Throwing off a lot of sparks when you’re sharpening on a grinder looks cool, but it actually might be ruining the tool’s temper.
As Larry Frye, my luthiery instructor, once explained, “The perfect cutting edge would be infinitely thin, infinitely smooth and infinitely strong — anything else is a compromise of one sort or another.” With that as our goal, every cutting edge we use must be an appropriately designed compromise. This isn’t as tricky as it may sound, since most tools will have the manufacturer’s best guess for the optimum cutting edge already ground and polished.
Truth be told, most woodworkers stay out of trouble until they start changing bevels and regrinding willy-nilly. Grinding should be done to repair an edge or to restore a bevel, because for most of us, it’s best to keep the existing bevel.
Grinding
Holding your chisel flat along the angle of the bevel against a well lubricated grinding wheel will do a nice job of repairing and sharpening your blade.
Grinding and honing are the two basic steps in sharpening any knife-edged tool. Grinding removes a significant amount of metal and sets you up for honing success. Improper grinding will generate excessive heat and change the metal’s temper — the term for the strength component of our perfect cutting edge. Using a lubricated grinding wheel and removing the minimum amount of material are keys to keeping your tool’s temper.
Before moving on in the honing process, make sure the back of your chisel blade is perfectly flat for a single beveled edge.
The first important detail in any single-beveled edge is that the back of the blade must be perfectly flat. Another general principle is to avoid changing the angle of the existing bevel. The bevel of an edge is the compromise of a steel tool relative to our goal of infinite thinness.
Honing
Start with coarser grain stones and continually move down to finder grain stone as you sharpen your blade to hone it.
Honing starts where grinding ends. After you have successfully ground the edge to the proper angle (thinness), while retaining the temper (strength), you must hone the edge to a mirror finish (infinitely smooth). Begin with a coarser stone and start to remove the grinding marks, honing the bevel perfectly smooth.
Move from coarse to ever finer grained stones, making the same number of strokes on both faces of the tool. Don’t skip a grit level as you hone; it will not save you time and will negatively affect your edge. The smoother you hone, the sharper your tool.
Your last polishing step should be with a leather strop. Then you will have reached your goal of a thin, smooth and strong cutting edge.
