Singularity's Sharpening Page





I mirrored Singularity's sharpening page on this server because it is so important to beginners and so much fun for debate by experienced people. So much information will lead to questions which is why email links exist! The original page is Here

And so to the article!


Nowadays, when you buy a new knife, it generally is "razor sharp", meaning it can shave an arm. But soon enough it gets dull, and I found that a lot of people are afraid, or do not know how to sharpen it.

But sharp is just a word that describes an experience, let's have a look at what is the meaning of sharp:


How "sharp" is this ? : Utilisation

With experience, one comes to understand that "sharp" may have different meaning, depending on the use of the tool. Knives and cutting tools have different main ways of being used:

The different types of cuts one can use

  1. Push cuts: The tool is used like a razor, the edge attacks the material perpendicularly, here tests have shown that the factor is to get an edge as finely polished as possible, meaning that the finer the grit of the abrasive, the better the cut is. A razor by example is used for push-cuts. Hatchets, axes, planes, or machete work uses push-cuts.
  2. Slice cuts: The tool is used to cut a slice, by pulling and pushing the edge on it's whole length, along the material, like if using a saw. There is little down pressure applied. Here, tests have shown that the polished edge, is not the most efficient, and that a coarser edge, cuts the material better. It is easy to understand why a coarser edge acts like a saw with micro-teeth. Kitchen knives are generally used as slicers, but the improvement from the coarse edge is rather better perceived on ropes, plastics, and other hard or fibrous materials than on soft food. Some tests on such material, have even shown that a coarse edge is more efficient than a serated edge.
  3. In between : Shearing cuts: But of course, these are extremes, and generally we cut things using a mix of both slice and pushing cuts. Cutting wood is an example where such a mix is useful, and more efficient that one of the previous types of cuts taken separately. The 45 degrees angled blade on the infamous French guillotine is said to bring as much as 40% more cutting power.

Other more practical examples of mixing push and slice cut, are the Khukuri "shearing cut" and Golok "draw cut", as shown on the picture, and compared to a machete push cut.

Knowing this, you will be able to adapt your sharpening to the tool you use, it is obvious that a hatchet is better with a polished edge, when a kitchen knife will be happier with a coarser edge, and a knife mainly used to cut rope, with a very coarse edge. But most certainly, you will choose a level in between. I often, for folders, polish a part of the edge, and let another part coarse (which is a very versatile solution, much better in my taste than the serated edges combos that some manufacturers are selling).


How "sharp" is this ? : Edge Profile and geometry

Now surely, it cannot be so simple, the edge, the steepness of the bevels, if any, or the presence of convex, flat or concave bevels, the number of bevels, and their size, the thickness of the blade, its outline (like a skinner, or straight, or recurved), and profile, change the penetration in the material to cut.

Some solutions like the convex edge, allow for good cutting, as well as good wear resistance and sturdiness. What is generally said about the convex edge is that is is normally thicker than a bevelled edge. Wrong: a convex edge or blade can be made as thin as wanted.

Example of some cutting tools profiles. The profiles are intentionally exaggerated.

Getting to work

Now that we understand what makes a knife sharp, or what makes us think it is sharp, because it is efficient, let's try to keep good things and improve bad factors.

finding the right angle

Common factory knives (1 &3 on figure) Generally, on commercial knives, there is a noticeable bevel formed by the cutting edge, regardless of the main bevel type. This does not help penetration in any material, for the same reason that airplanes' wings and boats' fans do not have any edge on their angle of attack: simple material penetration dynamics. But it does lead to this remark: like for wings, the speed will call for better profiles ( thin while resistant: axes, goloks, hatchets, any fully convex blade), while low speed cutting calls for a thin edge of any kind.

Bevels have the disadvantage that after some sharpening the edge will thicken.

I have owned many old knives, and I can tell you our grand parents never kept the original bevels, they were sharpening on all the surface of the blade, and they were maintaining the thinness of the edge.

So my recommendation here is to round these factory edge bevels as much as possible(4 on the figure). It will surely change the look of the knife, so this advice is not for collectors, but for users.

Puukkos (2 on figure) On Scandinavian knives, the edge bevel is huge, it is recommended to sharpen it all, and makes sharpening very easy, as the bevel maintains itself flat on the stones. Near the very edge, most people then just convex it very slightly (1/10th of mm).

Single bevel For these blades (chisels) that are flat on one side and have only one edge bevel on the other side, the secret is to sharpen the bevel, and remove the wire edge from the flat side (by sanding flat).

Convex blades(5 on figure) For convex blades, the important thing is to maintain the profile (or make your own like OldJimbo and myself) to its optimum efficacity. The sharpening can therefore go on a large amount behind the edge, for some centimeters, which will avoid the creation of a bevel, and a lot of work to remove it, by maintaining the profile.

Dents and over-sharpening Dents, are generally treated by flattening the edge where the dent is (edge perpendicular to stone), and then bringing this flat edge back to sharpness. Dents and over sharpening, may force you to rework the secondary bevels or the profile. It will get messy and scratched, but the knife will keep efficiency, and you can bring it back to some good looking by rubbing degressive grits sand papers on the whole length of the blade.

Angle(s) Of course also the angle is important, everybody can understand that a steep angle cuts less than a shallow angle. But a steep angle is more resistant to wear, so it is at the end your experience, the planned use, and the type of steel that will determine the angle.

But even maintaining a knife without trying to improve it, takes some time. The first thing I do is to rest the edge flat on the stone, and find the appropriate angle, by turning the handle. The right angle is just at the limit where there is space between the extreme edge and the stone, and where it touches. Then, when the angle is understood by vision and hands, I sharpen. I repeat the same process for the other side... Check the previous figure, it helps understanding.

For the last few strokes, it is good to go very light (feather light), with a bit more angle.

Apply the same number of strokes and same pressure on each side.

You will probably notice that most production knives are not symmetric at all, either in angle, or bevel size ! You can correct this.

Repeat all the process with a finer grit until the edge is adequately polished.

The wire edge

The sign of a working sharpening is the creation of a wire edge. The wire edge grips, when passing the thumbor a nail from behind the edge, and away from the edge. It disappears with finer grits, polishing, or burnishing (or cutting).

The sharpeners

There are loads of different systems to sharpen a knife. Some are guided, some are for use free hand, some high tech, some as old as the world.

The guided systems Good for beginners, the stone is guided though a guide fixed on the blade, they allow to maintain an angle, but on the disadvantages, it also maintains a bevel, which as I said is not efficient. Lansky, DMT and lots of other are available on the market. With experience, you'll find them useless and slow to deploy. I call those gadgets.

The semi-guided systems Two stones are set at the right angle (between 20 and 30 degrees to vertical, in a wooden or plastic base, possibly with a set of two angles available, the user holds the blade vertically against the stones. Spyderco Sharpmaker is the example of this kind, among others. This is more versatile than the previous system, as it allows sharpening any size of blade, as the guide is not fixed to the knife, but a reference to verticality. The double angled systems allow flattening the edge of the bevels of factory knives. They are a must for beginners, and can also be used free hand later.

Free hand sharpening: Whether it is a bench stone on which the blade is passed, or a stick passed on the blade, the system is the same, there is no guide, except the knowledge of the hands and eyes It may be disastrous when done by beginners, and a marvel when performed by an experienced person. One must learn to understand a blade's geometry, and to maintain one or many angles constant before mastering the technique. It gets tricky with knives whose bevels change steepness over the length of the blade like some puukkos, and then it is not about maintaining an angle, but a movement spanning from an angle to another! The results are in all the cases well worth the effort. The beginner will have to train on cheap knives.

Mechanized systems Well, lots of systems are available, from band systems, to circular whetstones systems, though grindstones, and my personal favorite, the angle-grinder equipped with a rubber padded hard plate, and sand paper... The danger is always to remove too much, to generate too much heat, or to make the blade fly in the air. Use at your own risk. But for blades that need a lot of work / rework, and if you have not studied Zen and the art of Japanese sword polishing :), they are the solution.

The abrasives

I have over the years used a lot of different abrasives, and here are my findings, starting with the most efficient.

Japanese waterstone

The Japanese waterstones are of two kinds: natural and artificial. The natural ones are getting rare, and the artificial are cheapo and efficient. As the name explains, they are soaked in water before use, and a sort of slurry forms on the top while sharpening. The slurry helps the stone cut better, and is also the sign the abrasive does not get clogged. The water also ensures that whatever is done, no heat will be produced that could destroy the temper. The Japanese waterstones are the fastest abrasives I know of for blades, even fastest than diamond! The only draw back, is that the material is soft and is abraded by the blade, so the stones need to be trued to flat after some time.

Studies from wood workers show that the advantage in speed and finish of the natural stones can only be seen on steels whose hardness is greater than 63 HRC, Which is fine for us because a 8000 [1] Awaseto costs some 150$  [2], a 2000 Aoto 70$, a 100 Binsu 40$!

They make now some new stones which are ceramic bonded (they are still not to be mistaken for pure ceramics as they do loose grit) but are a bit more resistant to wear, and almost as efficient.

There are multiple grits  [3]:

  • 250 to 350 are very coarse, for working on chips, and dents.
  • 800 to 1200 are the sharpening grit. Take care, though it is fine, it removes material faster then other abrasives.
  • 3000-4000 Medium finish grit.
  • 6000-8000 Finish grit, very close to polished surface.
  • Prices for artificial stones vary a lot, from 215 to 70$, most combination stones of the biggest size ( 8" or 20 cm long) are between 20 and 30$.

    Diamond:

    Diamond bench stones have been available for a while now. The surface in diamond grit held in a nickel coating over a steel plate. They remove material very fast, like a Japanese stone, but in my experience, they are expensive, and do not last long, because though the diamond is one of the hardest material on earth, the nickel bonding is abraded by the blades, and the diamond grit liberated, thus making the stone almost inefficient after a relatively short time.

    Reasonable size diamond coated bench stones are expensive, count 80 to 180$.

    Ceramics:

    I have also tried ceramic stones and bench stones. Here the problem is the opposite to diamond, they last forever, but they cut not that well, because the ceramic gets clogged, and needs frequent cleaning to work well.

    Ceramics are relatively cheap: 30 to 40$ for a bench stone.

    Leather strop:

    The razor leather stop is a tool designed to bring a fine polish on the edge. It can be used with other blades than razors, and the result is most of the time frightening!

    Count 30$ with green paste.

    Arkansas:

    Natural stone with a grit of 8000. See Ceramics. A stick is 10$, a bench stone 50$.

    Burnishers:

    The burnisher is a tool made of hard steel, or any other hard material, polished, and is used to realign the edges of a cutting tool. It often avoids sharpening, it has the advantage that it does not remove metal, just pushes it back in position. It works better on blades that are not too hard (less than 57 HRC), and gives them many more lives before re-sharpening. The tail of a hardened drill, or of a file if it is round, makes a good cheap burnisher. A small fine Arkansas can be used as a burnisher (though it removes a little material).

    Burnishing a knife takes a few seconds, and makes the edge like recently sharpened.

    The burnisher brings the question to know if it is better to have a knife with a hard edge, which will hold a long time, or a knife with a softer edge and a burnisher, which will also hold some good time? ...

    Impossibilities:

    It is impossible to sharpen a recurved portion of a blade on a flat stone. Worst that ruining the edge, it will ruin the stone. The only solution is a stick or hand-held small stone.

    Cheap and field emergency sharpening:

    Sand papers. Sand papers are not a stupid idea, whether glued to a flat wooden board, or to a mouse pad for convex edges sharpening, or in the pocket, they are highly efficient, at a very low price, and they take no space or weight. The mouse pad glued sand paper is a very efficient and easy solution favored by many outdoorsmen.

    Back of plates, or ceramic kitchen ware / pottery: You can use this as an emergency sharpening; it is ceramics after all no?

    Edge of car glasses, beer bottles/ glasses: The edges of car glasses are abrasive enough, and any glass surface can be used as a burnisher.

    Other blades, metal bits: The back of other blades makes good burnishers too (except for differentially tempered blades). Drill tails, file tails, screw drivers are good burnishers too.

    Stones: Stones can be used. Choose one of the right grit, and flat enough. River bed stones are generally great for this. works better with water.


    My choices

    At home, nowadays I have come to the following tools:

    1. Japanese waterstone King Combi 250/1000
    2. Japanese waterstone Cerax 1000/3000
    3. Japanese waterstone King 1000/6000
    4. Fine Arkansas (8000 grain) used as burnisher.
    5. A razor strop and green jeweler paste, for really fine polish.

    I use the waterstone 250 for reprofiling, removing dents. The 1000 grit is used for sharpening, slicing knives are left in that state, or brought to the 3000. Push cut knives are finished with the 6000, some others are even polished with the strop.

    The Arkansas as I said, I use as a burnisher, and a few passes of it bring back a razor sharpness.

    In the field, a double-sided small Spyderco ceramic is generally enough. It can too be used to burnish.

    I do not use anymore:

    1. Spyderco Sharpmaker (or only the ceramic sticks in the hand on recurved blades). Still, it has merits, and is the best guided system I have seen. I was useful when I was learning, but it imposes its geometry on the edges.
    2. DMT aligner, not able to sharpen more than 4" in a go, it lasted a week, I kept the stones:
    3. DMT Stones, I now use them to true my Japanese waterstones, as they lost they cutting power.
    4. Spyderco ceramic benches, medium, fine and extra-fine. Too slow, though the extra-fine is a good polisher. Gets clogged too quickly, and are difficult to clean well.
    5. Oil Norton stones, because oil is messy, and they clog quickly too. Some Nortons are waterstones though, and work as well as Japanese waterstones, I got one of these on a wood handle, and it is a marvelous profiling tool for big blades.

    Conclusions:

    Sharpening is a complex knowledge to acquire, because it is based on the understanding of the function of the edge that needs to be sharpened. I hope this article might help those that need to sharpen a bladed tool, and avoid them some mistakes.

    Singularity

    Notes:

    1] All the grit numbers in this article are Japanese

    2] $+-=Euro

    3] Japanese grits are not to mix with US grits, a 1000 Jap. is a 500 US, a 4000. Jap is a 1000 US

    PS:

    If there is demand, I may produce a how-to with pictures later...





    If you have questions, criticisms, or things to add - email me please.


    Jimbo

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