A portable journal turning lathe for machining a shaft at a job site, including a pair of clamping rings that are adapted to be centered at spaced apart locations along the length of the shaft to be machined. Extending between the clamping rings is a plurality of guide shafts which slidingly support a split ring clamshell assembly. A cutting tool carrier is affixed to the rotatable ring portion of the clamshell assembly so that when the movable ring is driven the tool carrier orbits the shaft being machined. A plurality of feed screws driven by a servomotor translates the clamshell assembly along the guide shafts.