Abstract: An apparatus and method for improving control over the milling force applied to a milling bit that is turned through a rotary drive to form a hole in a wellbore casing. A bit-weighting sub is applied between the tubing used to lower the rotary drive's motor into the wellbore and the rotary drive itself, the sub serving to take the weight of the tubing off the rotary drive when the motor lands in operative connection with the drive, and further serving to apply a known milling force to the drive (and thus to the bit) independent of the weight of the tubing. In a preferred form the sub includes a spring that is compressed against the drive when the tubing and motor are landed.