jackd

Hi all! I've been trawling through countless forums and post but i cannot find a solution. I'm looking at making a diy (or at least pricing and making a CAD model) of a Ultimaker on a slightly larger scale. NOT HEAPS! Just a little bigger. The X&Y extruder bars would be 500mm 6mm(d) Steel Rods and the outer X&Y would be X=600mm 10mm(d) and y 700mm 10mm(d). Z axis is a mystery, likely to be 12x3 700mm trapazoidal threaded bar: http://www.ebay.com.au/itm/111012363194?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649 Should I increase the Z support bars to 12mm or more? I though 10mm might be enough as well as extra support on the opposite end. So would 10mm thick bars be strong enough? I'm planning on using 6mm(+) plywood for the frame and converting to direct drive. Really just looking for advice on the strength of rods and any help would be greatly appreciated! Thank you for your time Jack.