Mortise Chisels

Posted by Easy WoodCraft on

Apr 5, 2017

Mortise chisels are distinguished by their thick, beefy blades and stout handles (right photo), both designed to handle the heavy mallet blows associated with chopping mortises by hand. But the blades are thicker for another, less obvious reason: The wide blade helps guide the chisel and keep it square to the sides of the mortise as it's being cut. This is a big plus, because it makes the chisel almost self-guiding once the sides and ends of the mortise are established. It's another key reason why this is absolutely the tool of choice for cutting mortises by hand.

Handle options

There are two basic handle options available for mortise chisels: tang-mount and socket-mount (right drawing). With a tang-mount, the tang on the end of the blade fits into a hole drilled into the end of the handle. Generally a leather washer is added between the base of the blade and the handle to serve as a shock absorber. On socket-style mortise chisels, a tenon is turned on the end of the handle to fit into the socket on the end of the blade. The disadvantage of a tang-style mortise chisel is that repeated blows will tend to drive the tang deeper into the handle. Even with ferrules installed at both ends of the handle, the tang will eventually make the handle split. This rarely happens with a socketstyle chisel.

The first time I chopped a mortise by hand, it was a disaster. I used a small, bevel-edged chisel that was relatively dull (I hadn't quite figured out sharpening yet). After half an hour of pounding on that poor board, I had a lopsided hole that looked more like a crater than a mortise. The width varied, the sides were rough, and the bottom was a jumble of torn slivers of wood. I had made three mistakes: I was using an improperly sharpened tool, I was using the wrong tool (I should have used a stout mortise chisel), and I was using the wrong technique.

Decades later, I find chopping a mortise by hand a pleasure—really. Sure, it's a bit physical, but it's very rewarding. I often will hand-chop a mortise if I've got only a few to make. It's actually quicker than going through the whole machine setup and test-cut business.

Define the ends and remove waste

After you've laid out the mortise and clamped the workpiece firmly to your bench (preferably over a leg), start by defining the ends. Position the mortise chisel (the same width as the mortise) so that the flat is toward the end of the mortise about 1/8" from the layout line (I'll explain why later). Then drive the chisel vertically into the mortise about 1/4" with a mallet (Step 1: drawing). Next, tip the chisel over so the bevel is down, and remove a chip (Step 2: drawing).

Once you've reached the desired depth of the mortise, start working across, cleaning out the waste at the bottom. Hold the chisel vertically and take light, paring cuts (Step 3: drawing).

Continue removing waste from the bottom, working your way toward the opposite end of the mortise. Take light cuts and pry out the waste with the chisel (Step 4: drawing). ShopTip: For a simple depth gauge, wrap a piece of tape around the blade of the chisel.

Finish cuts

Once the full width of the mortise is complete, you can make your finish cuts to the ends. By now you've probably figured why it's important to leave1/8" or so on the ends of the mortises while you chop out the majority of the waste.

Even when you're careful prying out the waste, there's a tendency to use the top edge of the mortise as a fulcrum for the blade. The result is that this edge gets crushed. If the mortises are stopped, the tenon will cover this, but the joint won't be as sturdy as it could be. But if the mortise is a through mortise, this will be highly visible. Leaving a bit of waste to be trimmed away as a final step yields clean, crisp edges (Step 5: drawing).

TIGHTENING A LOOSE HANDLE

Since mortise chisels really do take a beating, the handles can and do come loose occasionally. When this happens, here's an easy way to fix it. Whether the blade fits into the handle with a tang or a socket (as shown here), you can insert a small wood shim or two to take up the slack (left photo). Then simply hammer the handle back on. Another method that works well on sockets is to wrap a curled shaving around the "tenon" of the handle before driving the handle back in place.

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