Milos Glavonjic

A New 3-DOF Spatial Parallel Mechanism for Milling Machines with Long X Travel

It is well known that the shape and volume of the workspace are one of the greatest weaknesses of parallel kinematic machine tools (PKM). Hexaglide and Triaglide mechanisms are examples where workspace extension is achieved by elongating one axis as a principal motion axis that is a common feature of all Cartesian machines. With the idea of principal axis of motion in mind, a new 3-DOF spatial parallel mechanism for horizontal and vertical milling machines has been developed. In comparison with similar developed mechanisms it has several advantages such as: rather regular shape of the workspace (slightly modified block) similar to serial machines; greater stiffness by nature of the struts arrangement; good force and speed ratio through the entire mechanisms workspace. The paper describes the structure of the mechanism, modelling approach and simulation on a developed vertical milling machine prototype.

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.

Pose Measurement of Parallel Kinematic Machines with Serial Link Measuring System

For higher accuracy of parallel kinematic robots and machine tools in particular, several error models and compensation algorithms have been developed. The paper presents the approach for testing and control of Stewart platform-based machine tools and robots. The approach is based on serial link measuring system with six DOF in general, which may be used as an autonomous measuring device or in symbiosis with parallel kinematic machines to achieve Cartesian-based control schemes. Developed approach is demonstrated and verified on the example of developed planar parallel simulator with three DOF.