Re: Probably a dumb question....
William Duffield, on the Cohansey
>Scratch stocks work pretty well cutting the grooves to lay string inlay with the grain, but don't work any better than a dado plane without nickers when cutting across the grain. In these cases, you need something more sophisticated.
That's the reason for the complicated blade of the quirk router, but you don't have to go to that expense. I've heard tell of people who just took a pair of x-acto blades, put a spacer between them and mounted them in a handle, e.g., a pin vise or a slightly modified x-acto knife handle. It is relatively easy to mount one of these home-built tools in something like a compass or trammel points to make controlled, radiused curves. If the pair of blades are reground with a bevel on only one side (the inside), then there will be less need for putty or other wood filler after the inlay has been glued in place.
Instead of, or in addition to a router, a very narrow chisel, which can be ground into the end of a worn out triangular file, is also helpful in clearing the bottom of the groove. Since you are working with curves, you will want to use it bevel down, and might even consider grinding the edge into a fishtail, wider at the cutting edge than behind it. If you do your own saw sharpening, you should have lots of worn out triangular files in your "custom tool stock" box.