Analysis and modeling of green wood milling: Chip production by slabber

During the primary transformation of wood, logs are faced with slabber heads. Chips produced are raw materials for pulp paper and particleboard industries. Efficiency of these industries is partly due to particle size distribution. Command of this distribution is no easy matter because of great dependence on cutting conditions and variability in material. This study aimed a better understanding and predictionof chip fragmentation. It starts with a detailed description of cutting kinematic and interaction between knife and log. This leads to the numerical development of a generic slabber head. Chip fragmentation phenomena were studied through experiments in dynamic conditions. These experiments were carried out thanks to a pendulum (Vc = 400 m/min). It was instrumented with piezoelectric force sensors and high speed camera. Obtained results agreed very well with previous quasi-static experiments

Simulation of green wood milling with discrete element method

This work was carried out in LaBoMaP and PIMM at Arts et Metiers ParisTech. We acknowledge I2M laboratory for their technical support in Discret Element Method; Robert Collet, Louis Denaud, Guillaume Pot; PIMM and LaBoMaP technicians and Morgane Pfeiffer-Laplaud for their availability and advice.During the primary transformation in wood industry, logs are faced with conical rough milling cutters commonly named slabber or canter heads. Chips produced consist of raw materials for pulp paper and particleboard industries. The process efficiency of these industries partly comes from particle size distribution. However, chips formation is greatly dependent on milling conditions and material variability. Numerical simulation of chip fragmentation can allow some useful chip thickness prediction. In this complex situation in wood cutting, the utilization of the Discrete Element Method (DEM) is relevant. In this method, solids are modeled with spherical discrete elements linked by cohesive bonds. However the Discrete Element Method requires a previous calibration step with simple mechanical loading. For example the nature and the mechanical properties of the cohesive bonds must be determined. After an analysis of the different mechanical loadings in green wood milling, a complete study of green wood compression is carried out. This experimental study covers the strain rates range of 10-3 to 103 s-1 using a hydraulic compression machine and the Split Hopkinson Pressure Bar technique. Wood specimens at different moisture content states are compressed longitudinally. This study enables us to observe the viscoelastic and hygroscopic behaviour of wood. The experimental and qualitative simulation results show that elastic brittle beams are not well adapted to be used in quantitative green wood milling simulations

Analyse et modélisation du fraisage du bois vert.

In sawmills, logs are faced with conical rough milling cutters (slabber or canter heads). The chips produced during this operation are segmented in small chips, consisting of raw material for different sectors (pulp paper, particleboards and energy). Their valuation of paper pulp, which is the most remunerative outlet for these related products, requires a respect of defined particle size. This dimensional control requires a better knowledge of the chip fragmentation mechanisms. To this end, we conducted an experimental campaign of orthogonal cutting for green wood in order to study the influential parameters on chips dimensions and to define good chipping areas. The results of this study will be used as indicators to compare numerical simulations of fragmentation with reality. To help machine tool designers to better design their equipment, we have developed a fully customizable slabber. With this model, we studied the effect of slabber geometric and kinematic parameters on certain key variables in machining. Numerical simulation of the fragmentation of green wood chips should help to predict the size of chips formed. To this aim it is necessary to use a method taking into account the specificities of wood cutting. Here the relevance of the Discrete Elements Method (DEM) is studied. Finally to achieve quantitative simulations, the behavior of green wood in compression under dynamic conditions is studied.En scierie, les billons sont surfacés à l’aide de têtes de fraisage (slabber ou canter). Le copeau formé durant cette opération d’usinage se fragmente en particules nommées plaquettes, destinées à alimenter différentes filières (pâte à papier, panneaux et énergie). Leur valorisation en pâte à papier, qui représente le débouché le plus rémunérateur pour ces produits connexes, impose de respecter une granulométrie bien définie. Cette maîtrise dimensionnelle passe par une meilleure connaissance des mécanismes de fragmentation du copeau. Dans cet objectif, nous avons réalisé une campagne d’essais de coupe orthogonale sur bois vert afin d’étudier les paramètres influents sur les dimensions des plaquettes et de définir des zones de bonne fragmentation. Les résultats de cette étude seront utilisés comme indicateurs pour comparer les simulations numériques de la fragmentation avec la réalité. Afin d’aider les concepteurs de machine-outil à mieux concevoir leur outillage, nous avons développé un slabber entièrement paramétrable. Grâce à ce modèle, nous avons pu étudier l’effet des paramètres géométriques et cinématiques des slabbers sur certaines grandeurs essentielles en usinage. La simulation numérique de la fragmentation du copeau de bois vert doit permettre de prédire les dimensions des plaquettes formées. Pour cela, il est nécessaire de d’utiliser une méthode prenant en compte les spécificités de la coupe du bois. Ici la pertinence de la Méthode des Eléments Discrets (DEM) est étudiée. Enfin pour réaliser des simulations quantitatives, le comportement du bois vert en compression dans des conditions dynamiques est étudié

Green wood dynamic behaviour

La simulation numérique de l’opération de fraisage des grumes par slabber doit permettre de prédire les dimensions des plaquettes produites. Pour prendre en compte les spécificités de la coupe du bois. La Méthode des Eléments Discrets (DEM) a été utilisée. Cette méthode, basée sur le principe fondamental de la dynamique, présente l’avantage de bien gérer les contacts 3D et les fissurations. Cependant celle-ci nécessite une étape de calibration. L’étape de calibration vise à déterminer un jeu de paramètres du modèle DEM qui permet de reproduire le comportement du matériau voulu sous un chargement mécanique simple. Ici la principale difficulté réside dans le manque de bases de données et de modèles pour le comportement mécanique du bois vert à fort taux de déformation. La bibliographie montre deux types d’essais : les essais normalisés et les essais dynamiques. Les essais normalisés [ASTM D143] sont réalisés avec des éprouvettes normalisées à faibles vitesses de déformation sur bois sec ou au point de saturation des fibres. De nombreuses bases de données rassemblent leurs résultats. Les essais d’impact [Adalian 02], plus rares sont souvent réalisés sur bois sec, quelques fois sur bois saturé. Cependant il n’existe pas de bases de données relatives à ces essais. L’objectif est donc de réaliser des essais mécaniques sur bois vert permettant l’obtention d’abaques. Celles-ci étant nécessaires à la calibration du modèle DEM.Allocation Spécifique Normalie

Analysis of chip formation mechanisms and modelling of slabber process

During the primary transformation in wood industry, logs are faced with conical rough milling cutters commonly named slabber or canter heads. Chips produced consist of raw materials for pulp paper and particleboard industries. The process efficiency of these industries partly comes from particle size distribution. However, chips formation is greatly dependent on milling conditions and material variability. Thus, this study aims at better understanding and predicting chips production in wood milling. The different mechanisms of their formation are studied through orthogonal cutting experiments at high cutting speed for beech and Douglas fir. Within these conditions, ejection of free water inside wood can be observed during fragmentation, particularly on beech. As previously seen in quasi-static experiments, chip thickness is proportional to the nominal cut thickness. Moreover, the grain orientation has a great influence on the cutting mechanisms, so as the nominal cut and the grows rings widths. This chip fragmentation study finally allows the improvement of the cutting conditions in rough milling. In order to optimize machine design as well as cutting geometry, a geometrical model of a generic slabber head is developed. This model allows the study of the effective cutting kinematics, the log-cutting edges interactions and the effective wood grain direction during cutting. This paper describes the great influence of the carriage position on cutting conditions. The results obtained here can be directly used by milling machine manufacturers.Allocation Spécifique Normalie

Analysis of green wood chip formation mechanisms at high cutting speed

During the primary transformation in wood industry, logs are faced with conical rough milling cutters commonly named slabber or canter heads. Chips produced consist of raw materials for pulp paper and particleboard industries. The process efficiency of these industries partly comes from particle size distribution. However, chips formation is greatly dependent on milling conditions and material variability. Thus, this study aims at better understanding and predicting chips production in wood milling. The different mechanisms of their formation are studied through orthogonal cutting experiments at high cutting speed. Chipping observations are carried out on a Chardin's pendulum. This experimental setup, similar to a Charpy's pendulum, was designed to measure cutting forces in sawing. A piezoelectric force transducer records the cutting forces. It is synchronized with a high speed camera to observe the formation of chips. In this experimental campaign, wood specimens are machined with fresh and saturated beech and Douglas fir. Within these conditions, ejection of free water inside wood can be observed during fragmentation, particularly on saturated beech. As previously seen in quasi-static experiments, chip thickness is proportional to the nominal cut thickness. Moreover, the grain orientation has a great influence on the cutting mechanisms, so as the nominal cut and the grow circles widths. Digital image correlation is carried out in order to observe the strains fields during the cut for different cutting mechanisms. This chip fragmentation study finally allows the improvement of the cutting conditions in rough milling for slabber manufacturers