Flattening The Back Of Chisels And Plane Irons
Flattening the back of a chisel or plane iron is as important as sharpening the bevel. On a tool, the edge is created by sharpening the bevel and the back. Think of the back as a bevel of zero degrees. If the back is coarse, the edge will only be as sharp as the coarse back.
Diamond stones are great for fattening because they cut quickly and are very flat. I like to start by first testing the back with a fine grit diamond stone. By using a finer grit on a flat surface I can test whether the back is flat. The fine grit will put a uniform finish on the areas where it is in contact with the stone. I want to see that the back is uniformly abraded all the way to the edge. If there are any areas that are not abraded by the fine grit, I know that the back is not flat.
Once I have determined that the back is not flat, I move to a coarse or extra coarse diamond stone. I find the diamond stones cut fast and stay very flat which makes flattening an easier process. Once the surface is uniformly flattened with the coarse stone, I move to the next finer stone and repeat until I am at the extra fine diamond stone. At this point I will switch to my finest water stone for the final polish. If you do not have a set of diamond stones, you may use water stones for flattening. Because water stones don't stay flat like diamond stones, I suggest flattening the water stones periodically.