How do you tell when a knife is sharp?

There seems to be some confusion about how wire edges work. Usually a wire edge will cut paper well. It is very thin and has microserrations and this is good for cutting paper, but it will not stay sharp very long. If you can feel the uneven edge by strumming (I like that term by the way) you do have a burr, but you also have not removed enough material from the blade to get a full clean edge.

Color the sides of your edge with black felt pen. Using the flats of the stone do a few strokes to see where you are abrading the edge. I still think that you are not getting to all of it. Some of that "burr" may be leftover from a bad factory edge or previous blade damage. If you use the Sharpmaker by stroking alternate sides you should not feel an asymmetrical burr. You could create one using the edges of the hone and really high pressure, but I wouldn't expect it to be conspicuously asymmetrical unless you worked one side of the blade at a time. If you do a complete sharpening job with a Sharpmaker you will get a burr, but you should not be able to feel it. As you finish sharpening you use the flats and light strokes and that should mostly allign the burr with the middle of the edge.

General tips: Do NOT use the white rods until the very last few strokes if you are reworking a really dull edge. Use the edges of the grey rods for reprofiling then smooth your reprofiling using the flats of the grey rods. Do the felt pen test at this point to be sure you have worked your way up to the edge all along the blade. Ease up on your honing pressure and do a few more strokes to allign the burr. Don't do this by feel, you are doing this cause you know it is there. Now you are going to do some strokes with the spine of the blade tilted away from the rods (towards the center of the V between the rods). To remove the burr you have to hone lightly with your medium rods at about twice your normal honing angle. You are tilting the blade so that as you stroke the edge down the right rod the blade is nearly parallel to the left rod (and vice versa). You only do about 5 strokes per side, alternating sides. Now the burr you couldn't see is gone. This is really where you start following the Spyderco video. It assumes that you are not having to do major reprofiling. Work lightly on the flats of the grey, then the white rods. If you are working at the 20 degree angle and the edge does not get as sharp as you want, do a few very light strokes at 25 degrees by slightly tilting the blade spine away from the rods. Don't remove much or your edge will get obtuse, but this should give you a fine edge.
 
I remember the time when if it would whittle a chunk of wood or cut a carrot, it was sharp.

NOW.....If the hairs don't POP off with just pressure, I am going to the stone.

THEN....I had a worn out whetstone on a wooden handle from my mother.
It was free.

NOW....I have a new Sharpmaker because I.....needed....one.
It was NOT free.

Progress.

:D
 
do a search on the phrase "scary sharp" and you'll see some good stuff...maybe ThomBrogan can post the link here...

Basically there are 2 kinds of sharp. Working sharp and Scary sharp. Scary sharp is more of a feeling than an actual sharpness...:D

Working sharp means you can shave the hair on the back of your hand.


Another test I do is to run my fingernail down the edge of the blade - it should feel smooth.

The pen test is a good one too, but lacking a pen, you can use your fingernail again - this time "scraping gently" starting at the base, moving toward the tip with the knife exactly perpendicular to the top of your fingernail. If it catches and scrapes, it's sharp.

You can try this both directions to see if you have a burr on one side only.

Final sharpening should be on a strop to remove the burr.
 
I was watching Murray Carter sharpen a knife for a customer, which was an education in and of itself (e.g. his method of removing the burr was to lightly pull the knife perpendicularly along a piece of wood).

He was grinding it below 10 degrees a side, and he was showing the customer how to test the edge using the "thumb on the spine, three fingers on the edge" method I've seen several bladesmiths use.

He was gently sliding his fingers parallel to the edge (yikes!) and talking about how when you got it REALLY sharp, you would start to feel it bite into the surface layers of your skin as you moved them back and forth.

When he let me try it, I had trouble feeling exactly what he was talking about. I could sorta maybe feel what was more like a greasiness than the biting he was talking about. He would dull a knife let me try it, and then sharpen it, and let me try it. The hair test and the "biting into my thumbnail" test worked very unambiguously, but I still needed more practice to get a handle on the three finger test. Mr. Carter was pretty sure that once I got used to it that it would give me a really sensitive test for edge quality.

Mr. Carter is one of those annoying people who can freehand sharpen using a stone in one hand and the knife in another without even bothering to look at his hands. I was watching his hands, and it was like watching an industrial robot in terms of how stable he was able to maintain the angle between the knife and stone.
 
I think Herr Koster is referring to this link.

Be prepared to get some CrO compound, a buffer or a belt sander with a leather belt (your choice), and a hairy-assed gnat to really do this right.
 
To me, if you can actually SEE the edge(or, light reflecting off it), then the knife is not sharp enough yet. Once I get an edge that meets my qualifications(not being able to see light reflecting off it), then it will most certainly exceed my other test of lightly dragging it across my thumbnail. If it bites into my thumbnail just from movement and the weight of the the knife, it's plenty sharp.
 
How do you tell when a knife is sharpe?


The hair jumps off my arm when I flick it out!:D :D
 
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