@birchy, thanks.

I had seen that LinuxCNC link.
The final bit about using Blender got me looking at the internals of some of the data formats. SVG files seem fairly simple and I might experiment with using them as input to my JRuby GCode generating program. But it may actually be simpler to devise a piece of program that would produce a series of short straight lines for an arc.

Truth is I'm at the very beginning and I'm not sure what I will want. The purpose of my enquiry here was to ensure that I'm not re-inventing the wheel because I missed something simple, good and free.

I will probably try to do some milling on the lathe using a vertical slide as the Y axis and (just to make things complex) it will be the Z axis for 3D printing.

...R