|When you sharpen on a stone or sandpaper the hard flat surface of the abrasive gives accuracy over forgiveness. You either are touching the edge of your tool to the abrasive or you are not. Also, as you sharpen it's pretty common to apply pressure at the very edge of the blade and your bevel will slowly acquire a very slight belly. What this means is that the next time you sharpen there is a good chance your solid contact with a stone is actually a solid contact with the tool belly, not the cutting edge. Sometimes slurry gets caught in the gap between bell and edge and you sort of polish the cutting edge, but you don't remove much material and you don't get a burr. |
This issue applies no matter if you are free handing or using a jig. It's just pretty possible to find yourself spending lots of time sharpening a belly that has no effect on the cutting edge. Most annoying. If you are actually sharpening at the cutting edge, and there is no microbevel or flat to remove first you should be able to turn a wire edge in theoretically one stroke, in practice a couple more. Just polishing the edge means slurry is working but you don't have solid contact at the cutting edge.
There are three simple solutions to this when freehand sharpening (I don't know if there is a solution when using a jig other than shift the tool in the jig). The first is to hollow grind the bevel. The second is always get make sure there is pressure on the very tip of the tool as you sharpen. The third is to lift the tool slightly to ensure contact at the very tip and form a secondary bevel. In the first case, hollow grinding makes it easy to bear on the front edge but you also need a grinder (which I consider an essential shop tool). In the latter cases, over time you will form a belly on the bevel or a series of secondary bevels and each time you sharpen you need to raise the tool a little more. When it becomes hard to continue sharpening or the belly/bevels get so pronounced that the angle of cut gets too high, just go back to square one and hollow grind. Japanese tools are flat ground not hollow ground, but the softer backing material makes developing the belly harder in first place.
97% of the time when someone complains they can't sharpen and they spent hours on it, it turns out that they were sharpening the belly of the bevel and never came close to the cutting edge. This can be because they have their jig set wrong or they are applying pressure in the wrong place. Some teachers recommend embracing the belly and recommend a stroke that starts at the belly and eventually wipes out at the edge. This works but I think it's a waste of effort.
For a lesson in good technique, see my teacher Maurice Fraser explain how to do it consistently. Check out the free website and the video.
Next time I will talk about how stropping can mitigate some of the rounding belly issues.
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