Centre tap transformer?

[Jonathan]

More or less Amps wont hurt anything but less and the drives/motors will under perform when Max'd out, more and your just wasting electricty and money on more expensive transformer.

Too much less and it wont work clearly. A bigger transformer will (up to a point) help provide a more stable/constant voltage output and since the motors only only draw how much current the difference in power drawn from the mains is absolutely negligible so you wont notice a difference in for electricity bill. So get whatever you can find cheap (on eBay) greater than or equal to the required power rating.

With the above example a 160Va 2x20Vac transformer wired parallel would work giving 30Vdc @8A when rectified.

20*2^0.5-1.1=28.2V, so actually a bit less than 30V which is good since as you said don't want to be pushing the limit of the driver's ratings. Particularly bearing in mind the number of posts on here about those drivers going pop!

Working on 4 axis system with 4x3A motors giving 30Vdc @8A then approx 16000uF would be ideal, so 24000uF is a touch on the large side thou think it will just give more smoothing and not hurt anything. Thou should always test output voltage before connecting to drives.!

Yep, nothing wrong with using a bigger capacitor than calculated as it will improve the regulation of the PSU slightly. Wouldn't be worth buying a bigger one specially as the difference is so slight, but that doesn't apply here.

Also since this is for the lathe it's surely 2 motors not 4... either way the capacitor is fine. You'd be much better off with an m547 or similar 50V driver.