How do I sharpen mortising chisels?
The chisels on my hollow-chisel mortiser have dulled, making an already strenuous task more difficult. Because the bevel is on the inside, I'm not sure how to sharpen them. Can you offer advice?
—Philip Lewis, Wallingford, Conn.
If you've found mortising a strenuous operation even before your chisels dulled, Phillip, it's likely that yours were never properly sharpened to begin with. Mortising chisels, like many other bladed tools, often require an initial sharpening.
Pick up a mortise chisel sharpening kit like the one from Rockler, shown above (item #24727, $29.95, 800-279-4441, rockler.com). It comes with a quick-change handle as well as coarse and fine cone-shaped diamond hones capable of sharpening up to 1⁄2 " chisels.
Start by marking the inside bevel of the mortising chisel with a dark-colored felt-tipped pen; then clamp the chisel in your vise. Install the coarse cone in either the included handle or your cordless drill and turn the cone slowly in the end of the hollow chisel until you've removed all pen marks.
Remove the chisel from your vise and rub the outside edges of the chisel on adhesive-backed, 220-grit sandpaper applied to your tablesaw wing or other hard, dead-flat surface, as shown below. Rub equally on all four sides until the burr disappears (10–15 strokes). Don't worry about making the chisel smaller. This step shouldn't have to be repeated in the chisel's lifetime.
To hone the chisel, switch to the fine cone and sharpen for a few seconds until you raise a burr. Then remove the burr as before, this time with a 1,000-grit sharpening stone.
Once you've used these chisels, you'll never want to let them get dull again. So touch them up once in a while; skip the coarse cone and go straight to the fine cone and stone for a quick honing before returning to mortising.