Sharp edges are useful, whether on an axe or knife. Knives are a great tool in your home, garden – just about anywhere. It is a 2nd to last resort for self defense, just above hand to hand. A knife needs different types of edges for different tasks. You would not go about chopping a small log with fillet knife. In general terms, it’s good to be able to maintain an edge without resorting to a ‘professional’ sharpening business. I have been able to learn to get an edge on my knives at least as sharp as a service using relatively inexpensive guide systems.
The first thing to consider before sharpening your edge is determining it’s task. In general, the sharper the edge, the less sturdy it is and likely to dull. There are numerous ‘shapes’ to an edge that determine how sharp it will be and how sturdy it will be. A basic “V” shape is easy to develop but there are refinements to increase its durability by adding a second angle near the apex of the V called a ‘double bevel edge’, or by placing graduated angels along the edge then smoothing them into a curve which is called a convex curve and is one of the most useful edges combining both sharpness and durability. There are many, many others – see chisel edge, scandinavian edge, and others.
The first step in sharpening effectively is to maintain a steady angle when grinding away the steel that makes up the edge. This approaches an art form when done ‘free hand’ on a sharpening stone without some sort of guide. It also explains why there are so many types of guide systems. I suggest learning to minimally crudely sharpen free hand, in part to appreciate a guided system but to be able to do ‘well enough’ if no guide system is available.
When using a guide, copious attention can be applied to the small differences in angles needed to get your knife scary sharp. A bit fun if you like that sort of thing.
Early in the process, you may need to ‘thin’ the blade. If you just sharpen the very extreme edge of your knife, it will eventually have a tiny sharp edge in the middle of a thick chunk of metal. The thick part gets in the way of slicing. Thinning tapers the stout part of blade down towards the edge.
For most ‘fine’ applications found in food prep, a double bevel is used: 15 degrees is used for the main angle, and increased to 20 degrees on the edge face. The wider angle gives strength. This ia good compromise between super sharp and super durable.
If you’re butchering an animal, you want to use a fine edge for the skinning part, but a beefier angles of a double bevel like 25 – 30 degrees/ 20 degrees. Still sharp, but the edge is stronger and wont dull when hitting bone the first time.
To cut branches (you will need fire to cook), 30 degrees to 45 degrees works well, a bit less sharp if you try to fillet a fish but does well chopping small branches and the like.
A guide will, well, guide the blade angle as it moves across the abrasive stone (‘stone’ can be a traditional sharpening stone, ceramic, industrial diamonds applied to a metal surface, and in a pinch, sandpaper).
One way to guesstimate angles is to fold the short side of piece of paper to its long side, which gives you a 45 degree angle. Fold again like that and – presto – 22.5 degrees. If you are free handing, use that to help develop a feel for the sharper 20 degrees angle for a finer edge and 25 – 30 degrees for a sturdier edge.
A great learning tool whether using a guide or freehand, is to get a magic marker and cover the edges of the blade with ink. Inspect the ink after one or two passes of the stone to see if your removing metal from where you want metal removed. If you see that you are removing metal just at edge face, increase the angle a bit, and do the converse if you see that you’re removing the ink at the base of the edge (decrease the angle). Also pay attention to where the ink is removed across the entire edge – is the depth close to even or thick and thin as you look across the edge? Work on getting it nice and even.
Like finishing wood or metal, grits are important. Coarse grits remove a lot of metal and if used needlessly, will wear away your blade. Not used enough, you will spend too many hours using a grit that is too fine to thin the blade. When viewed microscopically, the edge of a knife looks like serrations. Coarse grits give a coarse serrations and fine grits give fine serrations. Using a leather strop further decreases the size of the serrations and some think that adds a bit of sharpness and durability as the serrations are less likely to break while cutting. Serrations can be helpful, for example, they tend to help with slicing chores by grabbing the surface better. Like everything, not too much and not too little is the goal. Lots of trial and error.
If the edge has not been sharpened in quite some time, you will likely need to start by thinning the blade with a coarse stone. If a lot of metal needs to be removed, do alternate sides after a few strokes so that the angles remain symmetrical on each side, otherwise you may remove more metal from one side. You know you removed enough when you feel the bur develop on the other side of the edge from the stone. The bur is easiest felt with coarser stones and less obvious as you move to finer stones. Run your nail slowly up the edge and if you feel it ‘catch’, that’s probably the bur. For field work, going up to about a 600 or 800 grit is sufficient.
You can get a bit fancier and use finer and finer grits, as well a stropping the edge on leather. You can get cruder as conditions dictate. In a pinch, using sandpaper stuck on something flat works. Remember, a sharpening stone is simply an abrasive to remove metal. It works very well because it is flat. Use a flat abrasive of any sort and you’re in business, from the finest Japanese waterstones (hundreds of dollars) to a piece of sandpaper. So in a prepper fashion, you may want to practice on an old knife found in the bottom of something, and start with 100 grit sandpaper at a 22.5 degree angle. Once you feel the bur develop on each side (note that you will lose the bur on the left side when you sharpen the left side and develop the bur on the right side!), move up to a medium grit and develop the bur as above and then move up to a fine grit, again developing the bur on each side. At this point, get some super fine wet/dry paper and further refine the edge. At this point you can stop having developed a nice V-edge, or you can increase the angle a smidge and gently develop the back angle to get a double bevel to give the edge a bit more strength, ie, longevity. And in a pinch, you can strop the edge against some leather – like and old leather belt.
There are some great resources on line that can give you more information than you want. See http://forums.egullet.org/