Recommended Starting Size
This CNC runs on inexpensive thin walled galvanized steel conduit found in 23.5mm, 25mm, 25.4mm Outside Dimension (OD). Stainless steel tubing (wall thickness approx .049″-.065″) is a good upgrade that increases rigidity and accuracy.
In the US 3/4″ thin walled EMT fits the 23.5mm parts, 3/4″ refers to the Inside Dimension. The price for conduit is about $8 US for the ~20′ needed.
Very large builds are possible, if the outer rails are supported along the span. The smaller the machine the more rigid it will be, the faster you can push it. I recommend starting with a foot print of 24″ X 24″ Outer Dimensions with a 3-4″ usable height.
Inspired by GeoDave and SteveCD, I gave it a shot.
Z Rails should have 5-8mm holes drilled 22.5mm from the bottom and and 75mm above that. If your nut traps don’t fit your rails thickness well Geodave made a nice parametric part here, http://www.thingiverse.com/thing:907882
File or sand down the inner and outer sharp edges left after cutting! Sharp edges make assembly difficult and can damage your wires. I have one of these and It was worth every penny. http://amzn.to/1SYicSL
Sample Cut list
3 Rails X = Usable Width +264mm (10.4in)
3 Rails Y = Usable Depth +264mm (10.4in)
2 Z Rails = Usable Height + 190mm (7.5in)
4 Legs = Usable Height – 13mm (.5in)
1 Lead Screw (T8) or Allthread = Usable Height + 76mm (3in)
Table will be 6.5mm (.25in) longer in each axis direction. (30in X rail will need a 30.25in table in the X direction).