Abstract: A machine adjustable boring bar assembly is disclosed for use with a rotatable spindle of a machine tool which accepts interchangeable tools. This assembly includes an elongated boring bar body interchangeably mounted in a rotatable machine tool spindle and with a transversely movable tool mount on the distal end of the boring bar body. First and second external sleeves are rotatably mounted on the body with a motion transmission connection between the sleeves and a tool mount to establish transverse tool adjustment upon selectively holding stationary or rotating in selected directions the two sleeves by means of pinion reaction members driven by motors. The motion transmission connection includes planetary gearing for a wide range of rates of radial motion of the tool mount so that this adjustment may be effected during the time when the boring bar assembly is mounted in the rotatable machine tool spindle and actually cutting a workpiece for step turning, undercutting, or taper cutting.