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

[Iwant1]

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?

Thanks for the insight Jonathan, seems as though I made a bad purchase with my profile rails and have decided to get new ones with medium preload and 4 rows of balls.

I think the sketchup discussions have come to an end, and now will be mostly about my build.

[Jonathan]

Thanks for the insight Jonathan, seems as though I made a bad purchase with my profile rails and have decided to get new ones with medium preload and 4 rows of balls.

Unless you're desperate to get this done, I hope 'new ones' doesn't mean brand new, not from eBay. You only really need to know which rails you have the day before the Z-axis is actually cut, so unless you're in a massive hurry until that time you can wait for something good on eBay.