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Al with the RC scale close isn't a very good option
I think what Mr. Robinson was talking about is most hand testers don't get closer than +-2 HRC and are usually set for thicker parts. As for those scratch testers they are in increments of 5 points. The Rockwell scale is like the Richter scale, in other words it's exponential. I couldn't live with a 5 point chance of being off. RC 56 is a LOT softer than RC 58! RC 60 is also a lot harder than 58. I can determine hardness with my diamond hone by feel better than that.
It is a non-linear measurement and I haven't found a study that gives precise numbers to the difference between 58 and 59, like is it 40% harder or is it 4x harder? I've seen some makers say that it's 10x because it scaled in decimal points like 59.7, but I haven't been able to find a scientific study that gives that precise value. Suffice to say that the 10x maybe very close. If anyone knows a study that does give the values I would like a link to it.
Now after confusing myself and probably you too Al about the hardness factors, I could add the RC 60 with D2 is going to be different with CPM 154 at 60. I prefer D2 as the Vanadium makes it more wear resistant. But 154 is tougher. Different steels mean different trade offs.
I did make my own tester files when I worked at my last job and they go from 55 to 61.2 in different increments that are not exactly even. Like I go from 57. 3 to 58 to 58.9 to 60, but they work, I don't make a knife under 58, nor will I buy one less than that with some exceptions. A 1080 fighter I will leave softer as edge holding isn't as important as toughness and flexibility. In other words, you don't want it to break if you've been attacked by a bear.
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