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3 finger test - I still don't get it
#1
I tried the creepy 3 finger test (3-ft) years ago and then developed my own different way that works well for me.  Nonetheless since this came up again here I decided I’d try again.
 
I just sharpened a blade with a A45 Trazact CP 337DC belt ~280 grit, then deburred on the rough side of a plain leather belt and a few swipes on my jeans.    The blade got nice and sharp, 185g on the PT50.  About right for a kitchen knife.  It shaves hair and all of the other obligatory tests without problem.
 
I can put 3 fingers on that blade and press WAY harder that makes any sense to a normal man, and my fingers just slide.  I’m sure I could press much harder, but common sense prevents me.   Pressing very lightly is just not that informative.  I just feels smooth.
 
Now, I know what it feels like to do that on a razor blade.  It has a certain resistance.  For super sharp blades I think I understand the 3-ft.  But that “resistance” is just creepy.  It has that "special" feeling because when your fingers are just bordering on epidermal leakage.  To me the 3ft makes no sense with super sharp blades anyway.
 
It seems to me that a blade has to be sharpened to about 100g or less before the 3-ft test really works well.  100g is sharper than I need or want the blade for edge retention purposes.
 
Here's an image of the blade at ~200X.  It takes a little practice to understand some of the microscope images.  Shadows can make grooves look like a super jagged saw edge.  It can be difficult to get the light just right.  Just zoom in on the image and you can see what looks like a very rough edge is just shadows.  Nonetheless, 280 grit does leave a nice "toothy" edge.  I only post the image to show that this is not a highly polished edge.

So, even though I gave it another honest try, I still don’t get it.  Anyway, it was an interesting experiment to try again, even though the whole idea of the 3-ft is against my better judgment. Rolleyes


Attached Files Thumbnail(s)
   
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#2
Sir

You have a "brain" - why would any thinking being use a subjective test (3-ft) when there are EdgeOnUp testing devices?

In lieu of EOUp, use telephone pages, wet tissue, damp tissue, single ply or double ply.

Before EOUp came into my life, the standard for me was adding machine paper rolls (they are woven without bias) the feel and the sound {if any} were my numbers.

Thank you Mike Brubacker!!

Rupert
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#3
Sir

What were you using to create the image (attached files) in your post #1 (3 finger test - I still don't get it)?

Again thanks

Rupert
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#4
It might help to know you are far from alone. The 3f test doesn't work for everyone. In fact, it hardly works for anyone. 99% of knife users have never heard of it, and if you try to explain it you will be met with looks of horror. 

I know you write well and true and honorably, grepper, so by default I'd say you're probably overthinking it. 

I don't think an A45 is capable of passing this test, so it's a great learning aid. You called it right when you said the edge feels smooth. That's the entire difference between tooth and no tooth. It either has tooth or it's smooth. 

Try a new, inexpensive, A/O belt of approximately 280 grit. It can even be much finer to simplify finding a belt. I would say up to about 400 grit. I almost guarantee you will not describe the edge as "smooth", and that's the difference.

Another great comparison would be CBN vs poly diamond compounds. Ask anyone with a straight razor and compounds. 1 micron CBN is a pretty comfortable shave. Even finer 0.5 micron poly is so opposite it's almost frightening to shave with. You have to be much more careful, because the edge is so much more aggressive. Even an im perceptual amount of slicing motion, and you're cut. After shave will also feel much different!

If it were called, "the slickness test", maybe everyone would understand exactly what it means. There is a very clear difference between slick (or "smooth") and anything else, so simply de-mystify it and call it what it is. Just replace 3f with "slickness".
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#5
Well, huh.  That makes much more sense. 

If you are feeling for a sharp, toothy, aggressive edge, the 3f works for me too.  That's feeling for a very particular type of edge, not just a sharp edge.  I've learned to feel that pulling a finger or thumb across the blade.  I can tell instantly super sharp, from very sharp, etc.

With a more polished edge, for the 3f to work for me, the blade has to approach razor blade sharpness.  But then as you say, you must be much more careful.  Anything less than razor sharp and my fingers just slide.  I know what would happen if I continued to press harder... All of a sudden the blade would cut and I'd suffer epidermal leakage.

If I now understand correctly, the 3f test is feeling for a very particular type of edge, not just sharpness.  I guess that's sort of interesting.  Rather vertical market though.  Not really general public stuff.  I mean, yeah, I suppose I get it.

Maybe somehow I missed just how specific a test this is.  I really didn't pick that up from that dude in his bathrobe on YouTube.  Maybe I need to watch the video again.

Mark, do I understand what's going on now?

Oh, and Rupert, I have a PT50B.  Couldn't do without it! And why then would I care about the 3f test? For two reasons; one because it's there.  Kind of like why climb the why climb the mountain thing, and two, because Mark said it works for him and I have a lot of respect for his experience.  I had written it off as rather kooky, but when Mark brought it again as actually credible,  it peaked my curiosity and I wanted to try and finally understand.

If I am correct, then the 3f test is widely misunderstood, and should only be for very experienced sharpeners.  Even then, and this is just IMHO, there are just as effective and less creepy and possibly bloody methods.  I think I may get it though.  Once you know what you are looking for, it's another real quick test for very experienced fingers.  But...
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#6
2xthumbsup  I'm so glad that made sense. Really, that means a lot to me. Let's run that up the flagpole and see what happens. That's how it applies to me. 

I'm humbled to be in this nucleus of guys who've spent their lives feeling edges. Everyone probably does about the same thing. When I first sharpened alongside Murray, I felt like everything was perfectly normal. We acknowledged each other's edges were sharp. It was not at all unlike the time I spent sharpening with Mr. Rupert. or a handful of other sharp guys. We are all much more alike than unique.

Grepper, I like how you put this; Quote " If I now understand correctly, the 3f test is feeling for a very particular type of edge, not just sharpness.  I guess that's sort of interesting.  Rather vertical market though.  Not really general public stuff.  I mean, yeah, I suppose I get it.

You have to admit, we are not general public. The sort of edges we produce take a Much more deliberate examination than the gp. We all shave a couple of arm hairs at a time. How any of us feel for "smoothness" has got to be fairly similar IMO. 

Yeah, we can polish an edge and split hair, but pushcutting sharpness is of use to me on my razor. That's it. Everything else needs tooth IMO, so it's always the second thing I feel for. Yes, right after I brush the edge with my thumb like everyone else.   

Grepper, I think you sum it up well with this, Quote;
"If I am correct, then the 3f test is widely misunderstood, and should only be for very experienced sharpeners.  Once you know what you are looking for, it's another real quick test for very experienced fingers.  But... " 

When I consider where the 3f came from, I know it is intended as the Most advanced move, but as you say, Grepper, "another real quick test", as it literally takes two seconds. 

I bleed every day anyway.   Big Grin
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