Sharpening philosophies

[dsvirsky]

Just a thought that’s been bouncing around my head for a while. Playing with my new ultra fine Sharpmaker stones last night sort of brought it to the forefront.

It seems to me that there’s two ways of looking at how to get the most out of the so-called "super steels." There are some (Cliff comes to mind) that want to squeeze every ounce of performance out of a blade: if the edge will catastrophically chip at an included angle of 18 degrees, they’re going to keep that bevel somewhere under 19 degrees. However, as much as we (myself included) may be enamored with the concept of "scary sharp," not all of us really need that level of performance from an edge. Would it not make more sense (for those of us in this second category) to sharpen a blade made from a steel with superior edge holding characteristics (e.g., S90V) so that the blade is sharp enough to do the job, but at an angle that minimizes chipping or rolling? In other words, to maximize the edge holding advantage of using such a steel, rather than getting a thinner, sharper edge that ends of needing to be sharpened just as often?

Disclaimer: This could just be self-justification since I can’t get my Apogee (S90V) to shave no matter what . On the other hand, the ultra fine stones turned my Starmate (S60V) into one super-slicin’ machine.

 

[Akwacko]

I used to really get into getting my knives "scary sharp" . I've since backed off to just "shaving sharp". The hair on my arm shaves off, but it doesn't leap off in fear anymore when I reach for my knife. I now go for a more durable edge.

Bruce

 

[Cliff Stamp]

The general rule for edge shaping is to grind right to the point at which if you go any lower it will start to fail, either by fracture or excessive deformation. Generally I ride all my daily using blades really tight to that point, which means that yes, they will get damaged if they get used beyond normal. For example I could sharpen my blades at an angle at which they could zip through a staple without excess harm (no visible damage), however I don't hit staples that often, and thus I will take the extra bit of sharpening time when it does happen because it allows a greater cutting ability.

It is just a matter of personal preference. The more obtuse you leave your edges the more durable they will be, and in general the far less metal removal you will have to do to fix edge damage. However this also means that you will be doing more work in general when you are using your knife because of the lower cutting ability. This is not a small amount either. I have taken factory blades and improved their performance on push cuts over 10:1 on woods and more than 2:1 on cord, by altering the edge geometry. How significant this is really depends on how much work you do with the knife. If you only cut a few pieces of rope and the blade takes ~50 lbs on a press cut, are you going to be really impressed with a blade that does it in ~25 lbs, all it did was in effect save you the effort of a cut or two. However if you have to do a few dozen cuts, well you now have a very different perspective, as you are looking at much less work.

The other point, and this is the more critical one, is that as noted, the thinner you grind edges, the more likely they are to get damaged and thus the more often you will be reshaping. This means the lifetime of the blade with be significantly shortened. A decent knife blade that doesn't get damaged, will easily last for years with only minimal wear with proper sharpening and upkeep techniques like steeling and stropping. However if you are grinding it often to remove chips and dings, you can wear it out in a year or two. Personally I grind mine right down to the break out point, which means that yes I do on occasion have to grind out damage, and yes they wear down rather quickly, but they cut very well and I don't mind buying new ones from time to time.

-Cliff

 

[db]

Sharpness is a wierd thing. At one time I had thought I had it figured out. Then I recieved a knife with a very thin main grind and a edge angle that was maybe 25 degrees per side. The damn thing out cut just about every knife I had with edges much thinner, 15 degrees and less. So, I have come to think edge angle is not the only factore in sharpness. What to do? Damned if I know.

 

[Jeff Clark]

db is onto a useful compromise. Unless I expect to pry with the edge of a knife I tend to reduce the angle of the main grind and only adjust the edge angle for optimising durability. If you use a knife as a knife rather than as an axe, cleaver, or crowbar you tend to damage the edge with incidental chips, dings, and small edge damage. These don't inpact very far back into the blade and whether the blade is .05 inch thick or .25 inch thick when you get .25 inch back from the edge has no effect on a .02 inch deep blade nick. You can get a lot of edge durability improvement by just sharpening your last .02 inch of the blade at a steeper angle and then having the rest of the blade's bevel at a lower angle.

I usually reprofile blades to a low angle (under 15 degrees) then I sharpen the last part of the edge thick or thin depending on use. For light duty the edge will be 15 degrees per side. For heavy duty it might be 25 or 30 degrees per side (and probably finished a little coarser to have some tooth). This combination will hold up better to things like occasionally cutting copper wire and yet works pretty well on wood or meat.

 

[Cliff Stamp]

Double grinding knives is an excellent idea, and it is one of the main reasons that convex bevels are so promoted as this is exactly what they do, the shoulders of the bevel are at a very different angle than the very edge. If convex bevels were compared to dual ground flat edges there would be a lot less hype about their abilities.

It would be really nice to see a production company offer this edge on their knives. They could even give it a fancy name for promotional purposes for those not familiar with double bevels. It would make sharpening much easier as all you have to do is touch up the final edge bevel, which as you noted Jeff is very narrow and will sharpen very quickly. No net loss in durability would be observed, except in very rare circumstances, and they could in fact make more durable edges.

Db :

...I have come to think edge angle is not the only factore in sharpness.

There are lots of factors that influence cutting ability besides the edge angle. The width of the angle and the width and angle of the primary grind also come into play, however at a much reduced effect. In rank of decreasing importance the cutting ability is controlled by edge angle, edge width, primary angle, primary width.

One of the least promoted interactions is how the primary grind influences edge durability. If you lower the primary grind you increase cutting ability, this means you can increase the edge angle and have overall no loss in cutting performance. Thus the lower primary grind allows a more durable edge.

Double flat bevels on an edge do the same thing. If you take say a 15 degree edge and grind a 10 degree shoulder, you can put on a more obtuse microbevel and have the same level of cutting ability as the full 15 degree angle. However the edge will be more durable as the part that takes the most beating will be at a more obtuse angle.

-Cliff