It is important to understand the connotation of the surface quality of the parts correctly, analyze the various process factors that affect the surface quality of the machined surface and improve the surface quality and improve the performance of the product.
Surface quality of machining, is refers to the parts after machining processed surface micro roughness, also called the roughness and the processed surface quality directly affect the processing of physical, chemical and mechanical properties. The working performance of the products, reliability, life in a large extent depending on the surface quality of the main parts. Therefore, correctly understanding the connotations of the surface quality of the parts, analyzing the machining process influence machining quality of the surface of various technical factors, to improve the surface quality, improve product performance has important significance.
1 factors affecting surface quality of machined surface
1.1 the influence of machine performance on the surface quality of machined surface
Effect of wear resistance on surface quality
Between a just finished the friction pair two contact surfaces, initially in the rough surface of the peaks of touch, the actual contact area is much smaller than the theoretical contact area and contact with each other in the a very large unit stress, the actual contact area to produce plastic deformation, elastic deformation and peak between shear failure, causing serious wear and tear.
Effect of fatigue strength on surface quality
Under the alternating load, surface roughness of Valley site easily lead to stress concentration, fatigue and grain production. Surface roughness value is bigger, deeper surface marks, lines at the end of radius more, the anti fatigue ability is worse. Residual stress on the fatigue strength of parts influence great. Surface residual tensile force makes the fatigue crack expansion, accelerate fatigue damage; and surface residual stress can prevent fatigue crack propagation, delaying fatigue failure.
Influence of corrosion resistance on surface quality
The corrosion resistance of the parts in a large extent depending on the surface roughness. Surface roughness value is bigger, valley of the accumulation of corrosion material more. Corrosion resistance is worse. Surface layer of residual tensile stress will cause stress corrosion cracking, reduce the wear resistance of the parts, and the residual compressive stress can prevent stress corrosion cracking.
1.2 factors affecting surface roughness
Factors affecting surface roughness of machining
Tool geometry reflect the tool relative to the workpiece feed movement, on the machined surface leave the cutting layer residual area, its shape is reflection of the tool geometry. (2) of the workpiece material properties processing plastic materials, by the tool of metal extrusion to produce the plastic deformation, coupled with the tool force cutting chip and workpiece separating the tearing effect, make the surface rough degree value increase. 3 cutting machining of brittle materials, cutting speed to have little effect on the roughness; plastic processing of materials, product crumbs tumor has the effects of roughness.
Factors affecting surface roughness of grinding
Main factors affecting the grinding surface roughness: grain wheel, the hardness of the grinding wheel, wheel dressing, grinding speed, radial grinding, feed rate and light grinding times, workpiece circular feed speed and the axial feed rate and cooling lubricating fluid, such as.
1.3 factors affecting the physical and mechanical properties of the machined surface layer
Cold hardening of the surface layer
The plastic deformation in the machining process for the cutting force, make character distortion, distortion, shear slip between grains, grains are elongated and fibrosis, or even broken, these will cause the surface layer of metal hardness and strength increased, this phenomenon is known as cold hardening (or strengthening) main factors. Effects of cold hardening are: cutting edge radius increases, to enhance the effect of surface metal extrusion, plastic deformation increased, lead to increased. Chill increase tool flank wear, flank friction and machined surface intensified, plastic deformation increases, resulting in chilled increase. The increase of cutting speed, effect time of the tool and the workpiece is shortened, the plastic deformation propagation depth decreases, chill depth decreased. The increase of cutting speed, cutting time of heat in the surface layer on the cut, will make the chill increases. With feed rate, cutting Stress increase, the plastic deformation of the metal surface intensified, chill effect strengthened. Workpiece material plastic is bigger, chilled phenomenon is more serious.
Microstructure change of surface layer material
When the cutting heat to be machined surface temperature exceeds the phase transition temperature, metallographic structure of metal surface will change. Main grinding burn, quenching and annealing burn. Improve grinding burn has two ways: one is as far as possible to reduce grinding heat generation; the second is improving cooling condition, try to make the heat produced with less transmission of the work piece. Correct selection of grinding wheel, a reasonable choice of cutting parameters improve the cooling conditions.
Residual stress in the surface layer
Surface residual stresses are generated: one is cutting in the metal surface layer generated residual stress, and in the metal layer residual tensile force. The second is cutting, the cutting area will produce a lot of cutting heat. The third is different microstructure of surface layer of metal gold phase changes, changes in the surface of the metal volume must blocked on and connected to the base metal, and the produce of the residual stress.
2 measures to improve the surface quality of machined parts
2.1 to establish a scientific and reasonable technological process is the basis of ensuring the surface quality of the workpiece.
Workpiece processing method on the basis of the scientific and reasonable procedure. Only by formulating a scientific and reasonable process rules, to the workpiece surface quality meet the requirements to provide scientific and reasonable methods according to, the surface quality of the workpiece machined meet the requirements become possible. Requirements of scientific and rational process planning is the process should be short, positioning accuracy, positioning norm choice as far as possible, the positioning and design datum coincidence.
2.2 a reasonable choice of cutting parameters is the key to ensure the quality of processing
Choosing the reasonable cutting parameters can effectively inhibit the formation of built-up deposition and reduce the residual area processing theory of height, ensure the surface quality of the workpiece. The choice of cutting parameters mainly include the selection of cutting tool angle, cutting speed and cutting depth and feed speed of choice. Test results show that in processing plastic materials if choose larger rake angle of the tool can effectively inhibit the product a built-up formation, which is because of the tool rake angle increases, reducing the cutting force cutting deformation, cutting tool and the chip contact length becomes shorter, reducing the base of built-up edge formation.
2.3 a reasonable choice of cutting fluid is a necessary condition to ensure the surface quality of the machined workpiece.
Choosing the reasonable cutting fluid can improve the friction coefficient between the workpiece and the cutting tool, which can reduce the cutting force and cutting temperature, so as to reduce the tool wear and tear, so as to guarantee the machining quality of the workpiece.
2.4 the choice of the final working procedure of the main working surface is very important.
Workpiece main work surface machining method selection is very important, because the final step in left on the surface of the residual stress will directly affect the performance of machine parts. Parts the main work surface processing method, the specific working conditions of the main working parts surface and possible failure modes should be taken into account.
Is closely related to the surface quality of the workpiece and its performance and the using performance of the workpiece is to ensure the normal operation of the machine is the premise of the design requirements, so in the process of machining workpiece, we also from economic benefits and other aspects to consider, can not only guarantee the machining quality of the workpiece surface, and avoid increasing the manufacturing cost of the parts, causing unnecessary losses. Only to understand and grasp the factors affecting the quality of surface machining to in the production practice, take corresponding technical measures, reduce parts due to the surface quality defects caused by the machining quality problem, so as to improve the use of mechanical product performance, life and reliability.