Once the blade is in rough polish, it is time to create a custom blade collar for it. This important piece of nihonto hardware is made specifically for each blade and is as complex as making a custom fit piece of jewelery. The function of habaki (鎺) are three-fold; the primary purpose is to secure the blade in the wooden scabbard without any pressure on the blade itself, the secondary is to provide a solid shoulder against which to mount the handle and guard, and the tertiary is to provide a stiffened and vibration-dampened flex zone across the transition from tang to blade and decrease the chance of failure at that critical intersection.
A standard habaki is fabricated from two parts; the jacket, which appears to be the entire habaki, and the machigane, a small compound triangular prism shaped wedge that closes the gap where the hamachi bridges the edge and the nakago no hagata of the tang. Most of the habaki is formed by forging, the final adjustments by filing, and the joining of the two parts by soldering or brazing. Once the habaki is fit, the blade is ready for a handle and the accompanying koshirae.
Forging the Blank
A correctly formed tang and blade are necessary prerequisites to making a functional habaki. The widest point of the knife must be at the area where the blade and tang meet on the spine, and a gradual taper in both directions as well as towards the edge ensures proper strength, balance, and the ability to assemble (and disassemble!) the koshirae. One of the main design points when creating habaki is that they should be quite thin at the front, especially near the spine as that is where they will slide as they are sheathed and unsheathed.
Copper is by far the most common material for habaki, but other metals such as silver, shibuichi, and shakudo are also used. Subtle variation in shape and proportion can be used to complement a blade or mounting, as can various patinas, hammer textures, or chisel marks. With the exception of the initial bar shape, most forging may be done cold in between cycles of annealing to soften the work hardened copper.
Filing & Shaping
Forging in advance can save quite a lot of filing time later, and some judicious filing before folding can save a lot of trouble after the habaki is closed up. While the goal is to get as close as possible to the final shape, it is generally a good idea to leave some extra metal as the bending may not go exactly as planned.
Bending & Fitting
The process of bending may require several cycles of annealing and forging depending on the accuracy of the original shaping. The sides are brought up and cold forged to the shape of the tang, being careful to work well back from the blade when hammering. When everything is fit well, the excess is cut and filed away and a small compound wedge called the machigane is forged and shaped to sit against the bottom of the tang in the gap where the habaki comes together.
Soldering & Finishing
Heated in a charcoal fire, hard silver solder is used to join the machigane to the habaki in such a way that the habaki is slightly too small to slide all the way up to the machi. Hammering the copper after soldering hardens the habaki as it stretches it to its final dimensions.