Thursday, March 15, 2007

Burr Management aka Sharpening

Sharpening is a mystery largely because a sharp edge lives in a world to small for the naked eye to see. We end up checking the results of our efforts on the wood and relying on process because we can't see what is actually going on without magnification.
Besides coming up with a process that works, (for me it generally relies on grinding the shape and then honing the bevel on waterstones) I've found that an understanding of what happens in that microscopic world can have great results in my full sized work.
On that micro level, even the most highly polished and finely shaped edge is really a series of jagged teeth, like a mountain range, made up of space as much as steel. In use, these teeth can break and bend, leaving a dull edge. The finer that I hone my edge, the smaller the teeth become and the less likely they are to break of or bend. It could be said that the sharper the edge is, the sharper it will stay.
The first key to getting the edge to be so sharp is to get the burr (the tiny unsupported edge of metal that bends over as you hone) to fall off on its own. This is done by switching from the front of the bevel to the back as you work your way through the grits of stones. What this does, is to abrade the tiny web of metal that is attaching the burr to the edge. Pretty soon it becomes so thin that the burr will detach. This is a very important opportunity to get an even finer edge. After the burr has fallen off, continue to pull the blade on the finest stone for a few strokes, keeping the bevel flat on the stone. Do the same on the back. The stone should be drying out and the slurry turning into a fine dry paste. This will polish the edge and burnish it to a higher grit than the stone is rated for. The edge should be ready to cut the end grain of a pine board to test for sharpness.
Often, after using a blade for a while, I'll notice a decline in the edges performance. It may be that the edge is dull only because those tiny teeth have deformed and seem to form another burr. So I go back to my finest stones and repeat the process of dropping the burr. It's a real time and metal saver to realize that you may still be able to keep the edge sharp just by managing the burr. Of course, at some point (sooner than we like to think!) the edge isn't just deformed, it's worn off, and the process starts over.
Choosing the correct geometry for the intended use will also extend the usefullness of an edge. More on this and microbevels soon.

1 comment:

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