Thanks I rather thought it might be the case, though good to know open bearings on supported round rail are not a no-no. The HiWin rail is okay priced but the bearings seem a bit steep. How about other brands? RS Components sell one called IGUS DryLin T which for 300mm is quite affordable (GBP14 rail, GBP16 Carriage, I can't do the pound sign on this Korean computer) but doesn't offer longer rails. There are many other brands too.