Discussion on the mechanism of sub-dry cutting (4)
a. In the case of the same amount of cutting and the same geometrical parameters of the tool, the most important factors affecting the cutting force are the workpiece material, the cooling condition during cutting and the tool wear. Among them, the physical and mechanical properties and state of the workpiece material have the greatest influence. The higher the strength and hardness of the material, the greater the shear stress, and the greater the cutting force. The workpiece material and tool material are consistent in this test. Under the premise of the same cutting parameters, the cutting force should be basically consistent. Therefore, the inconsistency of the test results is due to the inconsistency of the cooling method, which is that different cooling methods change some properties of the material in the cutting zone and the corresponding tool wear. Studies have shown that body-centered cubic lattice materials are prone to low temperature embrittlement during low temperature cutting. At this time, the mechanical properties of the material are affected by the internal lattice dislocations of the metal. At low temperatures, the internal dislocation heat energy of the metal is low, and the plastic deformation stress is higher than that under high temperature, thereby improving the material strength; and the low temperature brittleness is caused by the turtle. Cracked by. The yield stress required for the slip of the body-centered cubic lattice metal increases sharply with temperature, but the change of the twinning stress is small due to the difference in temperature. Therefore, the twinning stress is lower than the sliding stress at low temperature, and the low-temperature brittleness is preferentially caused by the twinning. of. It can be seen that the low temperature brittleness of the material being cooled by low temperature and strong wind cooling is the main reason that the cutting force is smaller than other cooling methods. At the same time, the low temperature reduces the cutting heat temperature, protects the cutting performance of the force tool, and delays the tool wear from the other side. A tendency to increase the cutting force.
b. In cold-air cooling cutting, the cutting temperature of sub-dry cutting is lower than that of ordinary cold-cutting, which is attributed to the lubrication of additives in sub-dry cutting. All the work consumed in metal cutting is converted into heat, and heat is mainly generated in the first deformation zone and the second deformation zone; when the workpiece material is sheared in the first deformation zone, the tool is sheared with the continuous movement of the tool relative to the workpiece. The shear stress increases with work hardening, and there is a large temperature gradient across the entire cutting width, usually around 600 °C. The cutting force is also greatly reduced; as the cutting continues in the second deformation zone, the chips flow out along the rake face and are subjected to the pressing and friction of the rake face, so that the chip temperature of the rake face of the tool is highest. Up to 750 °C; in the absence of lubricating fluid, high cutting temperature, the rake face and the chip are bonding friction or internal friction; when the lubricating fluid seeps into the chip, the contact surface between the tool and the workpiece, adhere A lubricating film is formed on the metal surface to reduce the friction coefficient between the chip and the tool and the workpiece, and to reduce the bonding phenomenon, so as to reduce the cutting force and reduce the heat of cutting. On the other hand, according to the cutting equation f=p/4-b+a (f is the shear angle, b is the friction angle, a is the tool rake angle), and decreasing the friction coefficient means that the eve becomes smaller and the synergy increases. The contact length between the rake face and the chip is reduced, the friction is reduced, and the cutting temperature is lowered, which is common to both the cutting fluid cooling and the sub-dry cutting. However, under high pressure and high temperature, the thickness of the oil film will be thinned and partially evaporated, and the unevenness of the metal surface will easily damage the oil film, which tends to be boundary lubrication. Therefore, the lubricating effect of the lubricating medium in the cutting process depends on the lubricating property of the lubricating medium and its permeability, film forming ability, etc.: the material in the cutting zone has low temperature brittleness and strong penetration of the additive during the sub-dry cutting process, so that it is cooled compared with the cutting fluid. The cutting temperature is lower.
2. Tool wear and machining accuracy
It can be seen from Fig. 5 that the influence of cutting method on tool wear and dimensional accuracy is from small to large in the order of sub-dry cutting, air cooling and natural cooling cutting, wherein the sub-dry cutting tool has small wear and high precision; natural cooling Cutting tools are severely worn and have low dimensional accuracy. Can be analyzed as follows:
a. In the cutting process, the most important factor affecting the tool life is the cutting heat of the cutting point. The increase of the cutting heat temperature will soften the cutting edge of the tool and accelerate the tool wear. Cold air cooling results in low temperature brittleness of the workpiece material in the cutting zone, reduces cutting force, reduces cutting temperature and prevents tool softening; sub-dry cutting not only prevents softening of the tool, but also reduces the contact between the tool, the chip and the workpiece. Friction, so tool wear is minimal.
b. Factors affecting the dimensional stability of the machined part are mainly the amount of change in the workpiece temperature and tool wear during the cutting process. The workpiece is deformed greatly and the tool wears quickly. The dimensional stability of the parts is poor. In the above-mentioned cooling cutting, the cutting temperature is the lowest, the cutting force is the smallest, and the tool wear is the least. Therefore, the dimensional stability of the parts is the best: after research, the sub-cutting The dimensional error is 5.5% of the natural cooling cutting size error; air cooling is 38% of natural cooling; under the same process conditions, the natural cooling tool wear value VB = 1.2mm, and the air cooling is only 0.42mm. It can be seen that sub-dry cutting can extend the service life of the tool and maintain the dimensional stability ratio of the machined parts, which is conducive to precision and automated processing.
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