Elsevier

Wear

Volume 271, Issues 3–4, 3 June 2011, Pages 590-595
Wear

Characterization of 3D surface topography in 5-axis milling

https://doi.org/10.1016/j.wear.2010.05.014Get rights and content

Abstract

Within the context of 5-axis free-form machining, CAM software offers various ways of tool-path generation, depending on the geometry of the surface to be machined. Therefore, as the manufactured surface quality results from the choice of the machining strategy and machining parameters, the prediction of surface roughness in function of the machining conditions is an important issue in 5-axis machining. The objective of this paper is to propose a simulation model of material removal in 5-axis based on the N-buffer method and integrating the Inverse Kinematics Transformation. The tooth track is linked with the velocity giving the surface topography resulting from actual machining conditions. The model is assessed thanks to a series of sweeping over planes according to various tool axis orientations and cutting conditions. 3D surface topography analyses are performed through the new areal surface roughness parameters proposed by recent standards.

Introduction

In the field of free-form machining, CAM software offers various machining strategies depending on the geometry of the surface to be machined. The surface quality results from the choice of the machining strategy and corresponding parameters (tool inclination, feed per tooth, cutting speed, radial depth of cut). Resulting machining time, productivity and geometrical surface quality directly depend on these parameters. In 5-axis machining, axis kinematical capacities as well as specific NC treatments alter tool trajectory execution, leading to changes in actual local feedrates. Moreover, as the tool axis orientation generally varies during machining, the resulting surface pattern can be affected [1]. The prediction of the 3D surface topography according to the machining conditions is an important issue in 5-axis machining to correctly achieve process planning and to link resulting surface patterns with part functionality.

With the advances in 3D measuring systems, it is now possible to measure machined surface patterns with enough accuracy [2], [3], [4] although there is no standard traceability [5]. A draft standardized project [ISO 25178-2] developed by the ISO Technical Committee 213 working group 16, proposes the definition of areal parameters as an extension of the well-known profile parameters [6], [7]. However, only a few studies try to link the surface roughness with surface requirements via areal surface roughness parameters. For friction in servo hydraulic assemblies, negative skewness and the lowest kurtosis values as well as the highest valley fluid retention index are found to have the lowest frictional characteristics [8]. The functionality of automotive cylinder bores is partially characterized by oil consumption and blow-by. In this specific case, it is more significant to consider Sq, Sk, Svk, Sds, Sbi to describe oil consumption and Sv, Svi for blow-by [9]. Concerning the fatigue limit, authors prefer to refer to Sq, Std and Sal [10]. Due to the lack of information concerning the influence of roughness parameters on surface requirement, a description of the 3D pattern obtained after surface machining is essential to bring out the influence of machining parameters on surface topography, and to afterwards link surface roughness with functional requirements.

In the literature, few formalized studies exist which aim at linking the surface topography with the machining strategy parameters [11]. Two standpoints can be adopted: the experimental standpoint and the theoretical standpoint. Based on surface topography measurements, most of the experimental methods attempt to establish the link between the feedrates, the machining direction, the tool orientation and the 3D topographies. Unfortunately, results are only qualitative; only a few of them clearly express the relationship between the machining strategy parameters and the surface topography [12], [13]. Adopting the theoretical standpoint, Kim described the texture obtained in ball-end milling from numerical simulations only accounting for the feedrate influence [14]. Bouzakis focused on the motion of the cutting edge. The author highlights the influence of the tool orientation, the transversal step and the feedrate on the machined surface quality [15]. Toh supplements this work by defining the best direction to machine an inclined plane [16]. In a previous work, we proposed to link the machining strategy in 3-axis ball-end milling with a 3D surface roughness parameter and to optimize the machining direction according to this parameter [17]. Kim proposed to simulate the 3D topography obtained in 5-axis milling using a filleted-ball-end tool. The envelope of the tool movement is modelled by successive tool positioning according to the feed per tooth.

Due to difficulties in measuring the surface topography for complex shapes, the need for models or simulations for predicting the machined 3D surface topography is real. However, if most literature works enhance the major role of the federate, the context of high speed machining is seldom considered. Actually, in multi-axis high speed machining the computation of the inverse kinematic transformation and the synchronization of the rotational axes with the translation ones impact the respect of the programmed feedrate which does not remain constant during machining. Therefore, it seems essential to integrate those local federate variations in a prediction model of 3D surface topography obtained in multi-axis high speed machining.

In this paper, a theoretical approach is proposed to predict the 3D surface topography obtained in 5-axis milling with a filleted-ball-end cutter tool integrating actual feedrate evolution.

Actual feedrate evolution is obtained thanks to a kinematical predictive model which accounts for the local variations of the velocity due to multi-axis high speed machining [18]. The modelling of the cutting process is only geometrical; material pull out is not consider here. The proposed model applies for complex surfaces for which the topography measurement is generally difficult. The topography prediction relies on the well-known N-buffer simulation method [19].

Based on simulations, the study finally aims at formalizing the influence of the machining parameters (feed per tooth, tool inclination, maximal scallop height allowed) on the 3D surface topography. For this purpose, the topography is characterized using the areal surface parameters. An attempt is made to propose links between areal surface parameters and the parameters of the machining strategy.

Section snippets

3D surface topography in 5-axis machining

Material removal simulation relies on the well-known N-buffer method [19]. The main difficulty is the integration of the effects linked to 5-axis machining within a context of high velocities. Indeed, the use of the two additional rotational axes leads to two main difficulties during trajectory execution: the computation of the Inverse Kinematical Transformation in real time to define set points corresponding to tool postures, and the synchronization of the rotational axes with the

Model assessment for plane surfaces

The model is assessed by comparing 3D surface topographies obtained by simulations to actual measured ones for different types of part. The first validation concerns the milling of a series of planes considering variable machining strategy parameters: the tool axis orientation, the programmed feedrate (Vf) and the maximal scallop height allowed (hc) (Table 1). In the proposed experiments, the tool orientation is defined by the tilt angle (θt) and the yaw angle (θn). A complete experimental

3D surface topography parameters

The complete experimental design is also conducted through simulations, considering experimental parameters defined in Table 2. As previously discussed, the feedrate is an essential parameter, as it actually conditions the 3D pattern (Fig. 8). Modifications of local feedrate during machining may affect the 3D surface finish.

Usually, the maximum scallop height allowed is one of the most used parameters in CAM software to define the 3D surface topography. As shown in Fig. 8 (case 4), a non-null

Conclusion

The objective of the present paper is to propose a method for characterizing 3D topographies of complex machined surfaces. For this purpose, a simulation model of material removal in 5-axis milling is developed and assessed. As in 5-axis machining, velocities are non-uniform during machining and vary linked to kinematical limits, the model is coupled to a velocity prediction model allowing the determination of actual local feeds per tooth. Simulations, compared with measurements, clearly

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