Why Not Razor Sharp?

[Vivi]

Why do I feel like I'm talking to a brick wall?

Why is it so hard to understand that a blade sharpened to a very acute angle (<20 degrees) will lose its edge faster than a blade at a more obtuse angle (40 degrees), all else being equal?

shallow grind = Thin metal = weaker edge

I don't understand how y'all can't get this concept.

I get this concept in theory.

What you have not demonstrated is how you have seen it applied to real life.

I ask this because I run my knives at much thinner edges than most people would be comfortable, and even under magnification seeing any sort of damage is extremely rare. I use a variety of steels, including D2, S30V, S60V, ZDP189, 13C27, AUS8 and so forth.

In theory, my edges are weak. In the real world, the only thing that I regularly see damage my knives is steel cored co-ax cable.

 

[jeep45238]

Some people will never understand that the "dull" 20* will still perform very well.

Ever think about why you can cut alright to very well with a dulled kitchen knife, but not with a dull butter knife?

If you want "tacticool" knives, go for it. My knives are users, and get used daily. I'm smart enough to realize limitations of my blades. My knives are not machettes, tin snips, pry bars, or screw drivers. I'm also smart enough to realize that whether or not you sharpen with a 20* angle or a 40* angle, the first time you make a cut the edge is dulled - and the thinner the edge, the less resistance there is on the edge.

It's all about selecting the proper tool for the job. You sharpen your knives how you want, I'll sharpen mine how I want, end of story.

P.S. - funny you're talking about "bubbaing" a knife, but your user name when pernounched is "bubba bonky."

 

[Vivi]

well that's awesome. But then the whole concept of "sharpening" it to the best ability of the blade to hold an edge is wasted isn't it?

No it is not, because the thin edge is still thick enough to not fail at the edge in the media I cut (Cardboard, food, plastic wrap, wood, fingernails etc.)

So you're saying you'd rather be cutting with a blade that has a tiny angle that has dulled, more than a wider angled that still is holding it's edge?

No, I'd rather cut with a properly sharpened and maintained thin cutting edge. I strop my EDC at the end of the day and sometimes give it 2-6 passes on a stone or ceramic rod to refresh the microbevel. The whole process takes about 1 minute. It's not time consuming to care for these edges, in fact, it's the opposite. You can reset the edge in literally 4 passes of a fine stone.

This makes no sense to me. Nor does reprofiling all your tactical knives to a "razor" edge of 10 degrees???? And then letting those edges dull????? What??

Tactical means different things to different people. To me it simply means a well built, reliable knife with modern features such as a clip and lock. I have no desire for 40 degree inclusive edges, even on my choppers and axes. It gives me no benefit over a 16 degree inclusive edge in my uses, all it does is make cutting less efficient for me. I do not let my edges dull, and have not stated such anywhere in this thread.

I don't get you people. Unless you're going to shave, literally shave with the thing, leave the poor blade alone. Hone it if you must, but leave it alone.

Well, I do use my knives to touch up a shave or haircut once in a while. Also, why not hone it to razor sharpness? Edge retention increases, the force required to make a cut decreases, and the range of materials you can cut increases since certain media requires a very sharp edge. What possible benefit do overly obtuse edges then present if the user does not require the "strength" they supposedly offer (Which is hardly ever utilized in a 3 inch pocket knife)?

I'm going to start calling this "bubbaing" a knife. Like those folks who take great old SKSs and stuff and add all sorts of "tactical" things to them and ruin them..? Same with people who take a perfectly usable 30-40 degree blade on a folding knife and whittle it down to 14 degrees and then complain when it chips and dulls.

Silly, verrry silly.

Where did you see me complain about chipping or dulling? I've been loving these types of edges since I tried them, and saying such quite frequently here for some time. The only time I had edge failure was when I purposely set out to do such, going beyond the scope of my normal cutting tasks to see when and how the blade would fail. I have no complaints about something I intentionally did.

A 40 degree edge is usable, but I would never pick it over a 16 degree edge unless I intended to baton through metal and rocks with said knife. I never do that with my knives, so the thin edge is more optimized for what I do. What is silly about optimizing my tools for my uses, based on observable data?

 

[thombrogan]

I ask this because I run my knives at much thinner edges than most people would be comfortable, and even under magnification seeing any sort of damage is extremely rare. I use a variety of steels, including D2, S30V, S60V, ZDP189, 13C27, AUS8 and so forth.

I haven't had the luck going too thin with S30V. Without a robust microbevel, I can't get it to do more than break down cardboard (even cutting twine is dulling unless a porcine microbevel is added ). What's your secret?

 

[hardheart]

Edges at higher angles take longer to wear and are more resistant to chipping. They are also 'duller' as they require more force to cut. The edge itself does not degrade at a constant rate, it slows as it dulls. After a session of cutting rope/cardboard/etc. with a low angle, fully formed, edge, the difference is noticeable, much less so if it was sharpened at a fairly high angle. Did the acute edge get really dull, or was the more obtuse edge just never really sharp?

You could cut cardboard with a crowbar all day long and not make it any 'duller', so that's fantastic edge-holding. You also won't have cut any cardboard at the end of the day, but maybe tore up a good bit.

 

[db]

Thom maybe try a shallow convex to the edge. I keep my S30 blade edge low with a convex bevel and haven't seen any issues with it. I should probably note my only S30V knives are custom knives RC 60. And like all my knives I've reground the edge to my liking.

 

[Vivi]

I haven't had the luck going too thin with S30V. Without a robust microbevel, I can't get it to do more than break down cardboard (even cutting twine is dulling unless a porcine microbevel is added ). What's your secret?

Maybe that's what it is, I usually go kind of steep of my microbevels. I take the back bevel down really thin, but then the microbevel is still 10 or 15 degrees per side.

I've had great luck going 4-6 degrees per side with that type of microbevel. I have others that are thicker, closer to 16-20 degrees inclusive with a microbevel slightly above that. That type of edge I do not like because it still feels too thick in cuts.

 

[thombrogan]

Thanks, guys. Unless I don't thin the edge out as much as I'd prefer, it gets chippy without the thick microbevel; even when it's convexed, db. I'll chaulk it up to operator error and continue using knives with other steels.

 

[jskatzoman]

I've never had a knife, axe, machete etc over 30-35 degrees. Depends on said blade and it's function. My kitchen knives and edc's are at a 20-25 degree angle (total) and don't need much maintenance at all. They are what I call 'uber' sharp and have only chipped when hitting metal (and even cut metal without chipping the edge)...which was worked out in no time. What people have to understand is, different strokes for different folks. I use my knives ALOT and don't have to spend much time tending to the edges...which many are convexed Are my machetes razor sharp?...no...are my Mora's...absofreakinlutely.

 

[db]

Thom sounds like your going to have to turn that S30V blade into your pocket saw. I'm guessing with a very coarse finnish you wouldn't notice the chipping problem.

 

[Broos]

I generally agree that a larger angle will degrade less with cutting compared to a thin angle. This theory is easily seen in industrial cutting, where increasing the work required to cut is not as important as maximizing the cutting tool life between sharpening or replacement. But that is not the same as edge retention, assuming edge retention is based on the amount of work it requires to cut a material. When a degraded thin edge will still cut with less force than a non-degraded thick edge, which one do you want to cut with? I'll take the thin edge that cuts better than the thick edge even when its dull.

Talk of angles is not complete without some reference to the thickness of the blade and edge. When I think tactical, I think a thick edge and a edge bevel above 20 degrees per side. Not a bad edge for general purpose use, but it bites compared to a thin blade/edge with a barely visible microbevel at 15-20 degrees per side. They may both have the same edge angle, but one will cut (most materials) much better than the other.

I've been thinking that the majority of the time when I sharpen, I am not sharpening the edge - I am sharpening off the edge. How you sharpen away from the edge is as important as how you sharpen on the edge when it comes to cutting ability and edge retention.

I've had good luck with S30V convexed, polished, and thin with the 15-20 degree per side final edge angle.

 

[Jeff Clark]

I'm the one who feels like he is talking to a brick wall. Unless you sharpen a knife so thin that the edge rolls over while you are working--thin stays sharp (the apex of the edge is thinner and the blade cuts better) for a longer period of time. It will cut better for every metric that I can think of longer if the edge bevel is more acute. What wears down most noticeably with wear is the apex of the edge. As that happens a more obtuse edge bevel gets wider faster. It is simple geometry.

Take a look at the test results in the last chart in this paper. That is how knives "wear". The data is from work Buck did in the late 90's prior to switching to a thinner blade grind that they called "Edge 2000". They decided that most users would see higher performance out of a thinner grind.
http://www.cutleryscience.com/articles/model_edge_retention.html


It is possible to have an edge too thin. I have done that by hollow grinding cheap Pakistani blades and really soft alloys. When your blade really starts to look paper thin (like a true straight razor) the edge will roll over, chip, or develop ripples. You see this most commonly by taking a hollow ground kitchen knife and hammering it through bones. Even with those thin geometries it takes serious abuse to see the problems. What doesn't happen is that the blades dull faster. The acute blades stay sharp longer or they conspicuously fail. The dimensions of most knives are such that failure is unusual with anything approaching reasonable use.

So as long as a knife is made with decent material and I don't expect to challenge it to failure--I hone it thin.

 

[theonew]

Excellent post JC, that might just knock a brick or two loose

 

[Broos]

I see the point and do not contest that thin edges generally cut best for people using knives. So I'll beat my head into the wall again, too.

There is a difference between the edge degradation, and the loss of cutting ability (edge retention). A thinner blade can experience more severe or deeper deformations of the edge (edge degradation), yet still cut using less force than a thicker edge. A good reason why edge retention comparisons of steels should be done with equal geometry to get a defensable conclusion (something that is obvious to me, though CS would not agree with it).

Edge retention is related to edge degradation, but not always completely dependent on it - that's where how thin the edge and blade is can make a difference.

Similar to some past comments about D2 - about how it won't get real sharp, but it will cut and cut. The edge may degrade, but cutting ability can stay high because of the nature of the degradation.

And if a thin edge always has better edge retention, then you would not see most industrial cutting machines use large edge angles (often 90 degrees). They not only don't deform as much, but they stay sharper too. The problem with applying this theory for hand cutting with a knife is that those large angle edges require more force to cut with, which is problem for some of you pencil-necked geeks (ie. people), but not a problem for those of us with arms that are akin to a hydraulic press (ie. a hydraulic press).

And I've wondered a couple times why anyone WOULD think a plot of edge retention would be linear.

Do a search for FFD2 if you want my opinion of CS's model.

 

[Vivi]

And if a thin edge always has better edge retention, then you would not see most industrial cutting machines use large edge angles (often 90 degrees). They not only don't deform as much, but they stay sharper too. The problem with applying this theory for hand cutting with a knife is that those large angle edges require more force to cut with, which is problem for some of you pencil-necked geeks (ie. people), but not a problem for those of us with arms that are akin to a hydraulic press (ie. a hydraulic press).

If you have arms as strong as a hydraulic press and use that much force when cutting with your EDC, I will concede that my posts may not apply to you.

 

[Jeff Clark]

When I directed readers' attention to the study by Cliff Stamp it was not specifically to evaluate his model, it was simply to look at how he plotted data collected by Buck Knives. The last plot on the web page illustrates that for ordinary wear (mildly abrasive wear) a thinly hollow ground 420HC blade can outperform a thicker grind on a BG42 alloy blade. To me that is like comparing graphite to diamonds and seeing that the graphite wins. It is pretty dramatic. The whole concept of a knife is to have an acute wedge with a narrow apex that can be forced through material (cuts through the material). Unless that wedge breaks down or gets jammed in the material, lower angles have a greater mechanical advantage than higher angles.

Now if you are cutting through steel belted radial tires you may want a more obtuse edge (probably convexed) to reduce edge rolling and chipping. You might even want that more obtuse edge if you are cutting gritty used carpet, but on clean material that is softer than steel thin usually is strong enough.

 

[knarfeng]

At the risk of poking my nose where it doesn't belong,
It was my understanding that the Edge 2000 effort resulted in more than an edge angle, but rather an entire blade profile. This profile is optimized for the CATRA test, which is essentially cutting cardboard. So the results are somewhat skewed as you are not merely evaluating edge angle, but blade profiles as well.

I believe the results might be more revealing if a medium were cut that is not dependient on the blade profile as well as the edge angle. One of the makers, I think it was Phil Wilson but may be wrong, has argued that manila rope cutting is better for judging the performance of steel than cardboard. I believe the argument might be extended to the evaluation of edge angles.

I am not saying that edge angle is not a major factor, just that the evidence presented by Mr. Stamp is a bit skewed, something that is often hard to prevent in using a set of data collected to support one hypothesis to suport a somewhat different hypothesis. The data was collected to support the development of a blade profile, not an edge angle.

 

[Jeff Clark]

I rarely cut rope, but I often cut cardboard. Buck was looking for a better hunting knife profile and edge. That is a profile for cutting meat, hide and sinew (some bone splitting). They weren't intentionally optimizing for cardboard cutting.

Most things I cut have some sensitivity to overall blade profile as well as edge angle. CATRA is not that bad a model for what I cut.

 

[knarfeng]

I rarely cut rope, but I often cut cardboard. Buck was looking for a better hunting knife profile and edge. That is a profile for cutting meat, hide and sinew (some bone splitting). They weren't intentionally optimizing for cardboard cutting.

Most things I cut have some sensitivity to overall blade profile as well as edge angle. CATRA is not that bad a model for what I cut.

Jeff,
I quite agree with that. I like the Buck blade profile and I do think that much of our cutting is represented by the results of that test. But, I must have gotten sloppy in my presentation because I think you missed my point. Buck did not just change the edge angle in the Edge 2000 effort. They changed the entire blade profile, as well as changing the edge angle. They did this because they found that the blade profile affected the CATRA performance just as the edge angle did. So they changed both.

So here is my point:
You and Mr. Stamp were citing performance data that resulted from changing the entire blade profile plus the edge angle as evidence of what happens when the edge angle alone is changed. You cannot draw valid conclusions from that. There is no data in that graph that shows what just changing the edge angle by itself did. And we know that blade profile plays a part in CATRA performance. So, based on Mr. Stamp's graph, you cannot say that merely changing the edge angle allowed 420HC to outperform the BG42.

Remember, these were Buck's tests to prove that changing the blade profile and edge angle gave them superior performance. They were not out to evaluate edge angles alone.

Highest Regards,
Frank R
former denizen of Devil's Gate.

 

[Jeff Clark]

Hi Frank, I get your point. If you look at that last diagram that I referred to the blade profile effects are most obvious as the cutting performance flattens out on the right side of the graphs. To compare apples to apples lets only look at the BG42 curves. As you pointed out the worn Edge2000 blade cuts better than the worn baseline edge. Some of that is due to the difference in honing angle, but much of it is also probably due to overall blade profile changes. The difference in performance in that region is about 1 on the depth of cut scale.

The place where the difference in honing angle does show up is on the left side of the graph. Note that the initial difference in performance is over 2 on the depth of cut scale. It looks like at least half of the initial higher cutting effectiveness of the Edge2000 is coming from its more acute edge angle. The more interesting thing is the difference in slope between the two curves. The curve for the BG42 edge honed at 20 degrees drops much more steeply than the curve for the BG42 edge honed at 13-16 degrees. That means that not only is the initial performance of the 20 degree edge lower than the performance of the 13-16 degree edge, it also dulls faster than the 13-16 degree edge. That was my point. Contrary to popular impression a more acute edge dulls slower than a less acute edge.