Friday, July 22, 2011

The Science of Knife sharpening

Ever since I switched from ceramic Kyocera knives to high quality steel knives, I have been learning about the art and science of knife sharpening. I have decided to boil down the things I have learned into a compact bullet list of items so that you can receive a quick revelation.

  1. Sharpening machines found at the kitchen stores ain't no damn good to God or this country. Don't buy them.
  2. 'Sharpening' steels don't sharpen, they hone a blade. Honing a blade means straightening a slightly warped edge. Sharpening steels only straighten an existing edge. They will not put a new edge on a dull piece of metal. If your edge is gone, a steel will do nothing for you. I don't bother with them. Why use a steel when can strop with leather or balsa wood? Either works better than a sharpening steel.
  3. Whetstones are for master blade smiths only. If you are an ABS Master Bladesmith, then God bless you for your skill in using these things. Otherwise, they are fairly worthless in maintaining an developing a new edge. Truth be told, very few MBS guys would ever use a stone to put the original edge on a blank piece of steel. Most use belt grinders or large water wheels to set the original edge. Since I don't don't have MBS skills, my Shapton Glass Stones are fairly worthless to me, and they are the very best that money can buy.
  4. The belt grinder is a truly marvelous piece of equipment. The belt grinder spins at 3,450 rpm, and sands down the edge of your knife quickly. The first time I developed a truly shaving sharp edge on a dull piece of steel, it was with a Central Machinery 1 x 30 belt grinder and an assortment of abrasive belts from This is a wonderful website, and I strongly recommend them. You can get belts as low as 20 grid, and belts as high as 12,000 grit. I have a wide assortment of belts. Get 'em, they are cheap.
  5. Better even than a belt grinder is the amazing Worksharp 2000. Don't bother with the 3000. The Worksharp is essentially a flat 150mm (6 inch) round plate on a DC motor. The plate spins at 1,750 rpm, or roughly half the speed of the belt grinder. The disk can be covered with an assortment of abrasives at various levels of grit. I have a honing kit that goes to 6000 grit. That level of grit adds quite a bit of polish on your knives when mounted on a device like this. The Worksharp is slightly better than a belt grinder because it puts a perfectly flat V edge on your blades. The belt grinder puts a slightly convexed V on your blades. All else being equal, a perfect V shaped edge is the sharpest edge you can possibly get.
  6. Leather stropping is wonderful, but it won't work without "loading" the leather with an abrasive compound. These are usually liquid detergent slurries of Silicone Carbide, Chromium Oxide, Boron Carbide, or diamond particles. I use several different compounds. I use leather belts on my belt grinder. I use flat sheets of leather on a simple clip board. I have used the great Russian leather strop. Interestingly, the flat sheet of leather on a clip board seems to work best. The Russian stop also works incredibly well. You will not achieve a truly mirror polish without 0.25 micron diamond spray on leather. Using this stropping technique, I have put mirror finishes on all my blades.
  7. There is one material better for stropping than leather. That is balsa wood. I was introduced to the technique of balsa wood stropping via the website. The technique is essentially identical to leather stropping. The only difference is that you load a flat plank of balsa wood with the abrasive compound, rather than leather. I had my first experience stropping on balsa tonight. It was amazing. Wushtof Ikon Classic knives that simply refused to shave hair suddenly became straight razors. With a simple low-pressure stroke, the 5 inch Santoku took a stripe of hair off my chest. Wow...
Basically, I am getting these four tools together into a sequence of ops that take a knife from bowling ball smooth to razor sharp. I am not finished perfecting my technique, but it will roughly look something like this.

  1. Flatten both sides of the blade edge using the Worksharp 2000. Move progressively from 200 grit, to 600 grit, to 1,200 grit, to 3,600 grit.
  2. After each grit level on the Worksharp, strop the blade clean using a leather belt on the 1 X 30 belt grinder. Make sure that belt is loaded with 0.5 micron chromium oxide.
  3. Use 6,000 grit and 12,000 grit belts on the belt grinder to add a slight convex edge to your otherwise perfectly flat edge. This will polish the edge to a fine level, and add toughness to your edge.
  4. Spray 0.25 micron diamond paste onto a block of balsa wood and strop.
If you go through those four steps, you will wind up with a gleeming mirror-polished edge that will easily shave hair off your face, back, legs or arms.