Not quite a new member but...

[JAZZCNC]

What kind of tolerance's would I expect to achieve with hiwin rails and 2010 ballscrews and would it be much worse with supported round rails and what kind of machine should I be building if I want precision?
Cheers, Charlie

Can't honestly answer that question because there's far more too tolerence than just the screws or rails you use.! Machine Design and build quality all play big part in this. I can tell you cheap chinese round rail is far inferior to profiled linear rail so if your chasing high tolerences then forget round rail.
Also If your on a budget then Chai at linear motion bearings is now selling Profiled linear rail at very good prices which makes using round type pointless. I've used loads now and they are good for the money. Not quite Hiwin quality but much cheaper and not far behind.

If you need high tolerences then you need a very ridged and stiff machine in which case provided the size required isn't huge then a fixed gantry machine is much better design.