Formation of surface roughness during cutting and its influencing factors
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Where: f - feed, mm/r; re - radius of the tip of the arc. When 2resink'r≤f≤(re/sink'r)[1-cos(kr+k'r)], the residual area is formed by the tool nose edge transition edge and the linear secondary edge. At this time, Rz=re[1 -sin(k'r+b)]×1,000
Sinb=1-(f/re)sink'r where kr,k'r is the tool's primary and secondary angles. When f>(re/sink'r)[1-cos(kr+k'r)], the residual area is formed by the tool nose edge transition edge and the two straight primary and secondary cutting edges. In this case, Rz = 1 f - re (tan kr + tan k'r )] x 1000 cotkr + k'r 2 2 When re - 0, the residual area consists of two main straight cutting edges. At this time, Rz = f × 1000 cotkr + k'r The roughness of the cutting edge of the tool is directly reflected on the machined surface. Therefore, the roughness of the cutting edge of the tool should be lower than the required surface roughness of the machined surface. In fact, the roughness of the machined surface is always greater than the height of the residual area calculated above. When cutting a brittle material or a high-speed cutting plastic material, the actual machined surface roughness is closer to the height of the residual area, indicating that the surface roughness is affected. There are other reasons. The build-up edge of BUE is due to the plastic flow of cuttings during cutting and the external friction between tool and chip exceeds the internal friction. Under the great pressure between the cutter and the chip, the cutting bottom and the front of the cutter are cold. weld. The effect of BUE on surface roughness is twofold: 1 It can sculpt longitudinal grooves; 2 It also adheres to the machined surface when broken off. The main reason is that when the built-up edge is in the growth stage, it is firmly bonded to the rake face, so the possibility that the edge of the built-up edge has vertical grooves on the processed surface is greater than that of the processed surface. Attached. When the BUE is in its maximum range and fades, it is not very stable. At this time, although it is sometimes grooved, it is more attached to the processed surface. Scale scales Scale scales are scaly burrs on the surface that have been machined. This is a phenomenon often seen when cutting with a high speed steel tool at low speed. Spurs usually form in the early stages of the build-up phase of BUE. Spurs occur even when there is no built-up edge, and at a lower cutting speed range. When the back angle of the tool is small, it is particularly easy to produce burrs. Spurs have a serious effect on the quality of the surface that has been machined, and it tends to reduce the surface roughness grade by 2-4 levels. The cause of scaly thorns is the cyclical change in friction on the rake face. If there is vibration during vibration cutting, the surface roughness will become significantly larger. The vibration is caused by the radial cutting force Fr being too large or the rigidity of the workpiece system being small. Other factors The compression of the residual area by the secondary cutting edge causes the residual area to be deformed in the opposite direction to the feed, so that the top of the residual area is misaligned to generate burrs, which increases the surface roughness. The cutting thickness of the transition blade arc portion varies, and the cutting thickness near the tip is small. When the feed amount is less than a certain limit, when the cutting thickness of this part is smaller than the minimum thickness that the cutting edge arc can cut, part of the metal cannot be removed, and the surface roughness will increase. When the brittle material is cut, chipping chips are generated, and when the chip is disintegrated, the crack is deeper below the processed surface and the roughness is increased. In addition, the condition of the chip, the precision and stiffness of the machine tool, etc., also affect the surface roughness of the machined surface. 2 Factors influencing the roughness The tool geometry The tool geometry parameters that have the greatest influence on the surface roughness are the cutting edge radius re, the declination angle k'r and the wiper. The radius of the arc at the tip of the knife has a double influence on the surface roughness: when the re increases, the residual height decreases, and on the other hand the deformation increases. Due to the greater influence of the former, the surface roughness will decrease as the radius of the arc of the tip of the knife increases. Therefore, under the condition of permissible stiffness, increasing the radius of the cutting edge arc re is a good way to reduce the surface roughness. The smaller the declination angle k'r, the lower the surface roughness. However, reducing the declination angle can easily cause vibration, so reducing the declination angle depends on the stiffness of the machine tool system. When k'r is larger than a certain value, the secondary blade will not participate in the composition of the residual area. If k'r is increased, the surface roughness value will not increase. The use of a wiper with a length slightly longer than the feed (k'r=0 on the wiper) is an effective measure to reduce the surface roughness. The use of an additional wiper to eliminate the residual area is a frequently used method for actually machining a workpiece. . The anteroposterior angle g0 has no direct effect on the surface roughness, because g0 is large and it is advantageous for suppressing the built-up edge and scaly spurs, and it increases. The radius of the arc of the cutting edge can be reduced, so an appropriate increase of g0 in the middle and low speed range can help to reduce the surface roughness. When v>50m/min, g0 basically has no effect. The sharpness of the cutting tool, the front and rear flank of the cutting edge, and the roughness of the cutting edge directly affect the roughness of the machined surface. In general, the roughness of the front and back flank of the blade should be 1 to 2 degrees lower than the required roughness of the processing surface. When the material of the cutting tool has a high affinity with the metal molecules of the material to be processed, the processed material easily adheres to the tool to generate a built-up edge and scale, and the metal adhered to the cutting edge separates from the surface to be machined. Create additional roughness. Therefore, if the bonding is serious, the friction is severe and the surface roughness is large. On the other hand, if the bonding and friction are not serious, the surface roughness is small. Cutting conditions Cutting speed v When machining plastic materials, the effect of cutting speed on BUE and scale marks is significant. Low cutting speed is easy to produce scales, low-speed to medium-speed easy to form built-up edge, roughness is also large. Avoiding this speed zone will reduce the surface roughness. In the processing of brittle materials, the cutting speed basically has no effect on the surface roughness because the built-up edge and scale marks are generally not formed. Thus, with higher cutting speeds, it is possible to increase productivity while at the same time making the surface roughness of the machining smaller. So the most important thing is to develop various new tool materials and corresponding new tool structures so that it is possible to use higher cutting speeds. Feed rate f It can be seen from the geometric factors that decreasing the feed f can reduce the height of the residual area. At the same time, the height of the built-up edge and scale can also be reduced, so that reducing the feed amount can reduce the surface roughness value. However, when the feed amount decreases to a certain value, it decreases again, and the plastic deformation is dominant, and the roughness value does not drop significantly. When the feed rate is smaller, the roughness increases instead due to the increased degree of plastic deformation. Depth of cut ap In general, the effect of depth of cut on the roughness of the machined surface is insignificant and can be ignored in practice. However, when ap < 0.02 ~ 0.03mm, because the cutting edge is not absolutely sharp but has a certain arc radius, normal cutting can not be carried out at this time, often the extrusion will slide over the machining surface and no chip will be cut and it will cause on the processing surface. Additional plastic deformation, so that the surface roughness of the processing increases. Therefore, the cutting depth must not be too small. However, excessive cutting depth may also affect machining accuracy and surface quality due to cutting force and sharp increase in cutting heat. The cooling and lubricating action of cutting fluid cutting fluid can reduce the interfacial friction in the cutting process, reduce the cutting zone temperature, thereby reducing the plastic deformation of the cutting process and inhibiting the growth of the built-up edge and burr, thus reducing the roughness of the processing surface. Degree of advantage. Materials to be processed In general, the better the material toughness, the greater the tendency of plastic deformation, and the greater the surface roughness in the cutting process. The effect of the material being processed on the surface roughness is related to the state of the metallographic structure. The precision and rigidity of the process system require low surface roughness after machining. Machine tools with high movement accuracy and high-stiffness process systems must be used. The vibration resistance is strong. Otherwise, even with good tools, the best cutting is selected. It is also difficult to obtain a high-quality machined surface for use. 3 Measures to Reduce Surface Roughness If the traces of the machined surface are relatively clear, indicating that the main factor affecting the surface roughness is the geometric factor, it should first consider reducing the height of the residual area. To reduce the height of the residual area, the first is to change the geometric parameters of the tool, increase the radius of the arc of the tip and decrease the angle of the declination k'r. Using a tool with a wiper with k'r=0 or a wide-blade planer and finisher are methods used to reduce the roughness of the machined surface during production. Whether to increase re, reduce k'r, or use a wide blade must pay attention to avoid vibration. Decreasing the feed rate f can also effectively reduce the height of the residual area, but reducing the feed rate f will reduce the productivity. Therefore, only when reducing the geometric parameters of the tool will cause vibration or other adverse effects will only consider reducing the Give volume f. If scales have formed on the machined surface, or if the groove is caused by a build-up edge in the direction of the cutting speed, then the built-up edge should be eliminated. It can be used with lower or higher cutting speeds, and with a smaller amount of feed, it can effectively inhibit the growth of BUE and burrs. In the middle and low speed cutting, increase the rake angle g0, while increasing the angle of some appropriate to inhibit the built-up edge and scales have a certain effect. Switch to a cutting fluid with good lubricating properties. If necessary, perform heat treatment such as normalizing and tempering on the workpiece to increase the hardness and reduce the plasticity and toughness.