Why the need for toothy edges if mirror polished edge push cuts so much better?

M

MichaelMyers

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Dec 10, 2015
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Why not just mirror polish all your edges and push cut all the way? If mirror polished edge can push cut straight through meat, ropes tomatoes and zip ties then why the need to use a draw cut and toothy edges? Most task like wood processing and chopping are push cuts anyway.
 
I'm starting to question the idea myself. On the other end of the spectrum, if polished edge cuts so much better then why do saws have saw teeth? I imagine a saw has saw teeth because it's the best form for the function.

On a smaller scale, would there be cutting tasks where a toothy edge would be so better?

I have no answers for you, just more questions.
 
Saws have teeth because they need to cut a kerf (remove material) to make clearance for the rest of the blade to slide freely through the cut material. If you think about it, saws can still use a polished edge at the tips of the teeth though. Generally, any kind of cutting action that utilizes a "push" or shearing type of cut will work better with a highly polished edge; any cutting action that utilizes a "draw" or sawing type of cut will work better with a bit toothier edge. There are exceptions to the rule though, of course. It's best to try both ways and use what works better for most things.
 
And some steels prefer a polished edge and some react better to a toothy one.

Fory tastes, I like how D2, M4, Sleipnir, m390 and s110v take a 400-600 grit finish. The finer grained/newer carbide rich steels listed will often go to 600.

The other steels that I like a higher 2k+ polish on are the "simpler" end of the spectrum, 1084, 1095, aeb-l, et al.
 
I tested this theory once with a butcher using belts. I sharpened two of the same boning knives, one I created an edge using a 320 & removed the burr. The next one I sharpened down to an A6 Trizact belt. The toothy edge was the winner after 3 weeks of use. However, I don't think the same goes for cutting fish where a more polished edge will work better.
 
What do you cut and how do you cut it? That's really what determines the optimal blade finish. Try cutting a bunch of cardboard boxes into pieces with a highly polished edge. It will cut really nicely... for a while. Then it will quit. Try the same thing with a rather coarse edge. Say 100 micron. The cuts aren't quite as whisper quiet, but it *cuts* and cuts well. See how long this edge lasts compared to the polished edge.

In my limited testing, I found the coarse edge to last roughly 3x as long as the polished edge in this type of use.

Jason B once described some CATRA testing he was involved in where the 320 grit edge lasted something like 3x (or more maybe?) longer than the polished edge.

It's all about what you're cutting and how you cut it.

Brian.
 
Blade & edge geometry matter as much as, if not more than, the finish on the edge. A thinner grind in fairly wear-resistant steel can do just as well with a polished edge as with a toothy one for some tasks, like cutting cardboard. My favorite for cardboard is a relatively acute & polished convex, finished to ~2K finish or so. Frictional drag is the enemy in heavy cardboard, so the thinness of the grind, finish on the bevels and the shape of the 'shoulders' of the bevels make a huge difference. More coarsely-finished bevels and edges are more prone to snagging & binding in cardboard, as are very crisp shoulders of V-bevelled edges; these things increase drag dramatically, especially when the apex starts to lose it's crispness. This is why I like a polished convex for that job; the cutting is scary-slick against the polished convex. Wear on the edge is minimized with a thinner edge grind, which reduces pressure exerted against the apex. Pressure would be higher with a more obtuse edge angle; this aspect has been verified by CATRA testing and resulted in thinner edge grinds being adopted by makers like Buck Knives.

If one prefers relatively thicker edge grinds, such as 30-40° inclusive or higher, a somewhat toothy bite helps initiate cutting in some materials, in order to overcome the resistance created by the geometry. This is well-demonstrated in cutting something like a tomato; if the edge is relatively thick but still has a toothy bite, it can do OK on a tomato as long as the tooth is still crisp at the edge. Otherwise, when that toothy bite goes away, the thicker geometry stops working on the tomato almost completely. When the edge geometry gets down to ~25° inclusive or lower in steels which handle this geometry well, the cross-sectional geometry makes the bigger difference and the chosen finish at the edge becomes somewhat less important and almost moot in some cases.


David
 
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Obsessed with Edges said:
Blade & edge geometry matter as much as, if not more than, the finish on the edge. A thinner grind in fairly wear-resistant steel can do just as well with a polished edge as with a toothy one for some tasks, like cutting cardboard. My favorite for cardboard is a relatively acute & polished convex, finished to ~2K finish or so. Frictional drag is the enemy in heavy cardboard, so the thinness of the grind, finish on the bevels and the shape of the 'shoulders' of the bevels make a huge difference. More coarsely-finished bevels and edges are more prone to snagging & binding in cardboard, as are very crisp shoulders of V-bevelled edges; these things increase drag dramatically, especially when the apex starts to lose it's crispness. This is why I like a polished convex for that job; the cutting is scary-slick against the polished convex. Wear on the edge is minimized with a thinner edge grind, which reduces pressure exerted against the apex. Pressure would be higher with a more obtuse edge angle; this aspect has been verified by CATRA testing and resulted in thinner edge grinds being adopted by makers like Buck Knives.

If one prefers relatively thicker edge grinds, such as 30-40° inclusive or higher, a somewhat toothy bite helps initiate cutting in some materials, in order to overcome the resistance created by the geometry. This is well-demonstrated in cutting something like a tomato; if the edge is relatively thick but still has a toothy bite, it can do OK on a tomato as long as the tooth is still crisp at the edge. Otherwise, when that toothy bite goes away, the thicker geometry stops working on the tomato almost completely. When the edge geometry gets down to ~25° inclusive or lower in steels which handle this geometry well, the cross-sectional geometry makes the bigger difference and the chosen finish at the edge becomes somewhat less important and almost moot in some cases.


David
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Awesome summation. Well said.
 
Did some work with a homemade tester that gave interesting results relative to this discussion. I need to go back when I have more time and really dial it in better, as I can learn a lot more from this unit than I have thus far.

My experience has always pointed to tailoring the edge to the use when practical. So items like my Chef's knife, hatchets, machetes get done to a bright finish and pocket knives tend to have a toothier edge to facilitate a bit of draw. That said, it is the material to be cut that determines the best strategy.

All else being equal - Fine edges chop and shave better as they have less variation along the edge = less friction. Used to draw cut they tend to dull more rapidly as they have less variation to be consumed before the edge is dull. They can also suffer penalties of needing more applied force to make the draw cut than a comparable toothy edge, more pressure on the edge = faster dulling.

Toothier edges draw cut better as they have more friction - also have more cutting surface due to the irregularities and so likewise tend to last longer when most cutting is done with a draw. Used to chop, they will dull more rapidly than the brighter finish as the increased drag causes the edge to experience more pressure.

Most uses will be a trade off, so a medium finish is a good compromise for a multipurpose edge. Most folk will likely favor the edge they can easily and reliably touch up with the gear they are most comfortable, even if that edge finish isn't the strongest performer in a test to failure. There is a bit of overlap regardless.

Anyway, back to the tester. It can be set to run variable length of draw or none at all, and make use of a static load or a dynamic one where either/or the amount of force need to make a cut, or number of cuts at a given amount of force can be tracked with fair amount of accuracy.
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One test involved setting the edge with a 4k Norton waterstone (Approx 1300-1400 ANSI). The load was 10 lbs static, 2" draw on 3/8" Manila rope. At the start and end of the test I measured dynamic load needed to execute a chop cut on the same material.

So - 12.5lbs needed to chop the rope.
90 passes with a draw cut at 10lbs static load before failing 3 passes in a row
16lbs to chop with the now "dull" edge
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Same knife prep'd at 800 grit ANSI yielded average:
16lbs to initial chop cut
161 passes before failure
26 lbs to chop with "dull" edge

over a 70% increase in number of passes with the rougher edge before failure. After failing to make the draw cut, the finer edge chop cut with about 40% less force than the rougher edge - in fact it equaled the force of the rough edge freshly sharpened.

This is the test unit:
https://youtu.be/2pxtGRaJT2I

Find the right strategy and you'll spend less time maintaining your edge, increasing the lifespan of the tool - edges/edged tools are a consumable. There is no one perfect edge finish, but some compromises work better than others based on usage habits.
 
Years ago I worked a shipping dock (incoming and outgoing freight). My coworkers and I were cutting stuff all day (various ropes and twines, heavy packing tape, plastic packing straps, heavy zip ties, plastic pallet wrap, etc, etc).

Some knife edges worked better for different materials.

I carried a fixed-blade and a folder. I kept a rough micro-serrated edge on my fixed-blade for cutting rope, twine, zip ties and packing straps, and I kept a smooth edge on my folder for pallet wrap and tape. A rough edge was no good on pallet wrap as it would snag, but a smooth edge sliced through it clean and easy. The same smooth edge didn't work very well on rope, plastic straps and heavy zip ties, but the rough edge ripped right through them.

That's my experience anyways.

I don't think it's any accident that serrated knives have been around for as long as they have. Sometimes serrations (including micro-serrations) simply cut better that a smooth edge.
 
Good discussion! Something that comes up allot. It really comes down to the task at hand and personal preference.
 
Beyond the typical responses of what works the best for which edge obtaining a more polished edged takes more time to do and sometimes all you need is sharp. After all it is what will get the job done. I move heavy machines (upwards of tens of thousands of pound) using forklifts, dollies, etc and a typical day may see me cutting cardboard, plastic wrap, zip ties, occasional scrapping stuff off, and who knows what else. I tried the whole polished edge in my work knives and it didn't last a day and took a lot longer to work through the grits. Going to a 600 grit diamond, fine india, or other media that is around there that still removes steel but leaves a bit of a toothy edge seems to be a quick route to a sharp edge which is all I need for the job.

This job has torn up edges in causing chips, dents, etc in the blade because sometimes you just have to abuse it the knife to get the job done. There are no safe queens, no let me grab another tool in the truck that's 10min away when I can do it in 5seconds with my knife. Were paid to do the job and if we have to slightly damage the edge so be it. We just fix that when we get home on a coarse stone in a minute or two. And if we start out with a slightly coarser stone we can repair minor damage from the start.

This is my reason for not dong polished edges. Figured I give you a more practical point of view of someone who uses and abuses his knife at work.
 
Thanks for the helpful info. So far what I gathered is polished edge cuts better and have better edge retention on a push cut while toothy edges cuts better on draw cut and have better edge retention. Then the question is why not push cut only with the polished edge since it push cuts so well? You can't use draw cut for chopping or wood carving but push cuts works everywhere.
 
Push cuts aren't the best for cutting rope, straps, cardboard, cutting up mats, etc. The edge dulls a bit and than push cutting is quite a bit more difficult. I am not talking in a controlled environment where everything is clean (but that will probably show it too), but out in the field dirty rope, straps, etc all these things are dirty and may be encrusted with stuff which wrecks havoc on a blade so push cuts tend to not do well in my experience. But toothy edges seem to last longer, you can attribute it to it acting like micro serrations or using more of the blade in practice by forcing yourself to draw more of the blade through the material as you cut instead of focusing on one section while push cutting. Either way practicality wise toothy edges seem to thrive in this type of environment.

Both toothy, polished, and any type of finish you can think of will all have their advantages and disadvantages. There is no "best", only what works good for you and what doesn't work good for you in an given application.

I also have to side with Killgar on uses for polished and toothy as it mirrors my experience as I work with a lot of the same stuff on a daily basis.
 
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MichaelMyers said:
Thanks for the helpful info. So far what I gathered is polished edge cuts better and have better edge retention on a push cut while toothy edges cuts better on draw cut and have better edge retention. Then the question is why not push cut only with the polished edge since it push cuts so well? You can't use draw cut for chopping or wood carving but push cuts works everywhere.
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It can take a long time to get that perfect polished edge working through the grit progression. Becuase i free hand sharpen, its not worth the time it takes to obtain that kind of edge compared to how long the edge lasts in use. 800 grit and an occasional touch up on a fine ceramic rod works, is fairly quick, and it cuts well.
 
MichaelMyers said:
Thanks for the helpful info. So far what I gathered is polished edge cuts better and have better edge retention on a push cut while toothy edges cuts better on draw cut and have better edge retention. Then the question is why not push cut only with the polished edge since it push cuts so well? You can't use draw cut for chopping or wood carving but push cuts works everywhere.
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Push cuts take more force than slicing cuts. Take a knife freshly sharpened and push it straight through a piece of rope, then do the cut using a slice. The slice takes less force, and the slice is what most people do for normal work. Cutting cardboard, carving, planing wood, chopping, and some other things take advantage of a polished edge. Think of it like this. Take a 2x4 and saw through it with a hand saw. Now imagine taking a piece of steel the thickness of the saw and just pushing it through the 2x4. It can be done, but it's much more difficult. It's just a function of how limited humans are in the amount of force they can generate. Hand tools look much different than power tools, because we just can't push them that hard.
 
MichaelMyers said:
Thanks for the helpful info. So far what I gathered is polished edge cuts better and have better edge retention on a push cut while toothy edges cuts better on draw cut and have better edge retention. Then the question is why not push cut only with the polished edge since it push cuts so well? You can't use draw cut for chopping or wood carving but push cuts works everywhere.
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The best knife that I have for cutting fresh baked bread was sharpened on a 120 grit belt. Took a good 10 seconds.
 
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