Sharpening effort

H

HoB

Joined
May 12, 2004
Messages
2,611
Did a couple of calculations yesterday to figure out, how much more effort (meaning relatively how much more material) I would theoretically have to put in, in order to rebevel an edge that was sharpened at 22 deg. to 20, 15 or 12 deg. Practical example: I have a knife that was sharpened factory wise to 22 deg. and I have the choice of either taking it to 15 deg on the sharpmaker or to 20 deg.
In my calculation I assumed a straight blade behind the edge bevel, which is clearly not correct for a saber grind, but should be a pretty good approximation for a full flat or hollow ground blade. The problem is pretty basic trigonometry but I fund the result quite surprising. I further assumed that you sharpen to ideal accuteness, without turning a burr. Both turning a burr and a saber grind would greatly aggrevate the general outcome.

Turns out that you would have to remove about 4.5 times more material per side (!) to take an edge from 22 to 15 degrees than from 22 to 20 deg. and over 8 times more material per side to take it from 22 to 12 than from 22 to 20 deg.
Meaning, if you work already on the edge and are ready to go to the whites after 20 strokes (each side) on the greys on the 20 degree setting, it would take about 90 strokes to achieve the same on the 15 degree setting.

Thought that might be interesting to some that are "into sharpening". I personally was just wondering, why it took me so much longer to sharpen a knife to 12 deg in comparison to one that I sharpen to 15. Now I know, I have to remove about 80% more material, meaning, if I spend half an hour on the stone for 15 deg, I have to expect to spend a full hour on the stone for 12.
 
I usually do the re- profiling on a lansky type, or free hand,, then to the Sharpmaker, its alot quicker.
You can get diamond hones for the Sharpmaker, but I don't have them.


B.G.
 
Look at a cross section of the blade to see how much material has to be removed, you'd be amazed.
 
When ever I really feel like sharpening I do it all on waterstones, but I've come to find out that I like the Sharpmaker angles the best: 15 and 20. Sometimes I put a 17 deg microbevel on a 15 deg edge. Found that's the best trade off between time spend sharpening and results. 12 deg takes just too long (now I know why!) and produces a large and potentially ugly edge bevel.

There are all these threads on this forum: which setting should I use on my xxx knife? Well here is the answer. If you don't like spending lots of time shapening and don't need straight razor like qualities, go with the 20 deg setting (which will still be shaving), because you will be spending potentially 4x (of course that number doesn't apply if your knife had an edge bevel of less than 20 deg.) less time per side no matter what system you use.

Edited for spelling
 
The numbers are enlightening, and funddmentally show the problem with angle tolerances in knives. If you are going to jig sharpen at 15 degrees and the NIB angle is 16, then you can hit it in no time. However if the angle is off just a little and up to 18, as you noted the honing time can make a huge jump. For a lot of people this would lead to a huge level of frustration and why thick edges blades often gave their steels a horrible name for ease of sharpening.

-Cliff
 
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