1.7*0.74*0.4m Mill / Router building...

[Jonathan]

I really thought 20 mm rails for the bed would be more than ample to carry the weight of the whole cnc machine

They will be ample to carry the weight since each of the 20mm bearings is rated for about 880N. That's not as excessive as it might seems though, see what I posted here:
http://www.mycncuk.com/forums/showth...near-bearings-...

The reason I advised using the bigger bearings is not directly due to the higher load ratings. It it because they have 5 rows of ball bearings, not 4. This seems to make them have a lot less play than the <=20mm bearings. So I'm not saying you have to get the 25mm bearings, just that it will probably be better if you do.

where on earth do you all get your aluminium from as I have ordered off ebay, it arrived today so bent it is unusable

I now use http://www.aluminiumwarehouse.co.uk/
Only issue with them is you have to buy 2000mm lengths. If there's something smaller you need let me know as I might have a bit...
What size did you buy out of interest, and who from so I don't make the same mistake?