Reply To: Chopping Mortises in Doug Fir
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Ragged mortises: welcome to the club! Three things can go wrong when chopping a mortise that give the “arby’s” effect you are talking about. First, the outer edges of the mortise, at the opening, can be chipped and ragged. Second, the walls of the mortise can be ragged. And third, the ends can be ragged. I have had all three problems, and have gotten better with all of them. As to the first, the outer edges at the opening seem to get ragged because the chisel does not go straight in and out. Any variation from the vertical when driving the chisel in or pulling it out, leaning it side to side, will damage those outer edges. I guess that is just practice. I know some folk start by chopping the full length of the mortise to only a mil or so, and I have found this helps me preserve the edges of the mortise. As to the second problem, any chopping will produce some tear in the walls. But the walls of the mortise can be torn too much, again, if the chisel leans this way and that as you chop. So keeping it vertical is crucial. Also, in some of the first videos I saw from Paul on chopping a mortise, he ended by dragging the chisel back and forth in the mortise to smooth it a bit. That can work if the mortise is tight on the chisel. It will not work very well if you end with a mortise more than a hair wider than the chisel. In later videos, he pares the walls down after the mortise is chopped. He uses a jig to help keep the paring chisel nice and straight. On this approach, you plan for the mortise to be a bit wider than your chisel right from the start. There is one other approach to smoother walls. When you drive your chisel in, you can level out the chips. As you lever, the edges of the chisel can smooth the walls a good bit, as long as the mortise is nice and tight on the chisel. In my (limited) experience, this works best if you use a real mortise chisel which is made for levering waste. When I chop mortises, as Paul often does in his videos, using a bevel edged chisel, I do not level very much since I am concerned about breaking the chisel, or at least damaging its edge. Some of my cleanest mortises have been done with old “pig sticker” mortise chisels I picked up since I can lever aggressively with them. Also, I found that with a bevel edged chisel, I got decent results with oak and even pine, and rather nasty results with Doug Fir. As to the final problem, the ragged ends, that is a bummer, and it is especially a problem in woods like Doug Fir. It is a difficult combination of very soft wood and very hard grain. Sometimes, though not always, the grain is ridiculously hard. Chisels dull fast when chopping a mortise, and doubly fast when chopping through some of the harder grain. So those dull chisels tear the wood as you try to chop straight down. One solution is frequent honing of your chisel when doing the ends of the mortise. My preferred approach is denial.