3-finger test

[napaknives]

Hi.  I'm a sharpener in California, and I have a question.  I tried the Murray Carter "3-Finger test" on my sharpened blades, and they don't feel very sharp, even though they test from 160 to 230 grams on my PT 50  What's up?

I hope that many sharpeners join this forum, as I am interested in improving my skills.  Thanks!  ............Dan

[grepper]

Welcome to the forum, Dan.

First, I would trust the BESS numbers.  160 – 230 is a nice, very sharp blade.  You can’t get numbers like that unless the blade is sharp.  I suspect that it shaves hair, cuts paper, hangs on your fingernail and passes all the other subjective tests methods without any problem.
 
People have dreamed up all sorts of ways of testing sharpness, and there are advantages/disadvantages with each.  I don't think there is a right or wrong way, but that it's really a matter of personal experience and preference.  You know; whatever works.  

Personally however, and this is just my humble opinion, I think there are some safety concerns with the three finger test.  It may work just fine for experienced folks, but it sure seems a little risky.  I know from (stupid) personal experience that a razor blade has a slightly almost sticky kind of feel, and then shortly after that you start bleeding.  I’ll feel a blade by gently pulling my finger perpendicularly with a slightly down angle across the blade, but never slide my finger along the edge.  Yikes!
 
Additionally, the three finger test would seem to be extremely subjective, and at the very least would require a lot of experience to really understand edge analysis via tactile feedback.  For example, a toothy edge is going to feel very different than a highly polished edge even when both are very sharp because the toothy edge is going to grab at skin and will have more sliding resistance, while a polished edge will ride on the surface of skin until it cuts.  You can easily demonstrate this on tomato, broccoli, onion or grapefruit skin.
 
In answer to your question, “What’s up?”, it’s difficult to say exactly.  Just guessing; it’s probably a combination of things like a toothy/polished edge, edge bevel angle, is there any burr on the edge, how tough are your finger tips, etc.  That kind of thing.
 
For crude, subjective quick tests, I’d stick with the well known methods of cutting paper, hang on a thumbnail or plastic pen, that kind of stuff, but when you really want to know, trust your BESS numbers. 
 
Obviously you are doing a great job of sharpening or you would not be seeing 160 – 230 numbers!

I'm curious, what do you think that it might mean?  Have you had good luck with the three finger test?  If you do the three finger test and conclude that a blade is sharp, can you relate that to a BESS number?

[napaknives]

Welcome to the forum, Dan.

First, I would trust the BESS numbers.  160 – 230 is a nice, very sharp blade.  You can’t get numbers like that unless the blade is sharp.  I suspect that it shaves hair, cuts paper, hangs on your fingernail and passes all the other subjective tests methods without any problem.
 
People have dreamed up all sorts of ways of testing sharpness, and there are advantages/disadvantages with each.  I don't think there is a right or wrong way, but that it's really a matter of personal experience and preference.  You know; whatever works.  

Personally however, and this is just my humble opinion, I think there are some safety concerns with the three finger test.  It may work just fine for experienced folks, but it sure seems a little risky.  I know from (stupid) personal experience that a razor blade has a slightly almost sticky kind of feel, and then shortly after that you start bleeding.  I’ll feel a blade by gently pulling my finger perpendicularly with a slightly down angle across the blade, but never slide my finger along the edge.  Yikes!
 
Additionally, the three finger test would seem to be extremely subjective, and at the very least would require a lot of experience to really understand edge analysis via tactile feedback.  For example, a toothy edge is going to feel very different than a highly polished edge even when both are very sharp because the toothy edge is going to grab at skin and will have more sliding resistance, while a polished edge will ride on the surface of skin until it cuts.  You can easily demonstrate this on tomato, broccoli, onion or grapefruit skin.
 
In answer to your question, “What’s up?”, it’s difficult to say exactly.  Just guessing; it’s probably a combination of things like a toothy/polished edge, edge bevel angle, is there any burr on the edge, how tough are your finger tips, etc.  That kind of thing.
 
For crude, subjective quick tests, I’d stick with the well known methods of cutting paper, hang on a thumbnail or plastic pen, that kind of stuff, but when you really want to know, trust your BESS numbers. 
 
Obviously you are doing a great job of sharpening or you would not be seeing 160 – 230 numbers!

I'm curious, what do you think that it might mean?  Have you had good luck with the three finger test?  If you do the three finger test and conclude that a blade is sharp, can you relate that to a BESS number?

---

Hi, Grepper,  Yes, my knives are sharp.  They cut paper and grip on a fingernail or pen.  I don't shave hair, because I have an aversion to skin bacteria.  Giving or receiving!  My belt progression is:  120 grit Trizact Gator; extra-fine Scotch Brite; and then a linen belt with 4 micron cbn.  Maybe the Scotch Brite belt takes away the toothiness.  I used to use a leather belt, but they stretch too much and took a long time to remove the burr.   

My set-up at the Farmers Market includes 3 Viel sanders, so I don't have to change belts.  I tend to get busy, and changing belts slows me down.  
  
The Scotch-Brite belts sure remove the burr!  Maybe the 3-finger test is bogus, I don't know.  But I can put lots of pressure on the edge with my fingers, slide them along the edge, and not cut myself..  I just want my customers to be happy.     .....................Dan                 Thanks for replying, Grepper.  I really appreciate it.

[grepper]

“Thanks for replying, Grepper.  I really appreciate it.”
 
No problem!  I really enjoy talking about sharpening and hearing what other folks are doing and their results.  The way I see it is that there is always more to learn.  In fact, it seems the more I learn, the less I think I know.
 
I think your post is very interesting.  I’ve always been frustrated by burr removal and keep looking for a better way to do it.  I had actually thought of using surface conditioning belts like Scotch Brite, but had dismissed the idea because I was thinking it was too aggressive and would probably dull the edge.  I can certainly see how they would remove burr.  What I don’t understand about it is how it relates to a grit size, what it does to the edge, does it form its own burr, that kind of thing.  You say it works well, so because of your post I’ll give it a try, do some PT50 testing and get some microscope images that I will post here.  Thanks for the suggestion!
 
Wow.  That’s a very interesting grit progression from two perspectives; one that you basically are sharpening to120 grit, and two, that you are using a Scotch Brite belt for burr removal.  I really wonder if the conditioning abrasive increases or decreases the scratching.  Checking it out under a microscope should prove interesting.
 
The Viel sharpens into the belt rotation direction, right?  Ever have a blade catch on the Scotch Brite belt?
 
I would think that 120 grit is very coarse.  I generally finish to 400 – 600 grit.  That said, sometimes I wonder if 400-600 grit is too fine, especially for general purpose knives for the general public.  A good, toothy edge works very well for kitchen use for example, and does a much better job slicing things like tomatoes, or hard shiny skin like broccoli.  I’m still undecided about finish grit level especially when you don't know exactly how the knife is going to be used.  I’ve been trying coarser grits and liked the result, so I can see why your customers might like what you are providing.
 
I imagine some of the blades you see at a farmer’s market are shockingly dull!  I’ve actually been surprised by knives that people that people have given to me to sharpen that they have actually been using.  I had one guy give me a chef’s knife that was literally as dull as a butter knife.  I couldn’t even imagine using it.  The thing wouldn’t cut; it would just bash and smash and crush its way through food.  So I can understand why 120 grit would work well in a market environment and make quick work out of reforming an edge.
 
I just did the creepy three finger test on a knife that was 215 BESS.  I’ll save that for another post.

I'm sure that you have learned a lot sharpening at a busy market, sharpening lots of knives in various condition.  I'm glad you are posting here, and appreciate you sharing your knowledge and experience.
 
I think there is a lot more we can discuss about this.  You have brought up some interesting methods and ideas that seem to be working well for you, so thanks.  The thing that I don’t like about your post is that now I have  more work and experimenting to do! 

[napaknives]

Hi, Grepper,    I will respond to a couple of your points now, and more this week.  First of all, I sharpen with the belt moving AWAY from the edge.  Going into the edge seems way too dangerous.  And yes, I get some pretty dull blades--1100 grams on the BESS scale.  So, the 120 grit belt gets me down to the apex very fast.  This saves a lot of time.  The Scotch-Brite belt smooths out the grind lines and thins (or removes) the burr. The linen belt insures that the burr is gone.  This process take one to three minutes; occasionally, it takes more time, but rarely five minutes.  I can be very busy, sometimes, and I don't have even four minutes to do a knife.  Customers usually want their knives back, sharp, in a short while.  A speedy sharpening process does me no good if their knives are dull!

It will be interesting to see your results from using a Scotch-Brite belt.  I use an "extra-fine".  Let's continue this conversation. I have time this week, and then I'm leaving on a 2-week camping trip.  Bye for now........Dan

[grepper]

Copy that, Dan. I'll look FW to your return. Hope you have an enjoyable camping trip!

Mark

[Bobbo]

I notice that there is a new version of the three finger 3.0 test available now for experienced users. The new version is 2.5  Sorry, couldn't resist.

[grepper]

Chuckle!  

I am a VERY experienced user.  I applaud your humor, but it's difficult to hear as it's the sound of one hand clapping.

[Mark Reich]

I notice that there is a new version of the three finger 3.0 test available now for experienced users. The new version is 2.5  Sorry, couldn't resist!

That is signature worthy! lol

Oh yeah, and 4u CBN has very little tooth. I'm sure they feel too smooth on the three finger, but that's what makes it a great test once you get familiar.

---

Grepper, the scotch brite belts are not for edge work. I'm pretty sure they'd round the edge.

They are for reconditioning blades to look like new, and I like them for that.

If you put a worn out heavy weight belt under your leather belt, you will never have to give it another thought.

[grepper]

Grepper, the scotch brite belts are not for edge work. I'm pretty sure they'd round the edge.

They are for reconditioning blades to look like new, and I like them for that.

If you put a worn out heavy weight belt under your leather belt, you will never have to give it another thought.

Yeah, that's what I've always used conditioning belts for.  I love the finish they create.  I was puzzled about using them for sharpening.  Maybe I misunderstood what was said about that.

And, put a heavy weight belt under a leather belt, why?  Just to have a stiffer leather belt?  Huh.  Just thinking about it, that's a cool idea.