pass until the holes pretty much join. Then I clean it up with a chisel, wood rasp, files and sand paper.
Comments
Looks like a variation of the technique I use to cut the slot on my 3-stringer headstocks. You drill holes all around the perimeter of the slot then use a chisel and hammer to cut out the wood in the middle. On my slots (which are narrower than yours), I drill holes about 1/4" apart down the center line for the slot, starting with small drill bits, and increasing the diameter of the drill bits with each subsequent pass until the holes pretty much join. Then I clean it up with a chisel, wood rasp, files and sand paper.