Blade Sharpness Question

G

Gadfly22

Joined
Mar 3, 2011
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I want my knife blades to pass certain consistent sharpness tests. The arm-hair shaving test is one. But my real test is cutting 8 1/2 x 11 printer paper (I have a stock of scrap I use): the cut that follows the paper grain has to be butter smooth and the cross-grain cut (the more difficult) has to be fairly smooth, with no snags for at least 3 inches.

I have very good knives with very good steels that easily pass the arm-hair test and the with-grain test but fail or stumble at the cross-grain cut. But those blades pass the other 2 test so easily that I know they are sufficiently sharp.

Any ideas why some blades would stumble at the cross-grain test? Blade width? Grind?

The specific knives that brought this question up were a Military with S30V (very very sharp but not great at the cross-grain cut) and a sub-$40 Rock Creek Pecos with some kind of proprietary steel (HWS-2K) that excelled at all tests.
 
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hold your knife straight up and down,apply the edge with the blade weight only to a plastic felt pen ,increase the angle of pen,until,knfe no longer bites ,make notes,its just a suggestion,
 
richard j said:
try newspaper and see how well they cut. try this test i used on a knife i made. the vid is kind of dark but i'm cutting free hanging newspaper with the edge of the knife up. http://knifetests.com/kII.html
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I tested the Military and the Pecos on newspaper last night also. Result: exactly the same fluid cutting. (I can't see your videos at work for some reason.) So I consider the sharpness of the 2 blades to be the same. It's just the mystery of the cross-grain cutting test -- where the 2 blades differ -- that I'm wondering about.
 
I'm going to do some additional testing on other knives where I've seen this behavior. It may turn out that it's a matter of the hollow grind versus the flat (or other) grind.
 
i'm not sure if the grind matters that much. the knife in the vid is k II and it is a chisel grind with a half convex edge. http://www.bladeforums.com/forums/showthread.php?t=585930

if i want to see how well an edge holds up i'll go chop some hard wood if the knife is large enough. k II for instance has held up really well to some hard ash. here is a link to a post where 5.56 posted a vid of me chopping on some hard ash and then push cutting some newspaper after. http://www.bladeforums.com/forums/t...-that-factory-edge-!!!!?p=9313785#post9313785 i made this vid after a buddy done some chopping on the same piece of wood. he was not beating on the wood hard enough so i had him take a vid of me.
 
you should up your sharpness tests. go for wittling free hanging hair and/or push cutting toilet paper! at the minimum you should be able to pushcut newsprint!

I'd suggest sending your knife to someone like richardj which knows what he is doing so you can feel how a really sharp edge feels and see what it can do. Hell, he will even tell you how about to get a sharp edge! (believe it or not, it really doesn't take all that long at all...)
 
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Either simply not sharp enough or small burr, but some extra work on the Sharpmaker fine stones seems to have made a big difference. Thanks.
 
The S30V Military is not lacking anything in terms of "ability to be sharp".

Some steels behave differently as they are sharpened. S30V has been known to give some people a little bit of trouble, but once you figure out how to gauge the sharpening process, you will find it is like any other steel.
 
Gadfly22 said:
Either simply not sharp enough or small burr, but some extra work on the Sharpmaker fine stones seems to have made a big difference. Thanks.
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Congratulations! If I were in your shoes I'd do a complete sharpening. Don't move from the brown stones until you can cut across the grain. Only move on to the fine when your not snagging across the grain. You'll be at another level of sharpness after the fine if you do it this way.

I just stay with the brown until I get the edge I want, then I just refine on the fine and sometimes on the UF rods.
 
richard j said:
i'm not sure if the grind matters that much. the knife in the vid is k II and it is a chisel grind with a half convex edge. http://www.bladeforums.com/forums/showthread.php?t=585930

if i want to see how well an edge holds up i'll go chop some hard wood if the knife is large enough. k II for instance has held up really well to some hard ash. here is a link to a post where 5.56 posted a vid of me chopping on some hard ash and then push cutting some newspaper after. http://www.bladeforums.com/forums/t...-that-factory-edge-!!!!?p=9313785#post9313785 i made this vid after a buddy done some chopping on the same piece of wood. he was not beating on the wood hard enough so i had him take a vid of me.
Click to expand...

Question: the ability of the knife to hold an edge like that is a function of the steel, heat treatment, geometry, grind, sharpening/honing... combination of all or a some of the above?
 
Magnaminous_G said:
Question: the ability of the knife to hold an edge like that is a function of the steel, heat treatment, geometry, grind, sharpening/honing... combination of all or a some of the above?
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All of the above with some factors becoming more important as use or the material being cut changes. For instance, wear resistance, which is also a property of a knife steel, assumes more importance when you cut abrasive materials. Another property is corrosion resistance, take CPM-M4, which by almost all accounts, is an excellent steel with fantastic edge holding but is not a stainless steel. A few people have found that CPM-M4 dulls quickly when cutting acidic foods.
 
Gadfly22 said:
I want my knife blades to pass certain consistent sharpness tests. The arm-hair shaving test is one. But my real test is cutting 8 1/2 x 11 printer paper (I have a stock of scrap I use): the cut that follows the paper grain has to be butter smooth and the cross-grain cut (the more difficult) has to be fairly smooth, with no snags for at least 3 inches.

I have very good knives with very good steels that easily pass the arm-hair test and the with-grain test but fail or stumble at the cross-grain cut. But those blades pass the other 2 test so easily that I know they are sufficiently sharp.

Any ideas why some blades would stumble at the cross-grain test? Blade width? Grind?

The specific knives that brought this question up were a Military with S30V (very very sharp but not great at the cross-grain cut) and a sub-$40 Rock Creek Pecos with some kind of proprietary steel (HWS-2K) that excelled at all tests.
Click to expand...

My experience is that thick blades with fairly obtuse V grinds do not crosscut paper very well, even if they're sharp enough to easily shave arm hair. Take the same blade and grind off the shoulder (convex it) and it'll likely crosscut the paper quite well. Again, in my experience a fairly toothy edge stropped up well will frequently crosscut paper better than one that's been high polished unless the polished one is refined to a very high degree. Speed of the cut and how much mass the cut-off side of the sheet has, all play a role. One of the tests I use is to start cutting cross grain and let the cut-off side flop over. If I can then continue to slowly finish the cut as the spent side drags over, I know I've got a pretty good edge.

Moisture content has to be considered too. I typically use catalog paper for quick cutting tests - split a catalog in half thats about the same grade as phone book paper - keep one half at home in the basement, and the other half stays at work where the climate is pretty well regulated. Always easier to crosscut the paper at work with the drier paper.

Related note re edge retention and Richard's tree chopping test - I brought a few machetes with me backpacking last Spring and tried them out on seasoned yellow birch and American beech. They both made it through the birch no problems, but the beech absolutely repelled both machetes - 1075 and 1055 steel respectively, convexed at approx 30 degrees inclusive, sharpened up on 800 grit sandpaper and well stropped. Despite having a few rolled spots on the bevels, and because of this neither machete could crosscut newspaper without snagging, both could still shave arm hair from the region of the blade where they'd seen the most abuse.

How the blade is used must factor in - give it to a greenie and they'll find a way to dull it up cutting fuzz sticks. Give it to someone that's got some experience and they'll hack down a bunch of standing deadwood, cut your fuzz sticks, and it'll still be sharp enough to shave your arm.
 
Edge & blade grind geometry make a HUGE difference. I'm coming to believe it's the biggest factor of all, in cutting performance.

I've come to like convex edge grinds for the reason pointed out by HH above. Smooth or round off the 'shoulders' of a V-bevel, and the difference is quickly seen (and heard). Polish the convex, and it makes it even more-so. Cuts through paper beccome a lot 'slicker' & quieter. The most noticeable difference is in push-cutting. Even on thin, small pocketknife blades, I've noticed this. Some have occasionally mentioned convex doesn't make much difference on small, thin edges like these; I've found it does make a significant difference, especially in edge durability. My best slicers are both Opinel blades (combination of thin blade AND full convex; it's a win-win). HH's point about seeing if the knife cuts after the paper has 'flopped over' is a 'next-level-up' test of sharpness. The Opinels mentioned above will cut in this fashion.

I've been testing my edges on phone book paper. I like using it, because even very tiny burrs will snag on it (sometimes those won't be as easily detected, when slicing heavier/thicker paper). It's been very useful for weeding out those tiny burrs, for me. Catalog pages do well for this, too.

I have noticed burrs on the shoulders of factory-ground V-bevels occasionally (see it more frequently on blades from Case; I've even resigned myself to expect it). Those can definitely snag or impede a cut through a sheet of paper. It's just another reason why I prefer the convexed edge (no hard, crisp shoulder means no additional burr to complicate things).

Second only to edge geometry, I think burrs & wire edges are the next most-common factor in impeding cutting performance (assuming the edge profile itself is fundamentally good and fully apexed).
 
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Gadfly22 said:
I want my knife blades to pass certain consistent sharpness tests. The arm-hair shaving test is one. But my real test is cutting 8 1/2 x 11 printer paper (I have a stock of scrap I use): the cut that follows the paper grain has to be butter smooth and the cross-grain cut (the more difficult) has to be fairly smooth, with no snags for at least 3 inches.

I have very good knives with very good steels that easily pass the arm-hair test and the with-grain test but fail or stumble at the cross-grain cut. But those blades pass the other 2 test so easily that I know they are sufficiently sharp.

Any ideas why some blades would stumble at the cross-grain test? Blade width? Grind?

The specific knives that brought this question up were a Military with S30V (very very sharp but not great at the cross-grain cut) and a sub-$40 Rock Creek Pecos with some kind of proprietary steel (HWS-2K) that excelled at all tests.
Click to expand...
Try using a carbon steel. Some steels can hold a finer edge than others. I believe the Sandvik steels in particular loves a razor edge more than most stainless.
 
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