How do I sharpen a Japanese pull saw?
I have a Japanese-style pull saw that's starting to dull. How do I sharpen it?
—John Eisenberg, Toledo, Ohio
It depends on whether the saw teeth were hardened during production, John. Some Japanese saws have impulse-hardened teeth, where a high-frequency heating technique hardens the teeth but not the rest of the blade. These teeth stay sharp several times longer than non-hardened saws; but because they are harder than sharpening files, they can't be resharpened. How do you spot impulse hardening on a saw? The teeth tend to have a bluish discoloration different from the blade body, and if you draw a file over the teeth, they'll scratch the file.
If your saw was not factory hardened, you can sharpen it using a specialty tool called a feather file. Feather files come in several sizes for different tooth counts. The two most common are a no. 3 (75mm) for dozuki saws (reinforced spine, fine teeth for crosscutting), and a no. 4 (100mm) for ryoba saws (dual-edge blade for crosscutting and ripping).
To sharpen with a feather file, clamp your saw in a vise with the teeth just above the jaws. Because these teeth are sharpened in an alternating left-right pattern, you'll sharpen every other tooth from one side, then flip the saw around and sharpen the remaining teeth. Push the file across each individual tooth 3 or 4 times, more if necessary to repair damaged teeth. After completing this step, file tiny secondary bevels on the tips to avoid having them break off during use.
Japanese saw files (set of 2): part no. 06Q10, Woodcraft Supply, 800-225-1153, woodcraft.com.