How To Simple tests for sharpness

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maximus83

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Nov 7, 2011
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What are your top 3 to 5 simple tests for sharpness?

I've searched the forum a bit, and I already use the common ones such as push/pull cuts thru newsprint, or resting the blade on a hard plastic surface like a Bic pen to see if it slips, or shaving arm hair. Interested to learn a few more. ;) Main interest is using ones that are simple and quick, and if possible, don't require putting an open blade on yourself and shaving off all your arm hair.
 
Snag-free, effortless cutting through phonebook page, to check for sharpness and for the presence of burrs. Then, to see how durable the sharp edge is, I'll make a few cuts into wood, or through cardboard; then re-test cutting in the phone book page again, to see if it holds up. If it does, I know the edge is stable and clean of burrs, and it will do whatever I expect of it in all my other normal cutting tasks. That methodology has never let me down. I don't rely on shaving sharpness alone to 'prove' an edge is ready for work; even thin burrs can shave, but they never hold up in more typical cutting tasks, sometimes even as simple as slicing through paper. So, testing the sharp edge for durability is just as important; probably more so, in fact.

After it passes that test, just for fun, I'll see how well it shaves hair from my forearm (it usually does). But, I'm also getting somewhat weary of the bare spots on (mostly) my left forearm. It looks odd to casual observers who may notice it, and the stubble itches underneath the sleeves of my shirts. And it takes a lot longer to grow back fully, than I anticipated, especially when I keep shaving the stubble off as well. Tough habit to break. :D


David
 
I think it is more logical to test on something that you will be using knife for. I test kitchen knives on tomatoes. If you cut cardboard, you should test on cardboard, etc. Mirror polished edge is better for shaving test, but for some other tasks toothier edge can work better.
 
One idea I saw from a knife maker on YouTube was to test a blade using leather. Use some fairly thick carving leather (say, three to four mm thick), hold the leather vertically (edge up), and see if you can push cut (not slice) into the edge.

It would be my guess that his idea behind using leather is that it simulates "real world" cutting tasks more closely than slicing paper or shaving hair. Perhaps it also provides more reproduceable results than cutting wood due to variations in density and grain.
 
Push-cutting into leather is actually pretty easy with an edge that also easily slices phonebook paper; more so with one that push-cuts phonebook paper. Anyone who's ever accidentally taken shavings or 'chips' off of a leather strop while resetting for the next pass finds that out pretty early. I have a lot of leather strops with divots in them (and cardboard, wood and mdf strops also, done the same way). Thinner geometry and polished edges/bevels also take it up another notch, with leather and push-cutting. Thick & heavy leather tends to be more grabby against coarsely-finished bevels, as increased friction comes into play in deeper cuts.

Slicing light paper, such as newsprint or phonebook pages, is more indicative of a very clean & defect-free apex, aside from just sharpness. Any shortcomings in the edge sharpness or uniformity, whether blunt, or with burrs or nicks, will either slip (blunt edge, rolled edge) or catch (nicks, burrs), and the paper deflects accordingly, making it easy to see and/or feel where the shortcomings are on the edge.


David
 
I use a hanging paper towel and cut off a strip from it. Then I tightly roll up the paper towel and slice through that.
 
Mine are a little more foundational.

1) Visual inspection. Looking at the edge under light to find reflective dull spots.

2) Once all reflective spots are removed, I'll rest various regions of the edge on my thumbnail and gently wiggle the blade side to side. If it sticks both directions, it's sharp. If it slides both directions, it's dull. But if it sticks in one direction and slides in the other, there's a burr rolled over on the side that's sticking.

3) Lastly, I'll see if it'll shave arm hair, and if it does, how high off the skin I can go with it.
 
One: Examine how well the edge (heel/belly/tip) sticks and slides laterally across a fingernail. Two: Carter's three-finger wiggle (heel/belly/tip). When on a jobsite or in the field, you are not going to hunt down scraps of paper to test with. Sharpen to a working edge, go back to work. At home, cuts against the grain on copy paper or newsprint, but only at the very end of a session. Having to dry the blade before cutting dissuades me from employing paper as a test medium within a session.
 
Obsessed with Edges said:
Snag-free, effortless cutting through phonebook page, to check for sharpness and for the presence of burrs. Then, to see how durable the sharp edge is, I'll make a few cuts into wood, or through cardboard; then re-test cutting in the phone book page again, to see if it holds up. If it does, I know the edge is stable and clean of burrs, and it will do whatever I expect of it in all my other normal cutting tasks. That methodology has never let me down. I don't rely on shaving sharpness alone to 'prove' an edge is ready for work; even thin burrs can shave, but they never hold up in more typical cutting tasks, sometimes even as simple as slicing through paper. So, testing the sharp edge for durability is just as important; probably more so, in fact.

After it passes that test, just for fun, I'll see how well it shaves hair from my forearm (it usually does). But, I'm also getting somewhat weary of the bare spots on (mostly) my left forearm. It looks odd to casual observers who may notice it, and the stubble itches underneath the sleeves of my shirts. And it takes a lot longer to grow back fully, than I anticipated, especially when I keep shaving the stubble off as well. Tough habit to break. :D


David
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What you describe--making a few cuts into wood--are you talking about a draw cut from the back of the edge to the front or are you talking a push cut like one was whittling wood? Murray Carter does a single pull cut, maybe two, through wood very lightly to ensure burr removal. That is what I am thinking you are talking about.
 
on_the_edge said:
What you describe--making a few cuts into wood--are you talking about a draw cut from the back of the edge to the front or are you talking a push cut like one was whittling wood? Murray Carter does a single pull cut, maybe two, through wood very lightly to ensure burr removal. That is what I am thinking you are talking about.
Click to expand...

Usually draw cuts (slicing), cross-grain in wood, with a KNIFE blade anyway. I specify that, because in the last few days, I've been fiddling with a wood chisel as well (using push cuts, as is appropriate for a chisel), and testing the edge in paper after that.

If I'm feeling pretty confident in the edge, I'll also exert some harder pressure when draw-cutting in wood, taking a deeper cut; more than would ordinarily be used in lightly removing a burr.


David
 
I do almost exactly what OWE has stated. If it passes the phone book - wood - back to phone book paper test, it cuts and holds up to all of my usual cutting tasks.
 
On my small 21 there are a few microscopic chips that are still big enough to cause snagging and drive me crazy.

Seems like no matter how much I sharpen it those chips just stay. Its frustrating having such a sharp blade that has mico-chips too small to see snagging on paper.
 
I test after my first strop on cottonelle toilet paper. I just fold a piece a few times and then run it down the edge. If there's any imperfection or burr it'll snag some of the fibers out of the tissue, otherwise it should slice cleanly through.

Otherwise it's just the usual tests.
 
adamlau said:
Two: Carter's three-finger wiggle (heel/belly/tip).
Click to expand...

I'm going to try this test. I also looked up what "cutting against the grain" meant in paper, I wasn't familiar with how you tell, and I tried that test and it was helpful. Also, I tried David's suggestion of push-cutting paper, then going to something like cardboard, then return to the paper to see if you get edge durability and no loss of fine push-cutting performance. I was pretty happy to find that a couple of my blades I recently got super sharp using only 2 grits, I was able to push-cut, then make some clean cardboard cuts, then return to push-cutting with no degradation at all.

Do you think that a combination of push and pull-cut tests, maybe alternating with cardboard, and the 3-finger test, is adequate for simple sharpness tests and can replace the need for "shaving" yourself? The only reason I've used the arm-shaving test up to this point has been that it's a REALLY good and reliable predictor of your overall level of sharpness on the blade, I found it was even better than push-cut ability. If I could find something to replace shaving tests and is still easy to do, I definitely would.
 
maximus83 said:
I'm going to try this test. I also looked up what "cutting against the grain" meant in paper, I wasn't familiar with how you tell, and I tried that test and it was helpful. Also, I tried David's suggestion of push-cutting paper, then going to something like cardboard, then return to the paper to see if you get edge durability and no loss of fine push-cutting performance. I was pretty happy to find that a couple of my blades I recently got super sharp using only 2 grits, I was able to push-cut, then make some clean cardboard cuts, then return to push-cutting with no degradation at all.

Do you think that a combination of push and pull-cut tests, maybe alternating with cardboard, and the 3-finger test, is adequate for simple sharpness tests and can replace the need for "shaving" yourself? The only reason I've used the arm-shaving test up to this point has been that it's a REALLY good and reliable predictor of your overall level of sharpness on the blade, I found it was even better than push-cut ability. If I could find something to replace shaving tests and is still easy to do, I definitely would.
Click to expand...

I've personally never liked (much) using a 'three-finger test' to check the edge. Most of the time, on my somewhat aged, dried-out and wear-toughened fingertips, it has often either deceived me or hasn't really indicated much at all, relative to what I can verifiy with actually cutting paper or other test media. And if my hands are wet or oily, as during some sharpening sessions, feeling the edge is even less useful to me. I still do feel the edge at times, just to see how (or if) the feel of it even correlates to what my other sharpness tests show me; often it doesn't. A more refined & polished edge in particular, which goes through paper or whatever like a laser, often feels like nothing much on my fingertip, though it's still capable of easily and painlessly cutting me. I sometimes don't notice how sharp the edge actually is, doing it this way, until I notice my fingertips are bleeding.


David
 
Same here with the three fingur test. I rarely get any use out of it and if my hands are wet just forget about feeling the edge. Lapedog before you resharpen the next time run the blade across the hone at a 90 degree angle and take off the very edge witch should get you down past the chips. Sometimes if you don't clean up the very edge you just keep sharpening the edge chip along with the rest of the edge and it is never removed. Also I never do a shaveing edge test it just doesn't give me much feedback about the edge. Heck you can easily shave with the burr.
 
maximus83 said:
Do you think that a combination of push and pull-cut tests, maybe alternating with cardboard, and the 3-finger test, is adequate for simple sharpness tests and can replace the need for "shaving" yourself? The only reason I've used the arm-shaving test up to this point has been that it's a REALLY good and reliable predictor of your overall level of sharpness on the blade, I found it was even better than push-cut ability. If I could find something to replace shaving tests and is still easy to do, I definitely would.
Click to expand...
I prefer three-fingers over shaving as I have no issues employing the method when digits and blade are both wet. I want to get in and out of a sharpening session as quickly and efficiently as possible and shaving requires me to dry the edge beforehand. YMMV. For me, testing on a fingernail and the pads of three fingers can be performed immediately off of a waterstone. Now if you have oily, wear-toughened fingers like OwE :) then you may be best served to follow along with his recommendations.
 
I often check the edge with my thumbs. I use both thumbs so that way I can feel if its catching more on one side or another.

Then I just test it on printer or lined paper. If it cuts that well enough then that is good enough for me. But I also cut down to the left and then down to the right. This is to check if it favours one side.
 
I do a simple test that I'm sure many have used, but remember, my edge is not quite as sharp as a lot of yours. I put the blade at a 90' angle to my thumb and drag it off, feeling the resistance. If I want to go one step further, I'll reduce the angle from 90', then if if bites, or catches at all, I'll figure I've done pretty good.
I like my arm hair where it is. :rolleyes:
 
Visual inspection, three finger sticky are the minimum.

Crosscut newsprint to within a half inch or so of the bottom and then see if shallow angle pass will catch the curl (coarse edges might not pass this portion and still be "OK"). Check to see if small strips can be cut from the flopped over piece without tearing it at the attachment point - coarse with a draw, fine edges with a press.

Shear cut paper towel, twist it up and lightly draw across.

Woodworking tools - treetop armhair and shave endgrain pine to a wet surface. Or just shave endgrain pine.
 
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