25° Sharper than 17°: Is this Possible?

PeteCress

PeteCress

Joined
Jan 2, 2023
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Just did two knives at 17° in the WorkSharp Pro that arrived today.

On an Edge-On-Up PT050b sharpness tester (as in https://www.sharpeningsupplies.com/Edge-On-Up-Professional-Edge-Tester-P1563C286.aspx) both tested in the mid-300's.

One knife was a pretty-good quality folder whose blade is S35AVN steel and the other was a fixed-blade of unknown steel type from my Cub Scout days a looooong time ago.

Couple years ago, I had customized that old fixed blade for opening boxes by grinding a cutting edge on to the first inch of it's false edge.

But 17° on the WorkSharp was not happening up there because it would have to take away too much metal - so I upped the angle to 25°.

Surprisingly, that edge tested at 150.

Is there any physics behind this or is it more likely measuring error and/or some other artifact of a noob's incompetance?
 
Depending on the steel, you can only make the edge so thin before it starts to fail. At 17° the steel probably isn't strong enough to support the edge. It may seem sharp but as soon as you put any pressure on it the apex is rolling over. The geometry of the blade may also play a a part here, depending on how thick the blade is right behind the edge.
 
Another Shepherd said:
Sharpness as usually defined and angle are different concepts. In particular, the Edge-On-Up devices are basically measuring the width or diameter of the apex, not the angle.
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No, they measure how much effort it takes to cut a standardized medium. There is no physical measuring of any part of the blade.
The PT50 Series of Edge-On-Up Sharpness Testers give the ability to measure and quantify the level of sharpness of any given edge. They work by measuring the force in grams required to cut through a standardized, calibrated medium.
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Arathol said:
No, they measure how much effort it takes to cut a standardized medium. There is no physical measuring of any part of the blade.
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The force to cut the test medium is in fact a surrogate for the width of the apex:

"BESS test media was developed in a rational sort ofway. Every effort was made to give BESS test mediarelativity to the user and the real world. With thatin mind, let's talk about the correlation betweenBESS scores and apex dimensions. We know that it takes 50 grams of force +/- 5 grams to sever BESStest media when measuring a standard DE razor blade so the relationship, apex radius in nanometers toBESS score, is very close to 1:1. Other tests and measurements conducted up to BESS scores of 300show this relationship to be essentially linear so with some confidence we can predict approximate apexdimensions using BESS score data. This, of course, comes as no surprise since the equation S = F/Apredicts just this result. We must be careful though because theoretical calculations and practice often collide at some point so while this information is useful in helping us to grasp the miniscule world of edges it is not yet absolute from a scientific perspective."

 
When cutting a thread all that matters is the apex. A fully apexed 25° blade can easily be sharper than a 17° blade that isn't fully apexed. Edge angle comes more into play when continuously pushing through material, like cardboard. Then the thinner wedge is easier to push through a material than a thick wedge.
 
Seventeen degrees per side, if accurate, is not a dramatically low angle and should not be causing edge failure on any decent steel in light use, which I think the BESS medium qualifies as. I say if accurate as the grind of the blade itself can subtract from the actual edge angle in some designs, such as when laying the grind on the "blade table" of the Edge Pro.

You may have a burr on the more acute edge.
 
Mr.Wizard said:
Seventeen degrees per side, if accurate, is not a dramatically low angle and should not be causing edge failure on any decent steel in light use, which I think the BESS medium qualifies as. I say if accurate as the grind of the blade itself can subtract from the actual edge angle in some designs, such as when laying the grind on the "blade table" of the Edge Pro.

You may have a burr on the more acute edge.
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I'd agree with this. Chances are there's a burr OR the sharpening didn't fully reach the apex OR the burr became so drawn out that when a wire edge was removed the steel torn off the apex left a duller edge. Angle ≠ sharpness. The angle is just the supporting geometry for the apex itself.
 
Assuming the edges were both apexed, keenness is ultimately determined by how well you deburr. It's usually easier to deburr a lower angle knife than a higher angle knife, because any rounding of the apex is going to have more dramatic results with a thicker grind, simply because it gets thicker faster than a thinner grind (same reason thinner grinds cut longer).
 
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