Sharpening question

rockrewls · Nov 22, 2003

What is the best angle to sharpen a knife that will be used almost exclusivly for skinning, with of course bones occasionally being a factor. I have thought 20 degrees would be best, is this correct.
Steel will be s30v,bg-42,154cm,s60v possibly 440c. What are the better techniques.
Thanks

VampyreWolf · Nov 22, 2003

it really comes down to knife geometry and heat treat.

The general opinion on it though is 40deg inclusive for general use, 30inclusive for hunting or food use.

I run a 30-35 on my fixed, my edc for work have around 30, and my beater(spyderco dragonfly) has about 12-15deg inclusive.

I ran my spyderco calypso jr(vg-10, flat grind, 1.75ish belly on a 3mm spine) on 20deg with a 30deg for the doublegrind edge... worked great until I hit a chicken bone.

Cliff Stamp · Nov 25, 2003

Occasional cuts into bone are no problem for a quality steel as steel is *much* harder than bone. As long as you can avoid chopping into the bone or twisting trying to break joints apart, neither of which should be a factor here, the edge angle can be as low as possible, ie. almost flat to the blade.

I have worked around pork, beef and poultry bones with no problems with edges of 10 degrees per side, and this includes some heavy (cutting) contact. As an extreme example of some bone cutting with a mild steel blade :

http://www.physics.mun.ca/~sstamp/knives/tension_bar_proto.html

I would recommend a primary edge bevel of <10 degrees per side to maximize cutting performance and a secondary edge bevel of 15 degrees which should be just barely visible. If the primary grind is fully optomized (meaning the edge is <0.010" thick) then you don't need the dual ground edge and just have it at 10-15 degrees per side, it will be barely distinct anyway.

-Cliff

berettaman12000 · Nov 25, 2003

I don't really think its a big deal with an exact angle, most will work just fine. I agree with the guy who stated that edge geometry is much more important.

Cliff Stamp · Nov 25, 2003

berettaman12000 :

I don't really think its a big deal with an exact angle, most will work just fine. I agree with the guy who stated that edge geometry is much more important.

The edge angle is a part of edge geometry, in fact it is the element that cutting ability and durablity is most sensitive towards. Edge thickness is next, but plays a far less reduced role, specifically it requires a much larger change in edge thickness to see the same magnitude of effect.

-Cliff

Roadrunner · Nov 25, 2003

A little experimentation should find the correct angle. Make it as thin as you dare, then use it for a bit and check the edge. As long as it hasn't chipped or rolled you're good, and if the performance isn't where you want it you can thin it out a little more. If it does chip or roll, raise the angle when you sharpen it then test again. I've used this method to find the performance limits of several of my khukuris and some of my hunting knives. It's worked well so far.

rockrewls · Nov 25, 2003

Ok let me see if I understand correctly here. 40 degress inclusive means 20 degrees per side I am assuming. And if I am understanding correctly I am to put a 20 degree inclusive edge on the blade first, and then put a 30 degree edge inclusive on the 20 degree inclusive edge is this correct? I dont understand why to be honest isnt the 20 degree straight edge better by itself? then the 30 degree on the 20 degree. Can someone explain this further. Some web site info regaurding this matter would also be excellent.
Thanks

Cliff Stamp · Nov 26, 2003

rockrewls :

... a 20 degree inclusive edge on the blade first, and then put a 30 degree edge inclusive on the 20 degree inclusive edge is this correct?

Yes, this is called using a secondary edge bevel, first used significantly by Joe Talmadge.

... isnt the 20 degree straight edge better by itself?

The small additional 30 degree edge adds a lot of durability. When edges chip or roll they do so to a very small extent, less than 0.1 mm in depth for most cutting. Even though this is barely visible, it can still seriously reduce the cutting ability of the blade.

By adding a slightly more obtuse bevel just along the edge you greatly reduce the likely hood of edge damage, and since this bevel is very narrow (just barely visible), it doesn't significantly reduce the cutting ability.

In addition, using a secondary edge bevel *greatly* increases ease of sharpening. When you apply the primary edge bevel of 20 degrees inclusing you are not really sharpening as much as shaping so you don't need to be concerned about precision nor accuracy, this is just a shaping step. This allows this part of the bevel to be applied very fast with a fairly coarse hone.

Now you sharpen the edge with the secondary bevel which gets applied very fast because of the already existing edge which is more acute. Putting a 15 degree bevel on top of a ten degree one is only a matter of a few passes per side that is all, just seconds.

When you resharpen you just hit the secondary edge bevel, after awhile it will thicken and you will need to reset the edge with the more acute bevel removing the secondary edge bevel completely and grinding a fresh edge.

-Cliff