If you cut a lot of mortise and tenon joints, like I do, a good mortiser can be a very handy tool in your shop. Like most power tools, when its cutters become dull, their effectiveness is greatly reduced. But unlike most power tool cutters, a mortiser’s cutter, comprised of a hollow-chisel and auger, is quickly sharpened using readily available shop tools and techniques. Another important reason to master this technique is that in working with oak, which Arts and Crafts furnituremakers have been known to do, our mortise-cutters dull quickly and frequently. It’s also a good idea to use this technique as a honing step before installing a new mortise-cutter in your mortiser. In this series of posts I’ll show you how easy it is to sharpen a mortise-cutter.
There are two-steps involved in sharpening a mortise-cutter: sharpening the hollow-chisel and sharpening the auger.
SHARPENING THE HOLLOW-CHISEL
Sharpening the hollow-chisel is a two-step process, and you’ve probably heard both steps referred to before: flattening and honing. When these techniques are used to sharpen bench chisels and plane irons they’re generally practiced in that order. But to sharpen the hollow-chisel, I prefer to do them in the opposite order.
Step 1: HONING THE BEVEL
The bevel is honed by removing material using a sharpening tool until a burr is raised. Several options exist for performing this initial step, including the use of large and small sharpening cones, course and fine sharpening cones, two-step sharpening cone systems, one-step sharpening tool systems, and, of course, sharpening stones.
Several ways also exist to implement these bevel honing tools. You can employ a simple handle or a brace or utilize your drill press set-up for approximately 250 RPM. I use either a brace or drill press depending on how much material needs to be removed.
There are a couple of things you should know before jumping in and honing the bevel. First, hollow-chisel manufacturers don’t readily reveal the angle of the inside bevel in their provided literature, but sharpening cone manufacturers do. Therefore, you probably won’t know the angle of the hollow-chisel’s bevel, but you will likely know the angle of the sharpening cone. One without the other is of little value. So the first time sharpening you should shade the bevel with a Sharpie to see how the angle of your hollow chisels and sharpening cones match up.
Shade the bevel. Then you’re ready to begin honing.
Begin by doing some initial honing with a course cone, chucked up in a brace, until you determine where material is being removed on the bevel. If you’re not removing material at the intersection of the bevel and outside edge, you will have to remove enough material until you’re honing on the edge. Visualize this as working on a chisel’s primary bevel before adding a secondary bevel. If this is the case, I prefer to chuck the course sharpening cone up in a drill press, because I may have to remove a significant amount of material. Remember to keep the speed low, and apply a light amount of lubricant.
If you’re fortunate enough to have your sharpening cone’s angle match your hollow chisel’s angle you’ll find shading gone at the intersection of the bevel and the outer edge. In this case, continue sharpening with the sharpening tool chucked up in the brace.
Once you’re sure you’re removing material in the correct place, continue honing until you have raised a burr. You should be able to feel the burr with your finger. When you have raised a burr on all four sides of the hollow chisel, you’re ready to move to the next step: flattening the outside edges.
Step 2: FLATTENING THE OUTSIDE WALLS
To remove the burr and simultaneously flatten the outside walls, I use an oil-stone. But which tool you use is not as important as your technique. As with bench chisel sharpening, whether you use oil stones, water stones, a low-rpm grinder, or whatever is not as important as ensuring your technique creates a pointed intersection of the flattened back (or outside wall in this case) with the bevel.
I prepare the oil-stone with a few drops of oil and then lay the first edge on the stone. Next, I gently pull it down the stone. When I do this I’m conscious of how much resistance there is between the cutter and the stone or how much effort the first stroke requires. I then push the cutter back up the stone and complete another pull stroke. If I have raised an appropriate size burr, which is normally removed on the first stroke, the effort required on the second pull stroke will be significantly less than the first stroke. This is confirmation that initially a burr existed, and it is now gone. Finish flattening the wall with 4 to 5 more strokes and follow this process on the remaining three sides.
If you have coarse and fine sharpening cones, you can repeat the honing and flattening steps with the fine sharpening cone to achieve a keener edge. If you don’t have a two-sharpening cone system, you can use a wet stone to achieve a finer edge. Once done sharpening the hollow-chisel, we’ll move on to sharpening the auger.