Mortiser

Also called: mortising machine, hollow chisel mortiser, chain mortiser

A mortiser is a machine that cuts a mortise, a rectangular socket chopped into a piece of timber to take a matching tenon, doing by machine what is otherwise done by hand with a mallet and chisel.

Two designs

Mortising machines work in one of two ways. The hollow-chisel type cuts with a square, sharpened, hollow chisel driven into the wood while a round drill bit spins inside it; the spinning bit bores the round part of the hole and clears away the chips cut both by itself and by the chisel's square corners, leaving a square-cornered hole. This design is built to do exactly one job: cut mortises. The chain type instead runs a cutting chain much like a chainsaw's; the part of the machine carrying the chain is plunged straight down into the timber to cut the mortise. Chain-type mortisers are generally considered far more hazardous to operate than hollow-chisel machines.

What it's for

Riley (1905) described the machine plainly: 'A Mortising Machine is used for making rectangular holes (mortises) in framing.' A mortiser attachment is worthwhile for anyone who does a great deal of framing work, and for continuous, production-style mortising, the heavy-duty slot mortiser, a machine built for repeated, high-volume cuts, is treated as the standard choice. Mortises can also be cut with other machines: a plunge router, a drill press, or, for cleaner and more precise slots, a long-hole borer built for the purpose.

In use

Ellis (1908) advised that on a hand-operated mortising machine the workpiece should rest on a strip of deal, a softwood board, so that the chisel cannot strike the machine's iron bed: 'set the work on a strip of deal, so that the chisel shall not be driven or fall on to the iron bed.' A combined rotary and oscillating mortiser can go beyond ordinary mortising to produce corner blocks and concave round or oblong pieces, and a punch-and-die attachment for punching holes in venetian-blind laths can also be fitted to a mortising machine as an accessory, though it is not standard equipment.

One maker's uses

Landis (1987) used a chain mortiser to cut a hole roughly two and three-quarter inches square straight through a benchtop, for fitting a bench stop, and built a custom mortiser that bores vise holes to a tolerance of three thousandths of an inch, matching interchangeable vise parts. He judged that old bench dogholes, the square holes in a workbench top that take a bench dog, were very likely drilled round first and then squared off afterward, either by hand chiselling or on a mortising machine, though this is his own inference about how they were made rather than a settled fact.