Structural analysis of a large moving gantry milling machine including its work support system and foundation

This paper describes the comprehensive analysis of the static structural stiffness of a large moving gantry milling machine of the type typically used to produce large components for the aerospace, shipbuilding and automotive industries. The need for high static stiffness arises from the requirement to produce parts to a desired size and shape [1] and although finish machining often takes place with small depths of cut and correspondingly light cutting forces, the resulting deflections can still be excessively large if the machine has inadequate static stiffness. The machining performance and component quality produced by these machines is heavily dependant upon their structural rigidity and it is therefore extremely important to maximise the stiffness of the entire structural force loop. Machines of the type described above are connected to the work-piece by the concrete foundations which support them and thus the machine's overall rigidity also depends upon the concrete foundation and the sub-soil beneath it. This research has investigated the contribution of each significant element of the structural loop to the overall static stiffness of the machine, as measured between the ram and the component support system, in this case, fixed workplates. Emphasis is made on the need to fully consider the concrete foundation and the subsoil beneath it [2, 3] when analysing the force loop for machines of this type. The predicted overall stiffness was compared to the measured value. The paper clearly shows how modern Finite Element Analysis (FEA) software enables the design of machine tool structures to be carried out quantifiably with significant confidence in the accuracy of the results.