Mathieu Ritou

Complete Analytical Expression of the Stiffness Matrix of Angular Contact Ball Bearings

Angular contact ball bearings are predominantly used for guiding high speed rotors such as machining spindles. For an accurate modelling, dynamic effects have to be considered, most notably in the bearings model. The paper is based on a dynamic model of angular contact ball bearings. Different kinematic hypotheses are discussed. A new method is proposed for the computation of the stiffness matrix: a complete analytical expression including dynamic effects is presented in order to ensure accuracy at high shaft speed. It is demonstrated that the new method leads to the exact solution, contrary to the previous ones. Besides, the computational cost is similar. The new method is then used to investigate the consequence of the kinematic hypotheses on bearing stiffness values. Last, the relevance of this work is illustrated through the computation of the dynamic behavior of a high speed milling spindle. The impact of this new computation method on the accuracy of a finite element spindle model is quantified.

Optimization of Process Parameters in CFRP Machining with Diamond Abrasive Cutters

The present paper proposes a methodology to optimize process parameters for trimming applications with diamond abrasive cutters. This methodology is based on the study of quality of trimmed surface, through material integrity and surface roughness, and on the study of cutting mechanisms. Their evolutions according to tool parameters and cutting conditions have been analysed. Results show that diamond grits size must be chosen according to the required surface roughness. Feedrate must respect cutting limitations due to CFRP removal mechanisms with abrasive cutters, which have been identify through analyses of specific cutting energy. Finally, a protocol in two steps is proposed to determine the optimum process parameters according to the application. Firstly, constraint functions due to respect of quality and to limiting cutting phenomena are defined. Thus, limiting values of process parameters are determined. Then, process parameters are selected in order to optimize productivity.

Investigation of CFRP machining with diamond abrasive cutters

Diamond abrasive cutters are more and more used for industrial applications of CFRP laminate machining, due to their resistance to abrasion and low cost. The objective of the approach is to improve productivity during trimming operations, by choosing adapted tools and cutting parameters. The objective of this article is to determine to what extent feedrate and spindle speed can be increased. To do so, specific cutting energy (Esp) and cutting forces are studied. Designs of Experiments and ANOVA are performed in order to analyze the influence of tool (grit size, level of nickel and diameter) and process (feed per revolution and cutting speed) parameters. It shows that high cutting speed is favourable to the cut due to thermal effects. Conversely, the study of specific cutting energy reveals that the feed increase is limited by diamond grit size and level of nickel due to the saturation of the tool intergrit space.

Implementation of a new method for robotic repair operations on composite structures

Composite materials nowadays are used in a wide range of applications in aerospace, marine, automotive, surface transport and sports equipment markets. For example, all aircraft’s composite parts have the potential to incur damage and therefore require repairs. These shocks can impact the mechanical behavior of the structure in a different ways: adversely, irretrievable and, in some cases, in a scalable damage. It is therefore essential to intervene quickly on these parts to make the appropriate repairs without immobilizing the aircraft for too long.

Cutting force model for machining of CFRP laminatewith diamond abrasive cutter

The article presents a cutting force model for trimming operations of CFRP laminate with diamond abrasive cutters. Those tools are more and more encountered on industrial applications of CFRP trimming, due to their abrasion resistance and their low cost. Contrary to endmills, they consist of a large number of cutting grits, randomly distributed around the tool. To tackle the issue, a continuous model of tool engagement is proposed. Validity of the approach is verified. A mechanical model of cutting forces, adapted to CFRP laminate, is then presented. The evolution of specific cutting coefficient in relation to fibres orientation is investigated through a piecewise constant model. It leads to the proposal of a sine model for the specific cutting coefficients. The simulated forces are in good agreement with the experimental results of cutting tests, carried out in multidirectional CFRP laminate for different fibres orientation and widths of cut. Cutting mechanisms are finally discussed depending on fibres orientation.

Bearings Influence on the Dynamic Behavior of HSM Spindle

The aeronautic industry requires high speed and high power spindles to obtain high material removal rates during long rough milling operations. The weakness of HSM spindle is the bearings, although high precision hybrid ball bearings have been developed to achieve this critical application. Inadequate use of spindles inevitably leads to shortened lifetimes. Choosing the operating conditions is a required step before machining applications. It can be achieved through either experimental tests or numerical modeling that leads to stability lobe diagrams. Stability of cuts relies on the dynamic behavior of the spindle, which is particularly due to the eigenfrequencies of the tool-shaft assembly. The frequencies depend on bearing stiffness that can change under operating conditions. That is why the impact of cutting conditions and bearing parameters on its stiffness are studied in the paper. A five degrees of freedom model of angular ball bearing is briefly presented. A complete bearing model is introduced. The originality of the approach is the complete technological modeling, notably of the radial expansions of inner and outer rings of bearing. A non-linear expression is established from continuum mechanics model. The influence of geometry of bearing, operating conditions and design parameters of spindle on the bearing stiffness are established and analysed. Then, modal analyses of the tool-spindle assembly are carried out in relation to the varying bearing stiffness. Finally, significance of the approach is demonstrated through the analyses of Frequency Response Function.Copyright