Contact zone analysis based on multidexel workpiece model and detailed tool geometry representation

A new method for analyzing the tool-workpiece-contact area in cutting processes is presented. To gain enhanced knowledge about tool-workpiece interaction, determination of chip thickness, contact length and resulting cross-section area of the undeformed chip is of major interest. Compared to common simulation approaches, where rotation-symmetrically constructed tool geometry is used, the new method uses a detailed three dimensional tool shape model for an extended and more accurate contact zone analysis. As a corresponding representation of the workpiece and its time dependent shape-changes a multidexel model is used. To prepare the geometric tool model, the contained BREP topology is built up within the simulation system using data from a STEP-file. First of all functional parts of the tool like rake and flank faces and cutting edges are labeled for further processing. In a second step the identified NURBS-faces are discretized for the application in material-removal calculation. This way a mesh is built-up based on triangle elements which maps the geometry of each cutting edge into a 2D parametric representation. In relation to rake face, each node is described by its position on the cutting edge and its perpendicular distance to this edge. To perform contact zone analysis each cutting geometry and a multidexel model are intersected in discrete time steps corresponding to a tool rotation of about three degrees. The intersection point of each dexel and the cutting geometry is calculated. Parametric cutting geometry allows for a direct computation of local cutting depth and contact length for each involved point. Based on the local values of contact length and cross section area of the undeformed chip the characteristic values for the entire contact zone are calculated and used to predict mechanical as well as thermal loads caused by the cutting process. To demonstrate the application of the novel approach, prediction of forces in slot milling of 1.1191 steel is presented.DFG/PP/148

Influence of Machining Parameters on Heat Generation during Milling of Aluminum Alloys

Thin-walled components, i.e. fuselage frames of airplanes, are prone to an unstable process behavior during milling. Therefore, tools with a chamfer between the cutting edge and the flank face are often used for such machining tasks. During milling, the chamfered area comes into contact with the just cut surface. This contact leads to process damping forces and the induced heat into the workpiece in this contact zone is increased. Furthermore, the amount of induced heat depends on the process parameters. At certain spots on the machined surface this may lead to a local overheating, which can reduce stiffness significantly. When this occurs during milling of a thin-walled component, the component is often regarded as reject. In this paper, the influence of chamfers and process parameters on the induced heat into the workpiece is investigated experimentally. In addition, a simulation which predict the temperature in the workpiece in dependence of the process parameters is presented.Ministry of Economics, Labour and Transport of Lower Saxony/ZW3-80134969DFG/DE 447/90-

Modeling a thermomechanical NC-simulation

This paper presents a method for a NC-Simulation based prediction of shape errors caused by thermal expansions in machining of complex workpieces. In the first part of the paper the basic approach of modeling a thermomechanical NC-Simulation for a faster and more precise process simulation is shown. Therefore, a fast dexel based material removal simulation including process models for calculation of localized heat flux and forces is linked to a FE model for simulation of thermal conduction in the workpiece. Interdependencies of thermal process and workpiece conditions are considered by a closed simulation loop. In the second part of the paper the modeling of each component is explained. To consider thermomechanical effects in material removal simulation the dexel based workpiece model is extended by additional information like temperature and deformation in every dexel. An inverse projection of the workpiece deformation on a triangulated tool model allows consideration this effect by deformation of the tool model. Thereby, a realistic shape of the workpiece can be simulated. In addition, the current cutting conditions like area of undeformed chip-thickness or contact length are changed. This results in diversified cutting forces and heat fluxes. For a realistic simulation of the thermal conduction the dimensions of the FE model have to be adapted by a time dependent virtual domain method. In the last part of the paper, results of the simulation are compared to measured data. The comparison shows that process temperatures in different workpiece areas are predicted accurately

Prediction of temperature induced shape deviations in dry milling

In this paper a model for a simulation based prediction of temperature induced shape deviations in dry milling is presented. A closed loop between Boolean material removal, process forces, heat flux and thermoelastic deformation is established. Therefore, an efficient dexel based machining simulation is extended by a contact zone analysis to model the local workpiece load. Based on the computed contact zone the cutting forces and heat flux are calculated using a semi-empirical process model. For a detailed consideration of the loads they are discretized and localized on the dexel-represented workpiece surface. A projection of the localized workpiece loads on the boundary of the finite element domain, taking into account the Boolean material removal during the process, allows the calculation of the current temperature and deformation of the workpiece. By transforming these thermomechanical characteristics back to the dexel-model a consideration in the machining simulation is possible. An extended contact zone analysis is developed for the prediction of the localized shape deviations. Finally, the results of the simulation are compared with measured data. The comparison shows that workpiece temperatures, workpiece deformation and shape deviations in different workpiece areas are predicted accurately.DFG/DE 447/90-2DFG/MA 1657/21-

Inverse determination of constitutive equations and cutting force modelling for complex tools using oxley's predictive machining theory

In analysis of machining processes, finite element analysis is widely used to predict forces, stress distributions, temperatures and chip formation. However, constitutive models are not always available and simulation of cutting processes with complex tool geometries can lead to extensive computation time. This article presents an approach to determine constitutive parameters of the Johnson-Cook's flow stress model by inverse modelling as well as a methodology to predict process forces and temperatures for complex three-dimensional tools using Oxley's machining theory. In the first part of this study, an analytically based computer code combined with a particle swarm optimization (PSO) algorithm is used to identify constitutive models for 70MnVS4 and an aluminium-alloyed ultra-high-carbon steel (UHC-steel) from orthogonal milling experiments. In the second part, Oxley's predictive machining theory is coupled with a multi-dexel based material removal model. Contact zone information (width of cut, undeformed chip thickness, rake angle and cutting speed) are calculated for incremental segments on the cutting edge and used as input parameters for force and temperature calculations. Subsequently, process forces are predicted for machining using the inverse determined constitutive models and compared to actual force measurements. The suggested methodology has advantages regarding the computation time compared to finite element analyses.BMBF/02PN205