Originally Posted by
Warren Mickley
When we use a mortise chisel, we ride the bevel for many of the cuts. This technique just doesn't work very well with a compound bevel.
Recently I bought a 19th century mortise chisel. It looked like it had not been used for many years, but the seller must have been reading blogs or something, because it was freshly ground to a long 18 degree bevel with a small micro bevel. Because of this I got a chance to try the micro bevel technique.
As one would expect the chisel rode the bevel poorly. One of the important aspects of a mortise chisel is its self jigging quality. When it rides on the bevel it is held securely at a constant angle. And as it moves down, the sides of the chisel engage the walls from previous cuts, also adding jigging and stability. But with the long shallow bevel, much of the thickness was cut away compromising the jigging. There is little gain from having such a thick chisel with nice wide sidewalls if much of it is cut away. Another thing I noticed with the blog configuration was that it had a poor heel that was well up on the chisel, making levering awkward. So three problems: 1) poor riding bevel, 2) poor sidewall engagement, 3) poor heel. And it would be harder to keep a constant angle on the bevel when sharpening. I don't think you will see a Japanese mortise chisel with this configuration.
Last night I altered this chisel to have a flat 25 degree bevel as you suggested, Kees. This worked much better: much more secure, smoother operation, which is easier on the edge, and more secure sharpening. I mortised white oak, jarrah, hickory, and hard maple. I prefer 30 degrees, but 25 is certainly a lot better than a secondary bevel.