Hollow Mortise Chisel Tune-up
I’ve always been geeky about sharpening things, not in the sense of polishing chisel backs to #32,000 grit, but having a good edge before going to work. Before using a router bit, I dress the edge with a diamond file, and I usually touch up the chisel before machine cutting mortises. Sharp is good, but with tooling used in a machine such as a mortiser, you hit a point where your efforts meet with diminishing returns. Here is a procedure that will take less than 15 minutes and will give you cleaner, cooler cuts.
This is a typical dull chisel. I found it in the cabinet in the base of our old mortiser, and don’t know what brand it is, or how old it is. It wasn’t completely trashed, but the end was discolored, and the grinding marks from the factory were still visible on the outer faces.
If you look at the pile of chips, you can see that they are fairly large, and some of them are scorched. There’s is a lot of friction between the chips and the metal as they work their way up the flutes of the auger bit. The chips rubbing against each other can get hot enough to burn, and once that happens, the smoke and resins in the wood start to get sticky. You can also see the chips in the flutes of the bit. If you can keep things cool, they won’t be there.
Here is a close look at the dull chisel, just about to undergo the first stage of the tune-up. The discoloration is mainly smoke damage, the chisel hasn’t been overheated to the point where the metal loses its temper.
The sharpening cone came from Rockler as part of a 3-piece set that includes a coarse and fine diamond coated hone, and a handle to put them in for hand honing. The set costs a little over $20, and I think it’s a good buy. The handle accepts hex-shanked screwdriver bits, and is comfortable to use for that purpose as well.
This is chucked in our drill press, set at it’s lowest speed, with the chisel held in a drill-press vise. It doesn’t take long to grind a good edge on the inside of the chisel. It takes me more time to get it locked in the chuck than to sharpen one chisel. All you need to do is bring it down until you raise a burr on the outside of the chisel.
The next step is to get rid of all the grinding marks on the outside of the chisel. I stick a piece of 150 grit sandpaper to thecast iron bed of one of the machines in the shop, and rub the faces of the chisel on it. I start with the clearance slot facing up and give each face the same number of strokes.
I do twenty-seven on each face because I like the number, but anywhere from 20-30 will do. The idea is to take an equal amount off of each side. Don’t worry about making the chisel smaller than its nominal size. You may have to go around 2 or 3 times, but this is a once in a tool’s lifetime operation.
After the sandpaper, switch to the finer cone at the drill press, and hit the inside again. Like the first round with the cone, you only need to grind for a couple seconds to raise a burr on the outside. Then work your way around the outside on a medium grit diamond (or any other) sharpening stone.
I tried seeing what going to a finer grit would do, but measurable improvements in the working of the chisel stopped about #1000 grit. You can keep going to finer grits if you want, but you’ll only have a shinier chisel, not a better working one. My grandfather used to call that “polishing the polishing”
The last step is to file a notch on each of the inside corners with a triangular file. I wish I could take credit for thinking of this, but it is a feature found on some chisels. I first saw it on the premium chisel set from Lee Valley. Depending on the hardness of the chisel, this filing may take a while, but I think the results are worth it.
This provides some more clearance between the outside of the auger bit and the inside of the chisel. I also think it makes it easier to plunge the corners of the chisel into the wood to start the cut. The benefits of doing this aren’t as great as the benefits of smoothing the outside of the chisel, but it doesn’t hurt, and in some species of wood may really help.
Once you’ve taken a chisel through this procedure, you shouldn’t have to go through it again. You will need to dress the edges occassionally. This won’t take more than a minute or two. Twirl the fine cone inside the end of the chisel by hand to raise a burr, then take a few strokes (I like 9 in this case) on each face with a sharpening stone.
Here’s a picture of the sharpened chisel, along with the chips it made. The chips are a lot smaller, and the chisel is a little easier to plunge into the work. There isn’t a tremendous difference in the amount of force you need to make the cut, but the bit and chisel will run considerably cooler.
Heat is the real enemy, and in addition to dressing the outside of the bit, the other important consideration is leaving a big enough gap between the bit and chisel. Most bit manufacturers and published advice suggest a gap that is way too small-a credit card or dime sized gap will likely smoke and burn. Set the gap a bit bigger, and you’ll enjoy much better results.