*warning* deep discussion on microbevels

Legacy Forum Archive thoughtstheoriesscience-related-to-sharpening
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[quote quote=“CliffStamp” post=14883]Thus if you do some experimenting you will find that the optimal geometry is the one which has just enough thickness to keep the knife from warping. If you go below this the knife will fail by warping that you will see. [color color=#ff0000]If you go above it then it will increase the rate of wear, deformation and fracture at the very apex[/color]…

Now as carbide volumes get high this means you have to take them into account because some steels have carbide loads which are so dramatic that if you under cut the steel and make it very thin there will not be enough steel to hold the carbides in place. It would be like trying to pour a slab of concrete 3" thick if you had 4" rocks in it. If you grind too thin for the carbides it is easy to see because the steel will just break apart. If you do it dramatically then it will form dramatic burrs in sharpening due to the heavy fracture.

This is a low carbide steel (5Cr15, 55 HRC):

vs a high carbide steel (S35VN/60 HRC, Peters):

Finished on the same stone to the same angle (<5 dps). Note how one forms very clean and the other clearly is fracturing readily. The stone was a 700 X Bester.
[/quote]

Ahhh that makes a lot of sense, thanks!

so what do you mean by having a thick-shouldered edge will cause it to "fracture"? I would think it would just wear away and become blunt, because the carbides would be supported in the wider steel matrix, right?

In your second picture, this is where the carbides in the steel have been ripped out because the dps angle was too low, right? Now… wouldn't the actual size of the carbides play a huge factor in this as well? Like s30v has very small carbides, which maybe could be supported in a lower dps angle, whereas D2 has larger, irregular ones so it could not? (I don't have the pics of each steel, but I have seen you post them before elsewhere)

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[quote quote=“razoredgeknives” post=14886]
so what do you mean by having a thick-shouldered edge will cause it to “fracture”? I would think it would just wear away and become blunt, because the carbides would be supported in the wider steel matrix, right? [/quote]

In general all abrasive wear is a very violent process which includes tearing, ploughing and fracture. Just take the point of a knife and rake it down the face of a piece of hardwood and look at the surface created. All of this happens on a micro-scopic scale as well but we just can’t see it and it looks like the edge is smooth, but under high magnification it will be dented, deformed, torn, fractured etc. .

This is also why blades will have a curious response in that if you use them until they are very blunt, < 5% of optimal the sharpness will start to oscillate. What happens is that as they fracture this exposes sharp edges which give an increased slicing ability and then these jagged edges themselves suffer wear and the sharpness drops again. If the carbide volume is very high this is increased as the carbides can be torn out and also exposed in large pieces.

Yes, but you have to take care what exactly the carbide size means. This sounds kind of silly as it obviously means the size of the carbides but you have to consider how they form. For example PM in general will produce very small carbides, especially those that form MC type (like vanadium). These carbides will be on the order of 1 micron, and thus some makers/manufacturers promote them on that basis. However this ignores the fact that as the carbide volume increases then the aggregates (clumps of carbides) increase dramatically in size.

PM steels can have aggregates on the order of 10 times the size of the individual carbides easily and the volumes can be so high there is little steel to hold them in place at high volumes. Thus a very high carbide PM steel could easily have a lower edge stability than a ingot steel with "larger" carbides.

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I have to ask both for myself and possible others what do you mean by “dps”?
I understand we have degrees minutes & seconds or we can decimalise it as often displayed on GPSs (and possible an engineers reference to gradians 400- 360Ëš) but I can find no reference to dps and since you have written so eloquently on this subject of thinning ,micro bevels , metallurgy and the Rockwell scale i should like to make sure I completely understand the reference to “dps” rather than possible making incorrect assumptions about the finer points made buy you.
It would be very helpful if you clarify what dps is referring to if it is an angle measurement it would simplify the text to stick to degrees since this is the measurement that the WE uses.
There was that disastrous episode on the space station when certain modules had been made using metric measurement and the other part it was to mate with was made using imperial measurement .
For my own part I struggle to grasp entirely what you are writing and would like to read it in more of a simpleton’s language.
Please do not take offence to this non is meant I have often been accused of explaining things in too technical terms whether it be navigation , trimming yachts or software.

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There has been a near constant source of confusion over what people mean when they say sharpening angles as it isn’t an accepted and consistent terminology.

I started referring to it specifically as degrees per side (dps) and degrees include (di) a few years back. After using them for a few months on a few boards I dropped the long hand and now just say 15 dps (or 30 di).

This is also in reference to measurement from the mid-line not measurement from the primary grind, though that is a less common point of confusion. This terminology hopefully is fairly clear and fairly precise.

Thus for example a standard Mora is sharpened at 10 dps, if you apply a 15 dps micro bevel it means the microbevel is 5 dps higher than the primary (or single) grind.

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Now I understand and dps makes perfect sense . The bevel terminology is another problem that often contradicts itself ; I quite like the Japanese knife terminology since as someone who does not understand Japanese the parts of the knife when referred to through labeled drawings seem to be a lot less ambiguous whether this is the case in Japan I do not know but all the reference to parts of Japanese knives seem more exacting than western terms bevel description been the worst of all.
I did start a post a few months to try to set a standard for bevel reference “Bevels -terminolgy” but it sort of fizzled out .I realise I misspelt terminology but you can cut & paste it into search to see what others & I had to say on it . I rather hoped we could standardise it at least in the forum .

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this article So I thought that this article was interesting (thanks for the research and works cited cliff) and I believe this has a large part to do with microbevels. It would seem like the microbevel serves 2 purposes: 1. Ease of Sharpening (use a low grit for back bevel and a higher grit for final bevel for quickest results) and 2. To ensure that there is enough of the steel matrix to support the carbides in the steel.

“As a point of clarification, carbides can be cut by a suitable abrasive, i.e., one which is harder than the carbides so it is not immediately obvious that a lot of carbide would so limit the ability to polish the edge. However as the abrasive cuts through the carbide there is an action/reaction force pair between the carbide and abrasive generated which in turn generates an action/reaction force pair between the carbide and the surrounding steel matrix, i.e., the non-carbide portion of the steel. As the edge angle of the cutting tool is reduced the carbides will be contained by less matrix volume and at some point there will no longer be enough matrix to sustain the action/reaction force pair and the carbides will get torn out of the cutting edge. Thus a steel with a higher carbide volume and larger carbide size requires a greater volume of steel matrix around the carbide to form a highly polished edge and keep that edge stable. A higher matrix volume around the carbides requires a greater angle as shown by Landes, Johnston and Elliot. As the abrasive is both harder and sharper there will also be less of an action/reaction force pair so the greatest edge stability will be found by using the hardest and sharpest abrasive as noted by Elliott.”

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Cliff, I see a lot of terms like “edge stability, toughness, wear resistance” etc… Could you please define these? Or anyone else that has a reference. Thanks

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The first time I saw anyone reference edge stability was Alvin Johnson (knifemaker) on rec.knives in the late 90’s. He did not call it that, but he noted that certain steels could hold an edge far better than others even if they didn’t have the wear resistance advantage. He just referred to it directly as the ability to hold a very high sharpness in very thin cross sections.

Roman Landes, metallurgist and knifemaker, defined it in his published work and it is the ability of the steel to resist deformation and fracture under micro-loads on the edge. An edge which has a higher edge stability will thus take less damage at a given load, or will take more load to take a given damage. He noted that :

-increasing hardness
-reducing austenite grain size
-minimizing retained austenite

increases edge stability and increasing carbide volume decreases it (as does the opposite of the previous properties). This steels which have very high edge stability are steels such as Japanese White steel, AEB-L, and similar steels often used as razor blade steels.

Wear resistance is the ability for a steel to resist being worn by an abrasive, i.e., how much steel is worn off in a given time under a given load and speed. A steel with a high wear resistance will have minimal steel removed in a given condition.

Wear resistance is broken down further into adhesive (steel vs steel) vs abrasive (steel vs something harder which can cut it directly). Abrasive is further subdivided into high load vs low load. High load abrasive wear is very dependent on toughness and thus S7 can have similar wear resistance to M4 under very heavy load but M4 will be superior under low loads when it doesn’t frature and just slow wears.

Ref : ftp://ftp.asm-intl.org/pub/MARC_Records/V07/asmhba0001587.pdf

Toughness is the ability of the steel to resist fracture which is basically the steel coming apart or separating. It is different from strength which is the ability of the steel to resist deforming where it changes shape but stays in one piece. For example if you bend a coat hanger then you have changed it shape but it is still in one piece, so you can checking strength. If you hit a piece of glass then it breaks and you can take the pieces and put them back in the same original shape but they are all apart, this is checking toughness.

I have given a very high level over view, these topics are quite involved, but hopefully that gives an understand of the basic meaning.

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Cliff,

Thanks a ton… That was very helpful and concise. I am sure I will be referring back to your post multiple times in the future :slight_smile: