A mortising chisel basically does one job, but it does it exceedingly well. While there are many ways to chop a mortise employing a variety of different machine tools, mortising by hand feels like “real” woodworking.
The first step in this process is accurately marking up the joint. Use a mortising gauge for this task to set both the mortise width and position. To set the depth of your mortise, simply place a piece of blue masking tape on the chisel back.
Position the chisel with two hands, then firmly tap the chisel with a joiner’s mallet to set the edge into the stock.
Now you have a decision to make. Do you chop to the full depth of your mortise in one process, or cut the mortise in “layers”? We’ll describe the layered process here. The layered approach will take five chisel cuts before you remove the first waste:
Cut 1: Set the chisel about 3/16" from the far end of the mortise. Tap the chisel lightly to set it — then hit it hard. It will take a few blows to drive it in to the first level.
Cut 2: Move the chisel about 3/16" towards you, set the chisel, and then give it three hard smacks. It will go a bit deeper than your last effort because of the space created by your first cut.
Cut 3: Again move the chisel about 3/16" toward you. Set and smack it three more times. You will drive it a bit deeper than for Cut 2.
Cut 4: Reposition the chisel as before, set and drive it to the same depth as in Cut 3.
Cut 5: Now position the chisel, back face away from you, 3/16" away from your last cut. Drive the chisel in, lay the mallet down and lever the chisel handle toward you. Take care to keep it aligned with the mortise sides. Then scoop out the waste.
To remove the second layer, go ahead and repeat the five cutting steps again.
Once the bulk of the waste is removed, square the ends of the mortise by placing the chisel on the marked line, with the chisel’s back face to the end you are cutting. Align your chisel against a square, then drive it down cleanly. NOTE: you will not get a flat-bottomed mortise chopping by hand. Make sure the depth is sufficient across the whole of the mortise. You’ve now completed your mortise.