One of the objectives of metallurgists as they are developing better HSS (high-speed steels) is to get a more uniform distribution of fine microcrystalline carbides throughout the HSS. This makes the steel far more wear resistant, so the time between tool sharpening is greatly extended. But it is not without its attendant difficulties, namely the sharpening of the carbide grains distributed throughout the HSS.
Most woodturners prefer aluminum-oxide sharpening wheels, which wear away faster than silicon carbide wheels, but don't heat the steel as badly. But, when the aluminum-oxide wheel encounters the microcrystalline grains of carbide in the HSS, the carbide isn't sharpened per se, it is worn down slightly, but definitely not sharpened. At some point, the micro-grain of carbide is sticking out of the steel matrix enough that the grinding wheel simply tears it out of the steel matrix. So, one can say that an aluminum-oxide wheel exposes new carbide grains, dulls them, and then tears them out! Perhaps an oversimplification, but close enough for everyone to understand. And it isn't optimal sharpening of HSS.
If you have ever ground a pure carbide cutter on a green wheel (silicon carbide) you will think CNB is the best thing since sliced bread. CBN wheels are far harder than carbide, and readily sharpen carbide. So, unlike with aluminum-oxide or silicon carbide grinding wheels, when grinding a HSS woodturning tool with a CBN wheel, as the microcrystalline carbide grains are exposed they don't get worn down, they get sharpened to the shape of the edge, and they don't get torn out of the matrix so much. Because CBN actually sharpens carbide microcrystals, on HSS the CBN wheel is a far superior abrasive than aluminum oxide or silicon carbide.
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