The Secondary Bevel

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The illusive Secondary Bevel, what and why is it?  The secondary bevel is another bevel on an edge that is at a greater angle of incidence than the primary bevel.  Huh?  Lets say you sharpen a tool edge to 25°, which as we know for an opposing bevel edge (bevel on each side) will be 12 1/2 degrees per side.  This will be a pretty sharp edge and will cut leather well; but it is a delicate edge and will require a lot of care, stropping primarily, to maintain it, still not too much of a problem for leatherworking knives, but it will eventually wear out, no amount of stropping will bring it back to bleeding sharp because the edge isn't there anymore.  This is the same for any edge, eventually it goes, and you have to put a new edge on the tool.  For the same thickness of blade, the "ramp" from the body of the blade to the edge will be longer the less the angle if incidence; 25° has longer ramp than 30°.  We can take the dull 25° edge and change the angle of incidence to 30° with very little removal of metal and still have a sharp edge that will have more metal behind it.  Of course, if you keep adding 5° every time you sharpen, you will eventually approach the geometry of a cold chisel (not terribly sharp, but tough as nails).  So what we do is use that 30° bevel (15° per side) and "ride" it back with subsequent sharpenings.  You will have to remove a little more metal with each sharpening as you "ride" the edge back, the heel of your bevel will increase each time you sharpen the secondary, but a whole lot less than going back to the primary.  Eventually it takes as much or more time to sharpen the secondary bevel as it would to just go back and sharpen the primary, or the secondary becomes in effect the primary (obliterating the original primary), and at this point you can add another (the "new" secondary) bevel at a higher angle, or drop back and sharpen at the original primary angle.  The nice thing about the secondary bevel, is that oftimes it requires using only a fine and then extra-fine stones to attain the edge because most of the work has been done when the primary was established.

This technique is used quite a bit when sharpening chisels and other working tools that may be sharpened a couple of times a day.  They often are micro-beveled at a degree or two increase over the primary and another degree or two over the last secondary when a new edge is necessary.  Other knives, especially head knives and other weird profiles are difficult to sharpen this way, but it can be done, although the increase of angle between the primary and secondary may have to be somewhat greater when hand sharpening.

I don't use the primary/secondary system on head knives.  I sharpen to a convex (teardrop) edge using a slack belt (although there isn't a lot of slack) and finish with leather or a sewn buff and .50 micron compound in the strop or the buff.

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