The single biggest problem with sharpening

C

Cliff Stamp

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The largest problem is probably just angle consistency, however there are many ways around that, using a jig or simply a marker on the edge. The biggest problem I have found is with the inherent damage on heavily used edges, which leaves the edge formed with weakened steel. The damage can be corrosion, but usually is fracture from impacts or fatigue.

Recently during some comparitive work I had noticed my sharpening results much more positive than usual. After some reflection it was obvious that this was due simply to more work being done on the edge. They were being examined under magnification [x20], and completely sharpened until all micro-bevels were removed, then given one more round (1-2 minutes) just to make sure.

Under this method I was seeing shaving edges with 1000 grit waterstones. In contrast the last time I tried to sharpen the Camp Tramp, I could not get a decent shaving edge even though it had a burr (cut off), then finished right up to CrO. The problem was that the knife had been used really heavily (a lot of throwing, and other heavy impacts, really gave the tip and edge a pounding).

This problem can be hard to notice because the edge will form well in regards to burr, catch on finger nail, even looks decent under magnification, however the sharpness will be low, and the edge retention worse. The only way to solve it is to recut all the weakened metal, and the only way to tell that this is happening (outside of really high magnification) is to see a sharp edge forming.

Nothing really new here, but as a possible solution, the next time you have a knife which is really refusing to take a sharp edge, go back to the most coarse hone, form a full burr, cut it off with a few strokes at a higher angle, then go back and remove this micro-bevel, repeat this a few times until the edge is shaving decently well even on the coarse hone. This is possible even with x-coarse DMT, and 100 grit AO belts.

The other well repeated point is that don't let knives get so dull in the first place. The more you sharpen the easier it will be as you will be preventing excessive damage.

-Cliff
 
I've had the same problem many times, where I finally had to sit down and really work on the blade with a coarse hone. I didn't like doing it, but it did solve the problem.

I have noticed the problem much less on my convex ground knives. I'm not sure why. Maybe the convex shape is more resistant to edge weakening, or maybe it's just that the wet/dry paper I use for sharpening removes enough material to completely reset the edge. Maybe a little of both.
 
I think when a convex edge is sharpened on abbrasives (not stropped on leather) there is suffient metal being abraded away to remove any weakened metal.

AS far as "Shaving" sharp edges, it is far more an issue of edge alignment than material property (i.e. taken past elastic weakening) or even edge finish.

As Cliff noted, even 100 AO edges will shave (though not super smooth like a polished edge.)

I find course edges from Arkansas stones shave well, and are still aggressive slicers. Going to hard Arkansas, and even black surgical grade stones, gives a smooth edge, better for wood work, less well suited for rope and cardboard work.

I have used Water stones, arkansas, ceramic, diamond, etc. I find that a good Arkansas stone (I use water not hining oil on mine) gives the best utility edge. A good double sided (soft/hard) Arkansas stone and skill are all that is needed to keep any knife sharp.
 
There isn't anything inherently superior to the convex profile in that regard, it is probably just a matter of frequency. When using the GB Wildlife hatchet frequently as a reference for chopping, I would sharpen it almost every other day. It never went much past shaving sharp and thus could be restored very rapidly with just a light honing on 1000 and then quickly to 4000 and the CrO. I never failed to get a crisp edge.

For whatever reason a few sessions past without sharpening, just rush to put it away. The chopping ability is also not significantly effected by a less than blazing sharp edge which is why it got so degraded. Then suddenly I had an axe with an edge that would just pass a thumbnail check, not dull by a lot of measures, but far below shaving. This one took a lot longer to restore, same profile. But just a lot of weakened metal, for those kinds of events it would be better to have a small 220 SiC hone to start with instead of the 1000.

The worst thing about this aspect is that it is hidden, you can't readily see it, and when it is bad, it is really bad and makes sharpening difficult. This is why when sharpening friends knives I will start out by completely ripping a new edge either with a file or on the sander. Old heavily used knives can have significant inroads of weakened metal which just continues to break and bend forming horrible edges until it is all cleared away.

-Cliff
 
On the other hand, if an axe is being used for fine tasks, like wood carving, or used for knife like tasks, then a very high level of sharpness is essential. Were it not for the very sharp edge on high quality axes, they would not be near as versatile. Any clumsiness or akwardness is made up for by the keeness of the edge, espcially as compared to most factory knives.
 
Yes, that was why in general it was kept very sharp. Plus even though the raw chopping power isn't significantly effected, sharper blades are less prone to glancing and just safer to use. Unless the axe is being used for very rough work, sod cutting and the like, the edge should be made as sharp as possible.

-Cliff
 
I pretty much agree with all that has been said. However I think there is a second biggest problem with sharpening and that is patience. Patience and using too fine a hone on too dull a blade.

How many times have you seen someone take very dull knife and rub on one side a few minutes, then the other and after a few minutes proclaim that they just can't sharpen a knife.

Ok, patience, too fine a hone on too dull a blade and just not understanding "the burr."

I guess it pretty much sums up to fundamentals.
 
Originally posted by gaben
What has happened to the metal when it is weakened? What has changed in its structure?
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Lots of stress fractures from being bent back and forth. When you lightly sharpen an edge, you are aligning it more than anything. This is especially so with kitchen knives that are steeled frequently, or chopping tools which are subject to high impact forces.

If you just give your knife a few passes on a Spyderco Sharpmaker (which has very fine stones) you are not going to remove a significant amount of metal in those few swipes, yet the knife will be razor sharp, becasue the deformed edge was re-aligned to correct any rolling.

To see another example of metal weakening in this manner, just take a coat hanger and bend it back and forth. You will notice that it gets easier and easier to bend the metal until it breaks. The reason it gets easier is that the metal is unable to resist deformation as it as stress fractures form when a metal is taken past a certain point (its elastic region).
 
Mike_Chandler :

If you just give your knife a few passes on a Spyderco Sharpmaker (which has very fine stones) you are not going to remove a significant amount of metal in those few swipes, yet the knife will be razor sharp, becasue the deformed edge was re-aligned to correct any rolling.
Click to expand...

Yes exactly, this is one of the problems with such sharpeners in general. Then can easily lead to edges which while very sharp can blunt very quickly in use. To Sal's credit however he does recommend using the coarse hones to hit the edge periodically, though the reason he cites is edge thickening. For light use knives, the edge weakening also isn't nearly the problem it is on heavier knives, which is why for such knives you don't want to rely on stropping or other edge alignment techniques for the majority of the sharpening.

Gabe :

What has changed in its structure?
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The steel can be damaged by corrosion, in which case the steel has been chemically changed to iron oxide which is very weak and brittle. It can also be damaged by impact which leads to direct stress cracks. This is when a force is applied so fast that the crystals can not move and thus they are just ripped apart leaving voids in the metal. When these voids grow and intersect pieces will break out of the edge.

As Mike noted it can also be weakened through plastic deformation, bending the edge until it takes a set and then repeating. If the crystal is deformed just a little, then the steel can snap back into place once the force is removed, like an elastic band, and hense the name elastic deformation. If the force is too large, or applied for too long, the crystal loses its ability to return to its previous shape and will end up with a weakned structure due to voids, or just really bad alignment of a crystal structure.

To undertand the latter just think of putting up siding on a house (or anything similar), the joints in the wood can not be in a straigth line as that is really weak. Now would you want the nails scattered all over the place, nor gaps in the wood. When steel is highy stressed essentially all of these things will take place which is when steel once bent gets easier to bend until it cracks / rips.

On an interesting note, the steel is actually stronger in the opposite direction of the bend. For example fold a piece of paper across the middle. Note then how it becomes much easier to fold again the same way whe the crease is formed. However now try to fold it the opposite way against the crease and it will be stiffer. This is because all the imperfections in the steel will resist moving in that direction as they have been ligned up to move in another, in effect making a pseudo-grain of sorts.

-Cliff
 
I;ve noticed this phenomenon as well. If you put a great shaving edge on with the coarse stone, you have a long-lasting sharp strong edge. If you can't get it to feel sharp on the coarse, you may get it sharp on the finer grits but it never lasts as long.
 
stripey357 :

If you put a great shaving edge on with the coarse stone, you have a long-lasting sharp strong edge. If you can't get it to feel sharp on the coarse, you may get it sharp on the finer grits but it never lasts as long.
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Yes, this is it exactly. It is often recommended that if you have trouble getting a shaving edge to just use a finer grit stone or proceeded right to stropping on loaded leather. This for the reason you noted might work for short term satisfaction, however won't give optimal performance. Note as well that the worse the steel, the more this is a problem, as cheaper steels are much more prone to extensive damage than the higher quality cutlery steels.

-Cliff
 
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