Before you sharpen a knife, ask yourself how you will use the knife? This determines the best sharpening angle, which is the angle between your blade and the whetting stone, says author Steve Bottorff in his book "Sharpening Made Easy." You use different sharpening angles depending upon how you plan to use the knife, Contrary to the rule of thumb, a 30-degree angle is not right for all blades; and the absolute sharpest blade is not always the best choice.
Botorff suggests that the importance of knowing the exact angle "is overemphasised. Knives are functional through a range of angles." More important is to control the angle, for a consistent edge.
Ceramic sharpening systems from companies like Spyderco make achieving a consistent edge easy; two ceramic rods sit in slots at angles of between 30 and 40 degrees, and you draw the knife straight downward, while pulling it toward you. But while a 30-degree edge is durable for your kitchen knives, you must adapt to achieve the 20 to 23-degree angle you desire for a sharp pocket knife.
Sharpness and Tests
There are four general degrees of sharpness. Which you require will help determine the angle that you use in sharpening.
The first is chopping sharpness, for heavy-duty implements like axes.
Next is slicing sharpness, good for heavy-duty cutting like you would find on a hunting knife. Botorff describes this as sharp enough to slice well through a sheet of paper with ease (using only the blade's edge, not the point).
Third is shaving sharpness, which Bottorff describes as sharp enough to shave some hair (but not all) off of your arm.
Last is razor sharpness, which will shave you like a Wilkinson Sword razorblade; this will shave all of the hair off your arm.
These are also good tests of a blade's sharpness, and a veteran knife sharpener likely has a bald patch of arm or leg, from testing edges.
In general, a razor's edge calls for a smaller angle during sharpening (under 20 degrees), while a hard-working edge, like on an axe, requires a larger angle. If razor sharpness sounds ideal, consider that you cannot chop a tree with a straight razor, nor shave your face with a butcher knife. A razor is a fine and delicate edge that dulls quickly; a barber must strop the edge anew between each shave. A wood cutter may sharpen an axe once a day or once every few days, because its edge holds up.
A 30-degree angle is an age-old rule of thumb, and creates a durable edge; but you require a more acute angle (a smaller one) for a very sharp edge.
Botorff recommends these angles, based on utility.
Use a 30-degree angle for heavy duty tools, like axes and chisel-grind knives.
Use a 24 to 25-degree angle for heavy-duty or chopping knives, like hunting knives and meat cleavers.
Use a 21 to 23 degree angle for kitchen and pocket knives; a 19 to 20-degree angle for even higher sharpness.
Use a 17 to 18-degree angle for shaving or razor sharpness, on fillet knives, craft knives, razor blades and so on.
Author Leonard Lee in the "Complete Guide to Sharpening" agrees with Bottorff that you need not be fussy about an exact angle. As Lee describes, if you imagine yourself attempting to peel off a layer of the stone with the knife, you will arrive at a fairly consistent angle.
Lee also suggests using the plastic spine clamp, the kind that comes on student report covers, to achieve a consistent angle. Attach this to the back of the knife, and hone with the clamp resting on the stone; your angle should be perfectly consistent.