Ichimonji Genno Japanese Hand Made Hammer by Kouzaburo - 45 Monme

According to information provided by Tsuchida san, Japanese genno (hammers) are traditionally classified into four size categories:

Mame-genno (豆玄翁): Refers to 20 to 30 monme (approx. 75–112.5 g). Used for driving nails and other small hardware.

Ko-genno (小玄翁): Refers to 50 to 80 monme (approx. 187.5–300 g). Used for striking oire-nomi (bench chisels).

Chu-genno (中玄翁): Refers to 100 to 120 monme (approx. 375–450 g). Used for versatile, general-purpose tasks.

Dai-genno (大玄翁): Refers to 150 monme or heavier (approx. 562.5 g and above). Used for striking heavy timber chisels (tataki-nomi / atsunomi).

Mame-genno are also well-suited for performing uradashi on smaller blades, such as miniature planes (ko-ganna), carving chisels, flat woodcarving knives, and kiridashi knives.

Furthermore, Tsuchida-san recommends using a 30 to 50 monme genno when performing uradashi on standard size (like 60，65，70mm) plane blades. Although this process is slower, he prefers delivering lighter, more frequent strikes to induce plastic deformation within the jigane (soft iron backing)—which gently draws the hard steel cutting edge into a minute curve—rather than using a heavier hammer to force the front of the blade to bend rapidly.

In my own observation and practice, while the two approaches may appear identical on the surface, the underlying mechanical intent differs: the former is focused on forcing the tip to bend directly in the direction of the blow, whereas the latter is focused on coaxing the jigane to elongate first, allowing the blade's edge to curve naturally in the direction of the force.