The habaki are the metal collar around the base of the blade. It holds the blade in the sheath so that the blade does not rub against the side, causing scratches, and also acts to transfer shock and force from the blade to the handle.
I use 1/8" thick copper as a base material. I measure the blade width and thickness at the spine and add in an eighth of an inch, then cut that amount from some 1" wide copper strips I have using tin snips. I then anneal with a torch and quench. Next, I measure to the midpoint on the habaki and mark out the width of the spine of the blade. I usually have my habaki go about halfway up the blade before the spine cutout, so I mark that point.
Since I'll need to bend the copper to fit the angles of the blade, I take a chisel with a 90 degree angle ground in and carefully incise lines where I need a sharp bend. Using the milling machine, I then remove the material for the notch, squaring up the cut if needed with a needle file. I then anneal and quench.
For the first bend, I just form it with my fingers -- with newly annealed copper, not much force is required, and with the incised lines, the bends are pretty clean. I then place the habaki on the blade and start forming it. I use a 4-oz cross pein hammer, polished of course, and a small anvil for this forming (anvil is also polished.) You have to be very careful with this process -- not only is there the danger of hitting and marring the blade, but remember this blade is pretty damn sharp, and you're holding it in one hand and hitting it with a hammer with the other. Use caution and care if you try this at home.
The forming is done by angling the blade and habaki so that they rest on the flat of the anvil. Strike the habaki with the hammer to get the copper to mate with the lines of the knife, and form the back to match any angles. You need as tight a fit here as possible. Anneal whenever the copper gets springy, I usually need two or three annealings before I'm done with this part.
When the habaki matches the shape of the blade, I take it over to the grinders and just neaten up the edges and brighten up the metal. To make the habaki internally match the shape of the notches on the blade, I make a wedge of copper and fit it into the the edge of the habaki so that the wedge will mate up with the notch when the habaki is in the proper location. If the spine notch was deeper than the thickness of the material of the habaki, you'll need a piece of copper in there to make up the difference. I use a low temperature silver solder, appropriate flux, and a propane torch to do the soldering on my habaki.
After soldering, I fit the habaki in place and make sure everything still fits. Sometimes in soldering a bit of solder will have gotten where it shouldn't have and needs to be removed, or when heated for soldering the habaki will have deformed. This is easily fixed with the hammer and a needle file, or my favorite file, a hacksaw blade. It's really just a very narrow file.
While the habaki is now made, it usually looks pretty ugly, with bit of the copper wedge sticking out, solder marks, scorch marks from the soldering and annealing, and the basis shape is just a square or rectangle. I take it over to the grinder and clean it up, remove any disfigurations, and shape it so that it has some style and grace. I then finish by going over it with a fine belt in the grinder, X16 or so, and get all the grind lines aligned and the angles either sharp or smoothed out, depending on what I'm looking for.
If I have the time, I'll make some habaki covers out of mokume-gane, but for now the copper looks OK, and will oxidize to some nice patinas.
So far, four habaki are completed, only eight left to go!