WoodNet Forums: Secondary bevels on mortise chisels may not be all that useful?

A while back, I posted the thought that the idea of a primary and secondary bevel angles on a pigsticker mortise chisel might not be that important. Usually, they are constructed so that the primary angle is about 20º, and there is a secondary bevel of about 35º. The idea is that the low primary angle will allow the chisel to penetrate more deeply, but that the higher secondary bevel angle will allow the edge to hold up to the chopping.

The reason that this didn't make much sense to me is that regardless of what the primary angle is, the wood is being cut by the secondary bevel, and the wood fibers being cut have no idea what the primary bevel of the mortise chisel is. So it didn't stand to reason that the geometry of the chisel at a point where it may not even be touching the wood would affect the penetration of the chisel.

The other reason that didn't make much sense is that Japanese mortise chisels uniformly do not have a primary/secondary bevel. The ones that I use and the ones that I have seen have a single bevel, usually about 35º. My feeling is that Japanese woodworkers wanted to get things done just as badly as western woodworkers, so if there was an advantage to a secondary bevel, they would have used it.

But that's the conventional wisdom: the primary bevel helps with the penetration of the chisel into the wood. Of course, there's nothing like real data. Except that I didn't have a pigsticker mortise chisel. I posted a query here on Woodnet, and a few days later I found out that OBG called up Tools for Working Wood and had them send me a Ray Iles 1/4" English mortise chisel! I was flabbergasted, and it just goes to show what a great guy he is.

So here's the experiment. The chisel came with a primary bevel of about 20º, a secondary bevel of about 30º, and a tertiary microbevel of 35º. I laid out a piece of scrap wood with a bunch of lines 1/4" apart to do some chopping. I sharpened the chisel on a Tormek, finishing up with the honing compound on the Tormek's leather wheel. Then I measured the length of the 35º bevel, which was 3/32", lined up the chisel on the line, and gave it four whacks with a big wooden mallet. I repeated the chopping routine four times. This is what the scrap wood looked like when I was done.

It's a bit hard to see in this picture, but hopefully you can see the differences in shading and the lines towards the cutting edge that represent the transition from the 35º tertiary bevel to the 30º secondary bevel, and the transition from the 30º secondary bevel to the 20º primary bevel.

I then took the chisel back to the Tormek and ground it down some more so that the secondary bevel got bigger. I repeated the above routine when the bevel was at 5/32" (honing on leather wheel, four whacks with a wooden mallet, repeat four times). I repeated this with bevel lengths of 7/32", 9/32", and 11/32".

I stopped at this point because after four whacks, the secondary 35º bevel was large enough that the primary 20º bevel wasn't getting to the wood anymore, whereas it had gotten to the wood with the other chopping runs. So I figured that the primary bevel really had nothing to do with the penetration of the chisel at this point. You can see the primary bevel not reaching the wood in this close up picture.

And here you can see how much bigger the secondary bevel is compared to where I first started.

So what were the results? I took a saw and sawed through the chops that I had made.

The black horizontal pencil line was drawn parallel to the surface of the board. There's some variation in the depth of the chops, but it's pretty clear to me that the length of the secondary 35º bevel did not really impact how deeply the chisel penetrated the wood to any great degree. If there was an impact, the first set of chops should have been deeper, and the chops should have become more shallow as the secondary 35º bevel became longer as I ground away at the mortise chisel.

So why would you have the secondary bevel? My guess is that the main reason is that it makes the chisel easier to sharpen if you have it. I certainly got a first hand look at how the sharpening of the chisel got harder as the secondary bevel got bigger. It took me much longer to go from 9/32" to 11/32" than it took to go from 3/32" to 5/32".

So why don't Japanese mortise chisels have a secondary bevel? With Japanese chisels, the sharpening is made easier because the hard steel layer which does the cutting is very thin, and most of the thickness of a Japanese mortise chisel is softer steel, which is very easy to grind away. So with a Japanese mortise chisel it's as if you have the advantage of sharpening just the microbevel without having to have a primary and secondary bevel.

There is a limitation to my experiment, and that is that I didn't have a more precise way of delivering equal blows to the chisel at each stage other than me trying to stay consistent with the chisel pounding. But I did try hard to do it consistently, and the fact that I did sets of four with each bevel length tends to even things out.

Anyway, I have to really thank OBG for sending me the chisel, and I'm going to send it to him now that I'm done with it. I can also say that the Ray Iles English mortise chisel is a really nice chisel to use, even for someone who really likes using Japanese chisels. Even though som amy things are different with the Ray Iles English mortise chisel compared to a Japanese mortise chisel, from the shape of the handle, to the length of the chisel and the blade, to the addition of the secondary and tertiary bevels, it was really nice to use. If anyone is considering a mortise chisel, you should strongly consider this one.

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Hail St. Roy, Full of Grace, The Schwarz is with thee.
Blessed art thou among woodworkers, and blessed is the fruit of thy saw, dovetails.
Holy St. Roy, Master of Chisels, pray for us sharpeners now, and at the hour of planing.
Amen.

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