I forgot to mention...I'm not keen on the placement of your Z-axis ballscrew. If a large force is applied parallel to Z, the axis will begin to skew (i.e. rotate in the Y-Z plane), due to the ballscrew placement being off center. The effect can be reduced by increasing the spacing of the Z-axis linear bearings, or go back to two ballscrews, or get the ballscrew closer to the center. That's why on the new machine I placed the ballscrew intersecting the Z-axis ram - it got it closer to central, but admittedly still not ideal.