Abstract: An excavator undercarriage frame is made to reduce the amount of machining necessary for supporting a swing bearing and ring gear on which an upper swing platform including the cab and operator's platform are mounted. The frame is made in two separate frame sections having top plates and formed peripheral skirts that nest together to form a planar support on which the base ring of a swing bearing and ring gear are mounted. The frame section plates are thin enough so that they will flex to conform to the base ring of the swing bearing, so it is not necessary to provide a machined surface for the swing bearing. The frame sections are welded together only around the peripheral edges and skirts, so that the distortion in the plates is substantially eliminated.