Twin Mortise Chisel: Nihon-Mukomachi-Nomi

Ever since reading the description of a twin mortising chisel in Odate’s book “Japanese Woodworking Tools” I’ve hoped to some day acquire one.   Its a truism that you can’t always get what you want, and because generally there is more than one way to make mortises a chisel like this definitely isn’t the first thing to go after if you’re trying to build a tool set. But it sure is nice.

I found this one through Yahoo Japan auctions, using Buyee to handle getting it out of the country. Usually I see these chisels half used up and part of a set of other chisels, but at the time I was looking this one popped up, a new chisel from old stock that had sat somewhere for a long time and offered as a single item.

The chisel widths are 6mm, with about a 4mm gap in between.

Up to now I’ve ben cutting twin mortises, but with a single 6mm chisel. Here they are side by side for comparison. The twin mortise chisel is a bit thicker, but it may be just the slightly larger size of chisel you can find between a cabinetmaker’s bench chisels and the larger timber chisels.

So I set about eagerly sharpening, hungry for that first try of a new tool.

For a first rough stone to take the steel to I’ve been using a Beston 400 grit, a nice affordable option. Its been wearing down quickly though, so I’ve lined up a Shapton Pro 320 grit stone to replace it when the time comes.

My 6000 grit Shapton on gass polishing stone is down to a thin little whisp of stone left. They are harder wearing stones but they won’t last forever. A synthetic Kitayama polishing stone is waiting behind it. I’m quite interested to see how it compares, I usually don’t bother with polishing the edge beyond 6000 grit except for sharpening my straight razor or seeing how fine a shaving I can pull with my kanna.

I did run into a disappointing observation with this chisel. The fork of this chisel was spread slightly apart, about 25 thou from the bottom of the slot to the cutting edge.

I gave this a lot of thought, tried cutting a few mortises, and decided the tolerances actually needed addressing for this tool to work properly. I don’t know if this is just something that happened in quenching or poor quality control, but without a fix the chisel would be hard to control and cut tapered mortise walls on the inside. It was disappointing to say the least.

But I didn’t start whacking it with a hammer right away. I have a habit of getting new tools and thinking something is wrong and then correcting it in error. For instance, have you gotten a kanna blade that wasn’t sharpened square to the sides in a new block? I’ve erroneously “fixed” that only to find that it was sharpened that way to match the block. Same thing with a double bladed marking knife…cutting edges that came from the factory at different lengths. Figuring that had to be wrong I “fixed” it and only then realized that it was supposed to be like that for a good reason. The blade has to tilt over a bit when marking so that the back bevel rides plumb against the straight edge, thus the different length of blade so the two cutting edges contact the wood evenly.

So definitely sleep on it before deciding to fix something you think is wrong, give it some real consideration and see how it works in the cut before making twice as much work putting the tool back to right.

Here’s another example, a two inch timber framing chisel I picked up at a small mountain antique store for $20 bucks. For comparison I’ve placed a 30mm timber chisel next to it, makes it look tiny, haha, but its not. I thought it was the most amazing deal until I got it home and noticed that the bottom of the chisel was way convex. Honestly I beat the hell out of this thing with a 3lb sledge hammer trying every technique I possess to get some of the bend out.

The marks on the side weren’t from my straightening attempts, some body had stuck this chisel in a knurled vise jaw and cranked down really hard. Can’t imagine why. It has a thick cutting steel forge laminated in there, wouldn’t budge no matter how I nudged the mild steel around it, and didn’t break even with me hammering the cutting steel directly, which I can’t recommend. Sometimes you need to just set the hammer down and leave it be, the chisel still works, just held at a different angle when chopping down.

So now duly considered of the dangers of ignorant hizumi I stuck my new chisel in a vise and tightened it where I needed to get a bit of deformation to bring the blades back in line. I did this slowly, by degrees of measurement with my dial guage, and thank god it worked. I had to bend the forks in 75 thousandths past where I wanted it to end up, which doesn’t sound like much when you’re measuring by the thou, but it was clear and visible just how much I was straining the chisel to the naked eye. The tool steel lamination on this chisel extends past the slot all the way to the heel of the chisel. I’d really love to see how one of these is forged, because they still manage to get the lamination to wrap the sides of each blade.

A chisel like this needs its own marking guage, one of my favorite little birdie things to make. Its real important to get a good fit of the beam to the guage block, make sure it comes through nice and square, just snug enough not to move when the keyed wedge is loose.

Nice and comfortable in the hand, easy to adjust, and it doesn’t hurt if it looks nice too.

I use tempered finish nails for the pins, sharpened to a chisel point with a file. The thickness of the nails gives you some adjustment room for getting it to exactly fit the chisel.

Now I really need to make some shoji, this thing is begging to be put to use. The appeal of this tool…double the mortises in the same time. Twice the glue area compared to a single mortise, or I imagine you can cut shorter tenons and shallower mortises and have the same joint strength compared to a single mortise. I love it and its  a classic tategu-shi tool.

Don’t despair if this isn’t a tool you can lay your hands on. How about making your own by slotting a stock bench chisel? I know I’ve seen chisels modified like that. You wouldn’t have to grind the slot to the full length of the chisel, just to the depth that you expect to be cutting mortises, probably no more than 1.5″. It would be a good little challenge to make accurate tool surfaces in hardened steel. When I was first starting out I definitely didn’t have money for all the specialty chisels and made my own 1/4″ dovetail chisel and fishtail chisel, even a little 1/8″ mortise chisel. They’re cheapo Harbor Freight steel, but I still use them and carry them in my chisel roll. They’re the mutts of my collection but give good service, so I take pride in using them and my work is the better for it.

4 thoughts on “Twin Mortise Chisel: Nihon-Mukomachi-Nomi”

  1. That American framing chisel is supposed to be convex on the back. You can’t straighten the hardened tool steel without cracking it. Next time maybe you could just sell it to a traditional timber framer before abusing it. If it’s a good chisel it could be worth $100 or more. If you want a flat back framing chisel, buy a Barr or Sorby.

    1. You can move hardened tool steel if it is laminated to a mild steel. But that is beside the point, I was giving examples of dumb things I’ve done to tools because of ignorance, exactly as you point out that it should be convex, I didn’t know that at the time. There are plenty of bloggers that only show perfect work, but we’re not perfect brother, don’t piss on my boots, I’m old enough to know if a tool is too good for me.

  2. Very interesting. Have you considered striking the twin mortise chisel like a tuning fork? Might be useful for more than cutting mortises…

    1. It does ring a bit like a tuning fork when struck, though I’m afraid the pitch will change a little every time it gets sharpened, lol.

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