17 October 2014

Knives: Sharpening angles

Personally I am an angle extremist as my main (vegetable) kitchen knife has a micro bevel of 14° (or 7 degrees per side).  Great but this time I wanted to sharpen a camping knife,  so which angle to choose?

Googling for data I get the following.  Looks good  ...  although 35° that's not much for a cleaver ... are machete in the same category as cleavers ... then I got it, these are sharpening angles, so the edges start at 24° to go up to 70° ...??? ... Welcome to the 21st century!

Second attempt and back to last century,  I took The complete guide to sharpening by Leonard Lee (1995).  I found a rather pragmatic approach:  for kitchen knives anything between 10° (5 dps) to 35° where the major aspect is finding an angle that is easy to reproduce when sharpening.
For camping knives he proposes 30° when whittling and 15° when mainly handling meat. I am not sure I agree there as I expect that when cutting meat from bones there is more strain on the edge. And a blade that doesn't 'bite' the bone too easily is more comfortable.

Looking at factory angles
Morakniv  knives do talk about angles, they are not in microbevels and propose 23° (12 dps) and 27° (14 dps) for their heavier 3.2mm (1/8") thick bushcraft knives that need to withstand batoning.  It is surprising they go to the American bushcraft market with anything thinner than 1/4". They are probably more hard headed traditionalists than marketeers.
Opinel knives, I couldn't find anything on their site.  Someone talked about 25° as recommended by them,  a sharpening angle so that's 25 dps.  It sounded so ... 21st century that I checked.  It's true, the picture shows the main reflection from the convex blade, but at 25° there is the reflection of the micro bevel.
For the old Opinel blades I can't see the factory edge anymore, but remembering the ease at wich they dented I guess 10° (5dps),  the blades were not yet convex then.

Now why do I think 50° (25 dps) is excessive.  When looking at plane blades I see angles of  25-35° used with the blade off center, that is not in cut but rather a scraping movement and regularly slammed with great force (6# of plane and the guy behind pushing with his two hands) against knots that represent a localized high resistance point. Apart from chopping bones there are few situations worse than that.

On youtube I found Canadian Cliff Stamp talking about Edge retention as a function of micro-bevel angle
and offering new perspectives on optimal angles.  He's someone who seem to spend his free time sharpening and cutting truckloads of wood, cardboard, rope and old carpets to assess his sharpening. This delivers interesting work and results.
What he says is that edge retention increases with smaller angles up to some usage-user dependent tipping point. On the graphic 13 dps does 4 times better than 25 dps.  I guess that the graphic stops at 13 dps because he uses a Wicked Edge sharpening gig to set precise angles. Thus losing the ability to go for low angles, the jig being a 21st century product and all that. The Wicked Edge minimum of 13 dps is a lot more than the Leonard Lee lower limit of 5 dps.
The reason behind the improvement are twofold:  low angles cut with less strain so less wear and they offer more 'meat' before the edge becomes too blunt.

So the micro bevel for the camping knife? Whatever I want,  maybe 30° (15 dps) to follow Leonard Lee, probably less.