Testing Sharpness with the Thumb

[Cliff Stamp]

I figure that anyone who would make such a statement knows not what they speak of.

Yes, like Leonard Lee, that is where the 1000 grit came from, in regards to a japanese utility knife specifically, he generally recommends 800-1200 for slicing knives. The grits will vary of course depending on the knife and the steel. It changes as well from one person to the next as you would expect as they cut different things and as noted the knives influence the result. KenCrest goes up up 1500, which is still fairly coarse. Ref :

http://www.japanesechefsknife.com/HowToSharpen.html

The influence of grit on sharpness and edge retention has been studied extensively both by by Furi and CATRA as well as extensive data by Mike and Joe, which can be read on rec.knives and early posts on Knifeforums/Bladeforums. I have noted the difference in detail in many reviews, usually on cardboard/hemp but on occasion have ran trials on food as well.

Another way to get a good slicing edge that feels sharp to the touch is to leave a rougher finish on the edge.

This is most likely what it causing the problem, most people are just familiar with the saw edge left by a coarse butchers steel and just judge a knife as blunt if it doesn't feel like that. I'd just do a demo on site. If the knives are sharp they will cut well and you also get to show off your mad cutting skills.

-Cliff

 

[Dog of War]

I've been testing edges with my fingers for close to 50 years. I wouldn't dream of using my thumb (that is the sign of an amateur). I use the pads of the index, middle and ring finger of my left hand. By spreading the fingers a bit I can do a pretty good survey of a three inch strip of edge at a time. What I am feeling is the drag on my finger prints. It gives me a pretty good read on how well an edge will do at slicing (draw cutting), particularly how it will do on things like leather or flesh.

Are you saying that using the thumb is the sign of an amateur because they - excuse me, we - aren't taking advantage of the extra surface area three fingers offers? Or are you saying that fingers are more sensitive than thumbs? Or is a totally different technique you haven't fully described? Whatever the case, it's still a crude test compared to other options.

 

[Cliff Stamp]

Whatever the case, it's still a crude test compared to other options.

I asked R. J. Martin this question some years back when I was figuring out a way to quantify sharpness. Since he was known for really consistent and high sharpness it seemed obvious to me he should have a way of measuring it. He felt it, as Jeff described. I like to cut fine paper on a slice/push, but that is just the empiricest in me. The problem with the feel method is that is is subjective and without the experience to know what a sharp edge feels like it is next to useless. However all you have to do is get a pack of Olfa blades or similar for a benchmark. Few people sharpen blades beyond that level anyway.

-Cliff

 

[D_R_Sharpening]

I have only one more constructive thing to add to this discussion...This is for the original poster (thread starter) "Noesis"...

If you want to talk to other professional sharpeners please see The Sharpnet Group on Yahoo. These folks, while sometimes brash at times, are very experienced and knowledgable in the subject of professional knife sharpening and will gladly aid you with any help you may need.

This isn't to say that there isn't very knowledgable members here at BF, because there is, but there's also a few "wannabe" knowledgable members as well that if left uncheked will be certain to steer you in the wrong direction. Readers beware!!


--Dave--

 

[huugh]

I wonder what do you identify as grit? FEPA, CAMI, USA or UK standards? Or something else?

 

[Noesis]

I like to cut fine paper on a slice/push, but that is just the empiricest in me.

I agree with this completely. The results of the thumb-testing method, while they may be accurate, cannot be verified by those unskilled at this technique; and of course objective verification is just as important as accuracy when dealing with non-enthusiasts. Paper slicing provides a visually impressive way to let the cooks and chefs know that you are not ripping them off. You can be absolutely correct in telling them that a knife is sharp, but they will raise an eyebrow the moment you tell them that they simply don't have the experience to notice.

 

[Cliff Stamp]

I agree with this completely. The results of the thumb-testing method, while they may be accurate, cannot be verified by those unskilled at this technique;

When I became semi-serious about sharpening/edge retention I went through many way of measuring it. I have settled on two primary ones now, both are objective measurements by cutting thread and light cord. You can use paper as well, specific to the type and how it is cut. Alvin Johnston folds a lip of a sheet so it forms a "L" shape turned sideways and cuts the part that sticks up just under its own internal tension. The higher the fold which can be cut the sharper the edge. I just pinch it and see how far it can be cut from the point it is held. Very sharp edges can push cut newsprint at 1-2" from the fingers.

Paper slicing provides a visually impressive way to let the cooks and chefs know that you are not ripping them off.

If you really want to impress them, sharpen it so that it doesn't require any draw at all. Most people have never seen a blade that sharp aside from disposable knives and for some reason they don't expect it from other knives. There is no reason than any knife (not defective steel) can't get that sharp. You can do it with coarse edges as well as polished. Most of the misinformation about coarse edges comes from people leaving them unfinished. They can shave readily if properly honed, even with really coarse grits like x-coarse DMT, 90 grit silicon carbide, 80 grit aluminum oxide, etc. .

-Clif

 

[Jeff Clark]

DoW, Re: "Are you saying that using the thumb is the sign of an amateur because they - excuse me, we - aren't taking advantage of the extra surface area three fingers offers? Or are you saying that fingers are more sensitive than thumbs? Or is a totally different technique you haven't fully described? Whatever the case, it's still a crude test compared to other options."

Mostly I'm joking. I think the main reason I don't use my thumb is because of the big scar that I have on the pad. Even back in junior high I was known as "the knife guy" so naturally I slashed the hell out of my thumb on a tin can lid. After 43 years it is still the biggest scar on my hands.

My real point is that feeling edges with your fingers or thumb is not as crude as you would think. For the most part it can tell you if a knife is sharp enough to get the job done. I often sharpen 50 knives in an evening. When I am doing that I don't stop and give them paper tests. I may try to shave with a few of them, but mostly I will just feel when they are good enough to stop. Back 35 years ago when I sharpened knives professionally I would bring along newspaper. I would demonstrate that the knife was sharp using the newspaper. It did not necessarily indicate that the knife was optimally sharpened for the customer's purpose, it was just something to impress the rubes. Back then I would sometimes get knives that could not take a razor edge. They were cheap chrome plated mystery alloy. I could not lower the honing angle without marring the chrome finish. So I would hone the existing edge as fine as it would go then roughen it using a medium-coarse aluminum oxide hone. Just by feel I could tell which edge would slice the paper. I usually got my highest praise from people who owned those chrome-plated knives. They had never cut so well. Years after I got out of the business those guys (well women actually) would call me up.

When I am seriously sharpening my own knives I test them by shaving with them and by cutting toilet paper. One of the tests I like to run occasionally is to shave my face with a freshly sharpened edge.

 

[Dog of War]

Mostly I'm joking. I think the main reason I don't use my thumb is because of the big scar that I have on the pad. Even back in junior high I was known as "the knife guy" so naturally I slashed the hell out of my thumb on a tin can lid. After 43 years it is still the biggest scar on my hands.

My real point is that feeling edges with your fingers or thumb is not as crude as you would think. For the most part it can tell you if a knife is sharp enough to get the job done.....

Just jerking your chain a little bit there too Jeff (though for all I know maybe somebody out there offers a Masters program in Knife Sharpening??? And lord knows I've stepped into it enough times on the web when I thought somebody was kidding.....)

Being a guitar player I'm also kind of limited on knife testing digits because of calluses, so left thumb and pinky fingers are my only choices. I'm interested that you consider it a better test than I tend to believe, even using it quite a bit myself ..... like Cliff I I guess I prefer paper or some other test medium. After a few minutes of holding a hone hand sharpening or even holding Sharpmaker in place my thumb does kind of lose its sensitivity.

 

[Noesis]

Just jerking your chain a little bit there too Jeff (though for all I know maybe somebody out there offers a Masters program in Knife Sharpening???.

That's funny you say that, Dog of War, because I was about to make a similar comment. If there is one, I want Cliff Stamp and Joe Talmadge as my thesis advisors.

 

[Jeff Clark]

Personally I don't think that spending time to get that Master's of Sharpening degree makes economic sense. It wouldn't pay much and there's plenty of chance for serious cuts. I think that DoW's musical vocation is much more promising:

""Now look at them yo-yo's that's the way you do it
You play the guitar on the MTV
That ain't workin' that's the way you do it
Money for nothin' and chicks for free
Now that ain't workin' that's the way you do it
Lemme tell ya them guys ain't dumb
Maybe get a blister on your little finger
Maybe get a blister on your thumb"

Those finger calluses are a small price to pay and the blisters are a small risk.

 

[matt321]

Forty years ago I opened my dad's pocket knife when he wasn't around and tested the edge by very lightly running my thumb pad along the edge. Before there was any sensation of pressure or drag I felt the sting of a deep cut accomponied by blood loss. I've never really been able to produce an edge like that. (I'm not a professional by a long shot.) I think it's because I'm not comfortable profiling my good knives down to very thin bevels.

.

 

[Jeff Clark]

Matt, It was probably much easier to sharpen your dad's knife. First it was thinner and second it was probably made from non-stainless carbon steel. It was all so much easier then. Get yourself something like a Case "Chrome Vanadium" (CV) trapper model. This non stainless carbon steel with added vanadium will let you compete with your old dad.
http://www.knifecenter.com/kc_new/store_detail.html?s=CA079

PS: When you're done honing be sure to strop it lightly on an old leather belt.

 

[Dog of War]

I think that DoW's musical vocation is much more promising:

""Now look at them yo-yo's that's the way you do it
You play the guitar on the MTV
That ain't workin' that's the way you do it
Money for nothin' and chicks for free

Only in my dreams! I would have never thought it possible you could have chicks for free ..... don't they always carry a stiff price?

Maybe what I enjoy so much about knives and guitar is that I've never made any money from either one, just do it for the personal satisfaction. Not that I don't like my job, but it's still 'work' you know.

 

[Greg]

Hmmmm,
interesting comments on polished or intentionally roughed edges. After using rough edges for years (I cook every day and have also taught cooking and knife skills for the kitchen) I now polish knife edges until bright and shiny, and find that they cut better and last longer than lower grit finishes.
I never use a steel and only cut on wooden boards. I notice Furi recommends and sells plastic boards. Research has shown that not only are wooden cutting boards easier on edges, they are also more sanitary.
I almost never cut rope or carboard.
Greg

 

[Cliff Stamp]

... find that they cut better and last longer than lower grit finishes.

I run very high polishes on some kitchen knives like paring knives and more coarse finishes on others, it depends on how the knife cuts through the food. It doesn't matter the media specifically, slicing rhubarb is the same as slicing hemp rope, and requires a different finish to be optimal than dicing the same material. Some steels inhernetly make a difference at low angles, but they are generally not common for kitchen cutlery like D2.

Almost everyone locally uses a rough butchers steel as it is traditional so when I sharpen knives for them I tend to leave them coarse no matter what the knives as since as soon as they hit the steel they are turned into a coarse saw anyway. Generally as the knife edges are reduced in thickness and angle there is also less of a need for lower grits to enable high cutting ability and it is mainly an issue of edge retention for slicing.

This is why for example guys like Alvin Johnston with knives that have edges at 3-6 degrees per side with edges barely 0.005" thick have no need for coarse edges and run very high polishes on everything. When your edge is that thin and acute you don't need a coarse edge as you don't need to actually draw the blade through anything, it just pushes right through.

-Cliff

 

[thombrogan]

This is why for example guys like Alvin Johnston with knives that have edges at 3-6 degrees per side with edges barely 0.005" thick have no need for coarse edges and run very high polishes on everything. When your edge is that thin and acute you don't need a coarse edge as you don't need to actually draw the blade through anything, it just pushes right through.

And that is how it should be!

 

[Cliff Stamp]

Yup, I took my small Sebenza, Delica, and Ratweiler to some rhubarb, 600 DMT finish on all knives, all shaved and could push cut newsprint. The small Sebenza with the modifed flat to the blade edge at 5 degrees could just pop off the rhubarb with light chops even with the coarse edge. I had it pushed over the side of a cutting block as I was attempting an edge holding comparison between S30V and ZDP-189.

No success with the edge holding comparison as none of the blades were effected, but the massive difference in cutting ability was fairly obvious. The Sebenza would have been better still if it was highly polished. However the Delica needed the 600 DMT finish as it needed a draw because the profile is to thick to push cut it well. The Ratweiler was just to check the resistance to corrosion and success it finally formed a patina and then the next day it was all burnished off again.

On some of the softer steels even when the edge is thin, coarse edges can still be of benefit for reasons of edge retention. On the grave and horrible day when you have to use AUS-4A instead of M2 this might come in handy. When someone gives you a set of knives to sharpen and you see that benchmark of quality "420 stainless" and you somehow have to turn that into a decent knife.

I have also started running multi-bevels on most of the chef's knives I sharpen now similar to how Joe modified the axis. Nedo Carver runs three distinct finishes from choil to tip which I have adapted to many knives and my Ratweiler now has a x-coarse, ultra-fine, medium finish, from choil to tip. The first inch handles all tough slicing, the part in the middle does chopping and carving and the tip does the brush cutting and slicing of loaves and fishes.

-Cliff

 

[psycho78]

I've used my thumb (running along the edge) to test sharpness since I started sharpening knives about 15 years ago (I'm 25).

I've found that if you are sharpening alot of knives and using this method, that the tip of your thumb will soon look like the face of a file, with many small grooves. If the edge gets into one of those grooves it may decieve you on how sharp the knife is because it feels like youre about to get cut. Thats the only limitation I think I have with this method.

I like the thumb method because I can clearly feel flat or dull spots on the edge and correct them. I can see/feel these things if I'm cutting something, but the thumb is a bit more personal method, and tells me more about my edges.

Personally, i dont wanna cut a bunch of random stuff to test my freshly sharpened blade, and dull it. Kinda defeats the purpose to me. I know what a sharp blade feels like, and I am confident enough in my skill to know that I can feel the thing with my thumb, know it's scary sharp, put it away in my pocket, and know it will cut most anything I put in front of it.

 

[Cliff Stamp]

Personally, i dont wanna cut a bunch of random stuff to test my freshly sharpened blade, and dull it. Kinda defeats the purpose to me.

If cutting a small amount of material dulls the knife significantly then it raises an obvious problem. A few cuts into newsprint, thread or light cotton doesn't even have an effect on the lowest grade of cutlery steel. However if feel works for you then that is all that is needed.

-Cliff