I've built this machine 100 different times in my head!!!

[Jonathan]

I spent ages today trawling eBay for profile rails and carriages and found that I can get Thk and Nsk sets at similar length for around £150 with p&p.

I've seen Zapp is very reasonable with his Hiwin rails and carriages, which work out at £180 for a brand new set, hgh15ca and 500 rails, compared to £150 for similar spec second hand Thk/Nsk on eBay. I don't understand how there can only be £30 difference, are Hiwin a cheap brand compared to Thk and Nsk.

That's because you only spent 'today' looking, you have to be patient to get the good deals - bidding not buyitnow. Yes I know, I used to think it's annoying when people say things like that, but it's worked for me in the end.

Does ZO, little preload, mean there is more play between rail and carriages then ZA, medium preload. I have no actual experience what any type of preload feels like, as I only have the second hand ones to go by, which you can blow from one side to the other.

If you can blow the carriage along, or it drops if you hold the rail vertically, then the pre-load is zero (Z0). When there is preload there is no clearence (play) between the rail and carriages, in fact the clearance is negative so it's an interference. This means that when the carriage is slid on to the rail the bearings distort slightly, so there is a force between the bearings and the rail - hence the name preload.

I'd consider having zero, or very low, pre-load a much bigger issue than only two rows of balls. You've got both...

P.S. Noticed there's two discussions going on here, the machine and sketchup - I can separate them if you want?