|Chip Formation: Theory|
Below is illustrated the basic geometry of two-dimensional chip formation. The model is two-dimensional for simplicity.
The material immediately in front of the tool is bent upward and is compressed in a narrow zone of shear which is shaded on the drawing above. For most analyses, this shear area can be simplified to a plane.
As the tool moves forward, the material ahead of the tool passes through this shear plane. If the material is ductile, fracture will not occur and the chip will be in the form of a continuous ribbon. If the material is brittle, the chip will periodically fracture and separate chips will be formed. It is within the shear zone that gross deformation of the material takes place which allows the chips to be removed. As on the stress-strain diagram of a metal, the elastic deformation is followed by plastic deformation. The material ultimately must yield in shear.
The figure below depicts the cutting area in terms of lines of flow. As the material flows from the bulk of the work piece to the shear area, it is violently sheared, and then continues into the chip section.