Causes of surface roughness in machining
The rough machined surface is mainly caused by the following reasons:
The first is the residual area, which is the area that the main cutting edge and auxiliary cutting edge of the tool remain on the machined surface after cutting.
The second is scale burr. Some plastic metals, such as low carbon steel, medium carbon steel, stainless steel, aluminum alloy, etc., will produce fish scale like burrs on the machined surface when they are cut at low or medium speed with high-speed steel tools, which is called scale burr. The generation of scales and thorns will increase the surface roughness of parts.
Next, there is chip buildup, which is formed by the accumulation of cutting chips on the surface of the cutting edge during the cutting process. When there is chip buildup, the protruding part may cut into the workpiece instead of the cutting edge, and draw grooves of different depths on the machined surface; When the chip tumor falls off, part of the chip tumor fragments may adhere to the machined surface and form fine burrs. Both of these conditions will cause the roughness of the machined surface of the part to increase.
Finally, there is vibration. During cutting, the process system produces periodic vibration due to insufficient rigidity, which will scratch stripes or ripple marks on the machined surface, and significantly increase the surface roughness value.
Factors affecting surface roughness in machining
In machining, what factors will cause residual area, chip buildup, scale thorns and vibration? Let's take a look:
The first factor is the cutting amount. Cutting parameters include feed rate and cutting speed, among which the feed rate has the greatest impact on the residual area, and the increase of feed rate will lead to the increase of residual area. The influence of cutting speed on surface roughness is mainly reflected in the generation of scale thorns and chip nodules. When the cutting object is plastic metal and the cutting speed is very low, it is not easy to produce chip buildup; When the cutting speed is very high, it is beneficial to reduce the plastic deformation, thus inhibiting the generation of scale thorns. Therefore, both cases will reduce the roughness of the part surface. When cutting brittle materials, the influence of cutting speed is small, because the material deformation is small, so the surface roughness value also decreases.
The second factor is the geometric parameters of the tool. The geometric parameters of the tool mainly include the blade shape and angle of the tool, among which the arc radius of the tool tip, the main deflection angle and the auxiliary deflection angle have a great influence on the residual area and vibration. Generally, on the premise of sufficient rigidity of the machine tool, when the arc radius of the tool tip increases and the main deflection angle and sub deflection angle decrease, the surface roughness value is small. However, if the rigidity of the machine tool is too low, the arc radius of the tool tip is too large or the main deflection angle is too small, it will cause vibration due to excessive cutting force, which will increase the surface roughness value.
The third factor is the tool material used for cutting. The size of the arc radius of the cutting edge and the time to keep it sharp are different with different tool materials. Cutting tools of some materials, such as high-speed steel, can be sharpened, but the holding time is short, so the surface roughness is small when cutting at low speed; However, for some materials, such as cemented carbide, the arc radius of the edge is larger after grinding, and the cutting surface roughness value is smaller at high speed. As long as the appropriate tool materials are selected according to the machining needs, the roughness of the machined surface can be reduced.
The fourth factor is the workpiece material. The higher the plasticity and the lower the hardness of the workpiece material, the more likely it is to produce debris nodules, scale thorns, cold hardness and other phenomena, and the greater the surface roughness. Therefore, the surface roughness of cutting high carbon steel, medium carbon steel and quenched and tempered steel is smaller than that of cutting low carbon steel; The surface roughness of steel workpiece is smaller than that of cast iron workpiece.