Why is a sharp knife cutting worse?

E

engblom

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Oct 23, 2021
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I have a strange problem. I am going through all 3 of the Spyderco stones (from medium to ultra fine) with about 13° angle, and then strapping with 1micron compound and finishing on clean leather. The result is truly sharp. Without any pressure I am easily shaving off the hairs of my arm. It is cutting through newspaper like nothing I have had.

BUT it is not gliding easily through an onion. As a matter of fact, it was better before I sharpened it. What could have been going wrong?

I redid the whole process, but this time I decided to give a totally new edge. So I first went through both side of the Sharpal dual diamond whet stone, just to get a new clean edge. Then the same process as above with the Spyderco set and strapping. The result is the same.
 
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Most good working edges will do better in real-world cutting tasks with an edge that's not so refined & polished.

For most tasks, a medium-grit edge with some bitey 'tooth' in it will initiate a slicing cut much more easily. By 'medium' grit, that would be something in the 220-600 range. In stropping these edges, I minimize the use of fine stropping compounds or avoid them altogether, as they can quickly take the 'teeth' out of the edge and overpolish it. Too much polish and not enough toothy bite will often create an edge that just slides across tougher materials and won't initiate the cut nearly as well. I only strop with bare leather, mainly for the purpose of cleaning the edge of any loose burr remnants and also to help align the finished edge straight. A medium-grit working edge needs nothing more than that.

My favorite edges are those created on either a Fine Norton India stone (320-400) or a Coarse or Fine DMT diamond (325-600). Then for some refinement and for occasional tuning up, I don't use anything finer than a medium Spyderco ceramic in just 2-5 featherlight passes per side, which can still leave that useful 'bite' in knives used for most tasks like kitchen use with meats, veggies, fruit, and utility tasks like cardboard, etc.

For the most part, a high-polished edge isn't very good at much besides shaving hair or other specialized push-cutting tasks, like whittling for example. For slicing cuts, an edge with some tooth in it is almost always much better.
 
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Obsessed with Edges said:
Most good working edges will do better in real-world cutting tasks with an edge that's not so refined & polished.

For most tasks, a medium-grit edge with some bitey 'tooth' in it will initiate a slicing cut much more easily. By 'medium' grit, that would be something in the 220-600 range. In stropping these edges, I minimize the use of fine stropping compounds or avoid them altogether, as they can quickly take the 'teeth' out of the edge and overpolish it. Too much polish and not enough toothy bite will often create an edge that just slides across tougher materials and won't initiate the cut nearly as well. I only strop with bare leather, mainly for the purpose of cleaning the edge of any loose burr remnants and also to help align the finished edge straight. A medium-grit working edge needs nothing more than that.

My favorite edges are those created on either a Fine Norton India stone (320-400) or a Coarse or Fine DMT diamond (325-600). Then for some refinement and for occasional tuning up, I don't use anything finer than a medium Spyderco ceramic in just 2-5 featherlight passes per side, which can still leave that useful 'bite' in knives used for most tasks like kitchen use with meats, veggies, fruit, and utility tasks like cardboard, etc.

For the most part, a high-polished edge isn't very good at much besides shaving hair or other specialized push-cutting tasks, like whittling for example. For slicing cuts, an edge with some tooth in it is almost always much better.
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A very succinct explanation. Thank you.
 
The polished edge is also getting dull fast, for some reason. It is really well stropped, so there should not be any burr left folding over. The knife is a Global GS-89. Is it a knife not able to keep a refined and polished edge well?
 
CROMOVA 18 seems to be designed to be primarily stain resistant and easy to sharpen. It is less wear resistant than VG-10 from what I read, so that means you will need to touch up the edge more frequently.
 
Chromova 18 is just Global's marketing name for AUS-6 steel, which has had quite a good heat reat btw.
From feedback from pro-Chefs i've found that a convex edge @ ~20 degrees inclusive with an 800-1000 grit finish performs best & longest for them.

Now most of these Chefs in the commercial kitchens over here have to use those HACCP colored cutting boards which in general are not exactly edge friendly.
If you use a good quality cutting board (like cheap HDPP plastic, medium range Hinoki wood, or expensive professional Hi-Soft boards) and avoid sand particles on that board as much as possible your edge will last even longer.
Provided that you use your knife only for kitchen cutting tasks, not for chopping and/or prying open tin cans.
 
I feel like I'm constantly cheerleading for Pete's Cedric and Ada channel with his bro science cutting the twisted sisal rope. He has a few videos where he's done polished vs "toothy" edges. I'm not sure a polished edge has ever had a meaningful win, and I'm pretty sure the best a polished edge has done is about as good as the toothy edge. And this isn't including that edge the guy I think from New Zealand sent him where it was coarse on one side and fine on the other but the sides were ground in opposing directions. He was getting Maxamet performance from M4 with that sharpening. Point of this rambling is unless you're trying to show off for Instagram I would just polish to 600 - 800 grit tops and leave it at that.
 
engblom said:
The polished edge is also getting dull fast, for some reason. It is really well stropped, so there should not be any burr left folding over. The knife is a Global GS-89. Is it a knife not able to keep a refined and polished edge well?
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Looking at online descriptions of the GS-89, they say it has an edge ground to 10-15 degrees. Assuming that's per side, anything less than maybe 12 per side is still pretty acute for a kitchen knife in an AUS-6 equivalent stainless at 56-58 HRC hardness (their published spec). Polishing such an acute edge to a high degree would likely leave the apex extremely thin and I'd bet unstable as well. When I used to attempt sharpening kitchen-grade stainless in a similar way - to very thin and highly polished - the edges tended to roll pretty easily in relatively light use and cutting performance would suffer for it.

With steel like this, I'd likely sharpen it at the factory angle using the dual diamond plate, since you mentioned having that earlier. Then, since you also mentioned having Spyderco ceramics, use either the Medium or Fine ceramic to apply a very minimal microbevel at maybe a couple degrees higher per side in only 1-3 very, very light passes per side. I've liked using this method because the microbevel adds stability to the edge and it performs much better in kitchen use, especially on a cutting board. When I finish edges this way for my kitchen knives, I don't strop them at all. The application of the microbevel, done very, very lightly, also works very well for deburring.
 
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I have been checking up a bit. Chromova 18 is NOT AUS-6.
knifebasics.com

Is CROMOVA 18 Stainless Steel good? - KnifeBasics

What is CROMOVA 18 Stainless Steel?CROMOVA 18 Stainless Steel CompositionProperties of CROMOVA 18 Stainless SteelCROMOVA 18 Stainless Steel HardnessCROMOVA 18 Stainless Steel Wear resistanceCROMOVA 18 Stainless Steel Edge retentionCROMOVA 18 Stainless Steel ToughnessCROMOVA 18 Stainless Steel...
knifebasics.com knifebasics.com

I will resharpen it in steeper angle, with maybe a bit less polishing.
 
engblom said:
I have been checking up a bit. Chromova 18 is NOT AUS-6.
knifebasics.com

Is CROMOVA 18 Stainless Steel good? - KnifeBasics

What is CROMOVA 18 Stainless Steel?CROMOVA 18 Stainless Steel CompositionProperties of CROMOVA 18 Stainless SteelCROMOVA 18 Stainless Steel HardnessCROMOVA 18 Stainless Steel Wear resistanceCROMOVA 18 Stainless Steel Edge retentionCROMOVA 18 Stainless Steel ToughnessCROMOVA 18 Stainless Steel...
knifebasics.com knifebasics.com

I will resharpen it in steeper angle, with maybe a bit less polishing.
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It has been described by expert sources as being similar or nearly identical to some versions of AUS-6 (or 440B as well), which is what I spoke of in my previous reply. See the linked info below, which mentions those comparisons. Sort of odd about the steel's composition, which shows relatively high carbon at up to 1.05% or so. If it were heat-treated harder to take advantage of that extra carbon, the edge strength (resistance to deformation) would improve and it should perform better than what you're seeing. But the mfr's spec of HRC 56-58 sort of wastes that hardness potential for the carbon. And based on what you're describing about the polished edge dulling quickly, it seems to behave like a lower-carbon steel at moderate hardness (mid-high 50s HRC). More typical kitchen stainless with half the carbon content (0.5% or so) and hardened to the same conservative spec would behave similarly if overpolished to an apex too thin to be strong, which I'm convinced is what's happening here. But they can work much better if the apex isn't thinned so much and if it's left with some tooth in it as I've described earlier.

 
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engblom said:
The polished edge is also getting dull fast, for some reason. It is really well stropped, so there should not be any burr left folding over. The knife is a Global GS-89. Is it a knife not able to keep a refined and polished edge well?
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Just because you stropped it doesn't mean there's no burr. I dealt with burrs and poor edges from them folding for some time. I was stropping on 3 micron then .5 micron on balsa wood. It was the balsa that caused the issue for me. The strop needs to compress enough to wrap slightly around the edge. I was successful in truly removing the burr by putting denim on top of my old strops and loading it with compound. The denim compresses a little. Also use a moderate force. If your keeping the right angle when stropping it will not round off your apex, only make it sharper, and more far more durable with no burr.

I sharpen usually to the x fine or xx fine dmt stones then 3 then .5 strops and my k390 is able to cut hanging hairs. I've never been dissatisfied with that edge as in wished it was rougher. I can get the concept but maybe it's the high carbide volume making it not so slick as it wears it exposes carbide and has a serrated ish cutting action rather than a smooth rounded apex
 
I haven't found stropping to be very useful; if anything, it will take off a biting edge. I've read that old straight razors were pretty soft compared to today's steels, so maybe stropping worked in old-timey barbershops when you went in for your weekly shave.
 
I've recently started stropping on a wood backed leather strop charged with 1 micron diamond paste, and I find it strikes a balance between a "smooth" and "bitey" edge that can slice paper and shave/pop hairs off my arm.
 
I think some people are stropping too much. I strop enough to get any final polish burr off. Or to correct a slightly used edge. But I also see people whack and drag the blade on a leather strip glued to a board. Not sure how you are keeping an edge angle to polish doing a whack and drag. That kind of stropping would probably also be a reason people loose the keen edge.
 
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