Rake Face Action of Cutting Lubricants: An Analysis of, and Experiments on, the Machining of Iron Lubricated by Carbon Tetrachloride

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A number of suggestions are current concerning the rake face action of cutting lubricants; that they reduce the shear stress between chip and tool uniformly over the whole chip–tool contact area; that they reduce the contact length between chip and tool; and that they induce conditions on the rake face of the tool in which the interfacial shear stress between chip and tool decreases rapidly with distance from the cutting edge of the tool. The present work describes experiments on the dry and lubricated orthogonal machining of high permeability iron which have enabled it to be decided which of these theories is correct. The workpiece flow pattern in lubricated machining has been recorded with sufficient accuracy to enable the slip-line field for the flow to be calculated. The shear-stress distribution along the plastic part of the chip-tool contact length has been deduced from the angles at which the slip lines meet the rake face. It has been found that even when the workpiece was flooded with carbon tetrachloride, sticking friction occurred between the chip and tool near the cutting edge of the tool, although good lubrication occurred in the region where the chip left the tool. It has therefore been concluded that the lubricant acted to induce a steep interfacial shear-stress gradient along the chip–tool contact length. The cause of the shear-stress gradient along the rake face in lubricated cutting is discussed in terms of the varying conformity between the chip and tool along the chip–tool contact length. Near the cutting edge of the tool, gross plastic deformation of the workpiece caused the chip to flow into intimate contact with the rake face of the tool, denying lubricant a path along the rake face to the cutting edge. A theory of the mechanics of orthogonal cutting is presented which explains how the shear-stress gradient along the chip–tool contact length causes the characteristic features of lubricated machining, namely the formation of thin and, in particular, tightly curled chips with a small area of contact with the tool rake face