Timber Framers Guild Forums General General Forum Questions corner chisel

I recently picked up a 3/4 corner chisel that needs to be sharpened. There is a small point at the inside corner and the two sides are uneven. Should the sides be at 90 degrees to the back edge or should they slope away from the point towards the outer edges? Any help or hints would be appreciated, Thanks, John.

John,

I have used corner chisels for years and am one of the "true believers" in them. That said, they are a pain in the seat cusion when it comes to keeping them tuned.

I think the "point" you find on a lot of corner chisels is the result of bad sharpening, often done on a bench grinder believe it or not. For that very reason, a lot of corner chisels are out there that have lost their temper at the cutting edge. Even if you sharpen it well, it may not keep an edge.

I start sharpening a used corner chisel by filing the edges square to the back. The best way is to stand the chisel straight up on a wide mill bastard and work it back and forth until it's trued up. At this point use a lapping stone to flatten the backs. Then use a file to re-establish the bevels. 25 degrees for softwood and 30 for green hardwood. You might consider going as steep as 35 for recycled oak or hickory. To keep the file from dinging up the bevel that you aren't trying to sharpen, make it "safe" by grinding off the serations on the edges. Always push the chisel from the socket end towards the cutting edge. Once the bevels are filed (use a finer file to finish) you will have a small wire edge on the bevels. DON'T break it off!

Lapping the bevels and honing the micro-bevel is a little harder to explain, but will be obvious as soon as you try it. Hold the chisel against the corner of your waterstone so that one bevel is laying flat agaisnt the stone on top and the other is draped over the side. You will need to swing the chisel out so that the bevel you aren't honing doesnt contact the side of the stone. Draw the chisel toward you reatedly until the wire edge starts to "loosen". Now rotate the chisel up 5 degrees so that only the cutting edge you want to hone is contacting the stone. Pull it towards you. Repeat as needed until the wire edge falls off and your micro bevel is honed. Now switch the chisel to the other edge of the stone and repeat.

Viola! You're ready to cut beautiful mortices.

Enjoy your sharpening experience and take your time!

Rudy

Dear John:
When I got my corner chisel, I was told it was suppose to have a small point at the corner where the two sides met. (I'm sure there are others who agree with Rudy. And I'm not saying he's wrong, just a different point of view.)
But it's my understanding that both sides should be even in length.
Otherwise his sharpening instructions are what I was taught also.
Good luck with your new (old) tool.
Jim

The pointy bit on a corner chisel is not intentionally created. It comes from sharpening the chisel on a stone that has an edge that is not a perfect 90-degree angle.

Take a look at most stones (even new ones) and you'll see that its long edges are a little rounded over. (Old stones are often very rounded, and that's probably why old corner chisel's have large nibs!)

A stone woth a rounded edge causes less steel to be removed from the part of the chisel that is closest to the edge of the stone (and more steel is removed from the chisel that is farther from the edge of the stone). Since less steel is removed at the inside corner than at the chisel's straight edge, a nib is produced.

Another way to visualize this is that the chisel simply becomes a mirror image of the shape of the stone that it is sharpened on.

The nib causes no particular harm, and probably has no substantial benefits either.

The two sharpened edges should be the same length, have the same angle, and the same bevel. But the edges do not really have to be 90 degrees to the length of the chisel--they could trail off somewhat without causing a problem in use.

My best
Robert