What’s your technique for adjusting tenon shoulders for a tight fitting mortise and tenon joint?
I used to suck at doing tenons - either through or stub - so I set out one day to keep doing them until I got it right. It took at least a dozen attempts, but here’s what I learned. Your mileage may vary.
Tenon accuracy starts with accurate measuring.All four edges of the tenon shoulders have to be co-planar and absolutely perpendicular to the stock being tenoned. This means that you can’t measure with a ruler - you have to gauge them with a marking gauge and it has to be sharp and locked down tight. It also means that you have to gauge from a single reference face and edge, and don’t assume that faces and edges are parallel and square, just because they’re fresh off the saw.
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| What’s your technique for adjusting tenon shoulders for a tight fitting mortise and tenon joint? |
Mark the shoulders with a knife, not a pencil.Pencils leave a visible mark, but you want a mark inscribed in the wood itself to sever the surface fibers. If your marking gauge has a knife edge, great. Otherwise go over its scratches with a sharp(!) marking knife and really slice deep. This slice is, in fact, the start of your shoulder.
Saw the shoulders first.If you cut the cheeks of the tenons first, they will flex as you cut the shoulders and throw things off just a hair. You want the wood absolutely stable as you cut the shoulders. I saw just barely on the tenon side of the line, knowing that I’m going to pare up to it in the last step. Depending on the quality of your saw technique, use a try square to keep the saw vertical.
Now saw the tenon cheeks.The question didn’t ask about the cheeks, but I always saw them a little heavy and trim them down to size later, especially if we’re talking about a through tenon.
Now pare the shoulders back to the line.Using a razor sharp (and I mean that) paring chisel, take a wafer thin slice off each shoulder to cut it back to the line you marked. Try to use your widest chisel and to do it in the fewest cuts you can manage. If your chisel is really sharp you should be able to do it with hand pressure. If you need to use a mallet, do the cut in one blow rather than tapping it.
(Optional) undercut the shoulders microscopically.Just before the paring cut I check the chisel for perpendicularity, then pull it fractionally (like 1–2 degrees) out of perpendicular, so the shoulder is barely undercut. I do this because I can never get a perfectly square cut and I’d rather err on the side of undercutting.
That’s it - there’s no magic involved, just lots and lots of practice and sharp tools. Of course you could say that about all of woodworking…
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