Definitions of edge, apex and bevel
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This is a follow-up to another topic (https://knife.wickededgeusa.com/forums/topic/strop-vs-lapping-film/page/2/) that was never continued.
Curtis referred me to definitions at the blog Science of Sharp about sharpness and keenness. The definitions are as follows:
A) The thinness of the edge (sharpness) as quantified by the edge width at 3 microns from the apex.
B) The edge width or fineness of the edge (keenness) as measured at the very apex of the bevel.
We started off with definitions of sharpness and keenness. I don’t 100% understand these definitions (English is not my first language) and my interpretation is as follows:
- Sharpness is simply the thickness of the edge (or 3 microns from the edge), i.e. of the knife at the very top.
- Keenness is the thickness bottom of the apex. The apex (not defined in the blog) is the triangle consisting of the edge as the top point and the two points where the sharpened sides of the knife transfer into the non-sharpened sides as the other two points.
Can anybody tell me whether this is correct?
Also, can anybody tell me what a bevel is? If I interpret Merriam-Websters dictionary correctly it is the slanted line from a bottom corner of an apex (in my above interpretation) to the edge. But Merriam-Webster’s gives multiple definitions and I’m not even sure I understand this definition.
Molecule Polishing: my blog about sharpening with the Wicked Edge
Mark, here is my hand drawn depiction of what you are asking. Even though it is rough and crude I believe I have the nomenclature correct and the labeling or identification of the parts correct. We’ll go with this until someone more knowledgeable weighs in and corrects it.
Marc
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Mark,
The way I interpret Todd’s definition in a practical sense is this:
You can have a very keen edge, measured by the width at the very apex (point where the two bevels intersect) but have a larger included angle. In this instance, the blade is keen but not necessarily sharp enough to cut whatever it is you’re trying to cut. If you thinned the bevel behind the edge to a lower included angle, then the edge would be both keen and sharp.
I visualize the bevels as two non-parallel planes, or surfaces. If they can be shaped such that they meet at a point, producing a keen edge or apex, which is simply the intersection of those planes. If, for some reason, the planes don’t actually intersect and the actual edge lies somewhere below the projected intersection point, the edge will be wider and will not be very keen. That scenario can happen on a blade that has a lower included angle and is somewhat sharp for certain tasks but would not be able to perform the HHT. Conversely, you can have a blade that is keen but too wide behind the edge to pass the HHT. The apex would be small enough to get in between the cuticle scales but would be too thick behind the edge to complete the cut. The width of the shoulders would start to distribute the force applied by the blade to areas adjacent the actually cut and would cause the hair to bend away instead of being cut.
So to pass the HHT, you need a blade that is both keen and sharp.
-Clay
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You can make a 90 degree inclusive angle very sharp, but it will not be keen enough to cut much of anything without a serious amount of force.
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But how could you get that scenario? If you sharpen your knives well, the bevels always meet, don’t they? Or am I overseeing something?
Isn’t that what we experience as we use a knife and the apex wears, the geometry hasn’t changed it just doesn’t intersect as closely, precisely …keenly?
Marc
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But how could you get that scenario? If you sharpen your knives well, the bevels always meet, don’t they? Or am I overseeing something?
MarcH is right that wear will produce this result. Another way it can happen is by incomplete bevel formation because you didn’t sharpen your knives well. To the naked eye, the bevels might look like they meet but actually fall short of intersecting at a point, which is why it’s important to use magnification when forming your bevels, especially if you’re trying to do it without creating a burr at some stage. The image below explains it better.
-Clay
Thanks guys! That explains it completely. And it has improved my English
.One reason the explanation Clay gave did not come up in my is that I always make sure to get a burr, and then gradually remove it (with finer grits and sometime stropping), so that the bevels always meet.
Molecule Polishing: my blog about sharpening with the Wicked Edge
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