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.
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
Shaping the Handle
Compressing & Swelling
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.