|Tanto lessons learned
||[Apr. 23rd, 2003|05:40 pm]
I've learned some things over the last few weeks working on tanto blades.|
1) before adjusting curvature or messing with the straighness of a hardened blade, temper temper temper. Despite the relatively soft bits from the hardening process, the stresses are just too much for an untempered blade, and it will go "ping." Lost two blades this way.
2) When doing your final polishing on a grinder, keep well away from the edge. Hand polish only. Ended up distempering two edges this way, should have known better. Fortunately it's relatively easy to re-harden, and with such a low carbon content on the 1050, grain growth is minimal. I can prove it, one of the rehardened blades was one of the ones that broke, beautiful grain on it. Sigh.
3) backs are hard. Getting all four or five (or 6 or 7) planes on a tanto or other japanese blade all straight while keeping the grinding lines sharp and without effecting the curvature of the blade is hard. lay out everything ahead of time, scribe your lines so they're visible, take it slow, and leave room for error (i.e. do the back first, it's the hardest (for me, at least) to get lined up and "perfect," then do the edge.)
4) Getting the sori (curvature) of the blade is a science. The more metal exposed at the spine during heat treatment the greater the negative curvature is, the less covered the greater the positive curvature will be. Angle the back, though, and since the metal has less mass than if you left it flat, it will produce a much shallower sori than if the back is just a flat plane. Leave too much off, you get a negative sori -- the blade starts to look like a sickle. Leave on too much, it bends the other way, and you end up with a U-shaped blade, or more likely a broken one as the stress of the curvature shatters then pulls apart the edge.
Sigh. Lets see how many more blades I can break trying to perfect this technique. So far, this time around, it's been 3 broken, 2 damaged, leaving me with three good blades and one more I'm going to try to get finished before showing them to a client tomorrow. I think over all for all the tanto blades I've tried over the years I'm at about a 50% failure rate -- which sounds horrible, until you discover that traditional Japanese bladesmiths have a similar failure rate on their swords, and they've all been through an 8 year appreticeship. Still, these are short blades, not long swords, and I feel I should be doing better.