Making a Handle for a Japanese Swordsmithing Hammer

Traditional Japanese swordsmithing hammers have rectangular eyes with no taper. The handles are not wedged but are held in place by a compression fit involving careful shaping, hand forged wood (kigoroshi, 木殺し), and soaking in water.

The wood is shaped a mm or two oversized, compressed by hammering, and then driven through the eye. When the wood is soaked in water (or in this case 100% pure tung oil), the cells swell back to their normal size and lock the head securely on the handle. This article will discuss the basic techniques for mounting hammer handles with rectangular Japanese style eyes.

Overview
Shaping the Eye
Shaping the Handle
Chamfering
Compressing & Swelling

Overview

Most of history was forged with very simple equipment made from found and natural materials. The hammer handle in the video below was shaped from a piece of the Green Ash tree that grew through my first blacksmith shop lean-to. I managed to get a piece of a branch when the trees were removed by the property owner and have made a couple of handles from it so far.

Traditionally the wood would be Japanese White Oak, the same material as the body of a kanna plane. Ash, Hickory, White Oak, Red Oak, Hard Maple, and other hardwoods with resilience and some spring would make good alternatives. Look for grain that is straight down the length of the handle, particularly near the head, and clear of knots, checks, and splits.

The procedure below consists of stages designed to monitor and control the proportions as it is reduced to proper size and shape. First the eye end is roughed to size with a slight taper, then the handle end is reduced to size but kept rectangular as well. Then the handle is moved to an octagonal shape by removing the corners evenly, the process repeated to create a 16-sided shape, and then finally to an oval by stages.

Mounting the eye is facilitated by shaping the wood 0.6-1.5mm taller and 0.6mm wider than the eye opening and compressing it by careful hammering before insertion. Wood really only compresses in one direction, from the outside towards its heart, so choosing and fitting the handle must take this into account. The wood cells are “re-inflated” to their normal size by soaking in water which firmly secures the handle to the head. Note that this is different than “over-swelling” a loose head, which only lasts as long as the wood stays wet.

Shaping the Eye

At this point the blank is squared off to the size of the widest point of the eye taper and the starting point of the taper is marked. The taper is very subtle and should become almost parallel at the point where the head will rest. Taper length can vary considerable for larger hammers and can be left longer for future repairs if needed. The length of a mukozuchi handle blank should be about 120cm.

The eye end is slightly tapered out to almost parallel and 2mm deeper than the eye and the corners chamfered. The handle end is tapered down into a straight rectangle the size of the finished handle.

Shaping the Handle

An oval template is carved into the handle end with a kiridashi. The handle should be straight rather than waisted of overly tapered, and relatively narrow even for a large mukozuchi.

The four corners are removed to form an octagonal profile using a kanna.

The eight corners are removed again to form sixteen facets.

The sixteen facets are removed to form thirty-two, and then the remaining high points are smoothed with a fine rasp.

The pommel end is rounded slightly and chamfered as it will sit against the pinkie finger in use.

Chamfering

After final size adjustment, the corners and end of the eye area are chamfered.

A small file is used to round a chamfer all the way around both sides of the eye to a 0.3-1.2mm radius.

Compressing & Swelling

After carefully forging the wood (木殺し, kigoroshi) to compress the remaining 0.6-1.5mm to the depth of the head, the handle is inserted into the head until the taper begins to stop movement.

Extra material should be left protruding above the eye to lock it on, particularly with larger mukozuchi hammers, this is safety insurance for the smith!

The tip of the wood is soaked over a day or two in water or tung oil to swell the wood cells back to their original size.

Once expanded the wood cannot fit back through the eye and should remain secure in use.

Island Blacksmith: Traditionally crafted knives from reclaimed steel.

With a few simple techniques, and careful adjustment it is possible to securely and permanently mount a hammer handle without use of wedges, glue, or epoxy for the purposes of historical swordsmithing.