While there are good-quality ball screws made in China, there are also some made there which aren't suitable for use in precision machinery, and are only useful for opening skylights, etc. You need to look at the specs for the screw you're thinking of buying, particularly the expected accuracy expressed in +/- mm per 300mm of deviation from perfect (or in decimal inches per foot). The best grade of rolled ballscrews (C7) have an accuracy of +/-.003" per foot, which is okay for most wood projects, but not great for mechanical parts in metal. For better precision, you need to go with ground ball screws - or acme screws, which are typically more precise than ball screws. Here's an article (probably more than you want to know) about the differences between rolled and ground ball screws: Precision Ballscrew Threads: Rolled versus Ground