CNC Machining accuracy and surface quality
CNC Machining accuracy and surface quality are key indicators of mechanical product performance, directly impacting their performance, lifespan, and reliability. CNC Machining accuracy refers to the degree of conformity between the actual geometric parameters (size, shape, and position) of a machined part and its ideal geometric parameters, while surface quality encompasses aspects such as surface roughness, surface waviness, and the mechanical properties of the surface layer. Controlling CNC machining accuracy and improving surface quality are core areas of CNC machining process research and crucial for ensuring product quality.
CNC Machining accuracy mainly includes three aspects: dimensional accuracy, shape accuracy and position accuracy. Dimensional accuracy refers to the degree of closeness between the actual size of a part and the nominal size, and is usually expressed by dimensional tolerance. The smaller the tolerance, the higher the dimensional accuracy. Factors affecting dimensional accuracy mainly include tool wear, feed rate fluctuations, and measuring errors of measuring tools. During the CNC machining process, dimensional accuracy can be effectively improved by rationally selecting tool materials, controlling cutting parameters, and regularly calibrating measuring tools. Shape accuracy refers to the degree of conformity between the actual shape of a part’s surface and the ideal shape, such as the roundness of a cylindrical surface and the flatness of a plane. It is mainly affected by factors such as the geometric accuracy of the machine tool, tool rigidity, and the workpiece clamping method. For example, the radial runout of the lathe spindle will cause roundness errors in the machined cylindrical surface. Therefore, it is necessary to regularly check and adjust the machine tool accuracy, and use rigid tools and reasonable clamping methods. Positional accuracy refers to the degree of conformity between the actual position of each surface on a part and the ideal position, such as parallelism, perpendicularity, coaxiality, etc. Its influencing factors include the parallelism of the machine tool guide rails and the positioning accuracy of the fixture. By improving the manufacturing accuracy of the machine tool guide rails and optimizing the fixture design, the positional accuracy of the parts can be guaranteed.
Surface roughness is the most intuitive indicator of surface quality. It refers to the microscopic geometric characteristics of the surface of a part, consisting of small peaks and valleys with small spacing between them. It is usually expressed as the Ra value, with the smaller the Ra value, the smoother the surface. Factors affecting surface roughness include cutting parameters, tool geometry, and workpiece material. Both low and high cutting speeds increase surface roughness, while moderate cutting speeds produce less surface roughness. Too small a rake angle or too large a clearance angle increases cutting forces and degrades surface roughness. Plastic materials are prone to built-up edge (BUE), which increases surface roughness, while brittle materials have relatively low surface roughness. Furthermore, factors such as grinding wheel grit, grinding speed, and feed rate during grinding significantly affect surface roughness. Fine-grained grinding wheels, high grinding speeds, and low feed rates produce lower surface roughness.
The mechanical properties of the surface layer are an important component of surface quality, including surface hardness, residual stress, and fatigue strength. These properties have a significant impact on the service life and reliability of parts. During the cutting process, due to the effects of cutting forces and cutting heat, the surface layer of the part undergoes plastic deformation and metallographic changes, resulting in changes in surface hardness, which is usually manifested as surface hardening (cold work hardening). Surface hardening can improve the wear resistance of the part, but excessive hardening can increase the brittleness of the surface layer and susceptibility to cracking. Cutting heat also causes thermal stress in the surface layer. If cooling is uneven, residual stress will be generated. Residual tensile stress will reduce the fatigue strength of the part, while residual compressive stress can increase it. Therefore, during the CNC machining process, the residual stress state of the surface layer can be adjusted by controlling the cutting temperature and selecting the appropriate cutting fluid. In addition, metallographic changes in the surface layer, such as the tempering phenomenon after quenching steel, can also affect the mechanical properties of the surface layer and need to be controlled through appropriate CNC machining processes.
There are various process measures to improve CNC machining accuracy and surface quality, and their selection depends on the specific CNC machining method and part requirements. For CNC machining accuracy control, error compensation can be used. This artificially introduces an additional error to offset the original error generated during CNC machining. For example, during lathe bed installation, adjusting shims can cause the bed to undergo a certain reverse deformation to offset the stress deformation during operation. Alternatively, error grouping can be used to group blanks or semi-finished products by error size, with each group receiving different CNC machining adjustments to narrow the CNC machining error range. For surface quality improvement, finishing and polishing methods such as honing, grinding, and superfinishing can be used. These methods can effectively reduce surface roughness and improve surface accuracy. For surface mechanical properties, surface enhancement processes such as shot peening and rolling can be used. These processes generate residual compressive stresses through surface plastic deformation, thereby improving the fatigue strength and wear resistance of parts. With the development of precision and ultra-precision CNC machining technologies, such as nano-cutting and ion beam CNC machining, CNC machining accuracy and surface quality have been further improved, providing strong support for high-end equipment manufacturing.
There is a close connection between CNC machining accuracy and surface quality. Higher CNC machining accuracy typically requires better surface quality, which in turn helps maintain the dimensional and shape accuracy of parts. For example, in precision fittings, high surface roughness can lead to uneven clearances, affecting fitting accuracy. In high-speed rotating parts, high surface roughness increases air resistance and frictional losses, leading to increased part temperature and, in turn, affecting dimensional accuracy. Therefore, during CNC machining, CNC machining accuracy and surface quality must be controlled as a whole. Factors such as CNC machining technology, equipment accuracy, and material properties must be comprehensively considered to develop a reasonable process plan to produce high-quality mechanical products that meet design requirements. Furthermore, as the manufacturing industry evolves toward intelligent and automated processes, real-time monitoring and adjustment of CNC machining accuracy and surface quality through technologies such as online detection and adaptive control will become an important development direction for future CNC machining.
