mortiser

A mortiser is a machine designed to cut mortises, which are the rectangular cavities that receive the tenons of mortise-and-tenon joints. The mortise must be precise in depth, width, and length, and the mortiser achieves this by driving a hollow chisel with a rotating auger bit at its center. As the auger bores, the chisel walls shear the wood fibers at the perimeter, and the auger carries the chips upward and out. The result is a clean-walled rectangular hole.

Mortisers come in two main types: the stationary floor-standing hollow chisel mortiser, used in shops for high production, and the benchtop or attachment mortiser, which can be mounted to a drill press or work table. The hollow chisel itself is typically hardened steel, sized from about 1/4 inch to 1 1/2 inches wide, with standard depths of 2 to 6 inches depending on the machine. The auger bit inside rotates at speeds between 600 and 3000 rpm, depending on the wood type and chisel diameter.

Common problems and setup

Mortisers require accurate fence and stop-block adjustment, or the mortise will be misaligned or wrong in length. Dull or poorly matched chisel-and-auger pairs produce ragged walls and excessive tearout, particularly in grain-prone woods like oak and pine. The chisel must be sharp enough to shear cleanly, not crumble or compress the fibers. A common mistake is feeding too fast, which heats the chisel and dulls it prematurely, or reversing rotation mid-stroke, which jams chips.

The term mortiser comes from its function: to form a mortise. Woodworkers who cut mortises by hand using chisels are also called mortisers, though the machine has largely replaced that skill in production work. In industrial settings, chain mortisers and hollow-square mortisers handle larger work and can cut multiple mortises simultaneously, but the basic hollow-chisel principle remains standard for custom and small-batch joinery.