Chisel sharpening 101
I got to class this week with two unfinished projects: a dovetail and some drafting. For those who read about how much I like drafting, you won’t be surprised that I went right to the dovetail. As you can see from the photo, it’s proud in some places and gappy in others. But, I’ve decided it not too bad for my first handcut dovetail. So, with that done, back to the t-square, set squares and eraser. There was lots of erasing.
The hands-on lesson of the week was sharpening chisels. After pounding on a few chisels to clear out the waste on my dovetail, I was keen to learn about proper maintenance. The back of the chisel should be completely flat and the front should have a bevel. The ideal angle is 25°, but between 20° and 30° works. The steeper the angle, the sharper the chisel; however, the chisel will lose this edge more quickly. It seems 25° is the happy medium.
The first step was to take the chisel to a grinding machine. This process is called hollow grinding because the wheel of the grinder creates bevel with a slight curve or hollow. The hollow is key for letting the wood cuttings clear the edge without unnecessary resistance. The shop grinders were setup with jigs that kept the chisels at the proper angle. When grinding, keep the chisel edge square to the wheel. Also, keep the chisel moving back and forth so that the wheel wears evenly. A rutted wheel would make future sharpening endeavours very difficult, if not impossible.
After running a chisel against the grinding wheel, the tip of the tool gets a burr, a very slight hook that curves back over the flat part of the chisel. This burr has to go, so out comes the water or oil stones. Our instructor chose an oil stone, but said it was just a matter of preference. In both cases, the liquids perform the same function: they draw chisel filings away to make for a smoother finish. Since our instructor is quite comfortable with sharpening, he didn’t use any jig to maintain the bevel angle as he moved the chisel back and forth on the stone. As with the grinding wheel, one must ensure that the stone wears evenly. Use a slight zigzag pattern as you move the chisel along the stone. After, say, five passes on the bevel side, turn the chisel over to the flat side for five more passes. Remember to keep this side flat to the stone. Repeat this process while diminishing the number of passes per side. Eventually, you will work that burr right off. We were warned never to try to grind off the burr at the wheel, which will ruin the flat edge on the back of the chisel.
As an alternative to hollow grinding, we were shown the Work Sharp 3000. This machine uses discs of sandpaper to sharpen chisels. Because the disc is flat, the chisel doesn’t get a hollow grind, thus no curve. Isn’t that curve important? It is, but there’s another option when working with a flat bevel. After working on the primary bevel of 25°, increase the angle to 30° and put a small—less than 1/16″ thick—bevel on the end. This skinny extra angle is called a “micro-bevel.” Like the hollow-ground curve, the micro-bevel helps with the clearance of waste wood. Also, when tuning up that chisel after working through some hard maple, you might be able to getaway with just honing the micro-bevel instead of the whole shooting match.
So, after our chisel-sharpening lesson, I couldn’t believe all the little steps that go into a seemingly simple process. (I didn’t even touch on the part about working through to finer sanding grits. Oh, man!) I figure I can expect the same myriad steps with the other things I’ll learn in this course. I suppose mastering the art of woodworking involves keeping all this stuff straight and staying sharp…sharp as a chisel?
Do you see anything missing from my introduction to chisel sharpening? What are your favourite sharpening tips or tricks? I’m keen to hear.
Jeff Dundas is our resident newbie woodworker. He’ll be sharing his insights and discoveries here on the Shop Hack blog as he learns to make sawdust.