Habaki is a non-ferrous collar for the blade that strengthens the base of the tang and holds the blade tightly in the scabbard. Often made of copper, it is composed of a large jacket and a small wedge that are forged and filed to shape and then soldered together before polishing and patinating. The topic of this post will focus solely on the geometry of the small machigane wedge as it can be a difficult component to study as well as illustrate due to its location and size.
This discussion will not cover all variations of the habaki and its construction, but will provide a general starting point based on my current work flow and understanding. As with all aspects of classical tanto geometry, the habaki is informed by both functional requirements and traditional aesthetics. The focus of this discussion will be the the method of calculating the compound triangular prism shape as it relates to the tang, edge, and hamachi.
For most knifemakers, habaki are one of the most difficult obstacles to the crafting of a classically styled tanto. Though requiring a venture into the realm of silversmiths, habaki are absolutely necessary for nihonto, both from the standpoint of aesthetics, and for the proper construction of a saya, or wooden scabbard.
Tanto HabakiのMachigane Form
Besides working at a small scale, the difficulty of creating machigane is that it is a compound triangular prism with unusual tapers and planes. Fortunately, each of those sides and corners is based on a real measurement that can be taken from the blade and tang with a little spacial-visual understanding. The simplest way to picture the size and shape of the machigane is to imagine extending the blade edge back from the hamachi along the bottom of the tang using copper instead of steel.
The bottom of the prism is a rectangle based on the width of the nakago no ha (edge of the tang), which should be a constant width. The top of the prism should be a peak that follows the direction the edge would take if it continued back into the tang, making allowance for the addition of some fumbari. The triangle face at the front should match the inside face of the hamachi (edge notch), and the back triangle face will be formed between the nakago no ha and the termination of the imaginary extended edge. Perhaps some photos will help clear things up…
Assembly & Soldering
While not the subject of this discussion, the set up for soldering provides some more views of how all the parts fit and align. Some notes on soldering include to clean and flux all surfaces (eg. file and borax paste), to use oxidized or rusted steel wire to create some tension on the parts, to place the solder inside and allow it to flow downwards into the joints, to keep the heat and exposure to oxygen under control, and to air cool before any further working.
One important point of creating habaki is that they must be formed and soldered just slightly undersize, sliding up and stopping with a few millimeters or so remaining before the machi. This is so when they have been soldered and are in a soft, annealed state, they can be cold forged to stretch them to the proper size, work hardening them at the same time.