Curved tools can be baffling when it comes to sharpening. The normal hurdles of keeping the blade flat on the bevel while stoning give way to a whole new array of techniques.
I thought that my recently acquired inshave would be a good place to start down this winding road.
The basic rules and goal of sharpening still apply, two polished planes meeting along a single invisible edge. The trouble comes in shaping the surfaces to get them evenly polished.
Some inshaves have a knife edge with no distinct bevel. I'll be honest, I've got no idea how to maintain the polish and geometry of these tools. I prefer to have a bevel on the outer edge of the tool that I can grind and hone. This not only gives me a chance to adjust the geometry, but assures me that I am reaching the actual edge during honing.
To cut properly, the bevel of the inshave must be ever so slightly rounded which allows the tool to enter and exit. Think of a spoon scooping out ice cream, the curve makes it happen.
I like to begin with the back of the blade. A hardware store carborundum burr helps knock out any high spots. I simply hone the surface with a diamond cone, or sandpaper on a dowel, until the high spots shine, then I abrade them with the burr, being careful not to grind too near the edge. What I end up with is like a very shallow hollow grind.
Once the grinding is done, I take some fine sandpaper wrapped around a dowel and chuck it in my drill to even out the surface and then finish off by loading the sandpaper with polishing rouge and going at it until it shines bright and even.
While I try to keep the surface flat, a little rounding will be ground out when I grind the bevel.
Grinding the bevel is easiest if the top edge of the tool lays nice and flat. I set the tool rest nearly flat and grind while holding the tool inverted. If the tool rest is too small or the tool doesn't lay flat, the inshave can be stuck to a plate with rare earth magnets to provide stability.
Once I have a proper grind, I hone the outer edge with small diamond paddles and finish with a leather strop. The leather strop rounds the edge just enough. A few strokes on the inside with a rouged dowel will finish the job.
Every tool, especially old ones, seem to have a sweet spot where the geometry and polish of the tool create optimal performance. I try not to drive myself nuts finding it on the first sharpening. A bit of use will tell me if the edge is too brittle to support the bevel angle or not quite right to work with the angle of the handles. Over time, repeated sharpenings will give me the opportunity to refine the job, and my awareness of the tools' abilities will foster a sense of value and familiarity that I'll feel whenever I reach for it.