Reply To: Freehand straight mortise?
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It sounds like a lot of folk have trouble keeping the chisel straight up and down when they mortise, but my biggest problem is a bit different: twist. I find that when I mortise freehand, my mortise is almost always a bit wider than my chisel. Sometimes it is a lot wider. It took a while to figure out what was happening. When you drive the chisel into the wood for your mortise, the chisel moves backwards away from the bevel. But if the chisel is twisted in the cut just a bit, “backwards” from the point of view of the chisel is no longer straight back in the mortise. Rather, backwards from the chisel’s point of view is at a bit of an angle with respect to the mortise. So the mortise wall widens.
This explains another problem I have. Paul generally cuts a mortise by starting at one end and working his way to the other. But I also tried by starting in the middle and cutting first one direction and then the other. But when I tried this, the resulting mortise and tenon usually ended up twisted. Why? The answer is the same. When I cut in one direction from the center, the chisel twisted one way and when I cut in the other direction from the center, the chisel twisted the other way. So half of my mortise was moving out of true in one direction and the other half was moving out of true in the other direction. The result was twist!
Once I realized that a twisting chisel was my biggest problem, and not whether the chisel was straight up and down, it took a bit longer to figure out why my chisel kept twisting. I could feel it twisting in my hand, and tried to resist it, but could not very well. The reason the chisel twisted is this. When I sharpen freehand, as Paul teaches, I do not generally get my chisels perfectly square. So one corner of my out of square chisel contacts the wood first and when I hit the chisel, it twists on that corner rather than driving in straight. Every time I hit the chisel,it twists a bit more in my hand and hence goes further off course, widening the mortise and making the resulting mortise and tenon twisted. I found that if my chisel is sharpened out of square just a hair, the chisel twists a hair and damages the mortise.
Knowing all this does not mean I can actually freehand a mortise very well, but I am working on it!