The mass of the gantry alone is of little concern. As has been highlighted, the mechanical advantage you get from ballscrew is significant, so the motor doesn't directly 'see' the mass of the gantry. You have to consider the equivalent inertia of the gantry, i.e. the mass the motor 'sees'. This combines the gantry mass with the inertia of any pulleys you use and the motor itself. Choosing large pulleys can have the same effect as adding 10's of kg to the gantry mass.

Ideally, you pick the motors to match the gantry. If the motors required aren't affordable, I'd sooner use undersize motors and have the machine acceleration be a bit less, than have the gantry less strong.

Quote Originally Posted by Robin Hewitt View Post
Just to make sure my poor old brain is still working...
Better than most ;)