Thanks for the info Dan13, I didn't know about backlash concerns for larger diameter rolled ballscrews. I think I will need a larger diameter so ground is probably better. I notice that lower grades are better too, so from one site they are C7 and cheaper, but the other site is C5 which is more expensive. That site does do the machining for me though, so maybe the higher price will even out over all.
The info is a common info and has to be confirmed with the particular ball screw manufacturer. My note about backlash is true for single nut with normal backlash or reduced backlash (preloaded). The trend now is to use a single preloaded nuts. The manufacturers are trying to avoid using double nut preloaded systems these days, as it's expensive and can only be supplied as a screw-nut preloaded and adjusted assembly.
Even if you need bigger diameter ball screws, I am not sure you have to go for ground ones, as we are talking of very small backlash rates for single nut. For a 40mm diameter ball screw the backlash (of single preloaded nut) would be 0.02mm. The standard backlash (no preload) would be about 0.04mm. I think these values can well suite your requirements, since you're speaking of C7 precision grade, which I think is about 0.05mm/300mm.
I think you can still adjust those nuts for ZERO backlash, but I believe the manufacturers wouldn't recommend doing this. Again, you need to check against the particular manufacturer regarding the particular product. As always, they can get you anything, depending on how much you're willing to pay.
Daniel
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Anyway it looks really much for me... 
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