A chain mortiser is a tool designed to cut square holes into a piece of wood, often to create mortise and tenon joints or post and beam fittings. Like other mortisers, the chain mortiser is designed to cut material in a square shape, which drill bits often cannot do. Unlike other mortisers, however, the chain machines can remove a significant amount of material very quickly, and the chain itself can be plunged quite deeply into the wood to create deeper cuts. The chain runs around a bar, much like the design of a chainsaw, but usually on a much smaller scale.
Two general types of chain mortiser machines exist: portable machines and stationary machines. A portable chain mortiser is often fairly lightweight and compact. A user will hold the machine to cut the square mortise, making this tool convenient for on-site jobs such as homebuilding. Stationary chain mortiser machines are much larger, heavier, and able to support the weight of wood to be cut. The piece of wood is placed on the machine's work surface and usually clamped in place. A hand lever, much like one found on a drill press, will be used to drop the chain and bar to the piece of wood on the work surface. The machine may feature a depth gauge to control how deep the chain cuts.
The chain will be looped around a pulley that is attached to a drive motor. As the motor spins the pulley at a high speed, the chain will rotate rapidly as well around the guide bar. The size of the motor will often dictate how rapidly the chain can be moved; in most cases, the faster a chain moves, the smoother the final cut will be. When the mortise chain is plunged into the wood, the result will be a square hole with a rounded bottom; the chain can then be plunged again next to the original hole to cut a flatter surface.
Larger jobs often require a chain mortiser over other types of mortise machines. The chain itself may be carbide-tipped for added durability and clean cuts, and for larger jobs, it is likely that several chains will be mounted next to each other to create a wider cutting tool. Each chain will be run around a separate bar, or around one bar with separate flanges to guide the chains. Depending on the type of chain being used, the cost of the cutting chain can be quite high; carbide-tipped chains are especially expensive, sometimes meeting or exceeding the cost of the machine itself.