A mortiser is a device or machine that cuts square holes in a piece of wood. It is often used to create a mortise and tenon joint, which is a method of joining two pieces of wood using a square slot on one board and a square insert on the other. A mortiser can be as simple as a special bit attached to a drill press, or as complex as a dedicated machine with a motor that rapidly cuts the necessary holes. A bit that attaches to a drill press actually consists of two components: the auger and the chisel.
The auger must be positioned within the hollow square chisel. As the auger rotates rapidly, it removes most of the material for the square hole. The square chisel then removes the material from the edges, creating angles at the edges of the hole. The result is a square that can be cut to a specific depth if the mortiser is prevented with a stop from going too deep. Many drill presses feature the capability to limit the depth of a hole to ensure uniformity if more than one identical cut is being made on several pieces of wood in succession.
A horizontal mortiser is a type of power tool that cuts mortises exceptionally well, though in a different way than other types of mortisers. A router bit is mounted on a horizontal plane; the bit itself spins rapidly, much like the bit of a drill press would, and the piece of wood to be cut is mounted to a specially designed table or jig that allows clamping at specific angles. This machine is often used to cut mortises in two separate pieces of wood, then a third tenon component is secured between the two pieces of wood to complete the joint. This type of mortise joint is known as a loose or floating tenon.
While a mortiser bit and chisel can be fitted to a typical drill press, some dedicated press machines will feature only mortise bits. The machine will function almost exactly the same as a drill press, but the work surface may feature jigs that allow the piece of wood being cut to be stabilized or otherwise fixed in position. This allows for greater stability during the cut and it also allows the user to perform the same type of mortise cut repeatedly on different pieces of wood.