Hi

Steel is three times stiffer than aluminium and for machinery stiffness is more important than strength in most cases. Any sort of deflection will add to the "Error budget" for the machine. Remembering that (Among others) the sum of all errors for misalignment, frame deflection, bearing slop and temperature have to be added together to calculate the maximum error.

Google search compare Steel to Aluminium: https://www.google.com.au/?gws_rd=cr...ared+aluminium

Regards
John