Chip,
I will be using 20mm diameter 5mm lead screws from Gary at Zapp. Once the paint on the mounts has dried I will fit the motor mounts with one bolt and clamp the end bearing in place to get a measure on the screws then get them made to fit. I already have the motor mounts and the bearings to suit the screws. The linear rail is 25mm and I think it was around £150 per meter. Its not cheap but its very good. I will be using 15mm Linear rail for the Y-axis because I have three lengths of it spare from an old job. It will also be more than enough for the machine.

Andy,
most of ther uk profle that I have seen has either 6,8 or 10mm wide slots. I dont know if the american stuff will still be in imperial sizes so it might not fit very well at all. The valuframe stuff is great stuff. Thats what I have used for my build except for the wider 275mm profle for the y-axis riser plates because the valuframe range only goes to 200mm wide I think. Normally if you are using a rack and pinion drive I would space it out from the edge of the frame by maybe 2mm and centre the drive pinion to the rack so that it would have a little bit of float. The good thing about rack and pinion is that it can float about a bit and not cause any problems and also if your gearbox/motor assembly is spring loaded it helps to keep the pinion in mesh and also lets it follow an uneven rack.

Regards

Ian