Identification d'un modèle d'efforts de coupe mécanistique et application dans le cas d'un contournage de cuivre pur

L'obtention des caractéristiques des pièces usinées ainsi que la bonne maîtrise du procédé d'usinage sont liées aux efforts de coupe. De nombreux modèles d'efforts de coupe ont déjà été développés, mais ils sont souvent appliqués dans le cas d'opérations d'usinage élémentaires (coupe orthogonale ou oblique), ce qui limite leur utilisation à la communauté scientifique. La méthode de discrétisation d'arête permet de généraliser les applications de ces modèles à des géométries d'outils plus complexes. Néanmoins, les applications restent généralement limitées à des opérations d'usinage simples (chariotage, dressage, fraisage flanc...) plutôt éloignées des besoins industriels. D'autre part, les modèles mécanistiques sont généralement critiqués car nécessitant d'être calibrés à partir d'un trop grand nombre d'essais. La présente étude propose de minimiser le nombre d'essais nécessaires à l'identification. Pour cela, les coefficients d'un modèle mécanistique ont été estimés par identification inverse, à partir de différents nombres d'essais de chariotage. Le modèle ainsi identifié est comparé, pour chaque couple de coefficients, à des essais couvrant une large plage de conditions de coupe. Ce modèle est ensuite appliqué dans le cas d'une opération de contournage. Les résultats obtenus par modélisation à partir de la trajectoire théorique, mais aussi de la trajectoire mesurée à vide sur la machine, sont comparés avec les efforts mesurés lors de l'opération de contournage

Mesure de champs cinématiques par corrélation en coupe orthogonale : Possibilités et précision de l'imagerie double-frame avec éclairage laser pulsé

Dans le but de valider les simulations de la coupe visant à prédire l'intégrité de surface, une instrumentation basée sur une caméra double-frame et un éclairage laser pulsé a été développée. A partir des doublets d'images obtenus par ce moyen d'acquisition, les dépouillements possibles par corrélation d'images sont détaillés, accompagnés d'estimations d'incertitudes de mesure

Kinematic Field Measurements During Orthogonal Cutting Tests via DIC with Double-frame Camera and Pulsed Laser Lighting

The measurement of machined-part strain fields induced by the cutting process remains a challenge because of the presence of highly intensive and localised strains. In this study, a high-speed double-frame imaging device with pulsed laser lighting is used in order to obtain sharp and highly resolved images during orthogonal cutting tests performed in an aluminium alloy. The displacement fields are then measured using a global Q4–digital-image-correlation (DIC) method and several strategies, facilitating calculation of the total displacements due to the cut, along with the residual strains in the machined part. Numerical procedures are developed to manage the removed material that disturbs the DIC. An automatic primary shear angle detection procedure using DIC is also proposed. Five different markings, which are produced via chemical etching and micro blasting, are applied to the observed surfaces. Their effects on the kinematic fields and the uncertainties are then studied. Three surface parameters are proposed as indicators for determining the surface preparation suitability for the DIC. The repeatability of the kinematic fields induced during the cutting process is studied, because of the ease with which testing can be performed. Finally, the plastically deformed layer engendered by the cutting process is measured using the calculated residual strains

Effect of rake angle on strain field during orthogonal cutting of hardened steel with c-BN tools

In the case of hard machining of steels, negative rake tools generate compressive deformation and high temperature under the cutting edge, leading to phase transformation or ”white layers”. The resulting surface integrity can be predicted by numerical simulations which may be validated by comparing simulated and measured strain fields. Recent high speed imaging devices have facilitated strain field measurement by Digital Image Correlation (DIC), even at high strain rates. However, the analyse is generally restricted to the primary shear zone and not to the workpiece under the machined surface. For this study, a double-frame camera and a pulsed Nd:YAG laser, generally used in the field of fluid mechanics, have been employed to record images during an orthogonal cutting operation of a hardened steel. The effect of the rake angle and the edge preparation of c-BN tools on the subsurface displacement field, which has been experimentally investigated by using DIC, are presented in this paper together with an analysis on the origins of the strains. The results of these measurements will be used to validate cutting numerical simulations or to improve hybrid modelling of surface integrity.WindProcess - ADEM

A generalised geometrical model of turning operations for cutting force modelling using edge discretisation

The knowledge of cutting forces is of prime importance to ensure the success of cutting operations, the desired properties of the machined parts and therefore the functionality of the workpieces. Edge discretisation is one way to model cutting forces. Traditionally used in milling, this methodology enables local changes in uncut chip thickness or cutting geometry to be taken into account and then gives suitable results in the three directions. A key point of this method is the geometrical transformation that enables the description of various tool geometries. This study proposes a geometrical model based on homogeneous matrices, whose main interest is to decompose the transformations step-by-step. The method, generalisable to all machining operations, is detailed for turning operations. Inserted cutters are modelled considering both the positioning of the insert and the local geometry of the insert. The cutting geometry and the edge are described using the same model in the machine coordinates system, allowing forces and moments to be calculated easily

Experimental and numerical assessment of subsurface plastic deformation induced by OFHC copper machining

Strain distributions in the machined surface and subsurface of OFHC copper workpieces were determined experimentally and through numerical simulations. An experimental setup, comprising a double frame camera and a pulsed laser, was developed to measure the displacement fields using the digital image correlation (DIC) technique; strain distributions were then calculated. A numerical orthogonal cutting model was also developed and applied in order to predict such distributions. Comparison between simulated and measured results enabled an understanding of the fundamental mechanisms of plastic deformation of the machined surface of OFHC copper

Improvement of Cutting Forces Modeling Based on Oriented Cutting Tests

In order to predict the characteristics of the machined part, such as geometry, surface roughness and fatigue or corrosion resistance, the cutting forces values should be known as precisely as possible. The edge discretisation methodology can be used to model the three components of the cutting forces. The results are generally considered as suitable, even if the considered cutting operation is complex, because the geometry is well described. Usually, the local cutting forces model is identified from orthogonal or oblique cutting tests and the local contributions are assumed to be independent of the orientation of the elementary edge in the reference plane Pr. However, when turning in the tool nose or with round inserts, the tool cutting edge angle Kr (or Side Cutting Edge Angle) evolves along the active cutting edge and the values of this angle are very small compared to that of 90° used in orthogonal/oblique cutting. For this study, a new elementary cutting operation, called “oriented cutting”, has been tested. In this configuration, the active cutting edge is rectilinear, without inclination, but oriented by an angle Kr different from 90°. In addition, cylindrical turning tests have been done. The measurements, performed in pure copper, show an influence of the tool cutting edge angle on the cutting forces. An interaction between Kr and the workpiece radius is also highlighted