I made this tool for a specific purpose but has proved to be generally useful. I've not seen anything like it described elsewhere, but I don't suppose it is completely original.
I needed to form an outside radius of several millimetres along the edge of the backplate of a firebox. In the past, I have done this by hand with files, checking frequently against a profile template, but that is tedious. I decided that I could flycut it instead.
Making the cutter was easy. It was rough ground to shape on the bench grinder, then finished using a small grinding wheel of the right diameter in my Dremel drill. There are lots of small wheels available for this tool, it is just a matter of choosing the right one. The problem was orienting the quarter-circle cutout in the cutter such that, when set into a fixed holder, the two tangents at the ends of the arc were horizontal and vertical. Without a proper cutter grinder, that seemed very hit and miss.
This holder can be used to set the angle that the tool makes with the work. The photographs are, I think, self-explanatory. The holder is in two parts, held together with a countersunk locking screw. The adjacent faces are made quite flat, and when the screw is done up tight, the cutter does not move. At least, not under the light cuts that one has to take with a tool like this.
It is sized for the Sherline mill, so the shank is MT1. The locking screw is M6, and it takes a 1/4 in. square tool, but the dimensions are not critical and it could be scaled as necessary.
Nick Baines • Model Engineering