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by rlonstein
971 days ago
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I don't disagree at all but this might be a problem with folk knowledge and terminology. I was taught to remove the burr or wire, you could feel it, and took it off with a steel or ceramic stick. My grandfather was a machinist and woodworker and I doubt he knew what the edge looked like. "razor" meant a pass with very high count fine (3k, 6k+) stones and maybe paste (I have green paste... I think it's Chromium Oxide ~45k) and leather strop. I'll do that for my straight razor, I don't for the kitchen knives. |
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Once you've ground an edge, there may be a burr or wire that you can feel. Secondary honing brings that edge to something so fine, you can't see it or feel it, but it's not the edge apex. There is either a wire edge so small you can't see it, a feather edge, or a foil edge. These edges after honing may even pass sharpness tests, but will almost immediately roll and lose sharpness. This is exacerbated by the modern "super" steels and stainless steels that are used in knives today, and that is a whole other avenue of discussion.
For example, take a pretty bog standard Global kitchen knife. This is a high vanadium content stainless steel, so it should be ground with a diamond or CBN wheel. Sharpening with conventional abrasives will actually erode the substrate steel around the vanadium carbides in the blade, giving the edge a toothed appearance that eventually results in carbide breakout during use. Because these knives are an "in-between" burr forming steel (which means they don't necessarily form a positive burr or negative burr) and because the vanadium carbides need to be honed away with diamonds, the recommended method for honing this knife would be to sharpen 12 deg per side edge angle, progressing up to 1000 grit CBN or diamond wheel while grinding into the edge, then de-burr the edge with a slotted paper wheel at 5 micron diamond paste, then with felt wheel running away from the edge, impregnated with diamond spray at 1 micron, and +0.8 degrees edge angle. This will remove the wire edge and deburr the edge down to the root of the edge apex. Then hone with slotted paper wheel with Chromium Oxide (which is about grit level 60,000 or 0.5 micron) on a paper wheel for the last pass. This should read about 40 BESS on a tester, and a SEM photo will show no foil or feather edge. That is about 10-15 BESS sharper than a safety razor, but that is what is required to get rid of the different type of burrs that will cause the knife to dull quickly. These degrees are very precise, free-hand sharpening will not end up with the desired effect.
Anyway, not far off what your grandfather recommended, but somewhat different. According to Dr. Kraichuk's testing, leaving the knife more dull than 50-90 BESS almost always has an undesirable edge burr, be it foil, feather, or wire. This burr will roll over almost immediately under use, bringing the knife to a "working sharp" level of 300 BESS, which is pretty bad.
I can't really summarize this book in a HN comment, but if you're at all interested in this research, check out that book. You can get it on a PDF from the author's website, http://knifegrinders.com.au but the author died suddenly so don't look for any support.