Discussion on the surface quality of the hottest m

Discussion on the surface quality of machining

in addition to the machining accuracy, the machining quality of mechanical parts also includes the working table quality of the control valve with the driving hydraulic system, which refers to the surface layer state of the parts after machining, and is also an important basis for judging the quality of parts. The failure of mechanical parts is rarely due to design errors, but mostly due to wear, corrosion resistance and fatigue of parts. Wear, corrosion, fatigue and other damages all start from the parts list. It can be seen that the appearance and quality of parts will directly affect the working performance of parts, especially the reliability and service life

with the continuous development of machinery manufacturing industry, higher and higher requirements are put forward for the wear resistance, corrosion resistance and fatigue strength of mechanical parts. Therefore, while speeding up the transformation of enterprises and studying the machining accuracy of parts, we must also explore the process factors and control measures that affect the surface quality

1 table and the influence of quality on the service performance of parts

1.1 table and the influence of quality on the wear resistance of parts

(1) the influence of part surface and roughness. Wear resistance of parts is a very important performance index. When the material, lubrication conditions and machining accuracy of parts are determined, surface quality plays a key role in wear resistance

no matter how precise any machining method is, the machined surface always has a certain surface roughness, and the surface roughness has a great impact on the wear resistance of parts. The lubricating oil film between them due to friction can be formed only when the pressure per unit product does not exceed a certain critical value. When the contact surface is rough, the pressure on individual protruding points will exceed the critical value, and the oil film will be destroyed, resulting in dry friction, which intensifies the wear between parts

but the smaller the surface roughness, the smaller the wear amount is not necessarily. In addition to the influence of surface and micro unevenness, the wear amount is characterized by small volume, which is also related to the scoring effect of worn metal particles, the extrusion of lubricating oil and the adsorption between molecules. Therefore, if the surface roughness value is too small, the wear will increase

(2) effect of cold work hardening of surface layer. The cold work hardening of the surface layer reduces the elastic and plastic deformation at the friction pair, so the wear resistance is improved. But it is not that the higher the degree of cold work hardening, the more wear-resistant. Due to excessive hardening, the metal surface will peel off, so the hardened layer should also be controlled within a certain range

1.2 effect of surface quality on fatigue strength

under alternating load, the concave part of surface roughness is easy to cause stress concentration and fatigue cracks. The larger the surface roughness value, the deeper the striation mark on the surface and the smaller the radius of the bottom of the striation, the more serious the stress concentration is, and the worse the ability to resist fatigue damage is

in addition, the physical and mechanical properties of the surface layer also have a great impact on the fatigue strength of the parts. For example, the cold work hardening of the surface metal of the parts helps to improve the fatigue strength of the parts. This is because the strengthened surface layer can reduce the deformation amplitude caused by alternating load, and hinder the expansion of existing cracks and the generation of new cracks. However, when the hardness is excessive, the metal becomes brittle, and the surface metal is easy to fall off, but it is easy to cause stress concentration and fatigue cracks. The surface layer has residual compressive stress, which can improve the fatigue strength of parts; while the surface layer has residual tensile stress, it will reduce its fatigue strength

1.3 the influence of quality on the corrosion resistance of parts

the corrosion of parts is caused by the influence of atmosphere and the action of various gases and liquids. The larger the roughness value, the more corrosive substances will be deposited in the valley. Due to corrosion and destruction, the protruding part of the roughness will finally fall off. Especially when corrosion and friction exist at the same time, the corroded protruding part will be corroded quickly due to friction, thus accelerating the corrosion process. Therefore, reducing the roughness value of the parts list can improve the corrosion resistance of parts

1.4 effect of surface roughness on mating quality

surface roughness will affect the stability of mating properties. Clearance fit, because the parts are easy to wear, the clearance is rapidly increased: interference fit, because the peak and valley are flattened, the effective interference is reduced: obviously, they all destroy the original required fit property and affect the reliability of the parts

1.5 other effects on surface quality

(1) for seals without gaskets, reducing surface roughness can improve their sealing performance

(∞ smaller surface roughness improves the contact stiffness of parts. 2 factors affecting the surface quality of cutting processing

2.1 residual and accumulation of cutting

during cutting, the tool tip moves a feed after the workpiece rotates

2.2 chip buildup

the outline of chip buildup is very irregular, and the length extending from the blade is inconsistent, which will make grooves on the machined surface, and some chip buildup fragments that fall off will adhere to the machined surface Machining the surface will affect the surface roughness value

the conditions of forming chip nodules mainly depend on the cutting temperature. When the cutting temperature is very low, the chip is in point contact with the front knife, and the friction coefficient is small, so it is not easy to form bonding; when the temperature is very high, the bottom metal of the chip is in a slightly molten state, and its lubrication and friction coefficient are also small, so it is not easy to form chip nodules. In the medium temperature zone, for example, when the temperature of cutting medium carbon steel is 300 ~ 380 ℃, the cutting bottom material softens, the bonding is serious, the friction coefficient is the largest, and the height of the chip formation reaches a large value

2.3 properties of workpiece materials

when machining brittle materials (such as cast iron), the chips become debris. Due to the collapse of the chips, many pockmarks are left on the machining table. The machining table often shows particle disintegration marks, leaving pockmarks, which increases the roughness. When processing plastic materials, the cutting trajectory on the processing surface is distorted. The reason is due to the extrusion deformation and tearing effect when the chips are separated but the production process of composite materials is still complex. The tougher the workpiece material is, the harder it is to separate the chips, and the rougher the machining table is

2.4 vibration during machining

when vibration occurs in cutting, obvious vibration marks will be generated on the cutting surface, which will increase the roughness of the workpiece surface and deteriorate the surface quality

3 measures to improve the surface quality of machining table

after understanding the factors affecting the surface roughness of machining table, we must reduce the surface roughness of machining table and improve the surface quality of machining table according to needs

3.1 reduce the height of residual product

reasonably control the cutting conditions, adjust the cutting parameters, and try not to form a medium temperature area, which can be effectively avoided. For example, in production, if a small feed rate or a large rake angle is selected for cutting, when chip buildup occurs, a very high cutting speed is often used to make the cutting temperature exceed the medium temperature zone, so that chip buildup can be eliminated

3.3 improve the performance of the workpiece material

the better the plasticity of the material, the greater the possibility of chip formation. Therefore, for medium and low carbon steels and some non-ferrous metals, corresponding heat treatment, such as fire suppression or quenching and tempering, should be carried out before finishing, so as to improve the hardness of materials and reduce the plasticity of materials

3.4 prevent or reduce the vibration of the process system

the low-frequency vibration of the process system generally produces surface waviness on the processing surface of the workpiece; while the high-frequency vibration of the process system will affect the surface roughness of the processing. In order to reduce the surface roughness value of machining, corresponding measures must be taken to prevent the generation of high-frequency vibration in machining

in addition, select the appropriate cutting fluid. The cooling and lubrication of cutting fluid can reduce the temperature and friction in the cutting area, reduce the roughness value and improve the surface quality

4 conclusion

because the quality of mechanical parts list is affected by many factors, it is necessary to comprehensively consider various influencing factors and put forward relatively practical and economical quality requirements of parts list according to the actual situation and needs