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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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I had just the problem you describe while making the trestles for my own bench last year; my first few mortises wound up about 1/4 inch too wide.
The problem I have with this idea (and Brians above) is that I can’t turn my brace as fast as your drillpress, so it takes half a day to create a mortise with Forstner bits. I found it faster to make a separate tennon of the desired size that I can use to check the mortise dimensions as I clean it out.
Brian,
Great tip. I’ll try that next time. One of the biggest challenges in chiseling out the corners is keeping the joint square. The chisel (even a corner chisel) will drift. And then you compensate or overcompensate and undercut.
Chris
After boring out the major part of the mortise with large Forstners as you describe, I then use a smaller Forstner bit to bore out the corners, leaving only a small amount to chisel by hand. It’s easier on the fingers.
BP