Quote Originally Posted by devmonkey View Post
A bit more research on different materials, Young's modulus (GPa):
MDF 4
SRBP (paxolin/tufnol) 6.5
Aluminium 69
Steel 200

The deflection of a rectangular section spanning two supports due to a downward force in the middle is inversely proportional to the Young's modulus * thickness^3.

So taking 16mm aluminium plate as a reference, thickness required for equivalent rigidity of those materials are:
MDF 41mm, SRBP 35mm.

Or another way, 25mm SRBP is equivalent to 12mm aluminium.

From this it is probably ok to conclude 25mm SRBP supported on 4 sides over rectangular bays in the machine frame that are 840x320mm is probably sufficient.
I would probably use one large piece as the bed, then a second piece as the sacrificial board.