At present, most of the parts machined on CNC milling machines are flat parts. The feature of the planar part is that each processing unit surface is a plane, or can be expanded into a plane, for example, the curved contour surface M and the perfect circular table surface N in the figure, and are planar after being expanded. Planar parts are the simplest type of CNC milling objects. 

Generally, they can be processed by the two-coordinate linkage of a 3-axis CNC milling machine. Parts with varying angles between the machined surface and the horizontal plane of the beveled part are called variable beveled parts. Most of these parts are aircraft parts.