A micro-adjustable differential screw assembly for providing fine adjustment of the axial and/or radial position of a cutting insert is disclosed. The micro-adjustable differential screw assembly can be mounted directly to a boring bar or to a mounting cartridge, which in turn, can be mounted to the boring bar. An axial micro-adjustable differential screw assembly provides fine adjustment of an axial position of the cutting insert, while a radial micro-adjustable differential screw assembly provides fine adjustment of a radial position of the cutting insert. The differential screw assembly includes a rotatable drive screw with threads of different thread pitches, and a non-rotatable adjusting screw with threads for mating with the drive screw. The difference in the thread pitches of the drive screw causes a greatly reduced axial and radial movement of the boring cartridge in relation to the relative movement of the drive screw, thereby providing fine adjustment in the axial and radial position of the cutting insert.