How to Make Deep Mortises When You Aren’t a Timber Framer
One of the defining characteristics of the heavy French bench is its joinery – the top and legs are joined with a joint that is one-half through-tenon and one-half sliding dovetail.
The sliding dovetail is the easy bit. Saw the walls. Knock the waste out with a chisel. Clean the floor with a router plane.
The through-mortises are trickier. It is more than 5” deep, 6” long and usually 1-1/4” wide. Typical furniture-making tools are slow and ineffectual. Here’s how I get around the problem.
- I bore out the majority of the waste with a corded drill driving a WoodOwl auger bit (usually a 1”-diameter bit). I bore halfway through the top, then flip the top over and bore halfway through again.
- Then I take my jigsaw and mount a Bosch Progressor blade in the chuck and kerf the waste right up to the knife lines that define my mortises. I do this on the face of the benchtop, then flip the benchtop over and repeat.
- Then I take chisels and pop out the kerfed waste hanging to the walls of the mortise. When the kerfs disappear, the walls of the mortise are pretty much square.
This process makes mortises that are square and fairly quick. It took about four hours to do the the four mortises in this massive workbench – start to finish. I’m sure a chain mortiser would make even quicker work of the job, but most of us don’t own one of those machines.
Tomorrow is the final day of the French Oak Roubo Project put on by Benchcrafted. Te workbenches we’re building here look amazing – I’ll post some photos of the completed benches tomorrow.
— Christopher Schwarz