chrismb wrote:That's one way, but this looks a little more controlled in a CNC machine. I'm pretty sure it is an actively-eccentrically driven curve of constant width.
I think it still has the template. It seems the template must have one more 'corner' than the cutter and I suspect the cutter must have an odd number of apex. Each sides of the cutter is an arc centred on the opposite apex. The apex of the cutter engages in a corner of the template at which point the cutter 'walks' around to the next corner cutting as it goes. At least that how I figure it.
One drawback with these as a opposed to broaching is that the holes are slighter bigger at the bottom, obviously OK on thin(ish) materials. An advantage is that they can do blind holes which is a bit more difficult with a broach without debris being lodged at the bottom of the hole.
I think I should try to make one the only real challenge being how to heat treat the tool steel. Maybe.... when I get a round tuit.