Retificação plana de aços para moldes e matrizes em várias condições de corte e diferentes técnicas de aplicação de fluido de corte

Abstract

One of the greatest challenges in grinding with conventional abrasives is to prevent the heat generated in the process cause damage in the work piece. It has been reported that about 85% of heat generated is during grinding be transferred to the workpiece because of low thermal conductivity of conventional abrasives and also the small dimensions of chips. A possible solution for this problem is to find an efficient combination of the cutting parameters. In this sense, this research investigated the peripheral surface grinding of three steels employed in the manufacture of molds and dies (ABNT VP100®, VP ATLAS® and N2711M) under different grinding conditions. Two grinding wheel materials, two different coolant delivery techniques were used for the cutting fluid (conventional and minimum quantity of lubrication-MQL), different flow rates of cutting fluid via the MQL technique and three values of equivalent chip thickness were employed. The surface roughness (Ra), microhardness and residual stresses of the workpiece material, as well as, the instantaneous electrical power of the grinding process were the output parameters investigated. Images of the machined surfaces were obtained by Scanning Electron Microscope to access the surface texture and to make comparison among the steels tested. The results showed that, in terms of finishing, the roughness increased with the equivalent chip thickness in the most tested conditions for the three steel materials. The best finishing was obtained by N2711M steel after grinding with the following parameters: lower equivalent chip thickness, alumina grinding wheel and MQL technique at a flow rate of 60 mL / h. With regard the finishing of VP ATLAS steel after grinding with alumina grinding wheel, the MQL technique was as efficient as the conventional coolant technique when grinding in the most severe conditions, irrespective of the flow rate used. The best results were obtained after the grinding with the MQL technique with lower flow rate, 60 mL / h, as well as for the N2711M steel. In relation of the surface finishing of the VP100 steel after grinding with alumina grinding wheel, the roughness increased with the equivalent chip thickness. For this steel grade, the MQL technique also outperformed conventional coolant technique in terms of roughness, however when using the highest flow rate of 240 mL / h. When using the silicon carbide (SiC) grinding wheel, the best surface finishing results were obtained for the VP ATLAS steel with the MQL technique at flow rate of 240 mL/h and, in general, the values obtained with the MQL technique were lower than those obtained for the conventional coolant technique. Machining with the SiC grinding wheel and the MQL technique at a flow rate of 60 mL/h generated the best surface finishing for N2711M and VP ATLAS steel grades at the more severe grinding conditions. With regard of microhardness values, they were lower than the reference value for most of steels in regions very close to the ground surface for the three steels materials, irrespective of the grinding wheel tested, in most of the conditions investigated. However, an increase in hardness next to surface were detected after machining VP100 steel with the conventional coolant technique. The lowest variation in the microhardness values for the three steels was obtained after grinding with alumina grinding wheel with the MQL technique at a flow rate of 150 mL / h. No increase in hardness was observed near the machined surface. No difference between performance of the conventional and MQL coolant delivery techniques was observed in terms of the steels topographies obtained by SEM and as function of the equivalent chip thickness. With regard the residual stresses, machining with the combination of the alumina grinding wheel and the MQL technique at the lower flow rate resulted in compression stress, independent on the equivalent chip thickness value for N2711M steel after machining with alumina grinding wheel. Similar behavior was also observed after machining the N2711M and VP100 steels with the conventional and MQL coolant delivery techniques at the higher flow rate, which is worth. The instantaneous electrical power increased with the cut chip thickness for all steels. The machining of the N2711M steel required the lowest electrical power during the tests, regardless of the grinding wheel employed. In general, the N2711M steel presented the best machinability among the steels tested based on the variables investigated, while the VP100 steel showed the worst machinability after machining with alumina grinding wheel under the investigated conditions. The machinability of VP100 steel was improved in terms of roughness, residual stresses and microhardness when using the MQL technique with the flow rate of 240 mL /h.