I'm trying to design a Z-axis for the CNC router I'm building to have 400mm travel. My current thinking is to use two of the 20mm linear rails from Zapp with two bearings on each rail...not the supported rails. I think the deflection of the rails themselves should be fine with this size?

The rails will be mounted to 15x160x600 alu plate so the bearings are 200mm apart and the rails spaced at 150mm (ish). I then intend to use another 15x160x600 aluminium plate to mount the router. This works out at over £50 in aluminium...ouch

Is this going to be strong enough? Using the 'gantry stiffnessv4' spreadsheet I've found on the forum and putting in these numbers I get a deflection of 0.58mm with 100N which is clearly unacceptable.

That's made me think instead of using aluminium plate for the router/bearing mount thingy I should use say an I section. I'm currently thinking of using 6x160x600 aluminium and fixing some 2"x0.5"x600mm 'strips' onto the edges of the aluminium plate to make an I cross section. I would mill a 6mm slot in the strips to fit the main sheet. According to the spreadsheet this reduces the deflection to 0.027mm which I think is fine....it's also a lot cheaper!

So, which option is the best - am I missing something here?

Thanks in advance!