Checking the sharpness

Michael, I use old phone books as a source for testing paper.  They keep dropping them off and I haven’t used a phone book to look up a phone number in many many years.  I don’t think it so much which paper you use as how it feels as the knife slices through the paper, and how it sounds.  It’s only a qualitative test but it gives a basis for comparison between knife edges you’ve sharpened.

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Brewbear

<p style=”text-align: left;”>To me, the question is how to check the sharpness while still mounted edge up in the clamp. Very much how how can definitively feel a burr along the entire length by feeling with your finger in an upwards edge trailing direction.</p>
My favorite thumb nail scrape works great out of the clamp because you can rely on your body for the reference feel. This is cumbersome upside down though. The 3 finger (Josh?) test works but is scary due to slip ups and stitches.

It begs the question, how do I definitely know when it is time to move to the next grit and  or am done.

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Brewbear

Yeah! I don’t think I’ll be attempting the three finger test, I have no desire to be called stumpy,  besides, I manage enought cuts without trying. Thank you tcmeyer for the video link.

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Richard

They say that your own neurological system will not allow you to cut yourself in “normal” type circumstances. Of course, like anything the details: no coughing, bumps, etc and you must feel in a tangential not lateral with your fingers. Actually just like you are feeling the burr.

For sharpness out of the clamp, I use a thumbnail scrape (only tangential), and I would bet my neuro (LOL) could be statistically close enough to BESS, sic., once calibrated. Put a fresh edge single ravor tangential to your thumbnail and you will feel a 6 DPS edge.

Its sort of the opposite of not being able to tickle yourself.

Enjoying NJ Indian Summer on the beach.

Tom

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Brewbear

I confess to never having tried the three-finger test.  I have some knives with very low bevel angles (foil-edge) that make that just too scary for me.  I do something just a little bit different.  If I have a bit of a callous on one of my finger pads, I’ll try to slice off the top layer.  If the knife isn’t sharp, it ain’t gonna happen.  This gives me enough control that there’s almost no danger of drawing blood and trying to work with a sore finger for a week or two.  If there’s no callous available, it’s the telephone book paper test.

The problem with any paper test has to do with the structure of the fibers in the paper stock and how stiff they make the paper.  Stiff stuff will slice pretty easily.  To prove this to yourself, try to cut a piece of facial tissue with your sharpest knife while you hold it as you would a piece of printer paper in the same test.  Good luck.  My telephone book paper will slice very nicely with an approach from a side edge.  Cutting with an approach into the top or bottom edges is hugely more difficult, as the paper would rather tear than slice.  There simply isn’t enough mechanical integrity in the cross-direction to stand up to the forces of an advancing edge.  Pick a type of paper and stay with it, once you know that only a sharp edge will cut it.

Paper stock is made on huge machines, which takes the wood pulp which has been ground up and mixed with water and adhesives into a wet, mud-like slurry.  The slurry is worked into a uniform thickness, after which the water is squeezed out and the resulting paper mat is dried, polished (if required) and wound onto full-width spindles.  I think some go up to 10 meters wide.  The forming process has a tendency to align the fibers in the machine-direction, giving the paper greater strength in one direction versus the other.

I recall some legends about Damascus swords which could slice through a silk scarf as it’s floated through the air.  Wouldn’t doubt that there’s one on youtube!  Sure enough!  There’s one from The Bodyguard (movie) that shows Kevin Costner dropping a silk scarf over the edge of a katana and it floats to the floor in two pieces.  Uh huh.

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Brewbear

I have used the “Three Finger Sharpness Test”.  With a very sharp knife edge you can tell when you just ever so gently place your three fingers against the knife edge.  It’s really amazing how the brain can sense the danger and you’ll inherently know not to even move or slide your fingers.  You can really get a good sense for “very sharp” VS “just sharp” VS “not so sharp”, with this method.  This along with slicing newspaper will give you a good idea if your knife was sharpened well.

The ultimate test is the “BESS Edge-On-Up Sharpness Tester“.  It removes the subjectivity and gives you a quantified objective evaluation of knife edge sharpness.  (That is once you learn how to perform the sharpness test procedure consistently).  It’s a good tool to employ to help you to learn what a truly sharp knife edge is and to help you to improve your sharpening outcomes by quantifying your results as you refine your sharpening technique and progressions.

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Brewbear, NotSharpEnuff, Haffner, Tommie, Scott DF

Tommie, the best most precise model can measure to 1 gram differences in the applied force needed to sever the testing filament.  It is also about twice the price of the least precise tester that only has 25 gram precision.  When you consider a BESS sharpness score of 200 grams or less indicates a very sharp knife edge it’s debateable if 1 gram or 5 gram precision is truly necessary.   Whether the reading is 200 grams, 175 grams or 176 grams it’s still a very sharp knife edge.  The test results within 25 grams is a good inexpensive tester, (the PT50C).  You’ll certainly know if your blade is not sharp, is sharp, or is very sharp with this budget model.

If you are an “edge snob” and your budget allows, then testing the edge sharpness within 1 gram is probably the meter for you, (the PT50A.)  For the price point truthfully the 5 gram resolution model, (the PT50B), is probably the most cost effective tester. After using my tester enough to get the hang of it, I can say it’s not easy to  perform repeated tests and achieve the same reading two tests in a row.  I have yet to do this.  If the successive tests at the same edge position yields +/- 10 to 15 grams, I think that’s as close a reading you can expect to get and a good indicator of the edge’s sharpness .

That being said it’s hard to really justify a model beyond the 5 gram, PT50B for home/hobby sharpening.  Even as a tool for the professional sharpener it’s still a good choice.  Unless you’re O.C. with an unlimited budget.  I don’t think with a hand-held test procedure you will ever see readings closer than 5 grams resolution in repeated testing.  If it were a hands off, automated mechanical tester I guess you could take advantage of the precision and resolution offered by the top of the line PT50A model.

The devices won’t lie to you.  You can’t fudge a better reading!  Be prepared for the unbuffered truth about the quality of your sharpening.  You will be able to quantify your results as your technique improves so you’ll know when you’re on the right track.

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Mike, Organic, Brewbear, NotSharpEnuff, Haffner, Dwight Glass