Is this normal? How long should my knife stay sharp?

T

trains are bad

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Jan 29, 2004
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I have a just a couple knives, and I was never happy with how sharp they were. Now I have a lansky and I'm happy with thier sharpness but not happy with how quickly they get dull.

Example: my Delica. Shaving sharp. I get a new aquarium and 20 gallon jugs of water. I invert the jugs and cut a slice in each one so it drains quickly. When I'm done my knife no longer is shaving sharp.

Same knife, hair popping sharp, only a few feet of corrugated cardboard, no longer shaving sharp.

I don't THINK I'm getting a wire edge, the knife stays relatively sharp for a long time after it loses its luster.

What all things contribute to a knife staying scary sharp under use?
 
Given what you've been cutting, it does sound like you are getting a wire edge when you sharpen the knife.
 
There could be a few things going on here. You might have a wire edge for sure. Also, hair popping edges usually disappear quickly after some work, and then you get a "working edge" that should last a good while. Cardboard is very abrasive to edges, and plastic will dull edges fast too.

Perhaps the issue is micro-chipping? How thin did you go on your edge angle? I really recommend going to radioshack and getting their pocket microscope thing. It lights up and goes from 60x-100x. Have a look at your edge and see of there is a wire edge. Since I got mine (cost like $15 Canadian) my edges have improved. Especially in the "wire edge" department. I know now I was leaving wire edges on my blades. I hadt the same problem as you, with the "sharp until I use it" type of thing. If you have a strop you may want to try stropping the edge after the lansky, just to help clean-up the edge of any wire.

I hope any of this helps!
 
No they should not lose their sharpness that fast. I strongly suspect a burr'ed initial edge. Does it shave smoothly on *both* sides of the edge?

In general a blade will stay sharper longer on the materials you are cutting if it is harder, wear resistance plays a part to a much smaller degree.

You might also be cutting with excessive force and or speed, and or twisting or torquing on the blade. So you could try cutting a little slower and/or with more deliberate straight lines.

Craylola made an excellent point about getting some way to look at the edge, that could help significantly.

Note sharpness does *not* decrease in a linear manner, all steels (and other blade materials) will blunt slower as time passes. It is closer to a log relationship.

-Cliff
 
Hmm that microscope sounds cool. I've always wanted to look at my edges at school with their microscopes but never did, because I'm not supposed to be carrying a knife anyway. Sigh.

I profiled the edge with the 20* angle, and sharpen a small 25* secondary bevel on it. It's a work knife and it seems to hold that 'working edge' really well like that.

I just like how it sails through rope at the beginning of the day, and starts to lose that effortlessness toward the end if I don't touch it up (which takes all of 3 minutes). If I don't sharpen it at all, it will stay reasonably sharp (just below shaving) for many days.

I like that log scale description of dulling.
 
Does lanskey make a 400grit stone? Right now I have nothing betewwn 280 and 600, which has always bothered me.
 
I agree, sounds like a wire edge to me if you notice that much of a decrease in performance after only a few feet of cutting through cardboard. Although you may be expecting a bit much, especially if you are sharpening a secondary bevel at 25*....that should make for a good long lasting utility edge, but a bit obtuse for shaving sharp.

You might also be experiencing a rolled edge in which case steeling it with a smooth steel would help. I get months out of a sharpening by simply steeling and stropping my blade once every week or two. Its really surprising how that will bring back an apparantly dull edge.
 
I have same thing with my relatively soft SAK that I removed the serrations from. But I am ignoring it until it no longer ....cuts.... easily. That first whick after the stone is really neat and I ....was..... refreshing on the 20 degree Sharpmaker quite often. Now I am just enjoying a nice ...regular sharp...blade and using it until it doesn't cut as well.
It seems to still shave (both sides) quite a bit past that "touch and bleed" stage.
My Spyderco in VG10 is really just about the same. I don't know if going to steel that cuts like a scalpel ....longer.... is worth the extra effort to bring BACK the edge.

:confused:

edited to add that when I plan on cutting CARDBOARD, I try to find a box cutter or a crappy knife.
 
You didn't mention which blade alloy your Delica uses. This has changed a lot over the years. You can still find them with AUS-6 (soft) or VG-10 (hard). It still sounds like you have a wire edge, but if your blade is AUS-6 it will dull significantly faster than one made from VG-10 when you are cutting cardboard.

The reason that I think you have a wire edge is that you are getting dulling when cutting plastic bottles. Those are not abrasive and shouldn't have a lot of effect on the blade. If you use a sharpening fixture at a single honing angle you will tend to get a burr. Steel is sufficiently ductile that abrading it at a fixed angle naturally leaves a residue at the edge of foil-like material that just folds from side to side as you hone. This problem gets worse if you work on one side at a time rather than alternating sides on every stroke. The solution is to do a few strokes edge-first at about double your normal honing angle. This should be done with a 600 grit hone or somewhat coarser. You do it lightly to not remove too much material. You should finish honing lightly at about 5-degree higher angle than your primary bevel. This will give you a fine edge with some lateral support to keep it from folding over as you work.
 
I carry a P.O.C. brand folder in one pocket for the cardboard and plastic, and in the other pocket I carry my 3 blade stockman for cutting the good stuff like food. :)
 
I'm betting on a wire edge. My beat-up OT trapper can do 28 linear feet of double corrugated without dulling out like that.
 
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