Hey Freff
Don't forget the inertial effect of the z axis starting and stopping and changing directions. Certain milling and drilling operations will make it bounce. The rod diameter sounds small for its length, especially for unsupported rod. Plus the force required to push a drill bit into the material will be significant. Take a drill bit of the biggest that you think you will drill in al and do it by hand on a scale and see how much force is applied. I think you will be surprised.
Is the X axis supporting the Y and the Z and cutter head? 12 lbs sounds lite to me if that is the case, but it you have weighed it, my bad.
If it is a gantry style machine, remeber that as the cutter head gets all the way to one side that x axis rail will see a large increase in load will the other side will get lighter.
Static load calculations don't always tell the whole picture.
Just my .002 worth.
Mike
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