How do you tell when a knife is sharp?

D

drjones

Joined
Sep 6, 2000
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Hi.

I'm not new to knives, but very new to sharpening. I know this may seem like a dumb question, but I subscribe to the philosophy that the only truly dumb question is the one not asked. That in mind...


How do you tell when a knife is sharp?

I guess that begs the question of "how sharp do you want it" or "what do you want to do with it?"

I recieved the Spyderco Sharpmaker in a passaround the other day and have been sharpening my Strider Tac GB with it. I believe that I have gotten it sharper, but it still will not slice paper too easily.

I know this could be due to the fact that it is such a large blade, but it only has a 40 degree edge. That's not exactly blunt.

I also question how much more I should sharpen it, as obviously, a sharper edge is also a weaker one.

Opinions, advice please!


Drjones
 
Are you sure you're removing metal from the edge? If not, it won't get any sharper (this would happen if the angle of the blade is not 40 degrees or less). You may have to reprofile it. I have sharpened pretty much all my knives that way, and they all push cut paper easily.
 
Dr. Jones,

Take a soft-plastic ballpoint pen. Hold the pen perpendicular to the blade and gently run the knife across it. Hold the pen at a 45 degree angle and let the weight of the blade catch into the plastic of the pen (with the edge facing the ground. The better the knife glides (the glassier the better) and catches (the stickier the better), the sharper it is. The tougher the knife glides or the more it slips on the 45 degree angle thing, the more work lies ahead of you.
 
Originally posted by thombrogan
Dr. Jones,

Take a soft-plastic ballpoint pen. Hold the pen perpendicular to the blade and gently run the knife across it. Hold the pen at a 45 degree angle and let the weight of the blade catch into the plastic of the pen (with the edge facing the ground. The better the knife glides (the glassier the better) and catches (the stickier the better), the sharper it is. The tougher the knife glides or the more it slips on the 45 degree angle thing, the more work lies ahead of you.
Click to expand...

When I hold the pen at a 45 degree angle, the blade catches right on.

I guess that's good? :D :D :D
 
When the hairs float off my arm without any tugging sensation.

be carefull if you try this method though!
 
I have axes at home that slice paper, I think it's more an issue of a wire edge, this can make a knife appear sharp but when you try to cut, it won't.

Do a search on sharp, or how sharp is sharp, or sharpening, there is a ton of stuff here, hell even search Sharpmaker, that'll keep you busy for a while, then when your done read Sharpening FAQ.

By then you should have your own answer, how sharp is sharp is a subjective question dependent on a persons paticular ability/skill at sharpening.
 
drjones,

I agree with the others here -- you might be getting a wire-edge.

Strider's S30V is very tough to sharpen, and from the factory floors, the guys, I've been told by a very reliable source, likes to put a working edge on it, not one that's shaving or hair-popping sharp.

It will take quite a while for you to raise a burr on the knife -- more like 100 strokes per side on the corner of the medium stones (and remember, after about 20 strokes, the corner's going to be loaded) to raise a burr. Work only one side of the knife, get a burr, then work the other in the same way. Finish-up with about 40 alternating strokes as described in the manual or on the video, then proceed with the next step.

You'll get your Striders to easily push through newsprint, or even as Sal demonstrated on the video, "shave paper" this way.

With Thom's tests (thanks, BTW ! I didn't know those tricks !), it's possible that your GB is so heavy that its holding itself even with a relatively blunt edge......

Allen
aka DumboRAT
 
I think I may have a wire edge. I have an idea of what it is, but what is it exactly?

I'll tell you that when I drag a finger across it to feel the edge, it feels kinda grabby from one direction, but not the other. Sound like a wire edge?
 
I would guess that your edge is only intermittently sharp. The blade's initial sharpening angle was close to 20 degrees per side and there was some significant wear on the edge. The result is that you have you remove a considerable amount of material to get all of the edge fully sharpened. When you sharpen with a wide bench hone this might be more conspicuous than with the Sharpmaker. A flat hone would only work on the high spots on the edge and you would notice wide low spots which did not get sharpened. Narrow hones (like Sharpmaker rods) can slip into the low spots better and provide some sharpening all over the edge. This gives you an edge that is somewhat sharpened all over, but can still be really uneven. It tends to mask the fact that you have not removed enough material to get a completely sharp edge. This kind of edge will slice a little then snag and tear paper. I think you have more sharpening to do.
 
Originally posted by drjones
I'll tell you that when I drag a finger across it to feel the edge, it feels kinda grabby from one direction, but not the other. Sound like a wire edge?
Click to expand...
That's a wire edge.

When you have gotten the very edge as sharp as can be, you've also gotten a very fragile edge, so thin it bends back and forth when you try to cut anything. You can strop it or steel it straight again, but it will fold over when you go to cut.

You can eliminate it easily with the Sharpmaker. Stroke the blade down the length of the rod, but at a blunter angle than if you meant to sharpen it -- in other words, tilt the back of the blade away from the rod, instead of perpendicular to the ground. Do this two or three times lightly on each side of the edge, then strop each side on a piece of cardboard or down your shirt sleeve or pants leg. This should break off the wire edge.

Now the wire edge is gone and a reasonably sharp but durable edge remains.

I like the way you feel for the wire edge. I run a finger down the edge myself, but be sure to run the finger away from the edge as you do it, not exactly parallel to it, which may lead to a very thin slice of skin ... !
 
Drjones,

You got some great sharpening advice. Nothing to add from my part, but to answer the question, "How do you know." ...

Arm hair shaving is a good first pass test.

Next on my list is a horse hair paint brush. You know - the cheap kind. If the blade edge is sharp enough to catch and pop off horse hairs at the tip of the brush you're there.

You see, the horse hairs are just hanging in mid air. You can't catch them with more pressure (like you can on your arm).

Steve
 
Thanks a lot, Esav. I'll do that and post results in this thread. :)

I can actually catch my fingernail on one side of the edge, but not the other. Of course I can't see the metal that its catching on, but its there...
 
If a knife is really sharp, the cut doesn't start burning until about halfway to the hospital to get stitches.

:(
 
Originally posted by Esav Benyamin

I like the way you feel for the wire edge. I run a finger down the edge myself, but be sure to run the finger away from the edge as you do it, not exactly parallel to it, which may lead to a very thin slice of skin ... !
Click to expand...

Well, I feel an edge the way most people do; blade spine resting on the tip of my index finger, and my thumb gently "strokes" the edge, perpendicular to it.

I don't run my finger at all parallel to the edge. My mom taught me better than to do that. :eek: :D

By "strumming" the edge from one side and then the other, I felt it "grab" my thumb a bit more from one side than the other. I.E.; when stroking from left to right, I didn't feel much, but when stroking from right to left, I felt the edge grabbing my thumb significantly more.

Is that clear as mud? :p

That was my first clue that I had a wire edge.

The next way I feel is to run the back of my thumb or index finger down the blade, from spine to edge, paying close attention to the edge. If my fingernail catches on the edge in the slightest, that says to me that I have a wire edge.

I gave it a few quick licks on the sharpmaker and took the wire edge off. (I think...may need a couple more...)

Thank you so much!
 
Dr. Jones,

The knife should pass both ballpoint pen tests to be considered sharp. The sharper it gets, the better it will do both. My best edge ever is on my ATR and it moves over the ballpoint pen as though moving across oiled glass. Even there, though, there's muy room for improvement.

Also, don't worry about the strength of a sharp edge. If you make the edge sharp by abrading away the steel, it will be sharper than if you use a butcher's steel to sharpen the edge.

Soon-to-be-Dr. Lee,

My pleasure. Got an Edge Tester from John Juranitch and then Steve Harvey told me that a ballpoint pen would work just as well and cost less. Normally, he gives me great advice and I ignore it for over a year. This time, I took his advice immediately (which was after getting and raving about my Edge Tester).
 
My favorite way of testing an edge for sharpness is to take the knife into the kitchen and go to work on fruits and vegetables.

Very few knives match the CUDA Maxx on everything from carrots to cauliflower to onions to tomatoes. That's partly a function of the thinness of the blade itself. But a very narrow edge helps, and the D2 will support it.

My large Sebenza was the first knife I used for all this with the same complete success, even though -- like the Maxx -- it wasn't arm-hair-shaving sharp. Or maybe my arm hair is just coarser than some of yours :D
 
Originally posted by thombrogan


My pleasure. Got an Edge Tester from John Juranitch
Click to expand...

I highly recommend the Edge Tester. It comes with complete instructions. It's essentially a 3-step process that will really give you a good idea of your blade's sharpness. Also, while you're at it the Razor Edge Book of Sharpening is a real good primer on edge geometry/sharpening/edge theory.
 
That's easy...Hand your knife to a dumb-ass and when you notice blood dripping off the finger that he just ran across the blade-edge, then you know that your knife is sharp.:D.
 
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