Is this why people use mortise chisels?

[Stanley Covington]

Of course, both of Warren's points have hit the nail on the head.

Try 30, and maybe even 35 degrees.

You didn't mention what variety of wood you were chopping.

New chisels are often a bit more brittle than the specified hardness at the extreme edge, and exhibit small fractures and/or chipping. They often calm down after being sharpened a few times. I think this is the same thing Bill said. This is not necessarily a bad thing. A new chisel that is too hard at the extreme cutting edge when new, may well improve after a few sharpenings, but one that is too soft and rolls an edge, or develops a burr, or, heaven forfend, dents in use will always be junk, unless it was burnt (lost its temper) during grinding, in which case it too may improve with a few sharpenings. Burnt cutting edges are not that uncommon in consumer-grade chisels, in my experience, but this trait is not consistent with decent quality control, and does not say good things about the manufacturer. Caveat emptor.

It is human nature to try to maximize chisel-work efficiency by driving the chisel in as far as it will go each pass. This results in the chisel becoming jammed into the wood, and putting high lateral pressure on the thin cutting edge. Wiggling and wrenching the chisel to free it will then create small fractures at the cutting edge, and chipping can easily result too.

Pay attention to the chisel's progress as it cuts the mortise, and stop short of jamming it into the wood too tightly. If you observe the pressure acting on your blade closely, you will notice that reducing your chisel's depth of penetration by 0.5~1.0 hammer whacks will make it easier to extract the chisel from the mortise each time, improving the rhythm of your work, speeding up the overall process, and ultimately extending the useful life of your cutting edges.

At one time my job required me to cut over a thousand small mortises everyday, and I learned these lessons well:
1. Rhythm is very important when you have a lot of mortises to cut;
2. Precision matters, and rhythm aids precision;
3. Consistency matters, and rhythm and precision aid consistency;
4. Like a dancer with a sore toe, a dull/chipped/poor-quality chisel is neither rhythmic, precise, nor consistent, so fettling one's chisel and keeping it sharper longer improves one's productivity (and reduces stress) significantly;
5. Using an oilpot makes it easier to extract the blade without a lot of wiggling and wrenching, and will improve rhythm, precision, and consistency;
6. Using a chisel to pry out chips and scrape the mortise's bottom will damage and dull the cutting edge quickly. Chisels are not designed for such abuse;
7. One's chisel is an extension of one's mind, so chisels have feelings too. Be kind to your chisel and it will reciprocate. An ornery chisel is as helpful as a whipped dog.

There are several different types of mortise chisels. I own and have used most of those but like the Japanese variety best. It is almost perfectly rectangular in cross section. With this design, the chisel's sides aid with aligning the chisel and cleaning the mortise's sides, significantly aiding precision and speed. Whichever style you prefer, check to make sure the blade's cross section is absolutely symmetrical across the blade's width measured from the blade's centerline, and that the angle between each side and the blade's face (flat) is less than or equal to 90 degrees. Irregularities will cause the blade to skew in the cut, gouging the mortise's sides, and making the mortise skewampus. Of course, you have seen this happen. If your chisel's blade, be it mortise, firmer, sash, butt, oire, tataki, or whatever, is not symmetrical, I strongly recommend you make it symmetrical before tackling many mortises.

The other dimension to check is the blade's width. It doesn't really matter whether the blade's cutting edge is precisely 6.00mm, or 5.96mm wide. What does matter is that the blade is EITHER the same width over its entire length, OR, that its width tapers slightly (just a little mind you) gradually being narrower near the handle than at the cutting edge. If the blade is wider near the handle, it will bind in the mortise. All is lost.

In addition, the blade's sides need to be very straight, not curved, or wavy. If the blade's sides are wavy, swollen, or hollow, it will not cut quickly and precisely, but will tilt, gouge, and bind. Adjust these dimensions if necessary.

When I was a young man, I was scolded by a old man for abusing my mortise chisels. Of course, he was gruff and abrupt, and I was prickly and easily offended, which did not help me see the error of my ways. But the undeniable fact was that, despite being blind in one eye and not able to see out of t'other, as they say, the old man still cut mortises quicker, more precisely, and with less apparent effort than I did.

I decided to test the concepts myself. If you doubt what I have written here, you would be wise do likewise. Time yourself on two series of ten mortises each, one set driving the chisel until it becomes jammed with each pass, and the second set stopping short of jamming. Sharpen the blade before starting the first set. Check the condition of your cutting edge after the first set, and take a closeup picture. Resharpen before cutting the second set. After cutting the second set, compare the condition of the two edges. Then compare the quality of the mortises. Then compare the times. All will become clear.

You should also repeat this test for the oilpot. You should then repeat it for a symmetrical and non-symmetrical chisel blade. Ain't science wonderful （ツ）

Two cents, and worth every penny.

Stan