Make round holes square
A power-tool woodworker cuts mortises with a drill press or benchtop mortising machine. However, both methods can leave excess material on the walls of the mortise that will weaken the glue joint. That's where a sharp chisel works wonders. Hold the chisel—using the widest one that fits comfortably in the mortise—with its back (the non-beveled side) against the mortise wall, as shown in the photo. Then plunge it down along the wall using only hand pressure. A sharp bench chisel shears off the waste left behind by the drill bit to flatten the walls of the mortise as shown in photo.