Automatic polishing process of plastic injection molds on a 5-axis milling center

The plastic injection mold manufacturing process includes polishing operations when surface roughness is critical or mirror effect is required to produce transparent parts. This polishing operation is mainly carried out manually by skilled workers of subcontractor companies. In this paper, we propose an automatic polishing technique on a 5-axis milling center in order to use the same means of production from machining to polishing and reduce the costs. We develop special algorithms to compute 5-axis cutter locations on free-form cavities in order to imitate the skills of the workers. These are based on both filling curves and trochoidal curves. The polishing force is ensured by the compliance of the passive tool itself and set-up by calibration between displacement and force based on a force sensor. The compliance of the tool helps to avoid kinematical error effects on the part during 5-axis tool movements. The effectiveness of the method in terms of the surface roughness quality and the simplicity of implementation is shown through experiments on a 5-axis machining center with a rotary and tilt table

Aide au choix du posage en usinage 5 axes continus par la modélisation du comportement cinématique des machines-outils

Ces travaux s'inscrivent dans le cadre de l'amélioration de la productivité en usinage 5 axes continus. La modélisation d'un centre d'usinage et de sa commande numérique est proposée en 3 axes et 5 axes continus. Un simulateur est alors réalisé. Il permet de prévoir la vitesse de déplacement sur chacun des axes et de modifier les vitesses de consigne afin de mieux respecter les besoins spécifiés par le programmeur FAO. De plus les sources de perte de productivité du processus sont identifiées. Dans le but de réduire le temps d'usinage, une optimisation de l'orientation et du positionnement de la pièce dans la machine est donc proposée. Pour cela des cartographies en couleur sont réalisées. Elles permettent à un opérateur de choisir aisément un posage optimisé. L'ensemble des modélisations et des optimisations sont validées expérimentalement. Un choix optimal de posage associé à une modification de la vitesse au cours du programme permettent de réduire de plus de 50% le temps d'usinage.These works deal with productivity improvement in 5-axis machining. A machine tool and its computed numerically control are modeled in continuous 3 and 5 axis machining. A simulator is then performed. It predicts the speed on each axis and can change the programmed feedrate to fit with the specifications of the CAM programmer. Besides, the productivity of the process is improved. In order to reduce the machining time, orientation and positioning of the workpiece in the machine are optimized and the results are summarized in colored maps. This tool allows the operator to choose easily an optimized setup. Both modeling and optimization are validated experimentally. The optimal choice of part setup associated with a change of speed during the program can reduce more than 50% the machining time

Kinematic modelling of a 3-axis NC machine tool in linear and circular interpolation

Machining time is a major performance criterion when it comes to high-speed machining. CAM software can help in estimating that time for a given strategy. But in practice, CAM-programmed feed rates are rarely achieved, especially where complex surface finishing is concerned. This means that machining time forecasts are often more than one step removed from reality. The reason behind this is that CAM routines do not take either the dynamic performances of the machines or their specific machining tolerances into account. The present article seeks to improve simulation of high-speed NC machine dynamic behaviour and machining time prediction, offering two models. The first contributes through enhanced simulation of three-axis paths in linear and circular interpolation, taking high-speed machine accelerations and jerks into account. The second model allows transition passages between blocks to be integrated in the simulation by adding in a polynomial transition path that caters for the true machining environment tolerances. Models are based on respect for path monitoring. Experimental validation shows the contribution of polynomial modelling of the transition passage due to the absence of a leap in acceleration. Simulation error on the machining time prediction remains below 1%

POLISSAGE AUTOMATIQUE DE MOULES D'INJECTION PLASTIQUE SUR CENTRE D'USINAGE 5 AXES

International audienceRésumé : Dans l'industrie de la fabrication des moules pour l'injection plastique, la réalisation de surfaces «poli miroir» est indispensable pour obtenir des pièces transparentes ou des surfaces dépourvues de rayures. Traditionnellement réalisé manuellement, nous proposons d'automatiser le polissage sur centre d'usinage 5 axes, moyen maintenant répandu chez les fabricants de moules. Notre méthode s'appuie sur l'utilisation d'outils classiquement utilisés pour le polissage manuel. D'autre part l'expérience des polisseurs nous permet de développer des trajectoires dédiées, basées sur l'utilisation de courbes fractales et de courbes trochoïdales. La maîtrise de l'effort de polissage est assurée par la mesure des jauges outils en effort. Notre procédé a été testé sur centre de fraisage 5 axes UGV Mikron UCP 710. Enfin la mesure des états de surface nous permet de comparer les écarts entre deux pièces polies automatiquement et une pièce polie manuellement. Abstract: In the plastic injection mold industry, "mirror-effect" polished surfaces are required for obtaining transparent parts or surfaces without scratches. Traditionally done manually, we propose to automate polishing on 5-axis machining center now available in mold manufacturers' workshops. Our method relies on the use of tools typically used for manual polishing. Furthermore, the experience of polishers allows us to develop dedicated trajectories based on the use of fractal and trochoidal curves. Polishing force control is done by measuring the tools load regarding its length parameter and its orientation. Our process has been tested on a 5-axis milling center Mikron UCP 710. Finally surface measurements allow us to compare the differences between two automatically polished parts and a manually polished one

Simulation of the behaviour of CNC machine-tools in three-axis milling of sculptured surfaces

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Study of polynomial path parameterisation for side milling of sculptured surfaces

International audienceThe present article focuses on side milling of wooden workpieces on a five-axis NC machine tool. Firstly, a path comprising points and axes is sensed on a manually produced prototype. To control the machine continuously, the path on the machine has to be programmed using a polynomial curve. The influence of parameterisation associated with a scatter pattern of points over the continuity of feed rates is studied. In order to avoid sudden variations in speeds that can lead to burning on the wooden workpieces, the relation to be respected between parameterisation and the tangents associated with interpolation points is demonstrated

Optimisation of workpiece setup for continuous five-axis milling: application to a five-axis BC type machining centre

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The precautionary approach and government action

International audienceCAM programs can generate cutting tool paths to be used by machining centres. Experience shows that CAM programmed feed rates are rarely achieved in practice during machining, especially when finishing free-form surfaces. These slower feed rates are due to the machines’ kinematic capabilities and behaviour of the numerical control (NC). To improve control over the machining process, applications need to be developed to predict the kinematic behaviour of the machines, taking the mechanical characteristics of the axes and NC capacities into account. Various models to simulate tool paths in linear and circular interpolation have been developed and are available in the literature. The present publication will first focus on the use of the polynomial model to simulate the behaviour of the machine when passing through transitions between programmed blocks with tangency discontinuities. Additional features are proposed to ensure enhancement of the match between the model and the machine’s behaviour. Analysis of machine behaviour shows that NCs do not always allow the axes to reach maximum performance levels, with an attendant loss in productivity. The present article proposes an optimisation procedure allowing control laws to be defined to reduce time spent in the transition. The contributions made by these optimised control laws are then evaluated, while impediments to their implementation are also considered