Abstract: A lathe for machining interior surfaces of revolution in hollow parts supported on a rotational axis by a head-stock and including a pair of transversely and longitudinally movable carriages arranged symmetrically on the work piece rotatable axis, the carriages each supporting at least one tool holder bar projecting along the work piece rotational axis and having a cooperating cross-sectional configuration by which the tool holder bars are supported along their length by engagement with each other. Alternatively the carriages may support turrets each carrying a plurality of cooperable tool holding bars in a manner such that the operative pair of tool holding bars derives similar support one from the other. In both embodiments, the cutting tools supported by the bars are disposed on a diametric plane inclined with respect to the holder bar in engagement plane to resolve all cutting forces into exclusively torsion on the tool holder bars.