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

Thachman said:
I'm really fascinated with this thread. I'm pretty new to sharpening but my whole life I've always used my thumb with a light swipe perpendicular to the blade to judge sharpness. After many many years you can just kinda tell how sharp a knife is by how much it bites into the pad of your thumb. Once it reaches a certain point of sharpness, can you really tell any difference past that point? What is the ultimate objective anyway?

For me, a knife is a tool to actually cut something I need to cut. Not my arm hair, not toilet paper, etc. For example, I field dressed 3 deer this past season with a cheap Gerber knife right out if the package and it cut through that deer flesh like butter. Afterwards, I sharpened it beyond the point it came out of the package because I can tell it has more bite on my thumb now. I'm sure it isn't as sharp as some of you can get your knives but for me it is more than sharp enough.

I have 200 dollar knives that don't feel any sharper. They look a lot nicer but they will all cut about the same.

What's interesting is people into woodworking have a similar bug. They (and I) will take great pride in holding tolerances of 1/64th an inch or less when in reality the wood will naturally expand and contract WAY more than that in the course of the seasons. Those tight tolerances are nice but they don't really matter. I kind of look at sharpening the same way. At some point I just have to say to myself it is sharp enough for what I need it to do.

I'd love to feel one of the blades sharpened by some folks on this forum to see what "sharp" means to other folks. Maybe I'm really missing out in life. I don't mean that sarcastically. I really am wondering if there is a dimension of sharpness that would blow my mind.
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You probably know sharp better than you think. If you know that you have removed the burr and the edge will shave equally then the last basic test IMO should be feel. Regardless of stone or grit finished at the edge should bite when you feel it with a thumb or fingers. If a edge does all these things plus cuts the way you like then I suspect you have some above average sharp knives :)


The side you are likely not in tune with yet is the technical lingo or what some like to call the rabbit hole. The edged tool has been in our hands for 200k years and we still don't know much about the cutting edge. We have devised tests for everything from edge retention to corrosion testing but once knives get in the hands of a individual all bets are off. Each cut, each sharpening, each knife, each test is all individual just like you, it might be close to another that looks just like it but its not going to act the same. Goes with your thinking of tight tolerances or making it better than it needs to be, but.... It never hurts to find out how sharp things can be :D (the realization of your lust for sharpness and a glossy wet polish is usually the time someone welcomes you to the "Dark side")


If you would like a knife professionally sharpened I would be happy to offer you my service for free. Send me a email if you are interested. Welcome to BF BTW.
 
I hear that Murray Carter has signed a sponsorship deal with Johnson & Johnson Bandaids brand...

TedP
 
Try paper wheels; they'll get your sharpening juices spinning... :cool::D

As well, handling the edge on a knife done right on the wheels can be bandaid inducing if the knife isn't handled carefully. :foot: My last experience with a paper wheel edge left me needing a few hours in emerg and stitches. :foot::thumbdn: So, I guess it was sharp enough LOL!
 
3 Finger test for Murray has more to do with polishing a microscopic "toothy" edge.
It slices just as good as a polished edge, but it retains a certain bite to it that you barely feel until you hit your finger's nerves.
What I have discovered using Murrays test is that it is a weird tingling feeling you get when you know you are in danger. It starts in the fingers, ends at the chest and you kind of instinctively stop. That edge however isn't the sharpest I've done. It was able to slice and ALMOST push cut toilet paper, and I want to say it isn't so much a "level" of sharpness but a finishing of the edge.
The problem with this type of edge is that abrasive materials will dull it's initial sharpness rather fast, carboard in particular. The steel I used to test this was 1095, 1080, 5160, s30v (barely), M390, AUS8.
The 3 finger test in my opinion just is too inconsistent to measure sharpness.

A good way to achieve this is to get some M390, sharpen it, hone it, and then strop it. You create a nice polished edge but microscopically it's still "toothy" due to the steel's behavior not so much sharpening or finishing.

For other steels, I found sharpening to around 3000 grit, and going for a 10,000~ grit stropping process tends to produce this type of edge.
 
As I've continued to use this test I find it more and more useful for all manner of edge types. A word of caution, some edges will cut the outermost layers of skin and right across the fingerprint ridges with little or no resistance. If an edge I've been working on doesn't feel very sticky I always take a close look at my finger pads before concluding anything. I agree this test is better at detecting the presence of microteeth but can be used on any edge with care. By combining how it feels with experience of whatever method/finish values I've applied to a given edge, it gives me pretty reliable feedback - short of a cut test anyway. Tells me way more than resting it on my fingernail or rubbing a thumb across the edge.
 
Luis G. said:
3 Finger test for Murray has more to do with polishing a microscopic "toothy" edge.
It slices just as good as a polished edge, but it retains a certain bite to it that you barely feel until you hit your finger's nerves.
What I have discovered using Murrays test is that it is a weird tingling feeling you get when you know you are in danger. It starts in the fingers, ends at the chest and you kind of instinctively stop. That edge however isn't the sharpest I've done. It was able to slice and ALMOST push cut toilet paper, and I want to say it isn't so much a "level" of sharpness but a finishing of the edge.
The problem with this type of edge is that abrasive materials will dull it's initial sharpness rather fast, carboard in particular. [/COLOR
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Thank you, as I've experienced the same results. DM
 
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I've found that as soon as you cut anything with a novelty edge it begins to dull.


True story.
 
while a non novelty edge hones itself while cutting. i'd love to learn how to produce that.
 
I just made a thread recently about how long an edge stays "hair whittling" and have to disagree that it really dulls that fast. Sure it goes under the ability to "whittle hair" rather quickly, but it doesn't dull any faster than an edge that wasn't refined to that level, so you still wind up with a working edge for just as long.

Also, I like the three finger test but find it's too inconsistent with how moist your hands are. For example, after using the waterstone with my skin very wet, no knife will pass the 3 finger test for me. However wait for my skin to dry up and they pass it with flying colors.
 
Czechmate said:
I've found that as soon as you cut anything with a novelty edge it begins to dull.


True story.
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Convex edges can retain their initial sharpness considerably more.

Honestly the best test is how it feels with stones.

Whether it's spyderco sharpmaker, dmt aligner, wicked edge, or edge pro, or benchstones. You can usually tell how sharp you've gotten by the feel of the drag on the stone.
I've never understood this until a year ago. I would talk to a traditional Japanese polisher (American toshi), I asked him how he knew when he was done with sharpening or grits. He would just say: The stones tell you.
 
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pwet said:
while a non novelty edge hones itself while cutting. i'd love to learn how to produce that.
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Do tell.


Luis G. said:
Convex edges can retain their initial sharpness considerably more.

Honestly the best test is how it feels with stones.

Whether it's spyderco sharpmaker, dmt aligner, wicked edge, or edge pro, or benchstones. You can usually tell how sharp you've gotten by the feel of the drag on the stone.
I've never understood this until a year ago. I would talk to a traditional Japanese polisher (American toshi), I asked him how he knew when he was done with sharpening or grits. He would just say: The stones tell you.
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What about when using a power sharpener such as the worksharp, paper wheel setup or belt grinder?...
 
Luis G. said:
Convex edges can retain their initial sharpness considerably more.

Honestly the best test is how it feels with stones.

Whether it's spyderco sharpmaker, dmt aligner, wicked edge, or edge pro, or benchstones. You can usually tell how sharp you've gotten by the feel of the drag on the stone.
I've never understood this until a year ago. I would talk to a traditional Japanese polisher (American toshi), I asked him how he knew when he was done with sharpening or grits. He would just say: The stones tell you.
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I've noticed a distinct change in feel, on the stones, when the bevel grinds down to the apex. The apexed edge seems to 'bite' into the surface when that happens, whereas previously, the bevels seem to 'skate' a little more (by feel, anyway). The change in feel is almost instantaneous, from one 'slick' pass to 'biting' on the next. Once I recognized what that meant, when feeling it, it's a great and not-so-subtle cue to stop honing and check for sharpness. If overlooked, and subsequent following passes are taken slightly off-angle (which erases the crisp apex), it's relatively easy to over-grind the edge. Real easy to overdo this on diamond hones with 'soft' steels like 1095 or simpler stainless.


David
 
the change i can feel quite easily on most stones, is when the burr is formed when i switch side, if it catches just when you touch the stone its there. this with light pressure of course, if you bear on the stone you'll feel nothing. easier when holding the stone in hand too.
 
I think you guys (pwet and OWE) have a very good way of describing those feelings because I think I've felt both distinctively. Especially with what pwet was mentioning and holding the hone in hand.

You guys ever notice the change in feeling as you establish the grind pattern of a new grit more and more, or when you lighten pressure and it seems to almost just polish the one you already have? I'm not so good at describing these things...
 
KennyB said:
I think you guys (pwet and OWE) have a very good way of describing those feelings because I think I've felt both distinctively. Especially with what pwet was mentioning and holding the hone in hand.

You guys ever notice the change in feeling as you establish the grind pattern of a new grit more and more, or when you lighten pressure and it seems to almost just polish the one you already have? I'm not so good at describing these things...
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YES, at least if I'm understanding what you're getting at. When switching grits, the previous 'coarse' finish can feel somewhat sticky on the next hone, UNTIL most of the coarser scratches are diminished (uniformly) across the steel in contact with the hone. When that happens, the honing suddenly feels smoother, and even 'slick' on finer grit finishing stages. In fact, before the next scratch pattern becomes uniform, it's often easy to feel the areas on the bevels that aren't quite 'there' yet, as the feel will alternate between 'sticky' on the coarser areas, and 'slick' on more highly-finished areas. Sort of like walking on pavement with isolated icy patches on it, in the pitch dark. Even without seeing it, you'll know what you're walking on, as you slip on the slick spots, and then the 'brakes' get applied on the dry spots. :)


David
 
KennyB said:
I think you guys (pwet and OWE) have a very good way of describing those feelings because I think I've felt both distinctively. Especially with what pwet was mentioning and holding the hone in hand.

You guys ever notice the change in feeling as you establish the grind pattern of a new grit more and more, or when you lighten pressure and it seems to almost just polish the one you already have? I'm not so good at describing these things...
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Yes, that is a great feeling. :D
 
KennyB said:
I think you guys (pwet and OWE) have a very good way of describing those feelings because I think I've felt both distinctively. Especially with what pwet was mentioning and holding the hone in hand.

You guys ever notice the change in feeling as you establish the grind pattern of a new grit more and more, or when you lighten pressure and it seems to almost just polish the one you already have? I'm not so good at describing these things...
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THATS THE GOAL! you want to get to the point you can "polish" with the stone, it means you have done all with that stone you can.
 
BryFry said:
Hey, am I the only one who when seeing Murery Carter run his three fingers along a razor sharp edge for the first time thought: WTF IS HE DOING?!?

LOL, I realize now that he is a pro, and it must be working for him, it's just that I was taught from childhood that you NEVER run your fingers down the edge of a blade. I've seen many guys do this test on one of my knives only to slice open their fingers (often badly) and then look at ME like WTF?!?

Am I the only one who tests an edge by running a thumb or finger ACROSS the edge, NOT along its length? Works for me, when I rake the pad of my thumb across the edge I can feel how sharp it is by how much I feel it catch on my skin. An extemely sharp edge will feel kind of "sticky" and bight right away without much movement at all. And doing it this way there is pretty much no risk of cuttin ones self no matter how sharp the blade is.

I also find that I can get a pretty good idea of the condition and sharpness of an edge just by looking at it and seeing how the light glints off the very outer most "edge" of the edge. The more glint you see at the very edge the less sharp it is. Ideally you do not want to see any glint that is separate from the rest of the edge, it should appear to be one continuous grind ending in a "zero point" looking edge. I use this method to visually inspect an edge that I am working on, sometimes there will only be one small spot along the edge that needs more work and I can spot it just by looking this way.

...Does that make sense to anyone else, or is it just me? :confused:


.
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makes perfect sense to me
 
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