Keywords: machining centre, frame, stiffness, experimental verification.

Modern high speed cutting machine tools require the design process to include not only durability, and stiffness criteria but also the dynamic ones, as they are vital for the shape and dimensional accuracy of the workpiece undergoing the machining process. The dynamic properties of a complex spring-mass system may be improved by both increasing the static stiffness indicators as well as decreasing the weight of movable modules. Both methods result in designing optimum bodies including the criterion of mass minimization and stiffness maximization.

The Machine Technology Department has developed an optimization method for machine tool bodies relying on the combined techniques of the finite element method and genetic algorithms [3,4]. The method was applied in the optimization process [2,3] of selected bodies of a newly designed milling machine centre [1]. We can say that a new concept for the design of machine tool frames has been proposed because up till now, frames are designed using either an analogy or an experimental method, simplified analytical models or making use of experienced designers only. In the literature there is little information concerning the results of the optimization of frames of machine tools and there is no information about results of experimental verification of optimized frames for machine tools. For that reason an experimental prototype of a new machine centre was made of metal and then verified in the course of the scientific investigation [5].

The paper presents a numerical model of two key bodies of a milling machine centre together with simulation and experimental investigation results. Comparative analysis of the findings obtained in the course of simulation and experimental investigation points to the fact that the result of such a comparison indeed depends on numerical values of the stiffness indicators.

In case of the stiffness indicator values, for which the displacement measured during the experimental investigation amounted up to several or more µm, the conformity of results was satisfactory, i.e. the results varied no more than 10-15%. Yet, for cases where stiffness indicators amounted up to several thousand N/µm the discrepancies were 100% and more. The direct reason for such significant differences was the high uncertainty of the measurements related to extremely minute displacements. This means, that verification of numerical models of bodies with a high stiffness is extremely difficult to carry out unless costly equipment is being used (i.e. laser measuring methods for small displacement).

1

"Design documentation of a Vertical Machining Centre", Search-Developing Centre of Machine Tools, Pruszków, Poland, 2006.

2

"Modeling Research on Frames Optimization of Saddle, Column and Headstock using FEM - Stage I. Report of a Developing Project", No R03 003 01, Silesian University of Technology, Gliwice, Poland, 2006.

3

J. Kosmol, P. Wilk, "An Approval of Machine Tool Frame Optimization using FEM and Genethic Algorithm", Proceedings of XLVII Symposium of Modelling in Mechanics, Wisla, Poland, 2008.

4

P. Wilk, J. Kosmol, "Optimisation of Frames of a Milling Machine Tool using Genetic Algorithms and the Finite Element Method", in B.H.V. Topping, (Editor), "Proceedings of the Ninth International Conference on the Application of Artificial Intelligence to Civil, Structural and Environmental Engineering", Civil-Comp Press, Stirlingshire, UK, Paper 13, 2007. doi:10.4203/ccp.87.13

5

"Experimental Research of a Testing Prototype - Estimation of Static Stiffness. Report of a Developing Project", No R03 003 01, Search-Developing Centre of Machine Tools, Pruszków, Poland, 2010.

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