if you are designing your robot in cad. it is a simple command of using Mass Proprieties. in autocad's case the command is massprop. the mass properties will tell you the volume of 3d objects all you have to do is take that volume and multiply it by how much that material weighs. like a cubic inch of aluminum weighs 0.0975 pounds roughly.
this site http://www.allmeasures.com/Formulae/ has spreadsheets and tables that let you just let you plug in your numbers and you have your weight.
and thats my tip of the day.