what size motor do i need

I am a newbe. I have done some reading and have a router/plasma table
almost done it will be driven by 424 oz in 34's geckos and deskcnc
controller board. I have two question the, the first is about power
supplys I have the amps needed but too much volts 70 I only need 40
can i put a zener on the thing to reduce the voltage? the second has
to do with the motor torque requirement. I questimated at the needed
torque, but the more i read the more im convinced that I may need to
bite the bullet and buy the (right motor) for the table. the table
moves and is heavy. I have an accurite fish scale and it says that
the it take 9lbs to get it moving and 8 once its going. that is with
the motors connected to the tabel through a belt. I have know way of
knowing the additional torque required to cut brass of aluminum. can
any you calculate or give me your best guess for the motor torque
required.I have looked over the setup page in desk cnc and have no
idea about motor speeds or step proccessing any help on that would
also be greatly apriciated.
Dave Scott