The Mechanism Of Material Removal In Cutting Graphite

Graphite is a good conductor of electricity with excellent chemical stability as well.Graphite and the finished products of graphite have the properties of high temperatureresistance and high strength, and graphite after special processing has the features of corrosiveresistance, good thermal conductivity and low penetration rate, and graphite after immersiontreatment has rather stronger gas barrier property. The unique properties of graphite make itmore and more widely applied, playing increasingly greater roles in industrial production. Asa new type of power generation technology of high efficient and environment protection, thefuel-cell power generation technology has drawn universal attentions all over the world. Thebipolar plate of the fuel-cell widely employs graphite bipolar plate, which has shown itsfavorable performance and advantages incomparable by other materials. And as a kind ofelectrode material, graphite also gradually replaces the copper electrode, becoming themainstream of electrode materials by electrical discharge machining. However, as a typicalfragile material, the graphite is hard to be machined, as the impact of cutters on graphite maycauses the graphite edge and corner collapse, and the machined surface would be hackythree-dimensional crushing surface with surface defects irregularly distributed and pits ofunequal sizes. Thus in the machining process such as the flow pass of graphite bipolar platesas well as precision and complicated graphite electrode, there would be low finished productsrate, quite long processing cycle, which is time and labor consuming. The graphite materialitself is of low cost, while the machining cost is quite high, which restricts the applications ofgraphite material, for example, the cost of graphite bipolar plate takes up about50%~60%ofthe total cost of the fuel-cell stack. To sum up, it has theoretical guiding significance on thepromotion of graphite material application and the improvement of graphite processingtechnology level to carry out the researches on the cutting material removal mechanism ofgraphite, moreover, the reflection of graphite’s special mechanical properties in machininghas very high academic research value.With regards to the problems of edge collapse, surface fracture and pit producing, as wellas the lack of theoretical guidance for choosing cutter parameters and cutting parameters ingraphite cutting, based on the ANSYS Workbench software, this article simulates the equivalent stress in graphite orthogonal cutting with different cutting thickness, and employsthe experiment of side coining the graphite plate with monocline pressure head to makequalitative and quantitative researches on the crack producing and expanding rules in graphitecoining with different coining thickness, and finally compares the coining experiment resultwith the simulation result to verify the accuracy of the simulation. It builds the process modelof graphite cutting, analyses the processed surface formation in orthogonal cutting of graphite,further studies the influences of cutter parameters and cutting parameters of graphiteorthogonal cutting on the processed surface quality, and optimizes the processing technicalparameters of graphite cutting. It uses the crack producing and expanding rules in graphiteside coining to explain the material removal in graphite cutting and the influence of differentcutter front angles on the processed surface quality. Meanwhile, it uses the stress distributionin graphite steady state cutting to explain the influence of different cutter front angles on theprocessed surface quality. Finally, it supervises and tests the cutting force in cutting process,and combined the cutting force analysis with the processed surface formation in graphitecutting to calculate the friction coefficient of the rake face of different front angle cutters incutting process.Based on ANSYS Workbench, we simulate the stress distribution in graphite steady statecutting, obtaining the equivalent stress distribution cloud chart of steady state orthogonalgraphite cutting with cutting depth of0.4mm by different front angle cutters and withdifferent cutting depth by front angle cutters of20°,0°,-20°. We judge the crack producingand expanding according to the equivalent stress distribution cloud chart, and measure thedepth and width of the breakdown pits, obtaining the width and depth changing rules undersimulated condition, and the results show that it is basically agree with the rules obtained inthe experiment of graphite orthogonal cutting and side coining.The conclusions obtained from the experiment of graphite side coining by monoclinepressure head mainly are:(1) At the initial loading stage, there is no crack produced on thetest-piece surface, with the increasing of loading, there is a tiny cut produced at the contactpoint of the test-piece surface with the pressure head.(2) When the load is increased to acritical value, the crack occurs suddenly and expands rapidly.(3) The initial crack expandstowards the test-piece inside and then towards the coining surface gradually, with a crack expanding path of circular arc.(4) With the increase of the wedge angle of the pressure head,the initial expanding angle of the crack increases linearly, and the crack expands to thetest-piece inside more, but the initial expanding angle of the crack has little relationship withthe coining thickness.(5) With the increase of the wedge angle of the pressure head or thecoining thickness, the pit width and depth formed on the test-piece surface increase linearly,and the increasing rate of the pit width is bigger than that of the pit depth.(6) When both thewedge angle of the pressure head and the coining thickness are relatively smaller, the crackexpands hardly towards the inside of the test-piece.The conclusions obtained from the experiment of graphite orthogonal cutting mainly are:(1)With the increase of cutting depth, the pit increases in number and grows in size, so doesthe edge collapse, the corner collapse is on as rising curve, the surface fracture rate increasesand the quality of the processed surface decreases.(2) In positive front angle cutters, with theincrease of the front angle, the processed surface quality becomes better. While in negativefront angle cutters, with the increase of the negative front angle of the cutter, the processedsurface quality becomes better, too. And the surface quality processed by cutters with a frontangle of0°is almost same with that of-5°,5°.(3) If the cutting speed increases, the roughnessof both surfaces processed by cutters with an front angle of positive20°and negative20°areon a decline curve, but the roughness reduction rate of the surface processed by cutters with afront angle of negative20°is relatively low, while with the increase of cutting speed, theroughness reduction rate of the surface processed by cutter with a front angle of positive20°is obviously bigger than that of negative20°, but it is not very big, the change of cutting speeddoesn’t greatly affect the quality of both surfaces processed by cutters with a front angle ofpositive20°and negative20°.(4) By employing relatively higher cutting speed, it can improvethe cutting processing efficiency remarkably, without affecting the quality of the cuttingprocessed surface; and by employing cutters with positive and negative front angle of a wideangle (no bigger than negative20°), it can improve the quality of the cutting processedsurface.The conclusions obtained from the experiment of graphite orthogonal cutting forcemeasurement mainly are:(1) The cutting force in graphite orthogonal cutting is equally highfrequency oscillating force, with periodic fluctuation features, and it looks like a zigzag shape. (2) When cutting by cutters with a front angle of positive20°, with the increase of cuttingspeed, the average horizontal cutting force is on a decline curve, but with small decliningrange, while the maximum horizontal cutting force is on a rising curve; the average verticalcutting force doesn’t change greatly, while the maximum vertical cutting force is on a risingcurve.(3) When cutting by cutter with a front angle of0°, both the average horizontalcomponent force and the average vertical component force grow with the increase of thecutting speed, the average vertical component force doesn’t grow by a large rate, but growlinearly. The maximum average horizontal component force changes in no regularity with theincrease of cutting speed, while the maximum vertical component force increases with theincrease of cutting speed linearly, but by a relatively small rate.（4）When cutting by cutterswith an front angle of negative20°, the average horizontal component force reduces with theincrease of cutting speed, while the average vertical component force changes in no regularity.Both the maximum horizontal component force and the maximum vertical component forcechange in no regularity with the increase of cutting speed.(5) The relationship between thefront angle of cutter and the cutting force is: the average horizontal component force is on alinear declining curve with the increase of the cutter front angle, while the average verticalcomponent force decreases in oscillation with the increase of the cutter front angle. And theaverage horizontal component force decreases by a bigger rate. The maximum horizontalcomponent force is on a slowly rising curve when the cutter front angle is-20°,-15°,-10°, itincreases by a relatively small rate, and is almost on a linear declining curve with the increaseof the cutter front angle afterwards. The maximum vertical component force also decreases inoscillation with the increase of the cutter front angle.