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 Post subject: Re: How do I know if my knife is sharp?
PostPosted: Wed May 02, 2012 9:28 am 

Joined: Wed May 02, 2012 5:14 am
Posts: 192
Location: Florida, USA, Earth
Arm hair: I can shave arm hair but need 2 or 3 passes. I think this is barely sharp enough. If the hairs pop of the skin easily with one pass I can stop sharpening.

Slicing paper: Printer paper slicing is pretty good and you can feel the difference in resistance when you slice. You need to start the slice by cutting into the edge of the paper, not stabbing the paper and cutting. If the edge doesn't cut into the edge of the paper you still have work to do. Phone book paper is what I prefer. Being much thinner the knife needs to be sharper to cut it smoothly. When you can cut into the edge of the paper and slice with virtually no resistance along the entire edge (heel to tip) your knife is sharp.

Murray's 3 finger test: I didn't think much of this test at first. I couldn't tell any difference in my edges when Murray was saying the difference should be easy to feel. It wasn't till recently I was only testing very sharp edges. I tried the 3 finger test on some kitchen knives I hadn't sharpened in a while. My fingers could slide along the edge and it would have taken quite a bit of pressure to cut my skin. I don't mean a whole lot of pressure but I could finally feel a difference. So now I'm using my 3 fingers to test sharpness before and after sharpening. I think there is a learning curve in being able to feel the difference in the levels of sharpness. I can feel the difference in a dull blade and a very sharp blade. Being able to tell the difference in a very sharp edge and a very very sharp edge takes a little practice and paying more attention to how it feels I think. With the 3 finger test you can know if a knife is sharp without obviously testing it by cutting something (except your fingers :shock: ).

Toilet paper I have given up on using (except for fun once in a while). I saw a video of Murray Carter sharpening a knife, then shaving his face. That edge shaved his face easily but wouldn't slice toilet paper cleanly. If a knife will shave and still not slice toilet paper I don't need to use toilet paper to test. I already know the knife is as sharp as I will ever need it. Like I said, I'll still do it once in a while. Usually I may try toilet paper after trying a new stone, strop compound, etc. just to see the ultimate level the new tools can obtain (with my skill level that is).


 Post subject: Re: How do I know if my knife is sharp?
PostPosted: Tue May 08, 2012 8:37 am 

Joined: Thu May 03, 2012 12:22 pm
Posts: 16
I spent a good bit of time yesterday trying to understand the 3 finger test. I tried on my not-super-sharp pocket knife and I could easily push and slide until my fingertips were white. I went home and tested some of my recently sharpened knives. I didn't "fear" sliding my fingers along any of them, though they were fairly sharp. I also tried it against a brand new razor blade; still no fear, and I could push pretty hard. This morning I sharpened another one of my blades. It felt distinctly toothy after the 400 grit, but much smoother, almost sticky, at the finer grits. When I called it quits, I could cleanly push cut receipt tape, and the weight of the knife alone was sufficient to cut 1/4" into an apple slice. I could tell the difference between sharper parts of the edge vs. those that needed a little more refinement. The sharper parts felt, again, smooth. But still it didn't grab into my skin with the 3 finger test. I did know not to push hard against the blade, though.

So I'm not sure what to make of this test. As I said the knife I sharpened this morning seemed sharp using paper tests. But at no point while doing the 3 finger test did my brain scream "STOP!" I wish I had a known reference point, like a knife sharpened by a "pro" to insane sharpness. The 3 finger test seems safe enough to perform, but didn't give me as much feedback as actually cutting something (e.g., paper).

 Post subject: Re: How do I know if my knife is sharp?
PostPosted: Tue May 08, 2012 12:27 pm 
Forum Moderator

Joined: Thu Apr 26, 2012 11:13 am
Posts: 3173
Location: CT
I cut the receipt paper, Netflix covers, etc. I have little shavings all over the place!! I shoot for that bitey, grippy, sticky edge.

I also do a Paper Towel test. Hold the Paper Towel between the thumb and index finger in the middle of the top edge of the paper towel and slice down the towel. If it can slice 4" cleanly with a 8 or 9" blade, I'm happy :) I had one (240mm Kikuichi Carbon Elite) go to 7" one time! I use the Brawny brand paper towel, so different brands may be easier or harder to cut. Then I fold the paper towel in half and cut into the seam. Should slice cleanly with the thicker material and cut both sides cleanly. Then I prep some food, cook something, then retest cutting receipt paper.

 Post subject: Re: How do I know if my knife is sharp?
PostPosted: Tue May 08, 2012 4:58 pm 
I like the hanging paper towel test also, a knife will do that good at 2k refinement... it's actually a rather good test to let you know if you've lost your teeth or not.

Sometimes when you get "up there" into higher grits, you'll lose the edge. It still seems sharp, cuts food decent, but it's just missing something, it probably means you lost your teeth. If your knife is above 2k, paper towel is a dead giveaway as to what's going on in the edge. If you've done things right, you should be able to slice a pinched, hanging paper towel at every grit from 2k to 16k, and beyond. What's a cool thing to do, if you like to deburr at each grit, just run the knife through a paper towel to both deburr and determine what's going on with your teeth-situation.

Every edge needs teeth, or it's not hitting on all 8 cylinders. Basically, you'll start finding out how to be more delicate at higher grits. As James Tiberius Bullman once said: "Slow and steady wins the race." :) If you fly through the sharpening, and you lose the teeth, you'll have no clue what went wrong and when it went wrong.

Normally only a couple of things cause your teeth to fall off. One is raising the angle while sharpening, then not staying at that angle, aka Wobbling, and the other is doing too many edge trailing strokes on a swarf filled stone. A big one that is easily preventable is getting rid of your burr from the low grit stones. After 2 or 4k you shouldn't really have to deburr again.

As for sharpening with synthetics stones, I like to start with a slurry, sharpen until the slurry loads up with swarf and becomes black, then rinse the stone clean and do a few alternating edge leading strokes.. the "zoop zoop" method. This helps weaken any burr, but also "pushes" the carbides back into the blade, rather than ripping them out; Plus using a clean stone to finish prevents particles from running into the edge, giving you the best chance at a clean finish.

 Post subject: Re: How do I know if my knife is sharp?
PostPosted: Wed May 16, 2012 1:47 am 

Joined: Tue May 15, 2012 7:49 pm
Posts: 443
Location: Amstelveen, The Netherlands
I use the finest cigarette paper. Especially the sound - or absence of it - will tell me if the edge is clean. A piece of leather with a thin layer of acryl paint - pigment: C2O3 - to confirm, by sound and scratch pattern, or rather its absence.

 Post subject: Re: How do I know if my knife is sharp?
PostPosted: Fri May 18, 2012 12:43 pm 
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Joined: Thu Apr 19, 2012 1:18 pm
Posts: 10864
Location: Madison Wisconsin
At work I always do the arm shave test and then I slice a big sheet of packing paper.

At home my favorite test is cutting up green peppers skin side up.

The 3 finger test rewards a toothy edge. Nothing wrong with that but the more you refine the edge the less it "bites" into your skin.

Mark Richmond
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 Post subject: Re: How do I know if my knife is sharp?
PostPosted: Fri May 18, 2012 2:00 pm 
I noticed on my Glass Stones that if I skipped 2k, it was much easier to lose a lot of the bite after 8k. If I included 2k, after 8k the edge would always have more bite.

A great man once said (recently) slow and steady wins the race, and that sharpening is a progression. J. Tiberius. ;)

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