|The grind begins
||[Aug. 28th, 2009|10:21 pm]
I started grinding. Blade profiling is done. Dimensions are 2.25" wide max x 25" of blade, an overall length of 29". Rambling is below.|
Grinding something like a sword takes patience, especially if you're as rough a forger as I am. Oh, I've gotten much better over the years, but even so when finished with the hot work, the sword was covered in little dents and ridges. Knocking off the scale fixed some of that, but there's still a number of spots that will all need to come off.
Unless they're doing primitives, most knifemakers take off the scale to get bright metal on their work. For a short knife, that's usually good enough. For a long bladed hand tool, one which it is reasonable to expect to cut through bone and gristle, even hit armor and not take damage, you need to do more.
Every time a piece of steel is heated above a point in an uncontrolled atmosphere, in addition to the iron oxidizing and forming scale, some of the carbon is also burned off. More carbon migrates out towards the surface from deeper in to replace that which was lost, and pretty soon you have a few thousandth's of an inch of decarburization.
This isn't entirely bad in and of itself, so long as you remove enough metal that it's not on the cutting edge. Heck, in many ways having a lower carbon, and therefore tougher, layer of steel on top on a harder inner core is a great idea, and it is the goal of san mai style construction as well as the whole Japanese lamination tradition from their swords to plane blades. The problem is that it's not controlled. It's uneven, both in thickness and, once you've ground away the scale, it isn't covering the entire blade. If you heat treated it like this, the steel would have different expansion and contraction rates during the heat treatment, and would finish their phase transitions with different densities, resulting in possible warping and even destruction of the blade if the stresses are too great.
The only way I've heard of confirming whether or not you've gotten rid of all your decarb was to etch the blade with nital (nitric acid and alcohol) and examining the grain structure. I don't even know where I can get nital or its ingredients nowadays, and I'm pretty sure buying some would put my name on even more watch lists than it's probably already on. I just keep grinding once I'm past my deepest ding, then use that as a baseline for the rest of the surface. If the blade warps, I did it wrong.
But, to start, profiling. By profiling first, I not only reduce the area I have to grind by a good amount, but can also get a better look at the blank to make sure I've not missed any cracks, warps, or other flaws. This is more important with pattern-welded blades, but with any sword missing a flaw now means a lot more work later.
Setup as always is critical to success. Over the last few years the rubber contact wheels for my grinders have taken a good deal of abuse, so a few weeks ago I started rebuilding them, adding in some thick leather to replace some of the missing rubber, and reattaching some of the floppy bits, and building up and reinforcing other damaged areas using that most wonderful of glues, Barge. It can't legally be shipped airmail. It's so toxic the warning label clearly states inhaling fumes can cause death. Inhaling said fumes is something I am not crazy enough to try, as reportedly "it must be used with good ventilation or else you will be chased around the workshop by the BIG FLYING PURPLE FUME MONSTER" This anecdote is enough warning for me.
I also found a fresh 36 grit 2" x 72" belt on the rack and put it on the big grinder. Yes, that's only 36 chunks of aluminum oxide embedded in a ceramic matrix in a linear inch of sandpaper. It's as coarse as I've found useful, designed for use on hard steels, which it removes at an amazing rate for good long time. "Use belts like they're free" goes the axiom, but the good thing about these belts is that they self-sharpen, so long as you're cutting a hard enough material, meaning that until you wear them down to the ceramic layer, you've still got a nice sharp belt. This one will probably last the entire sword, it finished profiling with hardly any wear.
I got out a container of water and a micro-fiber towel for cooling. The towel was an experiment, as using a meltable plastic to cool steel that was glowing a few seconds ago seems counter intuitive. As it turns out, it holds more than enough water when saturated to keep itself from melting. It certainly beats trying to rig up something to dunk 30" of steel into.
I put on a long-sleeved silk shirt, because white-hot splinters of steel moving at 60+ miles per hour hurt when they hit. Silk, because natural fibers don't melt to your skin like synthetics can, and because this is an old work shirt, retired due to worn out elbows. Kilt, steel toes, respirator, and ready.
I started with the most difficult and important section -- the spine. The spine of the sword is its thickest part, it anchors and lends strength to the cutting edge, it absorbs shock and transmits energy from the hand that wields it to the tip that pierces the target. I wanted a curve, slightly steeper at the tip, feathering out to an almost-but-never-quite-straight as it turns into the tang. Doing it right means that every little ding, every little warp now has to come out. If you find, as I just did as I was writing this, that you missed a tiny little dip in the blade on the spine near the tang, you have to re-grind the entire back to match, or the lines won't be right.
You can't really fix it later. The base grind defines the lines of the blade, sets its silhouette, and the angles of all its planes. The Japanese know it -- the base grind is the first method taught to sword polishing apprentices in the oldest surviving tradition, and until they get it right they are not allowed to move on to finer stones and more advanced techniques. It is so important that nowadays the swordsmith, the person whose name goes on the tang when its all is said and done, usually performs the base grind himself before sending it to the polisher.
The edge is defined in relation to the spine, its curve in harmony with the spine, but with more belly and a higher terminus. In order to accommodate this deeper curve, I did have to grind off a couple of inches of tip, but the blade looks all the better for it. I haven't tackled the handle yet, I'll leave that for almost-last, just before beveling and finishing the tip.
next up, sides and beveling.