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If you haven’t surmised it yet, one of the themes running through the Spring 2008 issue is the fact that accurate sawing has a lot more to do with accurate chisel work than anything else. When you cut a tenon shoulder, it’s the chisel that cuts the part of the joint that shows , the saw just removes the waste below.
Several readers have picked up on this theme, and they’ve also pointed out (politely, I might add) what looks like a contradiction in my instructions about chiseling.
In the article on the Stickley Tabourets, I’m chiseling the joint line for the half-lap joint with the bevel of the chisel facing away from the waste (you can see this on page 10). A few pages later (page 19) I’m chiseling the shoulder for a tenon with the bevel of the chisel facing into the waste.
Have I finally taken one too many sips of La Fin Du Monde?
Perhaps, but I did have a good reason for what I did , I just didn’t have the room in the issue to explain it. So here goes:
When you deepen a knife line by striking it with a chisel, there are two important things to consider. First is what shape the resulting knife line will be, and second is how much the chisel will shift when you rap its handle with a mallet.
The first part is easy to understand. Chisels are wedge-shaped. They have a flat face and a bevel. So when you knock the tool straight down into your work it makes a “V”-shaped cut that is a photocopy of this shape. One side of the V is straight up and down. The other side of the V is sloped.
The second part also has to do with the fact that chisels are wedges. When you drive a chisel with a mallet, it doesn’t want to travel straight down in a line that’s parallel to the flat face of the chisel. Instead, it wants to travel at an angle that is halfway between the bevel and the flat face. So if you have a 20Ã?° bevel on your chisel (as I do in the paring chisel shown in the articles), the chisel doesn’t want to travel at 90Ã?° (straight down), it wants to move at 80Ã?°. (This assumes you have wood pushing back equally on the bevel and the face of the chisel.)
This is why when you are chiseling out your waste between dovetails that the chisel is always trying to move toward (and even cross) your baseline.
Whew. With all that on the table, I can now explain why I did what I did.
When chiseling a tenon shoulder, the shape of the line created by the chisel is critical. I want it perfectly square so it will close tight with the stile. So I chisel the joint with the bevel facing the waste. If this so happens to shrink the overall length of the tenoned part by 1/128″, I can live with that. I want the joint to be tight more than I care about its final length.
When chiseling a half-lap joint, my considerations are different. This isn’t a show joint, so I just want it to be tight and structural. The shoulder line isn’t as critical. That’s why I chisel with the bevel facing away from the waste. The chisel will then drift into the waste a tad. So when I saw the joint, the notch made by the chisel will encourage the saw to cut a half-lap that is just a tad tight. Then I can plane the piece’s mate to get a perfect fit.
This might be a little fussy for you. If so, I apologize. A chisel seems so simple (it’s a steel and wooden corndog!), but it actually is a subtle instrument (like a corndog with chorizo inside). Play around with the tool. Try it with the bevel out and then with the bevel in. And let us know what you discover.
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