The main reasons for the residual stress in the surface layer of CNC machine tools are:
1. Cold plastic deformation. Numerical control machine tools under the action of cutting force, the surface metal produces plastic deformation, the surface produces tensile deformation along the direction of cutting speed, the metal character is elongated, while the base metal is in the state of elastic deformation. When the cutting force is eliminated, the base metal tends to recover, but is hindered by the surface metal, resulting in residual compressive stress in the surface layer and residual tensile stress in the inner layer.
2. Hot plastic deformation. Under the action of cutting heat, the local temperature rise of the workpiece surface is too high, while the substrate temperature is relatively low, so that the thermal expansion of the workpiece surface is hindered and a large thermal stress is generated on the surface.
When the cutting process is over, the surface temperature decreases, and its cooling shrinkage is limited by the substrate, resulting in tensile stress on the surface. The higher the cutting temperature of CNC machine tools, the greater the thermal deformation, the greater the residual tensile stress, and sometimes even cracks appear on the surface of the workpiece.
3. Metallographic structure changes. The high temperature produced by the cutting heat of CNC machine tools will cause the metallographic structure changes of the surface layer. Because different metallographic structures have different densities, the change of metallographic structure of the surface layer of CNC machine tools will cause the change of volume. When the volume of the surface layer of CNC machine tool expands or shrinks, it will produce compressive stress or tensile stress respectively because of the limitation of the matrix.
