SECTION 2.6
The Mechanics of the Cutting Action

An analysis of the mechanics of cutting reveals a couple of interesting things about the shearing process responsible for chip formation.

For any given metal, cutting tool, and metal-removal rate (or depth of cut), the amount of heat produced in the shear zone depends essentially upon the size of the shear angle. If we think of the plastic flow, or shear, as taking place along a single plane, the shear angle is the angle between that plane and the direction of tool travel. If this shear angle is small, the plane of deformation runs a considerable distance ahead of the tool, causing the layer of metal removed to be deformed into a short, thick chip. This is severe cutting action, requiring lots of power, and generating considerable heat due simply to the total number of metal atoms being slid over each other. In addition, the built-up edge will be large and floppy resulting in a poor surface finish.

If the shear angle is large, however, the shear path will be short. This causes the built-up edge to be small and controlled. It has been proven that the size of the shear angle is controlled directly by the coefficient of friction between the sliding chip and the tool face. The lower the friction, the less drag the tool exerts on the chip, and the larger the shear angle. In other words, if the friction between the chip and the tool face is reduced by the introduction of a lubricating type cutting fluid, then the heat coming from the rubbing friction will be reduced. More importantly, the reduction in friction causes the shear angle to become larger, allowing the chips to slide easier and consequently reduce the heat coming from the plastic-type deformation, the major source of heat. This is, of course, assuming that machining speed is held constant.