How to pass Murray Carter 3 finger test of sharpness?

[MichaelMyers]

My knife sharpened off a 600 grit AO stone is at a level where it will shave arm hair and slice newsprint. But sliding my 3 fingers gently on the blade produces nothing. I am in no danger of getting cut. Blade just doesn't bite in. So my question is how to pass the 3 finger test? Refine apex further? Go to higher grit polish? Toothier edge?

 

[bucketstove]

My knife sharpened off a 600 grit AO stone is at a level where it will shave arm hair and slice newsprint. But sliding my 3 fingers gently on the blade produces nothing. I am in no danger of getting cut. Blade just doesn't bite in. So my question is how to pass the 3 finger test? Refine apex further? Go to higher grit polish? Toothier edge?

Hi,
Is your 600 grit stone a water stone?
Is your last sharpening step stropping on "flexible" substrate ( leather+compound , sandpaper, buffing wheel )?

The way I hear it,
you need lots of experience to calibrate your fingertips
you're checking that the edge isn't rounded over,
that it won't slip when slicing cutting fiberous stuff like rope , or folded/rolled paper towel,
or meat , and since you always have your fingers with you, you're the meat

So, the way I figure,
if no strop and no waterstone
and you can shave and slice rope
then you're not pressing hard enough into your fingers
for the test to tell give you a pass/fail

I don't like this test,
I always wear clothes (fibrous material), so I would never have to check slicing sharpness on my skin
but then I finish sharpening on hard/solid stones
so not much chances of rounding over the apex
and no need to check on fabric/towel/rope

 

[MichaelMyers]

Its regular Aluminum Oxide lansky stone not water stone. I don't strop. The few times i tired stropping with either leather or newspaper, the bite gets worse. I just use mild pressure it should cut into my skin but it doesn't. It will cut hemp string. Haven't tried 1" manila rope. Maybe it just wasn't fully apexed?

 

[HeavyHanded]

IMHO it isn't just an end of process test. I often use it as I go to detect the burr forming when (re)setting a bevel.

The dulled, used edge will not feel sticky. In some cases neither will a highly refined edge, depending. If the edge is overly broad it won't feel terribly sticky.

I look for that catchy feel from a stone-deburred edge. Even if I strop or backhone on something and even if taken to 8k I still want it to immediately grab the pads on my fingers with light pressure especially with any sort of lengthwise movement. Even if the edge is cutting the tips of the print whorls it will still feel sticky.

You may be expecting too much from this test, or edge angle may be a bit broad.

Also, you have to use this test often and over time to get a "feel" for it.

 

[bucketstove]

Its regular Aluminum Oxide lansky stone not water stone. I don't strop. The few times i tired stropping with either leather or newspaper, the bite gets worse. I just use mild pressure it should cut into my skin but it doesn't. It will cut hemp string. Haven't tried 1" manila rope. Maybe it just wasn't fully apexed?

Hi
It is apexed,
Being able to shave hair and slice newsprint means you have apexed.

You can try calibrating on a tomato, rounded over edges wont slice/saw tomato with low force

 

[Obsessed with Edges]

Its regular Aluminum Oxide lansky stone not water stone. I don't strop. The few times i tired stropping with either leather or newspaper, the bite gets worse. I just use mild pressure it should cut into my skin but it doesn't. It will cut hemp string. Haven't tried 1" manila rope. Maybe it just wasn't fully apexed?

If your fingertips are anything like mine, you'll likely need something coarser to feel that toothy bite in the three-finger test. I'd bet the 600 Lansky stone isn't coarse enough for that, and might be a lot finer, depending on which generation it is. Older '600' Lansky stones were a relatively fine ceramic (purple/reddish stone in a dark blue holder), and that stone didn't leave much tooth on edges; it produces an edge approaching polished, but somewhat hazy. Not sure about the newer version; but based on your description of it still cutting arm hair and slicing newsprint, but with a lack of bite, it's likely not coarse enough either. Other variables, like glazing or wear of the stone (aluminum oxide stones are prone to it, pretty early in their life), can also make it perform like a much finer stone, with much less bite. In fact, Lansky's older aluminum oxide stones in the XC through Medium grits were very prone to glazing and lost their toothy cutting ability fast, based on the set I used.

To get a feel for that toothy bite, try a well-finished edge coming off a diamond hone; anything from ~ 320 - 1200 can produce the 'bite' that'll be obvious in the three-finger test. I'm sure a lot of other stone types would do it as well (depending on quality), but it's easiest to reproduce on a diamond hone.

As with any 'feel' test done with the fingertips, probably 70% of it also depends on individual skin as well, including factors like if the skin is 'thick' or not, soft or not, wet or not, or whatever. I've never had much luck in feeling that '3-finger-sticky' bite with wet or oily hands, for example, unless the edge is very coarse; with medium-grit or finer edges, I only tend to notice it when my hands are completely dry. And my 55-year-old hands aren't so soft anymore, either.

The same toothiness that's evident in the '3-finger-sticky' test is more easily seen, felt and heard in slicing thin, light paper, like phonebook pages or newsprint and catalog/magazine pages. To me, that's a much easier and more reliable test of the 'bite' in a freshly-sharpened, toothy edge. With a very clean apex, i.e., one with no burrs, rounding or overpolished spots, a more toothy edge makes itself known in how easily it 'grabs' paper on first contact with little or no added pressure, especially if it also can 'grab' the face of the page, and not just the edge; and also in the zipper-like feel and sound it makes as it's slicing through the paper.


David

 

[bgentry]

I've had a lot of trouble with this test also. Several times I've tried to slice into a tomato and found that my edge that didn't feel sticky on my fingers, bit right into the tomato quite easily.

The only time I'm really sure that I have a Murry Carter sticky edge, is when I stop my process at a fairly low grit and carefully deburr. For example, the edge I get from a DMT C (~320 grit) is quite sticky and easy to feel the bite. Anything above that and it starts to feel less sticky, or downright "slick". Yet, even these "slick" edges tend to slice food just fine.

I'm unsure how to use this test properly, even after trying it more than a hundred times. <shrug>

Brian.

 

[Rey HRH]

I don't have calloused hands, fingertips, or palms for that matter. I think I can tell if a knife is sharp with Murray's method but, for me, I slide my finger tip along the edge. Where the edge isn't sharp, my fingertip just slides along safely, where the edge IS sharp, the edge bites into the skin and I become acutely aware of the pressure I'm using as I continue to slide.

ETA: This is also how I feel for burrs (not that I intentionally do this to check for burrs). But for burrs, I can feel the raggediness of the burr cutting into my skin and when I look at the edge with a loupe, I can see the burr along that area.

 

[HeavyHanded]

...ETA: This is also how I feel for burrs (not that I intentionally do this to check for burrs). But for burrs, I can feel the raggediness of the burr cutting into my skin and when I look at the edge with a loupe, I can see the burr along that area.

As I get older and like to strain my eyes less and less I continue to slowly use tactile cues for more and more aspects, to be verified by visual inspection, but this saves me time and keeps the visual inspection downtime to a minimum.

I can tell when I'm getting there using this test - is nearly as reliable as going to an obvious burr. If the edge does begin to feel catchy I know I'm very close or just to it and will start looking. In that regard it is only helpful for setting the edge and subsequent progressions will kick over to feeling for the burr using a lateral rub. I am not to the point where I can tell if the edge has been deburred or not with this test but can see it being possible.

 

[HSC ///]

I'm no expert but let me offer what I know.
I've spent 9 days with Murray in his shop learning and watch him sharpen and he watched me and guided me.
It took me awhile to understand how light the pressure required is on the 6K stone.


Here is a test thread I did for myself - http://www.bladeforums.com/forums/showthread.php/1397540-sharp-knife-correlation-(possibly)

Its regular Aluminum Oxide lansky stone not water stone. I don't strop. The few times i tired stropping with either leather or newspaper, the bite gets worse. I just use mild pressure it should cut into my skin but it doesn't. It will cut hemp string. Haven't tried 1" manila rope. Maybe it just wasn't fully apexed?

Can you push cut paper? That's a little different than slicing.

I'd encourage you to ready Verhoeven's experiments on knife sharpening.
I've found that a loaded leather strop is key to my final edge.
Two light strokes per side.

The way I hear it,
you need lots of experience to calibrate your fingertips
you're checking that the edge isn't rounded over,
that it won't slip when slicing cutting fiberous stuff like rope , or folded/rolled paper towel,
or meat , and since you always have your fingers with you, you're the meat

So, the way I figure,
if no strop and no waterstone
and you can shave and slice rope
then you're not pressing hard enough into your fingers
for the test to tell give you a pass/fail

I don't like this test,
I always wear clothes (fibrous material), so I would never have to check slicing sharpness on my skin
but then I finish sharpening on hard/solid stones
so not much chances of rounding over the apex
and no need to check on fabric/towel/rope

Respectfully disagree here,
I don't think any sort of calibration is necessary, that's the point of the 3 finger test.
It's your natural nerve sensations at work here.

back to the OP question...what kind of steel are you sharpening? I've found it makes a difference
what kind of edge? acute or obtuse?

 

[bucketstove]

Respectfully disagree here,
I don't think any sort of calibration is necessary, that's the point of the 3 finger test.
It's your natural nerve sensations at work here.

Hi,
I am not sure what you disagree with , Is it my brain or my nerves?

 

[Daniel Fairly Knives]

Stropping will go a really long ways in refining an edge. If the blade gets duller when stropping try reducing the angle, with most knives you want to strop with the blade almost flat... matching the bevel works best but try this until you get a feel for stropping "to" the edge.

If you do overstrop (I'm talking angle) which is easy to do just bring up a burr again with your finest belt/stone/rod.

I find that a 6000-14,000 grit edge can easily pass the test so in my opinion the edge does not need to be toothy, just crazy sharp. It should pop hair, hang easily on the side of a bic lighter, cut a loosely draped piece of phonebook paper with no pressure holding the paper.

 

[MichaelMyers]

I'm no expert but let me offer what I know.
I've spent 9 days with Murray in his shop learning and watch him sharpen and he watched me and guided me.
It took me awhile to understand how light the pressure required is on the 6K stone.


Here is a test thread I did for myself - http://www.bladeforums.com/forums/showthread.php/1397540-sharp-knife-correlation-(possibly)



Can you push cut paper? That's a little different than slicing.

I'd encourage you to ready Verhoeven's experiments on knife sharpening.
I've found that a loaded leather strop is key to my final edge.
Two light strokes per side.



Respectfully disagree here,
I don't think any sort of calibration is necessary, that's the point of the 3 finger test.
It's your natural nerve sensations at work here.

back to the OP question...what kind of steel are you sharpening? I've found it makes a difference
what kind of edge? acute or obtuse?

Its O1 tool steel. About 40 deg inclusive. Parts of the edge will push cut magazine paper but not newsprint. It does clean slice newsprint in both directions. Do I need to go further in refinement?

 

[bucketstove]

Its O1 tool steel. About 40 deg inclusive. Parts of the edge will push cut magazine paper but not newsprint. It does clean slice newsprint in both directions. Do I need to go further in refinement?

Hi,

I think you need to do 1-4 light alternating edge leading passes at 40 degrees per side
followed by 1-10 light alternating edge leading passes at 20 degrees per side
all on the same stone you're using now (600 grit)
then check if you can push cut newspaper,

then do 1 pass per side
then check again
and repeat

usually it'll keep getting better at push cutting
until you go too far and it will get worse at push cutting,
at which point just do 1 pass at 40 degrees to deburr
and 5 per side at 20 degrees to back sharpen
and you're done

 

[HSC ///]

great steel to get scary sharp
personally I like 36 or even more acute at 30
I would strop with a loaded leather strop, that's my advice
then decide if you need more refinement.

did you get a burr?, did you draw the blade thru soft wood?
did you hone lightly edge trailing?

 

[cbwx34]

My knife sharpened off a 600 grit AO stone is at a level where it will shave arm hair and slice newsprint. But sliding my 3 fingers gently on the blade produces nothing. I am in no danger of getting cut. Blade just doesn't bite in. So my question is how to pass the 3 finger test? Refine apex further? Go to higher grit polish? Toothier edge?

Might be helpful to learn exactly how you're finishing your edge... what you're describing is typically what occurs with a stropped edge, but since you're not doing that, something else must be occurring. A 600g edge, (even if it's coming out finer) should easily pass.

 

[Wowbagger]

Yah, one of the things that drives me nuts about Mr Carter's videos (though generally I like him; don't get me wrong).
I don't know what all this finger slicing business is in aide of and really have no need of it.
First off many of us who work with our hands, read can sharpen our knives our selves, have abrasive grit embedded in the skin of our finger tips (maybe from handling the slurry covered stones. . . gosh youthink?) and so probably not the best surface to try the fresh edge on and expect it to come away unchanged.

I totally agree with Sal Glesser owner of Spyderco . . . testing the edge on one's finger nail or hard plastic will tell you ALOT more about the condition of the edge; in finer detail, including left and right bias and with zero edge degradation.

No . . . I suppose the finger test is ok for testing the sharpness of gardening tools but there are wilder skies than these when it comes to finely tuned precision edges. I am talking edges on fine woodworking paring chisels and finish planing hand plane blades but it applies nicely to pocket knives . . . heck kitchen knives too for that matter.

Watch Sal show you how it's done in his sharpening vid. I don't use the Sharp Maker, yet, but he knows his stuff.

 

[MichaelMyers]

Thanks to everyone for the suggestions. I figured the problem was a combination of not being fully apexed and a little too much convexity at the apex (from my imperfect angle control) which made it too obtuse. I got out the coarser 280 grit stone and did some heavy grinding with scrubbing motions to remove some of the convexity and produce a more substantial burr. Then remove the burr and did a few passes with 600 grit diamond to lightly refine the bevel. The edge feels nice and crisp now. Sliding along the edge at some point I can feel the edge bite and i can see the very top layer of skin sliced open (no blood). It does push cut magazine pages too. I am happy at this point.

 

[DeadboxHero]

Thanks to everyone for the suggestions. I figured the problem was a combination of not being fully apexed and a little too much convexity at the apex (from my imperfect angle control) which made it too obtuse. I got out the coarser 280 grit stone and did some heavy grinding with scrubbing motions to remove some of the convexity and produce a more substantial burr. Then remove the burr and did a few passes with 600 grit diamond to lightly refine the bevel. The edge feels nice and crisp now. Sliding along the edge at some point I can feel the edge bite and i can see the very top layer of skin sliced open (no blood). It does push cut magazine pages too. I am happy at this point.

yup, thats what the test is for, the most important aspect of sharpening, how clean and pointy is the apex, the test lets you feel how crisp and sticky the apex is

paper tests can be fooled and cut with an over polished edge ( rounded apex)

which means your not getting the knife to its maximum potential for cutting and edge retention

When I hand a knife to an experienced sharpener its the first thing they do

and they can tell me right away if I over polished, left some residual burr, or didn't fully apex

all from a quick simple test mixed with a deeper understanding about sharpening

of course
it doesn't tell you if there are chips in the edge though. (Thats where fingernails, paper and visual comes in)

but its just a quick test

 

[Rey HRH]

I'm no expert but let me offer what I know.
I've spent 9 days with Murray in his shop learning and watch him sharpen and he watched me and guided me.
It took me awhile to understand how light the pressure required is on the 6K stone.

I reread this thread and now I want to ask: how light IS the pressure required on the 6k stone?? Or any other stone for that matter.