Photos by Carl Duguay; Illustration by Len Churchill
A hollow chisel mortiser consists of a metal frame, a motor that rides up and down the frame via an articulating handle, and a chuck to which a mortising bit is attached. If you work primarily with mortises up to 5/8″, a 1/2 HP motor will suffice – otherwise opt for a 1 HP model. Features to look for include: a large work table (some have side extensions to better support long stock); a solid fence that is square to the table and easy to re-position without getting out of alignment; a quick-release tool-free hold-down to secure the stock; an accurate, quick-to-adjust stop for setting precise mortise depths; an adjustable handle that moves the column up and down smoothly; and a chuck that is easy to access when you need to change the bit. Some high end models offer titling heads for cutting angled mortises.
Proper alignment of the chisel and bit is important to ensure that mortises are drilled properly. The bit should be about 1/16″ below the chisel tips.
The chisels tend to dull faster than the bit. Use a cone sharpener to keep the chisels razor sharp.
A mortising bit doesn’t cut a perfectly flatbottomed mortise. Be sure to cut mortises about 1/8″ deeper than the tenon length.
Cut the first hole in the center of the mortise, the next holes on the right and left ends, and then work back towards the center.