In the process of metal cutting, the high-speed rotation of the cutting tool will cut the metal into the desired shape. However, in this process, when the high-speed tool is in contact with the metal surface, cutting steel or other plastic materials will cause the adhesion of metal materials on the front near the cutting edge of the tool, forming chip nodules. However, after the metal is cold worked, the strength and hardness are increased and the plasticity is reduced, and the surface hardening will occur.
Debris tumor
1. The cause of debris tumor
Chip buildup is the product of metal deformation and friction in the friction deformation area in front of the tool under specific conditions. When cutting plastic materials, the chip flows out along the front of the tool from the cutting edge, and the stagnant layer at the bottom of the chip is affected by the friction in front of the tool, and the flow speed slows down. Under the action of high temperature and high pressure, when the friction force is greater than the binding force of the stagnant layer, the metal of the stagnant layer separates from the chips and adheres to the front, forming chip nodules.
2. Influence of chip buildup on cutting process
When the metal becomes a chip deposit, it will produce severe deformation, so the chip deposit has a high hardness (about 2-3 times the hardness of the workpiece), which can replace the cutting edge for cutting, and has a certain protective effect on the cutting edge. The existence of chip buildup can also increase the actual rake angle of the tool and reduce the cutting force. This is a favorable aspect of the influence of chip buildup on the cutting process. The adverse effects of chip buildup on the cutting process are as follows:
(1) The existence of chip buildup will increase the thickness of the cutting layer, thus affecting the dimensional accuracy of the workpiece.
(2) The growth and falling off of chip nodules will increase the surface roughness of the machined surface and reduce the surface quality.
(3) When the chip mound breaks and falls off, part of the debris will flow into the tool workpiece contact area and form a "furrow" on the workpiece surface. Debris may also be embedded into the surface of the workpiece, causing hard spots and accelerating tool wear.
(4) When the chip buildup breaks, the cutting force also changes, making the cutting process unstable.
Based on the above, in general, especially in finishing, chip buildup is unfavorable to the cutting process, and measures should be taken to inhibit or avoid the generation of chip buildup.
3. Measures to inhibit or avoid debris accumulation
(1) Control the cutting speed and try to use a very low or high cutting speed to avoid the speed range of chip buildup. This is a good way to reduce the surface roughness value.
(2) Increase the rake angle of the tool and reduce the cutting deformation.
(3) Reduce the thickness of cutting layer and adopt small feed rate or small main deflection angle.
(4) Grind the front of the cutter to reduce friction; Use high efficiency cutting fluid.
2、 Cold hardening of machined surface
1. Causes of work hardening
In the process of cutting, the surface layer material produces plastic deformation under the action of force, resulting in shear slip between crystals, serious lattice distortion, grain elongation, fragmentation and fibrosis, which hinder the further deformation of the metal and strengthen the metal, and the hardness is significantly improved. The greater the plastic deformation of metal, the more serious the work hardening.
2. Influence of work hardening on the service performance of parts
In the process of machining, the cold work hardening of the surface layer of the machined surface is often accompanied by residual stress and fine cracks in the surface layer. The surface layer is the work hardening of residual tensile stress. While the cold hardening increases the microhardness of the surface layer of the part, the residual tensile stress will expand the micro cracks and cause the fatigue strength of the part to decrease, affecting the service life of the part. Therefore, it is hoped that the less the surface layer is the processing hardening degree of tensile stress, the better. The work hardening with residual compressive stress on the surface layer can improve the hardness and strength of the machined surface, delay and prevent the propagation of cracks, and thus improve the fatigue strength and durability of parts.