Edge retention and ease of sharpening

[Cliff Stamp]

It gets argued constantly that edge retention and ease of sharpening are at odds in a knife, that increasing one tends to decrease the other. The first serious opposition to this I saw was made by Alvin Johnson on rec.knives who presented the viewpoint that it was easier to sharpening high quality steels which gave better edge retention because there was actually more difficult to achieve a high sharpness on lower quality steels.

Back a few years ago I ran a blind comparison on a few blades from Ray Kirk and I noticed that if you stopped after cutting a given amount of material, the blades that stayed sharper long were actually faster to sharpen, mainly because they had not blunted as much and thus needed less metal removed to reset the edge. This carried over directly to extended and repeated use and the blades with better edge retention had far less sharpening time.

Recently I compared a small Sebenza in S30V and a Meadowlark in 8Cr13MoV (similar to 440C), on some cardboard. Both knives had the exact same edge profile as I had adjusted them both down to ~5 degrees and set identical micro-bevels. While there was a significant difference in the extent of blunting (surprise!) what was more significant was the responce to sharpening.

After the blunting, which had the blades at less than 25% of optimal sharpness, the Sebenza could be restored with a few passes on a smooth steel to align the edge, and some work on a CrO loaded strop and the edge would be restored to optimal sharpness. While the edge on the Meadowlark could be realigned on the steel just as fast, even with much more stropping (10+ passes), it could not reach optimal sharpness.

When the blades were then used peeling some potatoes and then doing some more stock work, it was apparent that the Sebenza had a much longer lifetime as its restored edge was of a much higher quality than the Meadowlarks which blunted significantly quicker. What I think is happening here is possibly due to the increased wear resistance of the Sebenza which allows the edge to be restored with minimal work as there is little metal lost. In comparison the Meadowlark needs to be honed on a decent abrasive.

As an extreme example, awhile back I compared a really low grade steel, CRK&T Point guard to some other knives on carpet and watched the micro-edge bevel on the Point Guard actually be cut right off by the carpet which again meant it needed much more work in resharpening, alignment wasn't helpful here as the edge has been cut right off.

The same type of performance is also seen in chopping tools. A large chopper or axe needs a really tough and hard steel to give good edge retention, and axes like the GB line and decent bowies made from solid carbon steels tend to have really high edge retention as they resist damage well and thus again sharpening usually requires a minimal amount of material removal.

As a side note geometry wise, by adding a relief it is also possible to both increase edge retention (by extending cutting lifetime) and ease of sharpening - thus in several respects these properties are not the polar opposite often presented. They mainly are in opposition only when you need to adjust an edge profile, or address edge damage.

What would be interesting would be to compare a few steels at different hardness levels and see at what point do the advantages set in. For example 1095 and M2 both at 65 HRC is kind of an easy win for M2, however 1095 at 65 HRC and M2 at 60/62 would be interesting as for example S90V at 60/62 vs BG-42 at 64 HRC.

Examine both rate of initial bunting as well as responce to sharpening, both in terms of alignment vs steel and honing with a mild abrasive.

-Cliff

 

[t1mpani]

It might also be interesting to try S30V in something other than a sebenza--since Chris runs that steel softer than most do.

 

[Cliff Stamp]

Yeah I have other S30V blades, I will do a Rat Trap comparison shortly, and will likely get Phil Wilson to reharden some.

-Cliff

 

[t1mpani]

Edge retention will certainly go up, but ease of resharpening/realignment might suffer. Of course, as you say, perhaps it'll deform less in the first place. Will be interesting to see.

 

[Don M]

Since the advent of diamond stones I have not seen a great difference between blades in how long it takes to restore an edge. A good steel (not soft!) responds fairly readily.

I will say, however, that edge profile (as you mentioned above) plays a very important role. A good edge relief makes the edge last longer and the sharpening go faster. A good convex edge is quick to resharpen on a strop, or if more material needs to be removed, on paper.

The knives I have today hold an edge better AND are much easier to sharpen than what I had many years ago, because of better alloys (with the exception of older Pumas and Bucks) and better edge geometry.

 

[Cliff Stamp]

Edge profile is crucial, J.J. of Razor's edge and Leonard Lee both discuss this at length, Lee does a better job of explaining exactly what you are trying to achieve and goes beyond flat relief grinds the J.J. recommends and mentions hollow grinds for similar purposes.

There are a number of makers who know how to grind really easy to sharpen edges as well, Phil Wilson and David Boye both grind their knives all the way down and then sharpen back so the edges are ~0.005" thick and resharpen trivially. A really nice deep hollow (Alvin Johnson) sharpens nicely as well.

You basically need to pick the steel so that it suffers the least damage (corrosion, deformation, and fracture) and then minimize its profile to get optimal ease of sharpening.

-Cliff

 

[kamagong]

I don't doubt that it is easier to touch up a more wear resistant steel like S30V than 420HC. But what about those who use a knife until it is blunt? Most of the people I know fall into this category. Once a knife gets blunt it is much easier to sharpen a steel like 420HC than S30V. Especially if the user is trying to sharpen the knife with an old-fashioned Arkansas stone. I know that modern steels are easily sharpened with a diamond stone, but casual users don't use modern sharpening materials.

 

[t1mpani]

Average steels are made for average users--and there's no insult intended in that at all. For the typical guy buying a knife at a hardware store, $25.00 for 420HC will probably serve him best. Won't rust, tough, and doesn't require him to be an avid sharpener/knife guy to maintain it. Enthusiasts, or others who demand (note, I didn't say need, I said demand) higher performance, will probably be more willing to buy the equipment they need and adopt the habits that will best serve them. I doubt someone who would be shocked at the price for a DMT benchstone is going to shell out $100.00 for an S30V knife.

 

[Don M]

The "typical" user who lets the knife get blunt before sharpening will have a lot of work no matter what the steel is. But as was stated, they are much less likely to have a highly wear resistant steel, so an Arkansas stone would work, albeit slowly.

A few weeks ago I sharpened some knives belonging to scouts in my troop. I literally have butter knives that are sharper than they were. One in particular I didn't even want to start with the coarse DMT, I took it to the belt sander first. It was a very soft steel, and formed a terrible burr with the sander (and feathered from the edge as well), but it was the only reasonably fast method given how blunt it was. I find this level of "sharpness" to be common with typical users.

 

[Quy4n8]

I agree with kamagong that many folks including myself sharpen only when a knife is really blunt. Maybe not the smartest thing to do, but I think many people do this. If one is using good sharpening devices (DMT’s or maybe water stones) is it that much more difficult to sharpen high end steels over standard stuff (say S30V compared to 420HC) when both are equally blunt?

As a side note geometry wise, by adding a relief it is also possible to both increase edge retention (by extending cutting lifetime) and ease of sharpening
-Cliff

Cliff, why is this so? I understand that having relief will make a knife easier to sharpen as one has less metal to remove, but why would this make the edge last longer? Thanks.

 

[Blop]

I guess it depends on the things you cut. It must be clear, that edge thickness and angle fit to the cutting object.

Another part is, the sharper the knife the less force is needed to cut. If you do some convexing on the edge you can secure the edge because if you cut into say a melone, the sides of the edge split the object and the edge is not touched.

If you look at the V angled edges it is to see, that a broad angle will come down to "dull" much quicker than a smaller angle. Compare Opinels, where this theory is put into reality.

My addition here is:

It was always hardest to resharpen kitchen knifes made of simple steel comparable in mixture to 420HC. The cheaper, the harder.

I guees you can expect better steel quality. Maybe you need a couple of blades custom made from differing steel to see. Instead of this you must expect the cheap knife to be bad in both: edge retention and ease of sharpening.

 

[Jeff Clark]

An accute bevel stays sharper longer because after the edge apex is worn the blade that is left is thinner (sharper) than if you had a more obtuse angle. Think of two triangles. One with a narrow base (with an accute apex on top) and one with a wide base (a less accute apex on top). Cut down the top of both triangles by a quarter inch and the triangle with the narrower base (more accute) will also have a narrower top flat. It is "sharper".

 

[Cliff Stamp]

Once a knife gets blunt it is much easier to sharpen a steel like 420HC than S30V.

The machinability is a lot higher for 420HC, but you can pretty much ignore this when sharpening with proper use of abrasives and micro-bevels. I give knives to my brother all the time and he rarely sharpens them and cuts metals (flashing) and fibreglass insulation.

When I get a knife back the edge will be visibly rolled, often small chips, and heavily worn. I recut the edge with a coarse hone, takes 1-2 minutes, give it a quick polish with a 1000 grit stone and check the edge stability under magnification and then reset the micro-bevel.

The entire process is less than five minutes with steels like S30V on a four inch blade. something like 52100 cuts the initial coarse honing time in half, so I am looking at maybe 4 vs 5 minutes. Time is a bit faster on knives which can be filed and the edge is heavily damaged.

The secret to a low sharpening time is start off with an x-coarse hone and then microbevel to actually hone the blade. If you are sharpening a strip of metal which is 0.1mm wide then any steel is honed extremely fast. The micro-bevel on a S30V blade is set in 1-2 passes, almost immediately.

If you don't have visible damage then you can just rehone the micro-bevel, maybe 1-2 minutes on the Sharpmaker if the knife is really dull. After every few of these sessions you recut the primary edge grind with the x-coarse hone to keep the sharpening time down and the cutting ability high.

Especially if the user is trying to sharpen the knife with an old-fashioned Arkansas stone. I know that modern steels are easily sharpened with a diamond stone, but casual users don't use modern sharpening materials.

You don't need diamonds to hone modern steels aggressively, you just need a coarse hone. I use a 220 grit SiC waterstone which much faster than an x-coarse DMT hone. SiC hones have been around a long time.

I understand that having relief will make a knife easier to sharpen as one has less metal to remove, but why would this make the edge last longer?

There are two reasons, the first one is that it allows you use the knife with more precision and less force thus it keeps the stress lower on the edge. The major one is that since the cutting ability is higher the knife has to be much blunter to reduce the cutting ability to the same level so it will appear to be much sharper even when blunter.

-Cliff

 

[Jeff Clark]

Arkansas stones have a mystique associated with them as the classic way to reach serious sharpness. A lot of people only own one or two Arkansas stones and nothing else. If they have a soft Washita stone that is their quicker hone and their hard Arkansas stone is for putting on a razor finish. The trouble is that even the Washita stone cuts very slowly when you have much material to remove. You just don't want to reprofile an edge with a Washita stone (much less a hard Arkansas stone). Typically some guy with a hard blade and Arkansas stones to work with will get real frustrated.

If you have a nice long coarse silicon carbide bench hone you can reprofile a real hard blade in a reasonable amount of time. It isn't as much fun if the steel is real hard. The blade likes to skip across the surface of the hone or it feels like it is scratching off steel rather than cutting it off. The harder steels don't feel as nice and take a longer time to work. It is also harder to get a pretty bevel when you are working the harder steel. It feels a little nicer if you use oil or water on your hone in this case. A medium-coarse diamond hone feels a little nicer and does a good job. A water stone is another nice choice.

I think it is true that for most people sharpening hard alloys is not fun. Sharpening soft alloys gives quicker results and just feels nicer. With the right equipment and experience the differences seem minor, but for some kid an M2 blade is going to be a real challenge of the will.

 

[klattman]

There are two factors that make sharpening difficult (for me anyway).

1. Large amount of material to remove.
This can be a pain on harder steels, but you do not have to do it very often, and a coarse enough stone usually solves the problem.

2. Once you have the back bevel formed, setting the micro bevel determines the sharpness of the edge. This is much easier to do on harder steels, since there is little material to remove and the steel can stand up to mistakes and not form a soft burr.

You can easily do #1 on a soft blade, but then getting #2 perfect is not easy.

This is why I hate most kitchen knives...

 

[Cliff Stamp]

The harder steels don't feel as nice and take a longer time to work.

Jeff have you ever compared the coarse SiC notron stone vs the coarse AO version. Goddard and others say there is a huge difference. There is no comparison between the coarse AO oilstone and the coarse SiC waterstone, the SiC one is many times over more aggressive.

-Cliff

 

[Jeff Clark]

I was specifically refering to SiC coarse bench hones since those seem to be the cheapest and most common. Additionally they are harder and cut more aggressively than aluminum oxide. I was trying to describe what I figured was a common type of experience. I've got one in my desk drawer at work.

I really prefer to use an aluminum oxide hone since it feels smoother and leaves a cleaner finish. It is also the type that I learned to sharpen with back 47 years ago. I only use oil on my Arkansas stones and my Norton combination India Stone (I think it is aluminum oxide for the coarser hone, but it feels kind of rough, I'll have to go look at the label). I also keep a Norton 4000/8000 waterstone in my office drawer. The Norton waterstone feels coarser and cuts faster than my King 1000/6000 waterstone so I wonder if the Norton uses a SiC grit.

For rough work I use a Shapton Professional (ceramic) 220 grit waterstone. That really cuts fast. I haven't intentionally compared the full array of Norton products. I have a sentimental attachment to the India stones. Back in college there was a summer when I couldn't find a part time job. I went door-to-door sharpening things. Scissors and garden implements I used a mill bastard file. For really dull knives I rough sharpened using a sanding disk chucked into my quarter-inch electric drill or a home made grinder in my garage. For hand work I used my old worn combination oil stone. For an optimized knife edge finish I used a Norton India stone and a little stropping on an old belt loaded with jeweler's rouge. It gave just the right amount of tooth to edges for the kitchen. I also had an India stone slip with rounded sides that fit the coarse serrations of the era. I really learned to hate serrations then since it was so hard to get them sharp and make them look new again.

Maybe I'll take a look at comparing some more Nortons. For the most part I just use my belt sander these days. When I rough sharpen by hand it is at the office or just an experiment.

 

[Carl64]

Altering the edge profile may make a difference, but I am still sticking with low hardness vs. wear resistance being the biggest factor in sharpening ease AND retention of cutting ability.

There is no reason to consider 420hc or 440a or AUS6 some kind of benchmark for sharpening and edge retention. If my dream steel is ever made, that blade will be the benchmark for how a knife should be. That will be "normal" edge retention and sharpening convenience. It's not so hard to imagine that, from that benchmark, another steel could be WORSE in almost all ways.

The logic is if it is easier to sharpen it must be easier to wear down. That decribes an opportunity cost type of graph. Here is one, and this is a standard graphical representation for nearly every choice in the history of the world whether it be production itself, the nature of the product produced, or anything else. Pretend they are labeled edge retention vs. sharpening ease:
http://mason.gmu.edu/~tlidderd/104/gifs/Fig2-2.gif
(this chart is applied to production, but it doesn't matter because it's always the same chart)

The claim that easier sharpening equals less edge retention is only true when wear resistance is the only variable in question, which assumes edge profile, strength-related ratings, and hardness are ideal. That would mean we were choosing among points A through F on the curve in that picture, choosing from the best possible options only to suit the job at hand or personal preference. No answer would be "wrong."

But we are not doing that. We are not on the curve. Most of the steels we have today are nowehere near the curve because sharpening and edge retention are about more than just wearing steel down. You want it to wear down into a particular shape to sharpen it. Resistance to bending and cracking are critical to grinding an object to shape.

In fact, in real life you rarely get exactly on the curve. Life is not perfect, and the accuracy of your equipment and many other factors will keep you just barely short of the curve.

We aren't even close with most of our production knives, not even a Spyderco made with s30v or a Sebenza. We are closer to the X spot on that graph. Everything is a compromise between 2 (or more) things, but that doesn't mean there aren't choices that do BOTH poorly. If you were making knives out of wood you would score pretty poorly on both variables. If you used a bad steel formula the same might happen.

And we are only talking about edge retention vs. sharpening. Concerns about general durability add another dimension. We may have to sacrifice both edge retention and sharpening to get a steel that doesn't break. But if a manufacturer does not understand why both edge retention and sharpening ease can be improved from 440a at the same time, they don't really even know what choices they are making.

 

[Cliff Stamp]

I was specifically refering to SiC coarse bench hones since those seem to be the cheapest and most common.

That is kind of funny, I have been looking for one, you can't buy them here.

For rough work I use a Shapton Professional (ceramic) 220 grit waterstone. That really cuts fast.

So does that wear at all, can you really lean into it?

For the most part I just use my belt sander these days. When I rough sharpen by hand it is at the office or just an experiment.

Same here.

-Cliff

 

[Jeff Clark]

I haven't noticed wear on the Shapton, but I don't use it extensively. I also don't worry much about wear on the waterstone as long as I am not sharpening a scandinavian grind or a chisel. Some minor curvature is desireable when you bevel other blade grinds. The old hone I used in my youth had about a 5 mm depression from end to center. It still worked fine.