OK, this is my take on it, and I'll preface this by saying this is strictly my opinion and is to be taken as such.
Most factory made blades are
sharp. They have a fixed bevel and a generally rough edge, which makes them grabby and enhances the "illusion" of sharpness. Unfortunately that illusion quickly dissipates as the micro-serration's quickly wear/break off and you are left with a dull blade. That edge is the one that easily cuts paper, because it grabs it and tears as it cuts. They are VERY fine tears (breaking through the paper fibers a few at a time) so it all looks like a cut.
Most/many (who knows) custom knife blades are
Sharp. The edge bevel is better refined with fine belts and possibly stone honing before the knife is shipped. This edge will cut extremely well, particularly things like rope, wood, paper, arm hair, etc. It's a great edge and will last a whole lot longer than any factory edge, because there are no micro-serration's to break off. Since the refined edge is actually a LOT sharper than the factory edge (even though the factory edge may FEEL sharper due to the micro-serration's), it is that hair POPPING edge you hear mentioned with good custom knives.
SHARP is what Tom puts on his TnT's. This is a wide, parabolic edge bevel that is highly refined in its finish. I use this on all my knives, and in my experience this is the best cutting and most durable edge available. What this edge does, besides cutting like the Sharp edge above, is to reduce friction through the cut. You can consider a knife edge to be much like a bullet, with the same ballistic qualities and needs in achieving penetration of a target. The nice thing about this parabolic geometry is that there is a very low amount of drag (friction) on the blade as it moves through the material being cut. Looking in only 2 dimensions, there is just the very edge itself and one point on the parabola. With a fixed angle bevel, that drag is all along the bevel and especially at the angled shoulder of the bevel, making the knife work harder to get through the material being cut. (This is actually one of the unstated benefits of a hollow grind when cutting SOME materials. In a fast cut, the sides of the blade may not even be touched by the target.) Low friction makes cutting easier, and makes the knife FEEL sharper than it might actually be. There is an added benefit of this edge, when you think about it a little. It is very strong, because it concentrates more steel just behind the fine edge. I've also found that because of its cutting efficiency, I can use steeper edge angles than you would use in a fixed bevel to get the same cutting effiency in most materials. This adds to the strength of the edge which in turn adds to edge retention as an added benefit. (Using this edge on an Ontario Machete, I've been able to chop down a large number of sapplings with considerable ease and still shave with the edge afterwards. I demonstrated this to some visting knifemakers while at the Blade Show. It's also why I can't keep an machete around the house for my own use. They walk off regularly.) A customer of mine bought his own small belt grinder to use for sharpening, and is now supplying (as Christmas presents this past year) a large part of South Texas with some of the best cutting machetes ever made.
The parabolic edge is achieved when the edge is pressed against a grinding belt, with the sharp portion trailing on the belt. Under tension, the belt deflects in a manner that creates a parabola (I think it's a parabola, but there are a whole lot of years between now and when I learned that stuff in school.)
On his TNT's Tom finishes off with a final polish/stropping on a medium felt wheel, giving that edge a level of refinement that makes it SHARP, SHARP, SHARP, rather than just sharp or Sharp. My TNT cuts better than any knife I've ever owned, and I go out of my way to abuse it.
Anyway, that's my story and I'm sticking to it...
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