When you are making big mortises, such as those in the Roubo-style Workbench in Issue 4, it’s a good idea to bore out as much of the waste as possible. This isn’t exclusively a power-tool perspective, either. A fair number of historical texts recommend this with large-scale joints (particularly in timber framing).
When I do this with a drill press, I prefer to use a sharp Forstner bit because it allows me to overlap the holes without the bit wandering or breaking. And it leaves fairly clean mortise cheeks. You see this technique all the time in other woodworking books and magazines. But there are always a couple details left out. What you normally see is the joint bored out and then the craftsman comes back with a chisel and cleans up the waste on the ends and on the mortise cheeks.
I have found this to be more work than necessary and an opportunity to botch the joint by undercutting the mortise cheeks in an effort to square them up. Paring mortise cheeks is a real skill. If you’re good at it, then great. If you’re like the rest of us, read on.
The trick is to use the Forstner’s ability to bore overlapping holes as much as possible. Don’t just overlap a little bit. Overlap the holes over and over again until the Forstner bit can move freely left and right in the mortise. This makes the most accurate, square-sided big mortises possible. Here are a few photos:
Once I define both ends of the mortise, I come back and split the difference between the holes. In the case of really big mortises I’ll split the difference several times until the holes overlap as shown.
Now clean up the little triangle of waste between the overlaps. Keep boring out these overlapping triangles clinging to the mortise cheeks until the bit will move freely left and right (check for this with the drill press turned off).
This is what the mortise looks like at the end. The bit has cut right to my scribe lines. This mortise is square. All I need to do is chisel out the corners , much faster than having to chisel the cheeks, too.
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