I used to have a couple of Thiel 158's, excellent machines. I remember the X axis being nice and free flowing, but the Y axis was always stiff. The Y axis is created by an overarm coming forward and backward with the spindle is attached, this all added to the weight and stiffness. Have you thought how you will get the stepper/servo drive to the Y axis? I am assuming you have a fixed table and not the XY fancy one.

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Mike