Other things have got in the way of my replying earlier, but in the interim I have been doing a lot of reading - mostly on the web, but a few books. Nothing I have read has made up my mind what kind of machine to build - or even what is needed to get the sort of accuracy I want. As an example of this, I was just looking at a Renishaw (dental milling) site where they said that too rigid a frame worked against accuracy. (something to do with compliance missmatch)

The upshot of this is that the only certainty is the need to machine parts to try different things out - I need a (CNC) milling machine to make one. As I enjoyed converting my Proxxon, hopefully I can find something that will both be useful, and become the basis of a conversion.

By the way, Ross77, the THK rails are fine. They have been swaddled up in protective paper against use in the, hopefully, not too distant future.

Mike