Murray Carter "Three Finger Sharpness Test"....what?

the bear grylls way of checking sharpness would be to shave your sack... after drinking your piss.
 
Interesting that this thread got revived. I just got a Spyderco Yojimbo 2 yesterday and examined the factory edge. It's got grind marks that look like ~320 grit or maybe slightly lower. It's also got the same tiny microbevel at the very edge that I was previously told is from the buffer used to remove the burr. That polished micro cuts phonebook paper rather cleanly and is overall quite sharp.

Doing the three finger test, if I use more than slightly light pressure, it does stick some, and I can see my finger pads move. But it doesn't really "feel" sticky, more like slick with a tiny bit of drag. This led me to use light pressure and the blade just slid over my finger pads, almost like a very dull knife would. I sawed back and forth 3 or 4 times (lightly) and felt no bite. Testing with my thumb pad, I felt a lot of bite, indicating a quite sharp edge. I was puzzled and felt like I'd learned nothing more about the 3 finger test. Just more data that doesn't make sense.

15 minutes later, after playing with opening and closing the knife a bunch of times, I was looking at one of my fingers that slipped off of the lock and kind of hurt. I happened to see out of the corner of my eye, my 3 finger pads I had been using. Looking closely, the middle one had a clear slice in it. Pulling it open, I could see red, but no blood came out. Then I looked at the other two. *Both* were also sliced, though not as deeply as the middle one. I laughed out loud at my ignorance and discovery.

It's been unseasonably cold here, and was probably 32F when I got home, wearing no gloves. My fingers were no where NEAR numb, but they weren't exactly warm and sensitive either. I don't know if that had anything to do with it or not, but this factory edge, which didn't feel very sticky, absolutely cut my skin. I'm still confused by this test, but I've learned a little something from my experience. :)
Brian.
 
bgentry said:
Interesting that this thread got revived. I just got a Spyderco Yojimbo 2 yesterday and examined the factory edge. It's got grind marks that look like ~320 grit or maybe slightly lower. It's also got the same tiny microbevel at the very edge that I was previously told is from the buffer used to remove the burr. That polished micro cuts phonebook paper rather cleanly and is overall quite sharp.

Doing the three finger test, if I use more than slightly light pressure, it does stick some, and I can see my finger pads move. But it doesn't really "feel" sticky, more like slick with a tiny bit of drag. This led me to use light pressure and the blade just slid over my finger pads, almost like a very dull knife would. I sawed back and forth 3 or 4 times (lightly) and felt no bite. Testing with my thumb pad, I felt a lot of bite, indicating a quite sharp edge. I was puzzled and felt like I'd learned nothing more about the 3 finger test. Just more data that doesn't make sense.

15 minutes later, after playing with opening and closing the knife a bunch of times, I was looking at one of my fingers that slipped off of the lock and kind of hurt. I happened to see out of the corner of my eye, my 3 finger pads I had been using. Looking closely, the middle one had a clear slice in it. Pulling it open, I could see red, but no blood came out. Then I looked at the other two. *Both* were also sliced, though not as deeply as the middle one. I laughed out loud at my ignorance and discovery.

It's been unseasonably cold here, and was probably 32F when I got home, wearing no gloves. My fingers were no where NEAR numb, but they weren't exactly warm and sensitive either. I don't know if that had anything to do with it or not, but this factory edge, which didn't feel very sticky, absolutely cut my skin. I'm still confused by this test, but I've learned a little something from my experience. :)
Brian.
Click to expand...

I think that may be why I (in part) tend to like edges finished a little higher in grit. They can still slice like a demon, but tend not to hurt much (if at all) when they cut me. I got cut by a brand-spanking-new factory edge on a Case folder a few years ago. Their factory edges are quite coarse, but still wickedly sharp when done right. One of the blades snapped shut on the tips of two fingers of my right hand, and neatly clipped 1/4" ovals of skin from the tip of each of my index and middle fingers. Hurt like heck, almost eye-crossing pain; bled like a stuck pig for a few minutes, then continued 'oozing' blood under the Band-Aids for a few hours afterward.

I don't worry too much about getting that 'sticky' feeling from an edge, though I've got nothing against it. It is kind of impressive in a nasty way, but I've never felt the need to get that particular 'feel' in order to know an edge is 'sharp'. Plenty of other ways to verify that... :)

BTW, I'm typing this with a fresh Band-Aid on my left index finger. Was doing some test-cutting in phonebook paper just a half-hour or so ago, and got a little reckless in my slicing. I knew I did it when it happened, but only felt the blade grab my finger a bit; no pain at all, but enough to bleed. I just rolled my eyes... It's sort of weird that I've gotten so used to it; more often than not, it doesn't scare or even startle me, like it used to. :D


David
 
kwazyjuice said:
There's nothing quite like a shorn scrotum... you should try it some time :) ~Dr Evil
Click to expand...


No.. there is nothing like it.. it SUCKS... when it starts to grow back... good GAWD the prickle and itch that ensue! Over a week of walking with cactus... YOU SHOULD NOT TRY IT SOME TIME OR EVER :)

And as far as 3 finger sharp test.. that's a good way to slice your fingers.. I could never do it without pysching myself out.. run my fingers lightly over a sharp edge?
 
I sharpened my knives using a sharpmaker and a strop for about ten years, my only way of judging sharpness was to shave my arms and i was more than happy to see the hairs fly off, thinking it was 'DA ULTIMATE' sharpness test.

I started to get interested in stupidly sharp edges something like 3 years ago and succeeded to get hair whittling edges, still using a sharpmaker and selfmade strops (balsa helped a lot getting consistently good results) but getting more and more towards overpolished edges, failing to realize that my edges lacked slicing agression and almost seemed dull when using the 3 fingers test or my thumbpad.

More recently I steered away from the sharpmaker and started to freehand using ceramics, diamonds, coticules and sic sandpaper used over a narrow and very firm woodbacked leather strop, i also stopped to strop my edges until mirrorlike finish in order to keep slicing agression, i still use a strop with black and white compounds but only to clean off burrs after i am done with sharpening, i still use a strop also to maintain an edge but once i feel the agression going away i put it back on sandpaper to rework a scratch pattern.

2 weeks ago i had to sharpen my Spydie santoku as i had to cut carpaccio so i decided to go mad and drop the edge angle a lot (im guessing 20-25 degrees) it was previously at 30 or slightly higher, once i was done with the grunt work using 220 & 600 grit i stropped with black compound for 2 minutes until no burrs then i realized the edge was whittling hair, gliding through phonebook paper, cut off finger pads ridiculously easy, treetop my rather fine arms hair 2 cm above skin and the three fingers test was scary to do. The edge was nowhere near shiny, it had a matte looking with a well defined scratch pattern, basically my santoku was about as sharp as my straight razor which is obviously highly polished and refined using a bare strop...

I guess i leveled up and it felt good to feel such raw increment in both sharpness and time to get there.
 
From my experience, the three finger test just has to be done, a lot, and in conjunction with any other impromptu tests you might care to use for it to be helpful. After some time much of the other "cross referencing" can be dropped away. I use feel and close visual inspection of the edge/skin contact point at the same time. I have yet to cut myself doing this test, but have sliced fine cuts across the tops of my fingerprint whorls - those edges still felt "sticky", but on a different scale than the less refined edge. If an edge has zero stickiness to it and isn't cutting my fingerprints with no shifting of the skin, no matter how well it might perform on other tasks, I can be reasonably sure the edge won't last - to me is a good indication its either overpolished or microconvexed or both.

Pretty sure I heard MC on a video say "if an edge is three finger sticky and shaves arm hair it is considered sharp". Not as sharp as it can get, to be sure, but for a practical reference point is pretty good. Have to keep in mind how many cutting tools pass through his hands on a given day.
 
Yeah, I'm not doing that. I know my knives will bite right into/through my finger. I use paper wheels, and I can push the edge straight through callouses on my thumb, just like cutting off a piece of soft cheese.
 
I'm surprised that after years of discussion no one has posted the video of Murray demonstrating this:

[video=youtube;2k1o70tMHYM]http://www.youtube.com/watch?v=2k1o70tMHYM[/video]
 
The 3 finger test of edge sharpness if done correctly, is safe, I've done it with shave ready straight razors, NIB Spydercos, and my edges that will can push cut toilet paper, and I have only been cut one time and it was yesterday as a matter of fact, I'm loving the "polished toothy edge" which is me sharpening on a coarse diamond, and getting it as clean and burrless from the coarse hone, then just stropping the edge to get a little polish and total burr removal , and I cut myself yesterday, and I was actually pretty happy about it lol. After using the 3 finger test for months and months , I finally got a knife so sharp it cut me before I realized it, The polished toothy edge is a mean mean edge. It can whittle hair, cut newspaper cleanly, etc. It cut's like a mirror polished edge, but it has bite like a coarse edge love it!
 
bgentry said:
Interesting that this thread got revived. I just got a Spyderco Yojimbo 2 yesterday and examined the factory edge. It's got grind marks that look like ~320 grit or maybe slightly lower. It's also got the same tiny microbevel at the very edge that I was previously told is from the buffer used to remove the burr. That polished micro cuts phonebook paper rather cleanly and is overall quite sharp.

Doing the three finger test, if I use more than slightly light pressure, it does stick some, and I can see my finger pads move. But it doesn't really "feel" sticky, more like slick with a tiny bit of drag. This led me to use light pressure and the blade just slid over my finger pads, almost like a very dull knife would. I sawed back and forth 3 or 4 times (lightly) and felt no bite. Testing with my thumb pad, I felt a lot of bite, indicating a quite sharp edge. I was puzzled and felt like I'd learned nothing more about the 3 finger test. Just more data that doesn't make sense.

15 minutes later, after playing with opening and closing the knife a bunch of times, I was looking at one of my fingers that slipped off of the lock and kind of hurt. I happened to see out of the corner of my eye, my 3 finger pads I had been using. Looking closely, the middle one had a clear slice in it. Pulling it open, I could see red, but no blood came out. Then I looked at the other two. *Both* were also sliced, though not as deeply as the middle one. I laughed out loud at my ignorance and discovery.

It's been unseasonably cold here, and was probably 32F when I got home, wearing no gloves. My fingers were no where NEAR numb, but they weren't exactly warm and sensitive either. I don't know if that had anything to do with it or not, but this factory edge, which didn't feel very sticky, absolutely cut my skin. I'm still confused by this test, but I've learned a little something from my experience. :)
Brian.
Click to expand...

this is exactly what happens to me every time i try this test. i decided it was not good for me a while ago but i try again every once in a while with this kind of result.
 
Mr. Carter probably had to make that video 10 times before he would stop laughing at that part, "your brain will tell you when to stop".. He's like, LOL SUCKAS GET CUT :D

I don't have the sensitivity in my fingers to do that properly and/or I can't get THAT sharp to "feel" real sharp.. but I'm not whittling hairs everyday either... :)
 
For me, personally, it depends on the application. For my swords, machetes, axes (big slabs of steel you swing to cut with) I prefer a completely polished edge, with little to no "bite". I use my belt sander with 0.5 micron stropping compound, and that leaves the edges mirror polished and ever-so-slightly "sticky". I find this cuts through targets well, lessening actual drag and making the edge last longer. I also grind the edge as thin as I can (about 20-30 inclusive) to enhance performance.

For utility, tactical, survival knives, I like the same edge as above, just with the inclusion of a micro bevel (about 45 inclusive).

Hunting knives change the game completely. I move away from the belt sander and use a Lansky system to set a 40 inclusive edge, polished up to a 2k grit "super sapphire" stone. This edge is as strong as I need it to be for splitting deer sternum, and is very, very sticky. I mean, you lay your fingers on it and move them LITERALLY a fraction of a millimeter and the "OH CRAP STOP!" Feeling kicks in. If you move them a centimeter, they will bleed. Slices deer hide like a blowtorch through snow.

My large slabs of steel and the like often fail, the three finger test. Yet they will glide through wood, cardboard, etc. and ALMOST tree top hair, yet shave extremely easily (it should be noted I have fine hair). While the hunting knives will tree-top, push cut newsprint like air, AND pass the three finger test with flying colors.

It confuses me so much, but it works well enough.
 
I know this has been a long post. but ive been sharpening frehand for years I believe if u really want a supierior edge u gotta get away from those sharpening cheaters! once u get some good practice in u will feel the angle. wich the edge is different for each knife u sharpen rember they are tools! a pocket knife for a farmer needs a completely different edge than a fillet knife for a fisherman. remember u must suit the edge to the need or your not doin u r job. if you really want a good look a your edge to learn what kind u really got get a low power kids microscope u can see exactly where you need to work. and get rid of the sharpeners after practice you will out preform them yourself
 
I thought Mr. Carter was talking about, after you made a jelly sandwich..... :D
 
I agree with several people in this thread, I think the MC method is stupid. I run my finger or thumb against the blade, not with it. That's a good way to open yourself up, and I would laugh if I ever saw someone do it infront of me. It's like walking around with your finger on the trigger of a firearm, then being surprised when you shoot yourself.
 
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