Hmm. And all these years I thought the center of spine to stone measurement was just about this much. If it wasn't this much, it was always that much.
OK trig wizzards, how do you hold that "measurement" anyway? Especially around the belly and up to the tip?
Take it from and old timer- thin the bevel out until the edge starts failing while you do what you normally do with your knife. Make it a little steeper than that.
That formula never fails.
Well, I've mostly used it when I want to grind a factory edge without re profiling, but when I want something static that I can keep as sort of a "reference" to come back to after a bit of grinding. So typically I figure the spine height that I need to match the factory angle, and then use spacers like pennies or something similar to make my static reference.
It's also useful if you want to know the angle of a knife that you've got from a customer.
I mean, in both cases, the marker trick would work... But I've found that there's less trial and error by using the calculations than marking the blade, trying an angle, raising a little, marking it again, and so on and so forth.
It just helps out a little bit. It's also fun for those academic kind of pursuits if you're messing with a blade. You can get it to some angle you like, and then what? How do you know which angle it was? What if you wanted to try it on a similar knife?
I mean, I know there's plenty of things people have done that makes knowing the exact angle kind of unnecessary, but it's still nice to know in a lot of cases. One thing I could think of is if you're posting here on BF for advice, and someone asks, "What angle did you sharpen to?" What if you don't know if it's 10 degrees, 15, or 30? The biggest reason I like to do it with spine height is that I can't estimate what angle I've got the knife at very well once you get down past about 30 degrees.
Really, the "trig" is just a more complex way of doing what a lot of people already do by determining angle with spine height. I remember watching or reading something that said with a 1" blade you could calculate the angle by raising the spine 1/16" at a time, and I learned the penny-stacking trick off of a Japanese cooking cutlery DVD. Using the trigonometry may make it seem like it's trying to be much more accurate than it needs to be, but there is actually a bit of simplicity to it when you realize that instead of messing around with markers or pennies, all you have to do is take a couple of measurements and do a calculation.
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