Testing for Sharpness

First, I checked for sharpness. It is commonly known that some ultra-0smooth edges will slice through non-fibrous materials better than fibrous ones. But if you think about it, even a piece of meat has fibrous tendons in it. Many vegetables can also be quite fibrous. I needed to find a test material that would be fibrous in nature but still require an extremely sharp edge to cut cleanly. There are many different ideas on the best was to check for sharpness. Some are better than others. At the fore font of sharpness checking demonstrate the sharpness of edges. The main problem is determining and quantifying how easily the edge shaves. Also you end up using more of a shearing cut than a slicing one. Remember, all the edges in the photos will shave hair easily. Slicing paper is also used as a test of sharpness. Most papers are very easy to slice smoothly, but a few delicate papers such as cigarette and onionskin papers are much more difficult.

Several of these papers also seem to be directional in the orientation of the fibers, making them easier to cut in one direction than the other. I tried several brands of paper and found that Zig Zag seemed to be the most consistent and the most difficult to cut across the short dimension.
As I started slicing paper, my misconceptions started surfacing. I had always thought that the smoother that edge was, the better it would cut- period. At a hardness of 58.5 HRC all the edges except D and F would slice the cigarette paper across the short dimension. Just as I had expected, it was most difficult with A, easier with B, easier yet with C and E, but been impossible to verify that all the teeth had been polished off without a microscope. The few teeth left on the edge would allow the blade to start the cut in the paper. That in turn, would mislead me into thinking that the stopped edges both sheared a sliced better than those having a tooth. The real problem with my theories started to come out when I started to look at how the edges wore. Here things can get very complicated.

Testing for Edge Wear

I sliced through corrugated cardboard for my wear tests. I used it because it was cheap, consistent, and available and dulls edges fairly quickly. I used a slicing type action to get through the cardboard instead of shearing it. Slicing is how most knives are used in the real world and dulls a knife faster than shearing. I sliced up 1x 2 squares with each edge finish and then examined them under the microscope. As I expected, edge A cut with the most difficulty while wearing to a larger sharply-toothed edge. It would still shave with difficulty but not cut the paper. B wore to something resembling A, after the same amount of cutting. The larger teeth had sharp edges and were not rounded off. Shaving was quite easy and it would still barely cut the paper. Then it happened. Edge C wore to something resembling edge D and would barely shave at all. Trying to cut the paper was like a bad joke. Remember, this was all done with the same blade.

Then, edge D wore to an edge that still looked like D but with a slightly rounded edge and no shaving at all. Something had happened between edges B and C. So now I had a problem. I thought the problem might be hardness, so I re hardened the blade and left it at 60.0 HRC. Then, the edges performed more like I would have expected. All the edges wore to a coarser tooth, except for D, which still rounded off. Edge C cut so well that I had slice up two additional pieces of cardboard before I started to notice any significant wear. When it did wear this time, it went to a coarser tooth with sharp edges. I was surprised that a hardness difference of only 1.5 HRC would make such a change.

I also did a little testing with a factory stainless. The tests were performed quickly as I was running out of time and only wanted to get a general idea of what was happening. The most important result was that all the edge finishes seemed to wear to a round smooth edge instead of a sharp, larger tooth. Another variable not even considered here is the actual angle at the edge. I am sure that varying the angle will affect the changeover point between the edges that wears to one having sharp teeth or one that becomes smooth. As you can well guess, my lecture at the seminar was a little fragmented and without a great deal of detail. The good part was that I learned a lot and have a completely new direction to explore.

Current Practice

Currently I am sharpening all of my blades to the finest tooth edge I can get that will not wear smooth. If the edge is continually wearing to a coarser tooth having sharp edges, the effective useful life of the edge is increased. If the edge is just rounding off, the useful life will be very short. For most of my steels and Damascus I am finishing with an Ultra Fine grit Spyderco stone. The blade hardness is usually about 60.0 HRC. You might think that 60.0 HRC is too hard and brittle for a blade. I salt bath mar temper and freeze my blades in liquid nitrogen, thus reducing the brittleness problem greatly but, that is the subject for another article. I have eliminated all leather stropping with compound.

A light strop on denim is used to clean the junk out of the teeth formed by the stones, but I want to leave the micro teeth in place. If I have the opportunity to make a knife that will only cut fleshy materials and never anything fibrous, I will consider stropping and polishing the edge. Please remember that these results are only preliminary. Different knives on a different day might show entirely different results. I have a great number of samples to make and tests to run now. This winter I will be running tests to try and figure out exactly what is going on. I will be including samples to investigate how steel choice, heat, treating, edge geometry and surface finish all affect sharpness and edge holding. If you have any comments or suggestions I would be glad to hear them. I am in many ways learning to sharpen all over again.