machine beds

[Tenson]

I've gone for 40mm x 40mm aluminium profile for the bed on my machine currently under construction. I considered 160mm x 16mm and 160mm x 28mm but thought it would not be stiff enough. I'd have thought a 10mm or even 20mm solid sheet would be rather flexible over large areas too, do you support it a lot?

I guess I'll still need a sacrificial bed on top of that, and I'm not sure what to do since that will stop me being able to easily clamp stuff down on the profile.

[Jonathan]

I've gone for 40mm x 40mm aluminium profile for the bed on my machine currently under construction. I considered 160mm x 16mm and 160mm x 28mm but thought it would not be stiff enough. I'd have thought a 10mm or even 20mm solid sheet would be rather flexible over large areas too, do you support it a lot?

How much 40x40 profile, what spacing etc? If you could post a rough drawing that would be ideal.
If using 10mm sheet I would put steel box section arranged in a grid beneath the sheet and screw it into that. On it's own 10mm would not be much good.

I guess I'll still need a sacrificial bed on top of that, and I'm not sure what to do since that will stop me being able to easily clamp stuff down on the profile.

If you're not using coolant than MDF is fine for the sacrificial bed so long as it is well supported (hence the question about spacing). You could make some plastic blocks to raise the part off the profile. HDPE will withstand coolant, but over the size of your machine the cost would be unreasonable. The good thing about MDF and HDPE is you can use standard wood screws to quickly clamp stuff down in any position and you can use any hole in the part for clamping so it's a lot less restrictive than T-slots. The compromise of course is a well supported aluminium bed will be a lot stronger. If you're serious about it I would go for strips of aluminium (http://www.aluminiumwarehouse.co.uk/...Bar/pages.html) bolted on to steel box section. Leaving a gap between strips gives slots which you can use like a T-slot for clamping. You should get as thick flat bar as you can afford, nothing is too much here really.
There are many many ways to make a bed so look at other machines around the same size...