Patent ID: 8528644

Claim:
In a hydrocarbon wellbore assembly comprising: a casing; a casing milling assembly including a rotary drive with a milling bit positioned at a hydrocarbon producing strata beneath the surface of the earth within the casing; a motor within the wellbore operably coupled to the rotary drive to rotate the milling bit to form a hole in the casing, and a tubing extending from the surface of the earth and coupled to the motor; an assembly for controlling the force exerted on the milling bit through the rotary drive during the milling operation, comprising: a no-go stop in the wellbore configured for supporting the tubing at a predetermined depth in the wellbore when the motor is operatively coupled with the rotary drive; and a bit-weighting sub coupled at an upper end to the tubing and at a lower end to the rotary drive, the bit-weighting sub having upper and lower portions that are mounted for axial movement with respect to each other to alter the length of the sub from an extended position to a retracted position to thereby alter the distance between the lower end of the tubing and the upper end of the rotary drive, and a device for biasing the upper and lower portions of the bit-weighting sub to the extended position to thereby exert a milling force on the milling bit independent of the weight of the tubing.