Force through ball screw based on stepper motor torque??

Hi guys,

This problem (if it is) has been bugging me for some time now. On
my CNC router, I can only get about 70-80 pounds of force on any of
the 3 axes. I am using the cheapo rolled ball screws 5/8 diameter -
.2" pitch and they are attached to the ball screws - direct drive.

The static load on any of the axis is less than 5 pounds of force.
In other words, to move any axis and overcome it's friction, it
takes less than 5 pounds of force.

I have two different brands/types of motors. Only one of them
is "modern" and it is a chinese made uniplolar 200 oz-in motor rated
at 3 amps. It is a six wire motor and does run real "zippy" - much
more responsive than the motor it replaced - an older round style.

Anyway, even though this machine is working good, I am using it as a
basis for determining the specs for some other machines I am planing
on converting to CNC. When I did the original calculations for
thrust force, I came up with much higher numbers than what is
actually showing up on the machine.

I am wondering if someone can run the calculations and tell me what
they come up with as to what force I should get on an axis with the
following:

Unipolar 200 oz in motor @ 3 amps, 5/8" x .2" pitch screw, direct
drive.

Oh, and the driver is running at 40 volts & 3 amps

Thanks folks!

Chris