The geometric characteristics of a machined surface encompass several aspects: surface roughness, surface waviness, and surface texture. Surface roughness constitutes the fundamental element of these geometric features. When a workpiece surface is machined using metal-cutting tools, the resulting surface roughness is primarily determined by the interplay and influence of three categories of factors: geometric factors, physical factors, and machining process factors.
1. Geometric Factors
From a geometric perspective, the shape and geometric angles of the cutting tool-specifically the nose radius, principal cutting edge angle, auxiliary cutting edge angle, and machining parameters such as the feed rate-exert a significant influence on surface roughness.
2. Physical Factors
Considering the underlying physics of the cutting process, the rounding of the tool's cutting edge-along with the subsequent squeezing and friction-induces plastic deformation in the metal material, thereby severely degrading the surface roughness. When machining ductile materials that produce continuous (ribbon-like) chips, a highly hard "built-up edge" (BUE) often forms on the tool's rake face. This BUE effectively acts as a substitute for the actual rake face and cutting edge, altering the tool's effective geometric angles and depth of cut. The contour of the BUE is highly irregular; consequently, it leaves tool marks on the workpiece surface that vary continuously in both depth and width. In some instances, fragments of the BUE become embedded in the workpiece surface, further exacerbating the surface roughness.
Vibrations occurring during the cutting process also contribute to an increase in the parameter values associated with the workpiece's surface roughness.
3. Process Factors
From a process-oriented perspective, the factors influencing workpiece surface roughness primarily include those related to the cutting tool itself, those related to the material properties of the workpiece, and those related to the specific machining conditions employed.
The surface quality of a machined workpiece has a profound impact on the functional performance of the finished part. Key metrics used to evaluate the surface quality of a machined workpiece include surface roughness, surface residual stress, and the℃of surface work hardening. Among these three indicators of surface quality, surface roughness is the most critical factor influencing the overall performance characteristics of the component.
The surface roughness of a component directly and significantly affects friction and wear; specifically, the rougher the surface, the more severe the wear. During the initial stages of wear, the microscopic asperities on the surface are rapidly flattened, resulting in a sharp increase in the rate of material loss. However, after a period of operation, the actual contact area between the moving surfaces increases, causing the rate of wear to slow down. If a surface is smooth and dense, the height and sharpness of its microscopic asperities are relatively low; consequently, smooth and dense surfaces exhibit greater wear resistance than rough surfaces.
Conversely, an excessively smooth surface hinders the retention of lubricating oil; this can actually lead to an increased coefficient of friction, causing the metal surface to overheat and potentially result in a "seizing" or "galling" phenomenon. During the cutting operations performed on a vertical machining center, process parameters-such as cutting speed, feed rate, and depth of cut-directly influence the cutting force. Cutting force and cutting temperature are two mutually interdependent factors: generally, a higher cutting force corresponds to a higher cutting temperature and, simultaneously, to more severe vibration within the vertical machining center.
Varying cutting speeds generate external excitation frequencies that differ accordingly. The closer this excitation frequency approaches the natural frequency of vibration inherent to the vertical machining center, the more likely it is to exacerbate the mechanical equipment's vibration.
To achieve optimal surface roughness values on workpieces during cutting operations, a detection system for monitoring cutting force and cutting temperature has been designed. This system aims to investigate the relationships between cutting force, cutting temperature, and the resulting surface roughness of the workpiece. By judiciously selecting process parameters-such as cutting speed, feed rate, and depth of cut-during the machining process, it becomes possible to control cutting force, cutting temperature, and mechanical vibration, thereby ensuring the attainment of the desired workpiece surface roughness.