A mortiser is a very specialized woodworking power tool used to cut square or rectangular holes in a piece of lumber, such as a mortise in a mortise and tenon joint.
Traditionally, a mortise would be cut by hand, using a chisel. However, due to the precision of fit required for a mortise and tennon joint, this is a difficult and time-consuming task for all but the most experienced woodworkers. Before the invention of the mortiser, other tools such as a table saw or router could be used for the task, but because these tools are not really designed for such a task, this is still difficult, and often the quality of the results depend just as much on the skill of the woodworker as cutting by hand.
The square chisel mortiser was invented in 1874 by Robert and Ralph Greenlee (founders of the Greenlee Company ). This device, similar to a drill press in many respects, combines the cutting of a four-sided chisel with the action of a drill bit in the center. The bit clears out most of the material to be removed, and the chisel ensures the edges are straight and clean.
For cutting small mortises, a normal drill bit can be used. However, for larger mortises, a common type of bit in modern mortisers is a Harry Watt square drill bit , in which a bit with a Realeaux triangle cross section is allowed to "wobble" while it rotates within a square with rounded corners (compare to the motion of a rotary engine). This type of bit removes all but the corners of the material to be removed, leaving very little to be cut by the chisel.
The Greenlee Company still manufactures mortisers, as do a large number of other power tool manufacturers. It is a common tool in the woodshop of professional woodworkers, but because of its specialized nature, many amateur woodworkers would not make enough use of it to justify the moderately high cost and space this tool takes up in the woodshop. Some smaller and less expensive models, suitable for cutting only narrow mortises are available.
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