Straightness of steel/alumunium box section

[Jonathan]

... if your 2 rails and blocks can still handle the load at 25% capacity then its not worth worrying about and save your y travel

They should do, they are rated for 862N/1372N which is well above the cutting forces. I'm not sure about the torque ratings of the bearings though which is probably more of an issue.

It dose look a bit narrow to me tho. dont forget that making it wider will reduce the torque arm caused by the long z axis. (fun this design stuff eh....)

It's 160mm wide and the centre distance of the rails is 100mm. I don't really understand which torque you're referring to that could make a difference to the Z rails?

From www.clickmetal.co.uk the 20mm aluminium plate (1220x160x20 total) will cost £88 inc. VAT and postage. I could get a 2 meter length of 6"x0.75" flat from www.aluminiumwarehouse.co.uk for £93 posted....I can't find anywhere nearby to get it :(