Mon Jul 04 14:43:57 CST 2022
Machined surface quality refers to the microscopic roughness of the machined surface after machining, also known as roughness. The surface quality of the machined surface directly affects the physical, chemical and mechanical properties of the workpiece. Product performance, reliability, life largely depends on the surface quality of the main parts. So what are the factors that affect the surface quality of metal cutting tool?
(1) Influence of wear resistance on surface quality. Just processed a friction pair between the two contact surfaces, the initial stage of rough surface on the peak of the contact, the contact area, the actual contact area is far less than theory in mutual contact of stress has a very large unit, making the actual contact area to produce plastic deformation, elastic deformation and shear failure between the peak of, cause serious wear and tear.
(2) Influence of fatigue strength on surface quality. Under the action of alternating load, the concave valley of surface roughness is easy to cause stress concentration and fatigue lines. The larger the surface roughness, the deeper the striation and the radius of the bottom of the striation, the worse the anti-fatigue ability. Residual stress has great influence on fatigue strength of parts. The residual tensile stress of the surface layer will enlarge the fatigue crack and accelerate the fatigue failure. The residual stress of surface layer can prevent the propagation of fatigue crack and delay the generation of fatigue failure.
(3) Influence of corrosion resistance on surface quality. The corrosion resistance of a part depends largely on the surface roughness. The larger the surface roughness is, the more corrosive substances will accumulate in the concave valley. The less corrosion resistance. The residual tensile stress of surface layer can cause stress corrosion cracking and reduce the wear resistance of parts, while the residual compressive stress can prevent stress corrosion cracking.
(1) Factors affecting surface roughness during cutting. ① Reflection of cutting tool geometry When the tool is relative to the workpiece for feed movement, the machining surface left a residual cutting layer area, its shape is a reflection of the tool geometry. (2) The nature of the workpiece material processing plastic materials, by the tool of the metal extrusion plastic deformation, coupled with the tool to force the chip and workpiece separation of the tearing effect, so that the surface roughness value increased. (3) The cutting speed has little effect on the roughness of brittle materials. The roughness of plastic materials is greatly affected by the nodules.
(2) Factors affecting surface roughness during grinding. The main factors affecting the grinding surface roughness are: grinding wheel grain size, grinding wheel hardness, grinding wheel dressing, grinding speed, grinding radial, feed and grinding times, workpiece circumference feed rate and axial feed, cooling lubrication fluid, etc.
(1) Surface layer hardening by cold work. Mechanical processing due to the cutting force of plastic deformation, character distortion, distortion, grain shear slip, grain is elongated and fibrosis, and even broken, these will make the surface layer of metal hardness and strength, this phenomenon is called cold hardening (or called strengthening). The main factors affecting cold-working hardening are: the radius of blunt cutting edge increases, the extrusion effect on surface metal increases, and the plastic deformation intensifies, leading to the hardening of cold-working. The wear of the tool surface increases, the friction between the tool surface and the machined surface increases, and the plastic deformation increases, leading to the enhancement of cold and hard. As the cutting speed increases, the interaction time between the tool and the workpiece shortens, the plastic deformation extension depth decreases, and the cold hard layer depth decreases. With the increase of cutting speed, the action time of cutting heat on the workpiece surface is shortened, which will increase the degree of hardness. With the increase of feed, the cutting force also increases, the plastic deformation of surface metal intensifies, and the cooling and hardening effect is strengthened. The greater the plasticity of workpiece material, the more serious the hardening phenomenon.
(2) Metallographic structure changes of surface layer materials. When the temperature of the machined surface exceeds the phase transition temperature, the metallographic structure of the surface metal will change. There are three kinds of grinding burn, quenching burn and annealing burn. There are two ways to improve grinding burn: one is to reduce grinding heat as much as possible; The second is to improve the cooling conditions, as far as possible to produce less heat transmission workpiece. Correct selection of grinding wheel, reasonable selection of cutting amount to improve cooling conditions.
(3) Residual stress of surface layer. The reasons for the surface residual stress are as follows: first, the residual stress is generated in the surface metal layer while the residual tensile stress is generated in the inner metal layer. Second, in the cutting tool process, there will be a lot of cutting heat. Third, different metallographic structure of the surface layer of metal has a change in the metallographic structure, the change of surface metal specific volume is bound to be hindered by the matrix metal connected with it, so there is residual stress.