The Paper Cut Test After Sharpening

cchu518

cchu518

Joined
Mar 6, 2013
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Hi Guys,
I'm a newbie knife EDC'er as of this spring and a newbie free hand knife sharpener.

I normally just use a Japanese whetstone sharpening method with a smith's tristone where I have the blade running perpendicular to the stone. I strop with either a kitchen steel or cardboard. It's always fun to do a paper cut test afterwards of course. But does the cut test actually prove if the knife is sharp or not?

For instance, a FFG Spyderco will tear through paper after a strop if you just do the samurai swing at the paper it will slice through it over and over. The same with a Skyline. However I have a Cryo that tears through rope and cardboard, packing tape...basically whatever I throw at it in addition to a drop blade Spyderco UKPK that for the life of me I can't get to slice through paper in the same way as a FFG blade. If you run the latter blades through the paper and push cut, it will go through paper fast though.

Whenever I put any of these sharpened knives to my fingernail they dig in hard of course and are all extremely usable in terms of cutting real world things just that some are better at the paper test than others.

Can someone explain to me the physics of why this is so just for my understanding.
 
We can explain it better in the Maintenance forum.
Please don't use General Knife Discussion as a catch-all.
 
Use newsprint or yellow pages soft paper.

Hold the paper between your thumb and forefinger.

Place the blade at a 90 degree angle to the paper, about one inch from your fingers, and gently push straight down.

If the blade slices cleanly and smoothly through the paper, it's sharp.
 
I've been looking for more information as to why some knives are better at that push test than others, not how to apply a paper test...
 
Toothy edge will cut longer and on coarser materials on the draw cut.
Polished edge will cut even delicate materials on a push cut.
 
I don't think I've seen anybody describe a reliable test of sharpness, there would be variations in the paper, how you hold it, etc. I read the description above and I can interpret it several different ways. I think for most real world uses of the knife it doesn't have to be sharp enough to easily cut light paper. The paper cut test, whittling hair, etc. is mainly for enthusiast interest.

I've cut paper lots of different times and it depends a lot on the type of paper and how you hold it. I think if you can cut light paper such as thin magazine paper or thin telephone book paper, and if you can cut it both ways, slowly, then the knife is pretty sharp. If you find a knife that won't cut the paper if you hit random spots along the edge, you can still get it to cut the paper if you hold the paper just right and try several times. If it cuts every time with less attention to exactly how you hold it then the knife is pretty sharp.

I would really like to see someone invent a more consistent test.
 
This is the cutting test routine that I run through, every time I touch up my edges:

  • Holding the knife nearest to the butt end of the handle, SLOWLY slice (draw cut) into the edge of the phonebook paper. Start at the heel of the blade, and utililize the full length of the cutting edge, all the way to the tip. Do this several times, from different directions, turning the sheet of paper and cutting into each edge of it. Look for slipping or snagging in the paper, and pay attention to where along the edge of the blade this happens. This check points out edge imperfections like burrs/wires that will snag, and incomplete apex which will slip without cutting.

  • After doing that cutting test, draw the edge of the blade through a piece of wood (cross-grain) two or three times.

  • Test the paper-cutting again, in the same manner as before. After drawing the edge through the wood, this will reveal how durable the edge is. If it has issues with burrs/wires or if the edge is just too thin for the steel, chances are it folded in the wood and cutting performance will degrade immediately. If the edge is fully apexed, clean and robust, then it should still cut well. Cutting might even improve; sometimes drawing through wood will clean up any remaining tiny burrs, and cutting becomes even smoother.

I've found that if my edges hold up well to this testing sequence, they'll also easily handle the rest of my other typical EDC uses (paper, food, cardboard, plastic, leather, wood). It's been a very reliable benchmark of sharpness, for me; therefore why I've settled into this routine. :)


David
 
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The blade geometry will effect how easy it will cut thru paper. If the blade is a thick edge with an obtuse (45 - 50 inclusive angle) then it will not cut as easily as a blade that is thin and with a 25 - 30 degree inclusive angle. It may feel sharp, and probably is, but will be harder to cut newsprint if the blade is thick and has an obtuse angle. Sounds obvious, but some people, including myself have had trouble with thicker blades, and thought "why isn't this thing cutting this paper, it feels sharp." Think of a blunt bow boat, and a deep v hull plowing through the water. Same concept with cutting thin flimsy paper.

Blessings,

Omar
 
I don't think that the ability of an edge to cut through a medium like newsprint is as dependent on edge angle as people may think. Maybe if you're talking extreme obtuseness (50 degrees or more), but I don't do that to edges. If an edge is clean, even, fully-apexed, and burr-free, it should push cut paper without a problem. Paper is a very good test of how well you've finished your edge, and the sound it makes tells you a lot about the level of refinement. But after you've sharpened enough knives--and if you are highly familiar with your gear and the steels you are sharpening--you'll know instinctively coming off whatever sharpening medium you're using (whatever stone or strop) what the edge will do to the paper, and you'll reach a point where there's no need to even test edges in that way anymore.

I think people might be confusing sharpness and refinement with geometry (slicing ability), and newsprint is indeed a bad test of slicing performance. Try a good, firm apple or sweet potato instead--but frankly, once you've sharpened all kinds of geometries, you'll instinctively know what the knife will do to those, too.

Just my two cents.
 
I fully believe that we are all running on caveman software. We all like sharp knives, and the standard litmus test, has been with paper, and hair. It used to be ok, if your knife would cut paper at all, and if it would bust a couple of hairs off your arm.

Now??? I want that thing to burn paper like a laser beam, and run half of the hair off my arm in one pass. Doing all of this, with no pressure mind you.

The point I make, is these are our tests. The level that your blade performs, is totally subjective to you, and your level of satisfaction. I guess when you cut enough paper and hair, you get a "feel", of when your blade is where you want it.

As far as geometry, I halfway believe that, and half don't. I have a flat grind Endura, that will get scary sharp. I also have a BM Adamas, that is .160 at the spine. Hard to get much thicker than that. But it will burn em every bit as good as my Spyderco. Sharp is sharp. YMMV, good luck
 
bdmicarta said:
I'm an engineer and I've thought about it a lot, but no luck so far.
Click to expand...
And the rest of us also. This is often the subject of "vigorous" debate, with different tasks being proposed, but some favor polished edges, some more coarse, etc. It's also hard to find a truly consistent medium.

Then there are those who favor "real world" testing. While they have a point, how can you compare butchering or quartering animals, etc., with varying amounts of dirt ground into their fur or woods that vary in hardness.

Probably the best you can do is to use a single medium with many trials to remove some (or most) of the variances. That is what CATRA does, but then some people complain it takes the human element out, such as side-loading the blade during use.

It's a much more interesting problem than most realize... Of course, most don't think of it like we do... ;)
 
My rule of thumb is: "Data in the real world beats lab data any day." From Jeff Hubbard a long term Buck Knives employee. DM
 
Magnaminous_G said:
I don't think that the ability of an edge to cut through a medium like newsprint is as dependent on edge angle as people may think. Maybe if you're talking extreme obtuseness (50 degrees or more), but I don't do that to edges. If an edge is clean, even, fully-apexed, and burr-free, it should push cut paper without a problem. Paper is a very good test of how well you've finished your edge, and the sound it makes tells you a lot about the level of refinement. But after you've sharpened enough knives--and if you are highly familiar with your gear and the steels you are sharpening--you'll know instinctively coming off whatever sharpening medium you're using (whatever stone or strop) what the edge will do to the paper, and you'll reach a point where there's no need to even test edges in that way anymore.

I think people might be confusing sharpness and refinement with geometry (slicing ability), and newsprint is indeed a bad test of slicing performance. Try a good, firm apple or sweet potato instead--but frankly, once you've sharpened all kinds of geometries, you'll instinctively know what the knife will do to those, too.

Just my two cents.
Click to expand...

I use the phone book paper test when sharpening knives, but could you elaborate a bit on the sound when cutting like (silently, ripping, noisily) and the speed used when cutting? Also, fast cutting usually provides results but I think slower cutting the paper demonstrate sharpness better.
I'll appreciate your opinion.
Thanks
 
I find if I try several cut tests on the newspaper (in addition to the pitch it makes as it cuts) I can get a pretty good idea how my edge is turning out. A fairly coarse edge can crosscut newspaper, but it makes a real gravely sound and might hitch a bit as it goes. Then a slow draw cut across the grain, seeing how the edge overcomes the curl and if it can initiate a new cut if it looses the crook of the curl. Then a circle cut or two, wavy cut, hold it up with a largish curl and see at what angle the edge starts to catch...
Lots of ways to use newspaper, helps to have working knowledge of how these characteristics translate to doing actual work.

HH
 
HeavyHanded said:
I find if I try several cut tests on the newspaper (in addition to the pitch it makes as it cuts) I can get a pretty good idea how my edge is turning out. A fairly coarse edge can crosscut newspaper, but it makes a real gravely sound and might hitch a bit as it goes. Then a slow draw cut across the grain, seeing how the edge overcomes the curl and if it can initiate a new cut if it looses the crook of the curl. Then a circle cut or two, wavy cut, hold it up with a largish curl and see at what angle the edge starts to catch...
Lots of ways to use newspaper, helps to have working knowledge of how these characteristics translate to doing actual work.

HH
Click to expand...

I still have to put lots of practice and learning which is not a bad thing.
Thanks
 
fmendez said:
I use the phone book paper test when sharpening knives, but could you elaborate a bit on the sound when cutting like (silently, ripping, noisily) and the speed used when cutting? Also, fast cutting usually provides results but I think slower cutting the paper demonstrate sharpness better.
I'll appreciate your opinion.
Thanks
Click to expand...

This is why I advocate SLOWLY drawing the blade through thin paper (like phonebook pages), while holding the butt end of the handle (I like to do this with knife held just between thumb and one finger). It's more difficult to influence the aggressiveness of cutting with the individual hold/technique, when held this way. The integrity of the apex (how crisp, how pure, how clean) will show up more here, and it'll effectively demonstrate both push-cutting and slicing effectiveness at the same time, using essentially the weight/geometry of the blade only.

More aggressive slashing and hacking at the paper is largely influenced by individual technique, much in the same way a golfer's distance and clubhead speed are influenced by the rhythm and fluidity of his own individual swing.

As an edge becomes both thinner (geometry DOES make a significant difference) and more polished, the sound when cutting paper will change from a louder 'ripping' sound (more coarse/toothy edge), to something approaching a whisper-quiet 'whoosh' in cutting the paper. Convexing & polishing the shoulders of the bevels will also tend to make paper-cutting quieter. I never realized how much the hard-edged shoulders of a V-bevel actually drag in a slice through paper, until the first time I smoothed & rounded them off, while essentially leaving the edge apex unchanged. Makes a big difference in the sound produced, and also in cutting resistance (more smooth/rounded & polished => much 'slicker' slicing).


David
 
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