edge retention of serrated vs straight : used carpet

[Cliff Stamp]

Quite some time ago there was a thread about cutting ability and edge retention of serrated blades, circa 1998, Mike Turber argued for serrated blades on cardboard from exerience cutting upon a lot of boxes redoing packages. Steve Harvey countered that a straight edged Boye knife would cut cardboard very well for a very long time.

Steve was the first person to point out that serrated vs plain edge comparisons ignored that the straight or plain edges are ground at twice the angles of serrated blades and some get as much as three times as obtuse, this makes for a massive difference in cutting ability and Steve was also not taken in by the myth of lower angles equals less edge retention.

I recall thinking it would be a good idea to compare the two with the same steel sharpened to similar angles. For various reasons I never got around to it, once of which being that it takes a lot of material to actually blunt serrated edges, and the second being that I usually don't buy them so those two conditions are rarely met simultaneously.

However recently I had a bunch of used carpet, and a Cara Cara which had both a straight and serrated section ground at near identical angles. I first checked the consistency of the carpet by using another knife to do five sample runs over random sections with rounds of 100 and 275 cuts, the performance was consistent to 5-10%.

The major sources of variance was the initial sharpness, I was freehand honing the blades for various reasons. Even just a hint of a burr totally scuttled edge retention, making a S30V blade worse than SAK. The edge retention is optimal with a blazing sharp edge which is well above shaving, no surprise, but the magnitude of the effect was enlightening.

The Cara Cara was used for three rounds of cutting. The sharpness was checked by slicing 1/4" poly under a 10 lbs load, the greater the edge length required to make a cut the lower the performance. The knife was also checked with steeling to see the effect of wear vs deformation plus examined under low mag (10x) to check visually directly.

In short, the plain edge section was comparable to a Dozier K2 and UK Pen knife in S30V, careful measurement or a very discriminating hand would be required to tell them apart (10-20% different). However the serrated section was in another class. After 275 cuts the Cara Cara's serrated section was sharper than the K2 and Pen after 100 cuts, more than a 200% improvement.

For sharpening, a small diamond rod was used to just work inbetween the scallops which were touched up with the medium and fine sharpmaker rods, then a light pass with a dowel with CrO. Total time was just a couple of minutes. The K2 was sharpened in similar time, freehand with a 200 grit SiC stone and diamond plates, fine and then x-fine then CrO on leather.

It should be noted though that while the serrated section was under the same vertial load as the plain edge knives during the poly cutting, it was harder to pull through the cord. While the others would do this easily, in fact if you just tipped them gravity would do it even on a light angle (10-20 degrees), the serrated section grabbed into the cord and required much more force.

However the mechanics of the body allow much more force to be applied on a pull with the arms than a straight push down, you can easily use your back and hips to pull directly, but pushing down is often not as easy to do depending on body position, thus even though the serrated blade requires more force in that way, it is largely irrelevant as it is easy to supply and thus an very efficient trade.

In short, the serrated section of the Cara Cara vastly outperformed the straight edge portion, and even allowed the blade to actually out cut other knives with more wear resistant and/or harder blades.

-Cliff

 

[blade14]

I hate to put you on the spot like this Cliff, but you've done a great job of tactically avoiding the thread I wrote a few days ago. I understand there are different knives and blades for various jobs but those of us who enjoy reading your reviews are interested to know which knives you prefer and what a layman knife connesour, like myself, might consider for EDC's, small fixed blades and large fixed blades. Any input would be greatly appreciated.

 

[kel_aa]

Is a "cut" whatever combination of push/pull cut that severs a piece of rug of a certain length, or whatever length of rug the given section of knife will reasonably cut in one action?

Since the horizontal load concerned with the rope testing is different, can you comment on relatively how much energy you expended? Say if your task were to cut a certain length of carpet, then roughly how much more tiring would it be to use a plain edge compared to a serrated edge?

 

[Cliff Stamp]

...avoiding the thread

Just considering it, it wasn't a trivial question. X vs Y is generally fairly simple, but what is "best" in general, wide scale, is more complex.

Is a "cut" whatever combination of push/pull cut that severs a piece of rug of a certain length, or whatever length of rug the given section of knife will reasonably cut in one action?

The knives were used with enough force to allow them to slice on a push with little slipping so with 100 cuts they all cut the same amount of carpet when normalized with their blade length.

I was also considering using them all with the same force, which would have led to comparing them after cutting different amounts of carpet on a given number of cuts, or the same amount of carpet with different amounts of cuts.

After talking to a few tradesmen, using them all with the same amount of force through the same amount of carpet cut would seem to be the most meaningful as a knife issue but would lopside the comparion from a steel perspective.

Since the horizontal load concerned with the rope testing is different, can you comment on relatively how much energy you expended?

While there are obvious differences in feel, the overall energy output was so low that even doubling it would not be meaningful. I used to do 3-4 knives at at time, so was doing 4x175 or 700 cuts in a row and fatigue just isn't an issue, carpet just isn't binding enough.

On some cutting like wood or hemp rope or even thick vegetables then this can be an issue because you can often compare 10 lbs to 100 lbs from one knife to another and thus you will notice a difference in effort and fatigue quite quickly.

-Cliff

 

[kel_aa]

Do you think you can say since the serrate edges have 30% more edge length, you can attribute 30% of the performance difference to that, and the rest to the serrate geometry [when used in this way]?

On some cutting like wood or hemp rope or even thick vegetables then this can be an issue because you can often compare 10 lbs to 100 lbs from one knife to another and thus you will notice a difference in effort and fatigue quite quickly.

Is this comment still in connection with straight and serrated edges of similar geometry, in which the effort of using the straight edge will increases much faster than that of the serrated edge? I don't know about hemp but it seems like in wood and thick vegetables binding would be the major energy drain and the actual sharpness of the knife minor.

 

[Cliff Stamp]

Do you think you can say since the serrate edges have 30% more edge length, you can attribute 30% of the performance difference to that, and the rest to the serrate geometry [when used in this way]?

The blunting isn't linear with the amount of material cut, so the effect would not be 30%, but you do make a critical point that to make a similar normalization the effective length of the serrations should be used. I'll repeat the work in the next for says and assemble a relevant table and include both for completeness. Thanks for pointing that out.

Is this comment still in connection with straight and serrated edges of similar geometry ...

Just in general in terms of effort to cut.

......in wood and thick vegetables binding would be the major energy drain and the actual sharpness of the knife minor.

Yes, for woods a knife has to be fairly blunt before that is significant so profiles are more critical than edge retention usually, this is really only an issue with carvers working on hardwoods or other finish work where you want to preserve the finish to reduce the need for sanding.

-Cliff

 

[the possum]

It should be noted though that while the serrated section was under the same vertial load as the plain edge knives during the poly cutting, it was harder to pull through the cord. While the others would do this easily, in fact if you just tipped them gravity would do it even on a light angle (10-20 degrees), the serrated section grabbed into the cord and required much more force.

However the mechanics of the body allow much more force to be applied on a pull with the arms than a straight push down, you can easily use your back and hips to pull directly, but pushing down is often not as easy to do depending on body position, thus even though the serrated blade requires more force in that way, it is largely irrelevant as it is easy to supply and thus an very efficient trade.

Actually this is one of the main reasons I won't use a serrated knife on critters anymore. Even if my arm is strong enough to supply the additional pulling force needed, the target may not be firmly anchored enough to make a difference. It will just move/get pulled over without getting cut. This becomes a real problem if a couple of the scallops have gotten a bit dull. Those few scallops will catch and pull the target, effectively stopping the cut, whereas a straight edge with a dull spot will just slide past the dull area and continue cutting with the remainder of the sharpened edge. I know I've mentioned it here before, but one time that really stands out in my mind I had to stand on the target with both feet in order to hold it in place, and use both hands just to pull the edge through. It did more ripping than cutting. It was not hair splitting sharp, which doubtless contributed to the edge catching like that, but a straight edge at the same sharpness has never given me that kind of problem.

 

[Cliff Stamp]

Even if my arm is strong enough to supply the additional pulling force needed, the target may not be firmly anchored enough to make a difference.

This is a fairly critical point about serrations in general, they work better when the media being cut is fixed or under high tension. Of course the serration type makes a huge influence here, some types are much more fluid than others, some of the more aggressive types can't readily cut ropes for example unless you actually stand on it and hold the free end tight. I should probably work some kind of ranking for this into the reviews, say a minimum tension necessary to allow a cut under a given blade length. It would change given what is being cut of course, but using one or two different materials (say a piece of cotton or canvas and a piece of hemp or poly) would provide a decent benchmark of both fluidity and aggression if the results were compared at both high and low tensions.

-Cliff

 

[sodak]

The blunting isn't linear with the amount of material cut, so the effect would not be 30%, but you do make a critical point that to make a similar normalization the effective length of the serrations should be used. I'll repeat the work in the next for says and assemble a relevant table and include both for completeness. Thanks for pointing that out.
-Cliff

I'm wondering if it's not so much the extra length (although that helps) as it is that serrations let you slice at most any (reasonable) angle which you hold the knife. Kind of like the way a Khukuri slices when chopping. Maybe. Seems to me that a serrated edge lets you also build up a little velocity from tip to tip or valley to valley, and that would help in penetration.

 

[Cliff Stamp]

The angles are definately an issue as they work similar to a recurved blade, or better yet something like a billhook which traps material which essentially requires much more force to be exerted against the material for a given force applied on a pull. You can really notice it cutting grasses with blades of different curvatures.

-Cliff

 

[yyElite]

quick question:

does the type of serration make a difference? I've seen serration that look like sawtooth (concave in toward to the spine of the blade) and others that look like a series of crescents (concave out).

 

[Cliff Stamp]

Yes, with serrations you are basically find a compromise between sawing aggression and ability to cut smoothly. Some patterns are very pointy and will saw things like plastics well, but are nothing but a hassle on softer materials. SOG uses one of the worst example of this type I have seen. Other patterns are very wavy and cut very smooth, Mission uses one of the best of this type I have seen. Spyderco has several patterns, they are of similar design but some are pointier and some are more rounded. Then there are the chisel tipped patterns which are very robust but have generally very poor cutting ability.

-Cliff

 

[Mitchell Knives]

Steve was also not taken in by the myth of lower angles equals less edge retention.

What would you consider the lower limit for the edge angle of a general use EDC knife that will only be cutting soft materials? What edge angle for a heavy use knife?

The edge retention is optimal with a blazing sharp edge which is well above shaving, no surprise, but the magnitude of the effect was enlightening.

Based on this information, would a knife sharpened and polished to a mirror finish be ideal for general EDC? I ask because you often hear people state that a semi-finished "toothy" edge cuts better than a polished edge. This may have some merit if you are cutting plastics or other material that is "slippery" on the edge. Otherwise, I'm not so sure.

 

[db]

I'm one who likes a little tooth to my edges. Many like a very polished edge I guess it all depends on how you cut and what you cut.Best way to figure out whats best for you is give both a try for a week or so.

 

[Cliff Stamp]

What would you consider the lower limit for the edge angle of a general use EDC knife that will only be cutting soft materials?

It depends on the steel greatly, fine grained steels can be taken down to 2-3 degrees and cut cardboard and ropes for a long time, bump it up to 5-6 for carving hardwoods, however coarse grained steels break apart at these angles and generally require 10+ degrees even for soft materials.

What edge angle for a heavy use knife?

It takes a lot to require more than 15 degrees, I have even seen blades of decent steel chop up caribou leg bone with this angle with no problems, and they will hack apart wood, including knots all day long. On softer steels, machetes for example, the edge will turn at this angle on harder woods so you can make the angle more obtuse to compensate. I just leave it lower and refile it every couple of hours when I stop to take a break.

Based on this information, would a knife sharpened and polished to a mirror finish be ideal for general EDC?

The thinner and more acute the knife is ground the less you need a rougher edge to enable it to slice. If you get knives ground efficiently enough they will pretty much push cut anything. However if you do a lot of slicing then a more coarse edge will give better edge retention. Steel type is an influence as well here, if the steel is soft and weak and rolls easily then a more coarse edge is usually of benefit for draw cuts.

A couple of general rules :

-push cutting favors a high polish / slicing favors more coarse edges
-thinner edges tend to reduce the need for slicing
-fine grained and very hard steels allow very low sharpening angles

-Cliff

 

[Mitchell Knives]

It depends on the steel greatly, fine grained steels can be taken down to 2-3 degrees and cut cardboard and ropes for a long time, bump it up to 5-6 for carving hardwoods, however coarse grained steels break apart at these angles and generally require 10+ degrees even for soft materials.

What steels have you found that respond well to a low sharpening angle? The CPM family of steels, particularly S30V sounds ideal due to the small grain size. How about D2 (not sure how coarse the grain is) or VG-10?

 

[Cliff Stamp]

The CPM steels should have a finer grain than the same steel without the CPM process, and I have taken CPM-10V down to very low angles with a custom heat treatment by Phil Wilson (62.5 HRC, full cryo) with no problems. I also have three customs by Alvin Johnson which work very well at acute profiles, one in 1095 (66 HRC), 01 (63.5 HRC), and M2 (65 HRC).

The S30V knives I have used, including a Sebenza, don't react well to such angles. I will hopefully be trying one of Phil's S30V knives shortly and see if that is any different. I have tried D2 and it doesn't work very well at those angles, D2 has one of the most coarse grain structures of the tool steels, and doesn't hold well at low angles neither do stainless steels in general.

This work was done about 20 years ago by Alvin Johnson on rec.knives, who found M2 works very well at max hardness and stainless steels don't including ATS-34 heat treated by Bos. Lots of others have found similar, Sodak recently has taken a bunch of knives to low angles (~10 degrees) and found problems with D2 and S30V.

-Cliff

 

[sodak]

I have taken D2 down to around 10 and it has chipped out badly. I tried this on D2 knives from 2 different makers, with different heat treats. I was trying to cut fuzz sticks from pitch pine that I bought at Walmart. This is very soft wood, but I was cutting and twisting (fairly hard) at the same time. Try to picture a twisting motion of the wrist exactly in the opposite motion of scooping ice cream. Anyway, I love D2 for it's edge retention, I think that I just found the lower limit of edge angle for those 2 knives. Ditto for S30V. Ditto for 420 HC. All chipped out badly.

Using the same angles, I have done exactly same on 52100 (actually Swamp Rat's SR101) and Cold Steel's Carbon V. Both held up well with little (CS) or no (SR) damage. That SR101 is **really** impressive at low edge angles. I haven't yet tried this with 1095 or CPM steels.

This was all found by accident. I used to love D2 at very acute angles, it cuts cardboard forever. But when I started cutting these fuzz sticks, I found that I now had a serrated blade. Not good. So I started trying a bunch of my favorite folders and fixed blades to see how they held up.

This is not necessarily an indictment of any of these steels, just a realization of their practical limits. I'm sure that a lot of this depends on heat treatment also. Once you grind them thin, you can't go back. No matter what the steel, when your thin blade *finally* dulls, 2 or 3 strokes per side on a DMT or ceramic hone, and you're back to uber-sharp.

It's a sickness...

 

[db]

I also really like a thin edge, but it's not for everyone. I'd guess that just about everyone would have a problem with an edge angle of 5 per side on just about all the factory knives. When you get down that thin you really need the hardness too and there are very few factory knives that can be taken down that llow. Wasn't it Joe Talmadge that said if the knife doesn't cut the way I want it to at 30 degrees,15 per side, he doesn't want the knife.

 

[Cliff Stamp]

I'd guess that just about everyone would have a problem with an edge angle of 5 per side on just about all the factory knives.

Five degrees means either the knife has a high hollow grind, or you are sharpening the entire primary grind, either is kind of extreme, however there are lots of knives that are not a lot more obtuse. Most single bevel grinds like puukkos are 9-11 degrees per side. Many of the full convex grinds like on Bark River are very acute at the edge, HoB just sent me the spec's on one he owns which was under 10 almost immediately.

Then there are the various working knives like Olfa which have very thin and acute bevels and are very much common working knives. There is even a guy locally who regrinds olfa blades thinner and the reaction to them was very positive from the carpenters who worked with them. The problem with low angles and steels is usually one of carbide/grain rather than hardness, though high hardness helps, but high hardness+coarseness doesn't. D2 at 62 HRC doesn't hold, but O1 at 63.5 does easily.

When you bring the angle down the focus of the knife changes to more of a pure cutting tool of course, unless it is expendable like the Olfa's, and you also tend to change techniques as well. It isn't for every knife, I would not put a 5 degree edge on a large bowie, or a general knife for outside wood craft. Knives like all edged tools need to be sharpened to a profile suitable for the expected use.

-Cliff