The best carpenters make the fewest chips.~English proverb, c.1500s
Japanese mortise chisels are called “Mukomachi Nomi” 向待鑿. I am unsure of the origin of the name, but the Chinese characters can be read as meaning “wait over there.” A curious name, it may refer to the shape of the transition from blade to neck, called a “machi” which is unique in Japanese chisels. I will simply call them “mortise chisels.”
Mortise chisels are single-purpose tools for cutting rectangular holes in wood for mortise and tenon joints, the oldest recorded wood joint known.
Unlike other Japanese chisels, and even Western mortise chisels, the sides of the Japanese mortise chisel are shaped square to the “flat” instead of being angled slightly less than 90 degrees. The surfaces of the sides are of course straight along their length, but are either flat or slightly hollow across their width.
Other varieties of chisels have sides angled inwards to prevent the chisel from binding in the cut. This is less than ideal, however, when cutting small mortises because it allows the chisel to twist inside the mortise scoring the sides and reducing precision. The Japanese philosophy is that the blade’s sides should shave and clean the mortise at the same time it is cutting it so the sides don’t require additional cleanup with a paring chisel. Its a matter of precision and efficiency.
The straight flat sides of the mortise chisel have a relatively larger surface area that can create a lot of friction in the cut making extraction difficult in some cases, so the standard maximum width is 15mm.
Many advocate using double bevel cutting edges for Western mortise chisels. I have no problem with double bevels for atsunomi used to cut wide, deep mortises because the double bevel tends to kick more waste out of the mortise hole than a single flat bevel, although double bevels are more trouble to sharpen. But in the case of the standard Japanese mortise chisel, I recommend using a simple flat bevel for two reasons:
The first reason is that, since sharpness is critical for precise work, and a flat bevel is quicker and easier to sharpen, a flat bevel is more precise.
The second reason is that a flat bevel tends to stabilize the chisel in the cut more than a double bevel blade can, keeping it from twisting out of alignment and gouging the sides.
The lubrication provide by an oilpot makes using a mortise chisel quicker and the final product cleaner and more precise. Please see my previous post on the subject. https://covingtonsons.home.blog/2019/05/09/the-essential-oilpot/
The mortise chisel is a specialist chisel for joinery, cabinetmaking and furniture work. It is not generally used by carpenters. Craftsmen that routinely use mortise chisels work to much tighter tolerances than most woodworkers, so a professional-grade mortise chisel must be forged and shaped to tighter tolerances than other chisels.
I only have one blacksmith with the skills and attention to detail required to make mortise chisels to my specifications. He thinks I’m a prissy pink princess. I think he’s a stubborn old fart. We’re like an old married couple（ツ).
If you need to cut lots of precise mortise holes quickly, then this tool will definitely improve your results and increase your satisfaction. It may not be the most handsome chisel in your toolchest, but you will come to rely on it more than any other for quality joinery work.
Standard widths for mortise chisels are 3mm, 4.5mm, 6mm, 7.5mm, 9mm, 12mm, and 15mm, but Sukezane won’t make 15mm mortise chisels for me anymore, dagnabit.
More than any other, mortise chisels are subtle, intelligent beasties, or at least they can be. I will talk more about what to look for in a good mortise chisel, as well as how to realize their Einstein-like focus to help you do better work, in future posts.
Links to Other Posts in this Series
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