Hi Malcolm,
As a lover of all things Acorn and a programmer for RiscOS and the BBC for 26 years I hate to say it, but RiscCAD is just not accurate enough for this type of work, I have spoken to Dave Buck on this subject many times and he is reluctant to do anything about it.
RiscCAD only works to 4 decimal places so its fine for structural and picture drawing but poor at CAM drawing, Oaks PDT is far better as it works to 16 places of decimal. PDT works fine on a SA-RPC but I have never tried it on an Iyonics, I won't pay Jacks prices.
If you want more detail contact me off forum.
Graham.