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Test for sharpness

tkluck's picture

Test for sharpness (post #104369)

For years, literally, I've tried to find a reliable test for sharpness. One that doesn't have any undesirable side affects.
The 'catch on your thumbnail' test doesn't tell you much. Neither does slicing a piece of paper, or a rag, or a rope, or raw meat (don't ask).
Someone mentioned sliding your finger gently along the edge. Peroxide seems to work well for removing blood stains from pine, by the way.
I'm back to shaving hair off of my arm. A prominent knife maker said in his book that is the best test, but a that professional knife maker would soon be shaved bald. He takes a swipe at a hanging hemp rope. Fine for bowie knives, but hard to do with a plane iron
Others recommend shaving the end grain of a piece of scrap. Doesn't seem as easy to judge, at least for me. Perhaps after all these years I'm a better judge of dry shaving arm hair than of carving.
Any Ideas? (Besides switching to leg hair where it doesn't show...)

Sardog's picture

(post #104369, reply #1 of 16)

TK


I was taught that running your thumbnail along the edge shouldn't feel any roughness.


And cutting the endgrain on a hardwood shoulld be easy and leave a shiny surface.


Hope that helps.


Jeff

hammer1's picture

(post #104369, reply #2 of 16)

Have a light source behind and over your head, then look directly on the edge. If it's not sharp, you will see a reflection of the light.

Beat it to fit / Paint it to match

Beat it to fit / Paint it to match

flairwoodworks's picture

(post #104369, reply #3 of 16)

There are a few methods I use, including the ones you use.  My favourite test is to take the blade (not installed) and use it to pare a shaving off the end grain of a soft wood.  You're looking for a glass-smooth cut with minimal pressure required (a little more reqiured for a 50-degree blade).  Soft woods are a better test than hardwood as they tend to crush if cut with a dull blade.


Looking at the edge under magnification works as well.


When I'm done sharpening, I know that my blade is sharp.  I don't bother testing it.


Chris @ www.flairwoodwork.spaces.live.com


 - Success is not the key to happines.  Happiness is the key to success.  If you love what you are doing, you will be successful. - Albert Schweitzer

Chris @ www.flairwoodworks.com
and http://flairwoodworks.wordpress.com

 - Success is not the key to happiness.  Happiness is the key to success.  If you love what you are doing, you will be successful. - Albert Schweitzer

derekcohen's picture

(post #104369, reply #4 of 16)

All the tests you mention are reasonable - in the appropriate context. There are some I might do, usually out of curiosity, but mostly I do not bother to do any (as Chris notes).


Consider that "sharpness" means different things to different people. Consider that bevel angle affects the ease of "penetration", which for me is a better term for sharpness. In this contect, the more acute a bevel angle, the greater potential for penetration. A "sharp" 50 degree bevel will not feel as "sharp" as a 25 degree bevel, and likely will not cut either arm hair or end grain as easily/well/smoothly as a lower bevel angle.


Does this mean that one must fall back on "knowing" when the bevel is honed to the max? This implies a sort-of mystical ability that comes to inhabit the experienced user. :)


No, it is not magic - it is simply belief that you have done it right and therefore it will be right. The knowledge is that the primary bevel was good, and that successive grits built on this. The check is a visual inspection for lght at the edge, be it from a poor primary bevel or remaining wire.


My final test is to rest the blade on my arm and watch the hair backing away in horror. No actual test cuts needed. :)


Regards from Perth


Derek

Lataxe's picture

(post #104369, reply #6 of 16)

Derek,


I confess to having applied the arm hair shaving test in the past; but I discovered two things:


1) As you mention, a high angle bevel will not shave hair as easily as a low angle bevel will, even when the blade is as sharp as one can get it (and certainly sharp enough for the tool-use).


2) A rough edge will shave hair too, especially if the roughness is such that it catches the hairs and holds them in place for the cut.  Then one also feels a tug of course.


So, I now do as you and Chris suggest - a proven technique to sharpen followed by usage.  If it's sharp, it cuts well.  I don't even bother with the softwood end grain cutting test these days.  The only issue is, do the required type of shavings or other slices of wood leave the workpiece in a smooth, contolled fashion without undue effort.  If so, the blade is sharp.


The feel of the actual cutting then also becomes the test of when honing or resharpening is needed.  First the required effort to make it cut gradually increases.  Then there is more difficulty in getting a very thin or fine shaving to start (even if once started it still cuts well).  Eventually the shavings become less shiny-surfaced and (in extremis) mild tear out or knap begins to occur on more difficult woods.


No doubt that Philip's finger-feeling method works - but not for me as I am an oaf and there would be blood all the time.  (I am feeling faint even now).


Lataxe

KiddervilleAcres's picture

(post #104369, reply #8 of 16)

Lataxe, Derek & philip, ALL,


Here's two that I use:


Inflate up a balloon and smear it with shaving cream; or if your sweet tooth is acting up you may want to substitute whipped cream!  Then shave the cream off the balloon - this will also test your nerves for fear of puncturing it............  In this regard I might recommend whipped cream should you have your mouth agape whilst shaving the balloon.  Sort of akin to firing a gun - you're not supposed to know when it fires!


Another test is to shave the fuzz off a peach, without scraping the skin on the peach.  Perhaps not as exciting as the previous method but if you can shave a peach the blade IS sharp!


Another fan of the Allman Brothers,



Bob @ Kidderville Acres


A Woodworkers mind should be the sharpest tool in the shop!


Edited 6/2/2008 9:41 am ET by KiddervilleAcres


Edited 6/2/2008 9:42 am ET by KiddervilleAcres

Bob @ Kidderville Acres

A Woodworkers mind should be the sharpest tool in the shop!

cherryjohn's picture

(post #104369, reply #9 of 16)

me?  i run my plane irons, jointer blades and  chisels across my tongue.......... a smooth cut is a sharp blade.  But dont expect me to judge the pies at the local Fair


 


Wicked Decent Woodworks


(oldest woodworking shop in NH)


Rochester NH


" If the women dont find you handsome, they should at least find you handy........yessa!"

Wicked Decent Woodworks

(oldest woodworking shop in NH)

Rochester NH

" If the women dont find you handsome, they should, at least, find you handy........"

Ron01960's picture

(post #104369, reply #10 of 16)

I see by your post the black flies have invaded your shop.

flairwoodworks's picture

(post #104369, reply #12 of 16)

John,


Does that work well?  Maybe I'll try that.  But I'd rather use your tongue than mine!


Chris @ www.flairwoodwork.spaces.live.com


 - Success is not the key to happines.  Happiness is the key to success.  If you love what you are doing, you will be successful. - Albert Schweitzer

Chris @ www.flairwoodworks.com
and http://flairwoodworks.wordpress.com

 - Success is not the key to happiness.  Happiness is the key to success.  If you love what you are doing, you will be successful. - Albert Schweitzer

philip's picture

(post #104369, reply #5 of 16)

T,
I believe there is nothing more reliable and convenient than a finger tip. Sure, one can examine an edge with various optical means, but these are inconvenient and beside, they don't indicate HOW sharp the edge is in a way that is useful for woodworking purposes.
Finger tips in unabused condition are extremely sensitive-think about it-what can a human actually feel with them? Ask a doctor....
I agree-the "catch on your nail" test is not much use other than to indicate that the edge has come to a point: might be sharp enough to cut electrical wires etc, but not good for paring or plane irons, for example.Likewise, scraping fingers ACROSS an edge is misleading-try it with a razor blade.
Shaving hairs off arms is fairly useful but not too practical either- (some folk are bereft of hairs on arms)and I have the same complaint against it- it may be a reasonable sharpness indicator but it doesn't tell you HOW sharp and WHERE the edge is sharp.
That is why I will test for sharpness by very gentle pressure with a finger tip or side of thumb that is moving WITH the edge. At no stage is a visible cut made in flesh-unless one is stupid or clumsy or is bumped by someone else (yes, seen it happen).
This test shows how sharp and where it is sharp (or not so sharp)- useful with long edges such as planer knives etc. The test also differentiates between the edge one can expect from a very fine stone or say the usual Norton India oil stone-you can feel the "roughness". Both will shave hairs- but one is a lot sharper than the other and they may have different uses for woodworking purposes.
You will aslo easily detect if the wire edge has been completely removed or is just "laying low" as can happen. It is not so easy to see a wire edge formed by very fine stones, but very easy to feel it.
So what is the actual sensation at the finger tip? Hard to explain without demonstrating but it is the amount of drag one feels as the edge begins to cut the skin-and no, my finger tips are not a mass of tiny cuts....
I have won not a few bets with this against those who claimed to have a sharp edge, and still don't understand why it is not universal. Certainly apprentices from the old British School were shown this way as a matter of course. I was shown this as a boy when pocket knife sharpening was also explained.
I now await an air strike.

Philip Marcou

Philip Marcou
Zolton's picture

(post #104369, reply #7 of 16)

Phillip,


Agreed completely about using the fingertips to test sharpness.


It's a learned skill. But once learned it takes just a second at assess the edge and get back to work.


For those who might want to try this, don't run a fingers along the edge. That way lies blood and gore. Hold a finger to the edge and scrape it gently perpendicular to the blade. I don't really do more than just touch the edge lightly, or "pluck" it (very lightly) might be the better term...


By practicing with a blade both before and after sharpening, you'll soon be able to ascertain which is sharp and which needs attention.


Zolton 


If you see a possum running around in here, kill it. It's not a pet. - Jackie Moon

If you see a possum running around in here, kill it. It's not a pet. - Jackie Moon

KeithNewton's picture

(post #104369, reply #11 of 16)

I always wear a magnifying visor when I sharpen, and have a good light right over the work.

Before I do the final polish, I drag the edge across a wood block to remove the wire edge, then do a final light touch against the mdf wheel charged with compound.

If the magnification and good light doesn't reveal any shine, I may do a light nail drag to feel for a tiny wire, and only touch the hair on the back of the wrist. If it will shave, you can feel it without actually shaving a patch. All you need to do is touch the hair about 1/8" above the skin. If the blade is dull, it will slide over the hair, as it pushes it over. If it is sharp, you can feel it grab as it starts to cut just a few hairs. That is all I need.

One last thing. Lots of kids learned from Gramps that the last step in sharpening is to wipe the blade across their palm away from the edge. I have heard and read this numerous times from some who think that this somehow improves the edge.

Back when Gramps sharpened the blade that he shaved his face with, he may or may not have known that stropping can leave a little line of waxy crud just back from the edge. If you don't wipe it off, it can be just enough to lay a hair down, rather than shear off. But I just can't understand how anyone can believe that the mere wiping a blade across your hand could improve the steel, or the cutting edge.

philip's picture

(post #104369, reply #13 of 16)

Admiral Keith,
You are introducing an element of inconsistency with your 1/8th hair : some folk have hairs that stand up like burnt corn stalks in a field whilst others have virtually none, or just a silky down, making it very difficult to cut one half way through. And what about the ladies ? What are they to do?

I have observed one or two Old Boys putting the finishing touches to those Stanley plane blades by use of a rapid hand stroking action imparted to the bevel side and back alternately, at great speed, so fast that there is ####slapping sound. I think it was more for effect than anything else but then again when the hands are ape- like one might expect the skin to be thick, rough and leather-like, possibly having a strop effect, or maybe removing the last vestiges of wire edge?

Philip Marcou
Philip Marcou
KeithNewton's picture

(post #104369, reply #14 of 16)

Hey Mook,
Well as usual, you are probably right. I hadn't thought of it like that. This may not work for all hominids equally.

I like to kid that I grew up so far back in the woods, that I was well into my teens before I stood fully erect, and stopped dragging my knuckles when I walked. Ha So I guess your milage may differ.

joinerswork's picture

(post #104369, reply #15 of 16)

Keith,


In Hasluck's Manual of Traditional Woodcarving, he mentions the habit of old-school proffessional carvers of stropping their tools on the palm of a hand prior to getting back to work after a pause for sharpening.  According to him, these guys' hands were pretty heavily callussed, from years of hitting the tool with the heel of a hand rather than a mallet.


A friend keeps a piece of a brown paper bag  on hand for stropping.  He claims it is just abrasive enough to finish the polishing of the edge.  I sometimes use the palm of my hand, a pant-leg, or the top of my shoe, as much to pick up any remaining oil from the stones as to wear those teensy-weensy metal crumbs off the edge.


Ray

KeithNewton's picture

(post #104369, reply #16 of 16)

Yea Ray, I know, and I give my blades a clean-up swipe, but to actually improve the edge with a bare hand, or leather without compound, would call for a metal with a hardness closer to mercury than tool steel.

I don't understand why some people just can't sharpen. I have used a couple of carpenters to help me with projects in the last couple of years that carry knives so dull that I could see the shine across the room. They couldn't even sharpen a pencil with them. I offered to sharpen it for one of them, and he would notlet me, for fear of cutting himself.