Mortise Chisels: Types, Sizes and Uses

A mortise is a deep, narrow hole cut across the grain and cleared by levering. A bench chisel would bend or twist doing that, so this chisel is built differently (see the plate above).

• Taller than it is wide. Lie-Nielsen makes these chisels taller than wide, with parallel sides, to keep them straight in the cut. The depth of the blade resists bending when it is levered back to break out the waste.
• Square sides that fill the mortise. The sides register on the walls of the mortise, so the chisel cuts a hole of its own width and does not wander. Narex tapers the width of its blades by 0.030 in from tip to shoulder, so the sides clear the walls behind the edge.
• A steeper edge. Heavy blows need a durable edge. Lie-Nielsen bevels its own at 30° and advises a further 5° secondary bevel; Narex grinds 25°; Veritas suggests 30–35°, or 35–40° for heavy use. How to hone them is in the sharpening guide.
• A handle made to be struck. Narex fits two steel ferrules to withstand mallet blows. Lie-Nielsen says its chisels should not be struck with a metal hammer. Japanese ones, like other Japanese striking chisels, carry a steel hoop and are made to be driven with a steel hammer — see the mallet guide.

Makers sell them in closely spaced widths, often 1/16 in apart in imperial sets and 1.5 mm apart in the smaller Japanese sizes, because the chisel’s width becomes the mortise’s width. The heavier traditional pattern, sold by Robert Sorby as a registered firmer, is used for the same job in larger work.

Choose the chisel from the joint, not the other way round. The width of the chisel is the width of the mortise and so the thickness of the tenon. A common guideline, given by Popular Woodworking, is a tenon no more than one-third of the thickness of the rail it is cut on for frame joints — which leaves the mortise walls at least as thick as the tenon — and as much as one-half where the joint is set back from the face, as in many leg-and-apron joints. These are starting proportions for common joints, not rules for every case.

A mortise is chopped in a sequence of cuts across the grain, each a little deeper, with the waste levered out between them. The chisel is struck with a mallet — see the mallet guide — and levered back against the waste, never against the finished end walls, which would crush them. Hand position, grain and the order of cuts are covered in how to use a chisel.

The same name is also given to the tooling for a hollow-chisel mortiser. It is a different tool: a square, hollow steel chisel with an auger bit turning inside it. A “chisel bit” sold for a mortiser or a mortising attachment is this machine tooling, not the hand tool.

Woodsmith describes how the two parts work together: the auger removes most of the waste and carries away the chips, while the four cutting edges of the hollow chisel square the walls and corners as the assembly is plunged. The auger is set slightly proud of the chisel’s edges; with too little clearance, chips jam inside the chisel and cause friction, heat and burning.

The hollow chisel is sharpened differently from a hand chisel: Woodsmith hones its inside bevels with a diamond cone, which keeps their shape, and then polishes the four outside faces on a stone with a few careful strokes.