Daniel Bachrathy

Surface properties of the machined workpiece for helical mills

Stability and surface errors are investigated numerically for milling operations with a helical tool. A detailed two degree of freedom mechanical model is derived that includes both surface regeneration and the helical teeth of the tool. The governing delay-differential equation is analyzed by the semi-discretization method. The surface errors are predicted based on the (stable) forced motion of the tool. New surface error parameters were introduced to characterize the properties of the spatial machined surface. The errors were calculated numerically for a given machine tool and workpiece for different axial depths of cut and spindle speeds. It is shown that both good surface properties and large material removal rate can be achieved by appropriate axial immersion in case of helical fluted tool. This phenomenon was proved analytically by means of the Fourier transformation of the cutting force.

Improving the stability of multi-cutter turning with detuned dynamics

ABSTRACT Multi-cutter turning systems bear huge potential to increase the machining productivity of cylindrical parts. The reason for this is that the application of multi-cutter turning heads enables to raise the material removal rate, since the rate is multiplied by the number of cutters involved. Multi-cutter turning technology is not only applied to increase cutting performance but also to ensure highest possible accuracy, which is one of the most important quality measures for cutting technologies. In this study, a further benefit of multi-cutter turning operations is shown, which is the possible elimination of adverse chatter vibrations by means of optimal tuning of system parameters. Based on numerical stability calculations, the stable machining parameter region can significantly be expanded for dynamically uncoupled and optimally tuned cutters. Results have been validated by measurements.

Stability properties and optimization of multi-cutter turning operations

Multi-cutter turning systems offer huge potential to raise productivity in the manufacturing process of cylindrical parts. The application of a multi-cutter turning head increases cutting performance, material removal rate, since the advance rate is multiplied by the number of cutters involved. Another reason to apply the multi-cutter turning technology is to ensure high feasible accuracy, which is one of the most important quality measures of cutting operations. Furthermore, adverse chatter vibrations caused by the regenerative effect can also be eliminated through the application of optimized multi-cutters. The aim of this study is to investigate the stability properties of multi-cutter turning operations. It is shown that the application of dynamically uncoupled cutters with identical dynamic properties will not improve the stability properties of the system, however, the usage of cutters with different modal parameters — especially finding the optimal ratio of stiffness values of different cutters — will expand stable area in the stability charts, thus ensuring more flexibility and a wider range of stable operational parameters available for the machinist. This study proves that the stability properties of a multi-cutter turning system using dynamically uncoupled cutters are equal to the stability properties of a system using a single cutter with many degrees of freedom.Copyright

Spectral properties of milling and machined surface

Machine tool vibrations cause uncomfortable noise, may damage the edges of cutting tools or certain parts of machine tools, but most importantly, they always have negative effect on the quality of the machined surface of workpieces. These vibrations are especially intricate in case of milling processes where complex tool geometries are used, like helical, serrated, non-uniform pitch angles, and so on. During the milling process, the arising vibrations include free, forced, self-excited, and even parametrically forced vibrations together with their different combinations. Regarding surface quality, the most harmful is the self-excited one called chatter, which is related to the regenerative effect of the cutting process. Its relation to machined surface quality is demonstrated in an industrial case study. The modelling and the corresponding cutting stability are presented in case of a helical tool applied for milling with large axial immersions. The extremely rich spectrum of the measured vibration signals are analyzed by means of model-based predictions, and the results are compared with the spectral properties of the corresponding machined surfaces. The conclusions open the way for new kinds of chatter identification.

Efficient stability chart computation for general delayed linear time periodic systems

The determination of the stability of systems with time delay is of high importance in many industrial and research applications, like cutting processes, wheel shimmy, traffic jams and even in neural systems, human balancing. A user friendly numerical method was implemented to analyse the general form of delayed linear time periodic systems with time periodic coefficients. The goal is to create a freeware Matlab package which is able to determine automatically the so-called stability chart, which illustrates the parameter range for which the given linear system is stable. The user has to define the governing equation by the time periodic coefficient matrices, the corresponding time delays, the orders of time derivatives of the general coordinate vector, as well as the range of the parameters and the resolution of the stability chart. The method is optimized for 2 parameters, which is a typical case in engineering applications, but 1 and 3 parameter stability charts are also supported and tested. The stability is analysed in frequency domain based on the Nrth order approximation of Hill’s infinite determinant. The parameter points where the number of unstable Floquet multipliers changes are computed by the Multi Dimensional Bisection Method. From these parameter points, another algorithm selects the stability boundary lines. The algorithm is tested by means of numerous examples.Copyright

A theoretical investigation of the effect of the stochasticity in the material properties on the chatter detection during turning

In this work the effect of the inhomogeneous material properties are investigated in regenerative turning processes by introducing white noise in the cutting coefficient. The model is a one degree of freedom linear delayed oscillator with stochastic parameters. A full discretization method is used to calculate the time evolution of the second moment to determine the moment stability of the turning process. The resultant stability chart is compared with the deterministic turning model.

EXPERIMENTAL VALIDATION OF CUMULATIVE SURFACE LOCATION ERROR FOR TURNING PROCESSES

The aim of this study is to create a mechanical model which is suitable to investigate the surface quality in turning processes, based on the Cumulative Surface Location Error (CSLE), which describes the series of the consecutive Surface Location Errors (SLE) in roughing operations. In the established model, the investigated CSLE depends on the currently and the previously resulted SLE by means of the variation of the width of cut. The phenomenon of the system can be described as an implicit discrete map. The stationary Surface Location Error and its bifurcations were analysed and flip-type bifurcation was observed for CSLE. Experimental verification of the theoretical results was carried out.

Optimization of the Robust Stability Limit for Multi-Cutter Turning Processes

Multi-cutter turning systems bear huge potential in increasing cutting performance. In this study we show that the stable parameter region can be extended by the optimal tuning of system parameters. The optimal parameter regions can be identified by means of stability charts. Since the stability boundaries are highly sensitive to the dynamical parameters of the machine tool, the reliable exploitation of the so-called stability pockets is limited. Still, the lower envelope of the stability lobes is an appropriate upper boundary function for optimization purposes with an objective function taken for maximal material removal rates. This lower envelope is computed by the Robust Stability Computation method presented in the paper. It is shown in this study, that according to theoretical results obtained for optimally tuned cutters, the safe stable machining parameter region can significantly be extended, which has also been validated by machining tests.Copyright

Algorithm for Robust Stability of Delayed Multi-Degree-of-Freedom Systems

Computation of the stability limits of systems with time delay is essential in many research and industrial applications. Most of the computational methods consider the exact model of the system, and do not take into account the uncertainties. However, the stability charts are highly sensitive to the change of some input parameters, especially to time delays. An algorithm has been developed to determine the robust stability limits of delayed dynamical systems, which is not sensitive to the fluctuations of selected parameters in the dynamic system. The algorithm is combined with the efficient Multi-Dimensional Bisection Method. The single-degree-of-freedom delayed oscillator is investigated first and the resultant robust stability limits are compared to the derived analytical results. For multi-degree-of-freedom systems, the system of equations of the robust stability limits are modified with the aim to reduce the computational complexity. The method is tested for the 2-cutter turning system with process damping.

VALIDATION OF CUTTING FORCE CHARACTERISTIC FOR COMPLEX TOOLPATH

In the design phase of the milling process, there is a great need for the prediction of the cutting force, the required torque and power of the spindle. These informations could be used to optimize the tool path and improve the material removal rate. In this work, we present our dexel based simulation software, its modules, calculations steps and the simulation method. Different force models were analysed to describe the specific force as a function of the local chip thickness. The models were fitted to the measured force data. Then the selected force model was validated in case of a complex tool path.