Quote Originally Posted by wilfy View Post
my question is do you buy machined ones for certain applications or do you always need to machine them?
Pretty much every application needs them end machining.

Plenty of people here, myself included, have bought ballscrews with end machining on eBay from linearmotionbearings2008. He will cut them to whatever length you require and can do custom end machining if required. He charges hardly anything for end-machining, so they are very cheap:
http://www.ebay.co.uk/sch/linearmoti...ngs2008/m.html

All the ballscrews he sells are C7 grade, which is 0.05mm per 300mm (not simply cumilative), or if for some reason you want nanometers per cm that's 1667. Howerer very few 'hobby cnc' applications require better although it's nice if you can get a higher grade cheaply.