I imagine that you are replacing the axis screws with ballscrews?
The lead and dia, of a ball screw is essentially a form of reduction that has to be taken into account, any increase in dia. or decrease in lead. (distance travelled/rev), will increase the torque of the motor by the degree of reduction. Just as any timing belt would.
The advantage of servo's is the maximum usable rpm which allows for higher lead ball screws.
I am helping my son CNC a 9x30 Lathemaster and we are using 400w Nema 23 AC servo's, with a .2 lead B.S. on the Z and a .1 lead BS on the X, I found a B.S. small enough to replace the X axis screw directly, this will be driven direct, the Z axis has a 2:1 timing pulley reduction, with the servo motor tucked back in the web below the tail stock end.
We have not yet reached the point of testing the performance.
I would tend to look at something higher torque for a 13x40.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.