Microbevel angle -- effect on performance, sharpness and retention?

[ColoradoDave]

I'm wondering if anybody has done any kind of testing (formal or informal) to determine if there's some optimum angle when applying a small secondary bevel or microbevel ...?

A few comments ... I've been thinking about this since over 20 years ago, when I was given a set of Case Crock Sticks. I believe these are the original Louis Graves design, and are set at 22° per side. This is also the angle I've seen on the Lansky Crock Stick products, and the Spyderco Sharpmaker isn't far off at 20° per side. But I recall seeing on AG Russell's site that he never agreed with Graves on the proper angle for a V-rod sharpener, and so I assume he has a different idea of what the "best" angle is.

The last year or so I've been "tweaking" the microbevel angle when I sharpen some of my knives, using anywhere from ~12° to side, up to 25°. I haven't done any controlled testing of either the initial sharpness I can attain, or edge retention, but just speaking subjectively, initial sharpness when using 20°-22° per side seems as high as with more acute angles. I've also noticed that, with many softer blades/steels, it's actually more difficult to attain as high a level of sharpness if trying to finish sharpen with a microbevel under ~20° per side.

So far, I've yet to convince myself that a microbevel more acute than 20°-22° per side has any advantage (though for some reason, I must keep hoping it will.)

Would be very interested in everyone else's opinions, experience and conclusions.

 

[Cliff Stamp]

If the finish is very rough (x-coarse DMT), then the micro-bevel angle is very critical as it sets the depth of the teeth. The deeper the teeth the more aggression on a slice and the longer the edge retention, in extremes with both, the performance just goes to retarded levels and the blade just cuts ignoring all dulling efforts (for details, see the Mel Sorg review and the work with the reprofiled blade, and the posts on self-sharpening).

If the finish is very high, then I have not been able to detect any different in sharpness from very low angles (~3-5 degrees) to moderate ones (20-22) degress per side. Since this is a change of 5x with no effect, obviously you can conclude it has little to no effect on sharpness. It also doesn't significantly effect cutting ability. I have tested side by side blades with micro-bevels (1/10 of a mm in width) and been unable to tell them apart from feel or performance measuring forces with the single beveled blades (hemp, wood cardboard) . The secondary edge was often double the angle of the primary.

In regards to durability the effect can be extreme because most damage is often very slight, and thus the whole edge can behave as if it was ground at the secondary angle, thus it can make sense to chose 20 vs 15 for the secondary finish. Of course once the micro-bevel gets large enough to see, then the cutting ability starts to degrade and the durability starts to rise accordingly, however this then isn't obviously a "micro" bevel anymore.

I have been for some time meaning to compare the initial sharpness, and edge retention on hemp with the finish of both Spyderco rods at both edge angles. I actually meant to do that awhile ago when I did the medium/fine comparison :

http://www.physics.mun.ca/~sstamp/knives/sharpmaker_grits.html

But I got sidetracked looking at something else.

-Cliff

 

[ColoradoDave]

Thanks for the informative reply, Cliff.

Please feel free to correct me if I'm wrong here ... but I'm seeing (or believe I'm seeing) an interesting and complex interaction of variables here. The edge established when applying the microbevel of course is what initiates cutting. Yet if the microbevel is kept sufficiently small, the grind or profile of the primary bevel immediately behind the microbevel also has a very significant effect on the amount of force required. Given a sufficiently small microbevel, cutting performance is essentially the same as if the primary grind were continuous to edge. I'm assuming this may be due in part to the way in which some (most?) materials being cut tend to pull apart, as well as just the lessening of resistance to being pushed into the material, the smaller the actual microbevel.

If the finish is very rough (x-coarse DMT), then the micro-bevel angle is very critical as it sets the depth of the teeth. The deeper the teeth the more aggression on a slice and the longer the edge retention ...

I went back and re-read the Sorg review, and wanted to ask a couple questions to clarify what you're saying here. If I read the review correctly, the secondary bevel was actually established by the coarse abrasive AO belt on a slack grinder, and was quite acute.

Considering now what happens when you apply a microbevel using a much finer abrasive such as a ceramic rod, to a blade sharpened with a much coarser abrasive ... Am I visualizing this right, in thinking that a more acute microbevel will yield more aggressive slicing performance, due to leaving deeper teeth at the very edge than would a more obtuse added microbevel? Or are we concerned with the depth of microserrations relative to the sides of the edge? It would seem to me that deeper lateral microserrations might result in greater "toothiness" during early stages of edge wear; and that, in turn, would seem to suggest the advantage is with a more obtuse microbevel.

Thanks again.

Dave

 

[Cliff Stamp]

The edge established when applying the microbevel of course is what initiates cutting. Yet if the microbevel is kept sufficiently small, the grind or profile of the primary bevel immediately behind the microbevel also has a very significant effect on the amount of force required.

The total force applied is just the linear sum of the force required to rupture the material, and the force required to push it out of the way to let the rest of the blade pass through.

Thus as you make the edge sharper, by removing debris, aligning, it and removing any burr, you reduce the amount of force needed to cut into the material. This is essentially not dependent on the angle, or thickness, just the quality of sharpening.

Now as the material is cut the blade needs to progress through to reach the uncut material so so to exert a force on it and cut it. In order to do this is has to mash all the rest of the material out of the way, this thicker the blade the further it has to move the material and thus you can reduce the force by making the blade slimmer.

If the microbevel is very small, as you noted the total cross section increase is going to be insignificant and thus the increase in mashing force (to be really technical) is also going to be insignificant.

Since the sharpness is the same, push cutting wise, that aspect of force is also near idential and thus the total force is unchanged. Hence micro-bevels when push cutting have basically no drawbacks.

[if you want to get complicated there are other forces involved due to surface friction and drag - but they are generally *far* less significant]

... the secondary bevel was actually established by the coarse abrasive AO belt on a slack grinder, and was quite acute.

Yeah, I never intended to actually do any cutting with it, because common knowledge said all such edges are burred, but when I checked it under mag it was clear, so on a lark I did some cutting and quickly regretted it because it failed to blunt significant, and I only stopped because I really didn't want to do another round as it would have been 2000+ cuts.

... a more acute microbevel will yield more aggressive slicing performance, due to leaving deeper teeth at the very edge than would a more obtuse added microbevel?

Yes, you can see them at really low angles they get soo large it starts to become like Cold Steels serration pattern. Besides the huge increase in aggression they also get weird self-sharpening effects. I have seen this increase even with mild steel, though you can't go as thin there as with I did on the D2 blade as the material is weak and it just collapses when you try to cut the rope.

Titanium is odd in this respect though, it doesn't form such an aggressive pattern when belt finished. I suspect it may be because of the high oxidation rate.

Of course it should be noted that if you nick a 3-5 degree edge into a piece of anything hard you will knock a good sized piece of the edge out. You really want to be careful with these edges unless the blade is *really* hard and thus very strong.


-Cliff

 

[davebolton]

Hi there,
My thoughts on the subject are pretty simple based on testing from our shop. The slighter the angle, the sharper the edge and the more prone to damage. The more obtuse the angle, the fatter the edge and less prone to damage. This geometric relationship applies to both micro and macro bevel.

 

[ColoradoDave]

<RE: SORG D2> ... I never intended to actually do any cutting with it, because common knowledge said all such edges are burred, but when I checked it under mag it was clear, so on a lark I did some cutting and quickly regretted it because it failed to blunt significant, and I only stopped because I really didn't want to do another round as it would have been 2000+ cuts.

<g> The more of your reviews I read, Cliff, the more I've come to distrust the "common knowledge" I carried around with me for years. That particular review would make for some interesting discussion, no doubt more tests, but I'll let you save that for the future. I'll chip in for beer if you want to get your brother over there for a 2000+ cut marathon session.

Also meant to comment on your "Sharpmaker grits" findings when you posted that earlier. I would imagine it would be extremely difficult when looking at slicing performance to isolate the effect edge polish has on the push cut component of a slice cut. Still -- and maybe just note this as a request/suggestion for yet another future project -- I would appreciate hearing your thoughts on choosing grit and level of finish for various cutting chores. Probably make a good, thick chapter for a book.

My thoughts on the subject are pretty simple based on testing from our shop. The slighter the angle, the sharper the edge and the more prone to damage. The more obtuse the angle, the fatter the edge and less prone to damage. This geometric relationship applies to both micro and macro bevel.

I would be very interested in more specifics about your testing, hopefully to try to understand what you've done to realize better performance from a more acute microbevel. Despite what I've personally observed, and even with Cliff's corroboration of same from his experience, there's still the lingering question in my mind as to whether or not a more acute microbevel might indeed be made sharper than one that's more obtuse. Intuitively at least, it still seems that this relationship should hold. Assuming a reasonably polished edge, I'm wondering if perhaps effects of steel alloy, hardness, sharpening media and technique for applying the microbevel might result in different findings.

Perhaps what bothers me here is that the somewhat obtuse microbevel -- meaning, 20°-22° per side -- almost seems like the proverbial free lunch. Since I know there isn't some government subsidy to explain it, I'll probably always have questions.

Dave

 

[FoxholeAtheist]

I also read the Sharpmaker grit thread, and have an additional question about that... I've seen the "ultra fine" stones advertised, and have often wondered if they would really be worth getting. I've read that the diamonds are good if you want to remove material quickly, but I can see myself justifying the less expensive UF stones to my fiancee more quickly than the diamond rods.

 

[Cliff Stamp]

"ultra fine" stones

...be worth getting

I don't use the Sharpmaker much, I don't even own one. I have bought them for friends, even for myself but gave it away as a gift as it was too easy to use and therefore get a blade very sharp with little effort. It was thus *really* difficult to have an incentive to further my freehand skills with the Sharpmaker.

Anyway, the point of all that was that my Sharpmaker skills are obviously not above average and even then, I have consistently been able to bring blades under 100 g on the thread with the regular fine rods. This is *very* sharp, approaching "Tree topping" shaving ability so I would wonder about the practical advantage of a finer finish.

I think I'll pick up the ultra-fine rods just out of curiosity to see just what happens if you take a pass or two with them to finish.

...when looking at slicing performance to isolate the effect edge polish has on the push cut component of a slice cut.

To be complete I should record the force used both vertically and horizontally on the draw. It would not be very difficult to do, just angle the blade and note the degree at which the rope moved. You could also then study the length of the draw as a function of the angle and see how the slice paramaters were effected as a function of vertical and horizontal load.

the somewhat obtuse microbevel -- meaning, 20°-22° per side -- almost seems like the proverbial free lunch.

I avoided microbevels for a long time because I assumed it had to decrease cutting ability, plus it just didn't seem "neat". But after comparing the best blades I have seen with more acute angles, sharpness in extremes is much more determined by quality of finish, edge angle just helps if you are sloppy.

Consider again sharpness as a measure of force, what is needed to cut the material and what is needed to press it out out of the way. When the bevel is just a fraction of a mm wide (thickness of ~ 0.001"), it doesn't take much force to separate materials this much, and if the angle is 20 degrees or 10 it doesn't matter because in any case that part of the force is much smaller than what is necessary to actually cut the material so it has no significant effect on the total.

Of course once the edge thickens this is no longer the case, as then the wedging force can get larger than the cutting force and as noted there is still the issue of tooth size with coarse finishes.

-Cliff

 

[ColoradoDave]

Excellent discussion, has really helped me visualize and understand what's going on here. Thanks to all.

Dave

 

[Jeff Clark]

I'm posting a comment here just to delay the time when Spark might delete it.