In the previous article, we have discussed the surface finish, the surface roughness as one of its characteristics, it can be used to evaluate the quality of machines and mechanical parts. Here we’ll introduce the factors affecting surface roughness and how to improve roughness.
Surface roughness, often shortened to roughness, which is defined as the shorter frequency of real surfaces relative to the troughs, is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. It is an important index to tell whether a machined part is qualified or not. It has an impact on the wear resistance, quality, motion accuracy and service life of the parts in the process of use. It’s significant to obtain appropriate surface roughness in the CNC machining process.
When a cutting tool is used to cut a workpiece, there will be a residual cutting face on the surface, with micro geometric error, feed rate, main/auxiliary deflection angle and the radius of the tooltip arc, which will affect the size of the residual face. Adjust the feed rate and angle in the machining process can reduce the deformation and cutting area of the part. In addition, the lubricant and cutters selected should conform to the characteristics of the material, to reduce the plastic deformation and inhibit the formation of tool burr and scale. It is also an effective measure to reduce the surface roughness by increasing the rake angle of the tool to reduce plastic deformation
Grinding is used for fine machining. The hardness of abrasive grains is very high, and working at high speed. It can obtain high machining accuracy and surface roughness. Because of the advantages of grinding, in the specific machining process, the temperature can reach 1000 ° C to 1500 ° C, which will deepen the plastic deformation, and the negative front angle grinding of abrasive particles is relatively thin. When grinding, most of the parts are extruded. In the plastic deformation process, the side of abrasive particles will produce plastic heat flow, and then draw a small roughness on the parts, and the high temperature will further deepen the surface roughness.
3. Cutting parameters
– Feed rate: decrease the feed rate, the height of the residual area of the machined surface becomes smaller, which is beneficial to reduce the Ra value of the roughness.
– Cutting speed: When cutting plastic materials, when the cutting speed is less than 5 m / min or more than 100 m / min, it is not easy to generate chip accretion, which is beneficial to reduce the Ra value of roughness. When the cutting speed V is 20-25 M / min and the cutting temperature is about 300°C, the friction coefficient between the chip and the front face of the tool is the largest, and the height of the chip accretion is the largest, which increases the Ra value.
In the machining process of mechanical parts, the following methods can improve the surface roughness of the parts.
1. Cutting condition: appropriately reduce the amount of surface machining. Cutting plastic materials with high cutting speed; reducing feed rate; using high efficiency cutting fluid; improving the kinematic accuracy of machine tools, enhancing the rigidity of process system; using ultrasonic vibration cutting, etc
2. Select the geometric parameters of the tool: appropriately reduce the deflection angle of the pair and increase the radius of the arc of the tooltip, and if necessary, grind the polishing edge. It makes the cut become easier and reduce surface roughness. Select the tool material with small affinity with the workpiece; carry out oxygen and nitriding treatment on the tool; limit the wear amount on the auxiliary blade; select the fine-grained cemented carbide as the tool, etc.
3. Control the vibration of the machine tool: start from reducing the friction and extrusion between the tool and the workpiece, sharpen the tool edge, add cutting fluid, and properly heat treat some workpiece materials with good toughness.
4. Choose a reasonable processing technology: in the process of precision machine parts processing, machining technology is also very important. If the process is unreasonable, it may affect the processing quality and production efficiency. Many precision machined parts need finish after rough machining.
5. Different selection of raw materials: mechanical equipment parts are made up of many different raw materials. According to the different density of raw materials, the selection of cutting tools and machine tools in the processing is directly related to the surface roughness. The distribution of carbides in alloy elements should be fine and uniform; the easy cutting steel should contain elements such as sulfur, lead, etc.; the workpiece should be tempered to improve hardness and reduce plasticity; the particle size of graphite in cast iron should be reduced.