A practical procedure for conceptual design and testing of machine tool structure

Abstract

This paper discusses the initial development of a machine tool and its structure (concept, calculation, design) and the verification of the prototype. The topics studied include two issues: static rigidity and dynamic stability. For static rigidity several experiments and modelling studies using the finite element method have been carried out in order to identify the model parameters. In this way differences between models of bolted joints, slideways and the cross-section of the structural elements have been determined. The model is formed by design documentation and later verified through experiments on the prototype of the machine. The approach is different in the case of dynamic stability. The model is not made on the basis of design documentation or static calculations, but by experiments performed on the prototype. This relates to an oriented transfer function; parameters are determined by fitting experimental transfer function curves. With this model, the stability is analyzed under different machining conditions. Specific features of this methodology are as follows: • • The finite element method is used for qualitative comparison of different machine tool structure concepts during the conceptual and design stages. Only after completion of the prototype may the parameters of the prototype model be adjusted for the purpose of obtaining quantitative indicators. • • Dynamics are analyzed by parameter identification of the oriented transfer function model. The dominant degree of freedom is naturally selected by experiment and not from hypotheses about the behavior of structures obtained from mathematical manipulations such as expansion of the model according to the finite element method. If necessary another machine tool structure may be modelled; in this way hypotheses are drawn about the stability of the reconstructed prototype. Such a procedure has been applied and verified on the machine tool structure of a horizontal machining center. Results for static rigidity and dynamic stability have been obtained from the model and experiments performed on the prototype. The following techniques have been used: • • finite element method for qualitative identification of static behavior, • • self-excitation of the machine, • • digital signal processing on the FFT basis, • • smoothing of curves and digital filtration, • • function fitting of the transfer function (modal analysis), • • coefficient calculus and oriented transfer function, • • stability assessment of the fitted model under different machining conditions, and • • modelling of the regenerative machining effect by cutting. Necessary tests have been done by instruments required for the use of the above techniques. Such a combined static-dynamic criteria procedure for structuring a machine tool enables efficient follow-up of all results and facilitates necessary future expansion, the utilization of universal equipment, the combination of modelling and experiments, and the synthesis of simple models of the examined machine with behavior identical to the machine. The well-known machining system dynamic stability theories are applied to such models.