Application of the stability lobes theory to milling of thin workpieces, experimental approach

The optimisation of cutting conditions in High Speed Machining (HSM) requires the use of a vibratory approach in order to avoid a fast deterioration of the tool and of the spindle, as well as a loss of quality of the surface rough- ness. We suggest a transposition of the method of stability lobes to the case of the milling thin parts, which is very typical from the aeronautical manufacturing context. After having modelled the dynamic behaviour of a blade and of the cutting efforts in side milling, we describe the zones of machining instability. An experimental validation permits us to emphasise the transition from stability to instability, in accordance to our theoretical results. The experimental profile is then compared with a computed profile. A decomposition of the different situations of contact between the tool and the part permits to show the influence of back cutting in the model. Tests of machining permit then to quantify its role. The objective of these works is the definition of a quick methodology for deter- mining the optimal cutting conditions in a given industrial machining configuration

Influence of material removal on the dynamic behavior of thin-walled structures in peripheral milling

Machining is a material removal process that alters the dynamic properties during machining operations. The peripheral milling of a thin-walled structure generates vibration of the workpiece and this influences the quality of the machined surface. A reduction of tool life and spindle life can also be experienced when machining is subjected to vibration. In this paper, the linearized stability lobes theory allows us to determine critical and optimal cutting conditions for which vibration is not appar- ent in the milling of thin-walled workpieces. The evolution of the mechanical parameters of the cut- ting tool, machine tool and workpiece during the milling operation are not taken into account. The critical and optimal cutting conditions depend on dynamic properties of the workpiece. It is illustrated how the stability lobes theory is used to evaluate the variation of the dynamic properties of the thin- walled workpiece. We use both modal measurement and finite element method to establish a 3D rep- resentation of stability lobes. The 3D representation allows us to identify spindle speed values at which the variation of spindle speed is initiated to improve the surface finish of the workpiece

Integration of dynamic behaviour variations in the stability lobes method: 3D lobes construction and application to thin-walled structure milling

Vibratory problems occurring during peripheral milling of thin-walled structures affect the quality of the fin- ished part and, to a lesser extent, the tool life and the spindle life. Therefore, it is necessary to be able to limit these problems with a suitable choice of cutting conditions. The stability lobes theory makes it possible to choose the appropriate cutting con- ditions according to the dynamical behaviour of the tool or the part. We introduce the dynamical behaviour variation of the part with respect to the tool position in order to determine optimal cutting conditions during the machining process. This general- ization of the classical lobes diagram leads us to a 3D lobes diagram construction. These computed results are compared with real experiments of down-milling of thin-walled structures

Experimental study of thin part vibration modes in machining

The machining of thin walls generally generates milling chatter, that damage surface roughness and manufacturing tools. Stability lobes which include natural frequencies are successful in case of tool chatter. When milling thin webs models are less adequate, because the interaction with the tool disrupts the behaviour of the work piece. The modal approach generally used for stability charts may be not adequate enough because of neglecting the tool and the work piece contact. This paper presents the experimental phase of a work aiming at analyse vibration modes of a thin web during machining. A finite element calculation shows the influence of a contact on natural frequencies of the part. For a better investigation, field displacements of the work piece are analysed. This work eventually aims at better knowledge of the contact between the tool and the part to improve the hardiness of models

Experimental characterization of behavior laws for titanium alloys: application to Ti5553

The aim of this paper is to study the machinability of a new titanium alloy: Ti-5AL-5Mo-5V-3CR used for the production of new landing gear. First, the physical and mechanical properties of this material will be presented. Second, we show the relationship between material properties and machinability. Third, the Ti5553 will be compared to Ti64. Unless Ti64 is α+β alloy group and Ti5553 is a metastable, we have chosen to compare these two materials. Ti64 is the most popular of titanium alloys and many works were been made on its machining. After, we have cited the Ti5553 properties and detailed the behavior laws. They are used in different ways: with or without thermal softening effect or without dynamic terms. The goal of the paper is to define the best cutting force model. So, different models are compared for two materials (steel and titanium alloy). To define the model, two methods exist that we have compared. The first is based on machining test; however the second is based on Hopkinson bar test. These methods allow us to obtain different ranges of strain rate, strain and temperature. This comparison will show the importance of a good range of strain rate, strain and temperature for behavior law, especially in titanium machining

Experimental study of coated carbide tools behaviour: application for Ti-5-5-5-3 turning

The goal of this paper is to study the relation between the input data (conditions and geometry of cut) and answers (wear of tool, forces and cutting temperatures) when machining the Ti-5-5-5-3 alloy treated. This study has shown that the cutting process is different and that the slip forces are preponderates. Compared with other materials, the specific cutting pressure is higher and does not vary according to the cutting speed but depend on feed rate. Moreover, both edge preparation and feed rate have an influence on cutting force direction. Besides, cutting temperatures are high and almost similar to those provided by high speed machining with low cutting speed. Finally, we have shown that failure modes are different from those obtained when machining other titanium alloys. Built-up edge is the most deteriorating phenomenon and no flank wear was met in our study context

Chatter Control by Spindle Speed Variation in High-Speed Milling

High-speed milling operations are often limited by regenerative vibrations. The aim of this paper is to analyze the effect of spindle speed variation on machine tool chatter in high-speed milling. The stability analysis of triangular and sinusoidal shape variations is made numerically with the semi-discretization method. Parametric studies show also the influence of the frequency and amplitude variation parameters. This modeling is validated experimentally by variable spindle speed cutting tests with a triangular shape. Stable and unstable tests are analyzed in term of amplitude vibration and surface roughness degradation. This work reveals that stability must be considered at period variation scale. It is also shown that spindle speed variation can be efficiently used to suppress chatter in the flip lobe area

Links Between Machining Parameters and Surface Integrity in Drilling Ni-Superalloy

In aerospace industry, the manufacturing of critical parts (high energy components) requires an important validation process to guarantee the quality of the produced parts, and thus their fatigue lifecycle. Globally, this validation consists in freezing the cutting conditions using metallurgical analysis or fatigue trials, and a test on the first article. This process is extremely complex and expensive. In this way establishing the correlation between the cutting conditions and the surface integrity will help us to optimize the manufacture of those parts. In this article, by the means of an experimental method, we define a domain of validation by combining the cutting conditions according to the classic criteria established by AFNOR E66-520 norm (Couple-Tool-Material) and the criteria of surface integrity for the drilling of a Nickel-base superalloy. The experimental device consists in drilling a Ø15.5 mm hole on a 3-axis milling centre instrumented by a 4 components Kistler dynamometer (Fx, Fy, Fz and Mz), a spindle power sensor “Watt-pilote” and three accelerometers placed following the directions X, Y and Z. Scanning Electron Microscopy (SEM) observations, micro-hardness tests and topographic measurements with an optical profilometer, are carried out to characterize the metallurgical state of the holes manufactured. Finally, correlations were respectively made between the cutting conditions, the recorded signals and the metallurgical state of the holes

Optimization of pocket machining strategy in HSM

Our two major concerns, which should be taken into consideration as soon as we start the selecting the machining parameters, are the minimization of the machining time and the maintaining of the high-speed machining machine in good state. The manufacturing strategy is one of the parameters which practically influences the time of the different geometrical forms manufacturing, as well as the machine itself. In this article, we propose an optimization methodology of the machining strategy for pockets of complex forms. For doing this, we have developed analytic models expressing the feed rate of the cutting tools trajectory. Then, we have elaborated an optimization method based on the analysis of the different critical parameters so as to distinguish the most suitable strategies to calculate the cutting time and define the machine dynamics. To validate our results, we have compared them to the experimental ones and also to those found in literature

Application de la théorie des lobes de stabilité au fraisage de parois minces

L'optimisation des conditions de coupe en fraisage UTGV nécessite l'utilisation d'une approche vibratoire pour éviter une détérioration rapide de l'outil et de la broche, ainsi qu'une perte de qualité d'état de surface. Nous proposons une transposition de la méthode des lobes de stabilité au cas du fraisage de parois minces, proche du contexte de fabrication aéronautique. A l'issue d'une modélisation du comportement dynamique d'une pale et des efforts de coupe en fraisage de profil, nous décrivons les zones d'instabilité d'usinage. Une validation expérimentale nous permet de mettre en évidence et de préciser la transition stabilité – instabilité puis de la corréler avec nos résultats théoriques. Le profil expérimental est ensuite comparé avec un profil issu d'une modélisation numérique incorporant le talonnage. La comparaison avec des essais d'usinage où le talonnage est contrôlé permet de quantifier son rôle. L'objectif de ces travaux est orienté vers la définition d'une méthodologie rapide de détermination des conditions de coupe optimale pour une configuration d'usinage industrielle