Hey guys, I just reground a pocket knife (Otter brand from Germany, carbon steel) for fun. It is ~7 thousands of an inch thick behind the very edge which flexes when you cross it over your fingernail. I reground the whole primary bevel right to the apex and polished the whole blade. Afterwards I put a quite big edge bevel on it (~8dps on a sic stone and a 15 dps microbevel on the medium SM rods). I had to destress the edge 3 times before I was able to form a nice and sharp apex.
I wanted to test what this thin knife can handle. Cutting soft wood without knots went quite well, then I cut through a knot and the edge was extremely damaged. I have some (quite bad) pics for you
I guess for cutting only vegetables and stuff it is ok, but for other tasks it is definately ground too thinly...
It looks like the edge deformed behind the apex which points to the edge angle being too low for that steel/application. The apex angle looks fine as it is intact in multiple sections.
Depending on the steel, 8 dps cutting through hard knots with a decent amount of force could over load the steel. Just keep in mind that the stiffness of a steel is cubic with thickness and the strength is quadratic so for example if you went from 8 to 10 dps the stiffness doubles. The reverse is true as well of course, if you go from 10 to 8 you half it.
This is one of the things which isn't that nice about the way the physics works because the cutting ability and edge retention are linear with angle and will tend to increase as the angle decreases, but the strength/stiffness decrease far faster which means you quickly can see durability problems. However there is a big jump between cutting hard knots and fruits, a lot of materials will fall between those two extremes.
Steel makes a difference as well of course, a high apex stability steel at 66/68 HRC has a much higher strength than a high carbide steel at < 55 HRC thus the range of angle applications is influenced by steel as well, but to a much smaller extent.
Thanks for the information. I just wanted to try out how this thin knife performs. It gives you a lot of feeling for edge angles/geometry in terms of different applications. I did another test with a Wetterlings axe. I sharpened it at ~10dps. It deformed heavily when I chopped wood with it (torque power). Afterwards I made the edge a little bit more robust (maybe 17dps). Absolutely no deformations, chipping and still shaving armhair with ease after a lot of heavy wood chopping/cutting.
10 dps is low for an axe unless it is soft wood and/or you are pretty skilled. A full size felling axe is typically ~15 dps for most wood, heavier for knots or very hard/frozen wood so 17 dps is likely close to optimal.
I would note though that take care in judging what angles are required from new knives as they often have less than ideal steel on the edge. In general you often get significant benefit from just repeated sharpening so your true optimal angles are likely a little less than what you find is necessary on a new knife.
But nice work anyway, observations + rationality = knowledge.
_centurio_ wrote:
I guess for cutting only vegetables and stuff it is ok, but for other tasks it is definately ground too thinly...
Well, there is quite a bit of room above cutting vegetables and below wood knots! As you said yourself, your knife was fine on soft wood until you hit the knot.
Having acquired a set of calipers today, I measured my Persistence at ~0.010" at 1mm from the edge, ~0.007" at the ~7 dps edge-bevel. I also run a 15 dps micro-bevel (UF rods) and had no issue cutting through the clamshell packaging the calipers came in. As I don't plan on cutting through any wood knots, that kind of grind is perfectly viable for my EDC knives, and the resulting performance gain is rather large. Large enough for my re-profiled 8Cr13Mov Persistence to obviously dominate any of my more stock knives in cutting performance.
So I think your edge line will flex when you run it over your fingernail?
As a point of consideration, in many places this is critical as a sign the knife is ground properly to cut well, the German Herder knives are devoted strongly to this and heavily praised for cutting ability, ease of sharpening, because of it. Of course it is critical to match the geometry to the steel and task at hand hence why those knives come with warnings not to cut bones and similar.
Cliff, i know :) I also own a Herder Buckels which measures about 0.005 behind the cutting edge. Just wanted to know how Steel Drakes knife compares in terms of flexing to mine.
It is quite sad that there is only 1 company left in Germany that I know of which still produces this kind of knives. The cutting ability of these is simply amazing.
_centurio_ wrote:
It is quite sad that there is only 1 company left in Germany that I know of which still produces this kind of knives. The cutting ability of these is simply amazing.
I think the real issue is demonstrated by what happened with Spyderco put out the Nilakka. With the original grind it was one of the highest performing Spyderco knives (or in fact any knife) I had seen. Now there were a few issues with durability but with a little work to remove the coarse scratches and put on a high polish and then a true-micro bevel to stabilize the apex it was simply fantastic on soft to moderate materials. However there was an over promotion of durability issues because the knife as-boxed could not cut cross grain harder woods and/or knots. But lots of wood working tools do this, all finished carpenters have separate chisels for that. However for knives we often have the one-knife rule and it makes manufacturer have to build in heavy tolerances.
I will say though in the last few years in the West there has been a proliferation of kitchen knives which are radically reground. Now they are not at Herder levels but they are commonly in the 0.005-0.01" range with edge angles of 10-15 dps. In the extreme, when both of these collide you end up with an edge of 0.005"/10 dps with a micro-bevel and the cutting performance/sharpening is very nice. But of course if you try to slam that through a turkey knuckle the edge will ripple. But this goes back to the days where people had chef's knives and a cleaver, a chefs knife wasn't supposed to do everything and thus it could do a reduced set of tasks very well.
0.010" 1mm above the edge is exactly what my damaged knife measures. So I think your edge line will flex when you run it over your fingernail?
Indeed it will. As I said though, the hardest task on edge-stability that I typically put my EDC knives through is cutting through clam-shell packaging, and this geometry hasn't had an issue standing up to that. As a nice bonus, my Persistence now push cuts through two layers of clam-shell packaging with less force than I would have had to use on it cutting through cardboard with its stock geometry.
Basically, since geometry has such a large influence on cutting performance, I wanted to run a geometry that is just sufficient to handle my EDC tasks (with some safety margin) without excessive risk of edge failure. How lean that is depends on what tasks (and safety margin) you have in mind.
old singing straight razors were ground very thin it was said you would pluck your finger on it and could hear it "singing"
Of course because of such a thin grind this vibration could actually crack the edge and ruin what could have been a very fine shaving instrument
I hqve an old straight razor that a buddy gave me. He gets them so sharp on an oilstone and leather strop that you can just touch a hair to the edge and it will split in two with an audible "ping" from the blade.
