On-machine wire electrical discharge dressing (WEDD) of metal-bonded grinding wheels

Metal-bonded diamond wheels, due to its strong grain retention and thermal conductivity properties, are generally used for grinding difficult-to-cut materials, such as high-performance ceramics. On the other hand, the poor dressability of this type of bond limits its application. This study aims to evaluate the use of a wire electrical discharge machining (EDM) principle for truing and dressing metal-bonded grinding wheels. Through the EDM process, the electrically conductive grinding bond is eroded, so that grain protrusion can be generated. For evaluating this dressing process, a wire electrical discharge dressing unit was designed, manufactured, and integrated into a universal cylindrical grinding machine. The dressing process is carried out using the grinding oil also as dielectric fluid. High material removal rates were achieved. Cylindrical plunge grinding tests on silicon nitride workpieces indicated that in comparison to conventionally dressed wheels, smaller cutting forces and wheel wear are achieved by using EDM-dressed grinding wheel

Abrasive waterjet machining of three-dimensional structures from bulk metallic glasses and comparison with other techniques

Bulk metallic glasses (BMGs) are a promising class of engineering materials, but they can be difficult to machine due to high hardness and a metastable structure. Crystallization due to machining can have negative effects, such as a decreased load-bearing capacity of fabricated parts, and thus should be avoided. Here, a Zr-based BMG was machined using abrasive waterjet (AWJ), electrical discharge, ns-pulsed laser engraving, and conventional dry-milling techniques. Characterization of the processed material indicated that AWJ preserves the amorphous phase and provides the combination of speed and flexibility required to rapidly fabricate small three-dimensional parts, while the other techniques did not achieve these goals. As proof-of-principle, a screw, similar to an orthopedic implant, was rapidly machined from the BMG using AW

Influência do sistema de alimentação de fluido lubri-refrigerante na retificação com rebolos de CBN

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-graduação em Engenharia Mecanica.O emprego de altas velocidades de corte no processo de retificação e a utilização de rebolos de CBN são tendências cada vez mais evidentes dentro do universo da usinagem. O processo de retificação caracteriza-se pela geração de grande quantidade de calor em função do atrito e deformações plásticas, onde a temperatura provocada pelo atrito aumenta com o aumento da velocidade de corte. O calor desenvolvido no processo é conduzido para a peça, para o grão abrasivo, para o ligante e para o meio, podendo acarretar danos metalúrgicos à peça e ser responsável por um desgaste prematuro do rebolo. Dentro deste contexto, o fluido lubri-refrigerante apresenta-se como um recurso indispensável para os processos abrasivos. A eficácia do fluido lubri-refrigerante depende de fatores, como a pressão, a vazão, o projeto da tubeira e assim também da velocidade de escoamento do fluido na saída da tubeira. Na retificação, devido às altas velocidades de corte empregadas, uma grande quantidade de ar é deslocada em torno da ferramenta rotativa, sendo formado um colchão de ar que reduz a eficiência da penetração do fluido na região de corte. Dentro deste contexto, diferentes concepções de alimentação de fluido vem sendo desenvolvidas, visto que uma otimização do processo pode ser alcançada quando uma maior quantidade de fluido é forçada para dentro da região de corte. Na indústria, é comum o emprego de tubeiras convencionais, como as de jato livre. No entanto, para otimizar o processo com este tipo de tubeira são necessárias altas pressões e velocidades de saída do fluido para vencer o colchão de ar, o que muitas vezes se torna economicamente inviável. Uma alternativa interessante é o emprego de tubeiras do tipo sapata. Com a sapata é possível aumentar o efeito de bombeamento do fluido para a região de corte, ao mesmo tempo em que é reduzido o efeito do colchão de ar. Isto ocorre porque a sapata é ajustada por retificação ao rebolo, funcionando como um raspador (scrapper), impedindo que o ar deslocado atinja a região de corte. Com o objetivo de avaliar a influência do sistema de alimentação de fluido lubri-refrigerante no processo de retificação, foram empregadas tubeiras dos tipos jato livre e sapata para diferentes condições de alimentação. Foram utilizados dois tipos de rebolos de CBN, sendo um com ligante resinóide e outro vitrificado. O material ensaiado foi um ferro fundido nodular com designação GGG70. Como fluido lubri-refrigerante foi empregada uma emulsão com 5% de óleo em água. Os resultados obtidos mostram que com o emprego da sapata é possível alcançar melhores resultados de desgaste radial do rebolo e qualidade da superfície retificada mesmo para menores vazões de fluido

Influence of Workpiece Circumferential Speed in Wire Electrical Discharge Machining

In Wire Electrical Discharge Machining (WEDM) a specific wire run-off speed is applied to compensate wear and avoid wire breakage. Since the workpiece generally stays stationary and short discharge durations are applied, the relative displacement between wire and workpiece during one single discharge is very small. In Wire Electrical Discharge Dressing (WEDD), however, the workpiece rotates at very high circumferential speeds, since dressing is usually performed at grinding speeds. In this work the influence of relative speed in WEDM is assessed. Single discharge experiments were first carried out using circumferential speeds up to 80 m/s. The plasma channel was found to easily slide over the anode, creating elongated craters, and higher removal per single discharge was measured for higher relative speeds. A thermo-electrical model was used to help understand the process, showing that higher melting efficiencies can be reach as relative speeds are increased. The model was also used to predict damages to diamonds grains used in metal bonded grinding wheels. It was found that even if the plasma channel slides over a diamond due to the relative speed, graphitization is not likely to occur.ISSN:2212-827

Applying Wire Electrical Discharge Dressing (WEDD) to Improve Grinding Performance of Metal Bounded Diamond Wheels

In order to improve grinding performance of metal-bonded diamond wheels, a proper conditioning method has to be used. In this work, wire electrical discharge dressing (WEDD) is evaluated. With a self-designed WEDD-device the dressing process can be carried out inside a grinding machine, reducing non-productive time. Moreover, the in-process WED-dressing was assessed, showing potential for future applications. Dressing experiments indicated that high erosion material removal rates can be achieved and, in comparison to conventionally conditioned wheels, a better grinding wheel topography is generated. Finally, a model to calculate the axial dressing feed rate in WED-sharpening is proposed.ISSN:2212-827

Modeling and simulation of electrical discharge machining

Electrical discharge machining involves complex physical processes occurring simultaneously, which lead to the removal of material from both electrodes. During the discharge, material melts, evaporates and is ejected when the plasma channel collapses. Modeling the erosion process can help to better interpret and predict results, which are otherwise difficult to understand. In this work, the material removal rate is predicted based on the simulation of single discharges. It was found that a better correlation with experimental results is achieved when the latent heats of fusion and evaporation are taken into consideration, as well as temperature-dependent thermo-physical properties of the workpiece.ISSN:2212-827

MHD printhead for additive manufacturing of metals

Within the scope of additive manufacturing of metal parts, a droplet generator for metal melts is proposed. Its magnetohydrodynamic (MHD) working principle is shown and the actuation force available for droplet generation is calculated. The design of such a printhead is shown to consist only of very few, non-moving parts and therefore it is of great simplicity. First results gained with a prototype printhead show that successful droplet generation is possible, but that pollution of the melt with oxides and sparks developing during the droplet generation process have to be overcome to fully exploit the potential of the proposed print head.ISSN:2212-827