I have sharpened many kinds of Japanese knives. Some are souvenirs purchased while the clients were in Japan. Others are from Vancouver Japanese knife shops, such as Ai & Om in Vancouver Chinatown and KnifeWear in Vancouver.
Generally speaking, Japanese carbon steels are harder than most stainless steels. However, my equipment can easily sharpen them. I use four Swedish Tormeks, water-cooled and running at a slow 90 rpm, with CBN (Cubic Boron Nitride) wheels, then deburred and honed with a variety of pastes, including diamond. Grinding and honing angles are calibrated within 1/10 of a degree using Vadim Kraichuk’s smartphone software. Knives are held in Tormek knife clamps and are guided along guide bars.
Traditionally, Japanese knives are sharpened by hand. The documentary film, “Jiro Dreams of Sushi”, shows the chefs freehand sharpening knives nightly in the alley. Angle control comes from experience and muscle memory. However, I read somewhere that for freehand sharpening, the angle can deviate by up to 3 degrees.
Angle control is critical to being able to sharpen a knife as sharp as possible. Otherwise, a higher angle would round off the edge. I had to source components from 18 different manufacturers to build my Japanese knife sharpening setup. But the results speak for themselves. I think it was worth it – and it seems my clients think so, too.