Thermal Deformation Of The Workpiece
During the cutting process, the thermal deformation of the workpiece is mainly caused by the cutting heat, and the thermal deformation of some precision parts and large parts is also affected by the ambient temperature. The heating of the machine tool workpiece can be divided into two cases:
Relatively Uniform Heating
The lathe is like the CNC machining of general shaft workpieces. The workpiece is uniformly heated. During the cutting process, the cutting heat gradually increases the temperature of the workpiece, and the diameter also gradually increases due to the expansion, so that the actual back-feeding amount of the tool also gradually increases. Cylindricity errors occur after the lathe has cooled the workpiece. If the precision of the workpiece diameter is high, it will become smaller due to cooling shrinkage, or even be scrapped out of tolerance.
Especially when processing slender shafts between two centers, the linear expansion caused by thermal deformation will elongate the workpiece.
If the distance between the centers remains unchanged, the workpiece will be bent and deformed by the pressure of the two centers, which will not only make the T . The cylindricity error of the piece occurs, and there is a danger of being thrown out in severe cases.
[Example 3-1]Example of calculating the elongation of the workpiece when the temperature rises
Turning diameter d: 40mm, length L = 1500mm, the material is 45 steel machine tool smooth bar, the known suffocation temperature is f. = 20cc, due to the influence of cutting heat during turning, the temperature of the workpiece rises to f=60°C, and the elongation of the workpiece is obtained A/.
Solution: Look up the table to get: 45 steel linear expansion coefficient d. =11.95×lO-6 gate C
According to the calculation formula AL= a/L(f, – to)
=11 95 x l0 mountain/℃×150(X)×(60-20)
=0.6954mm
Uneven Heating 2
According to the structural characteristics of the workpiece shape, the heating of each part of the workpiece is uneven. During cutting, the B side does not participate in cutting, and the cutting force of the lathe only acts on the A side, and generates cutting heat to increase the temperature, so the temperature of the A side is higher than that of the B side. When the workpiece is cooled, the A side is generated due to shrinkage Anti-arching error.
Thermal Deformation Of The Tool
The thermal deformation of the tool is mainly caused by the cutting heat. Although only part of the cutting heat is dissipated to the tool, due to the small size of the tool and the small heat capacity, the cutting part of the tool usually reaches a very high temperature and a large amount of thermal elongation. For example, when turning with high-speed steel tools, its The temperature of the blade can reach about 7000C, and the thermal elongation of the tool can reach 0.03-0.05 mm, which has a certain influence on the machining accuracy. This effect is obvious in the early stage of cutting, but after reaching the thermal equilibrium state, the thermal deformation of the tool tends to be stable, and its effect is relatively small. Therefore, in the continuous operation of batch production, special attention should be paid to the change of machining dimensions at the beginning of cutting.
Thermal Deformation Of The Machine Tool
During the cutting process, the temperature of each part of the machine tool will gradually increase due to the influence of internal and external heat sources, but due to the asymmetry or uneven heating of the machine tool structure, it will produce uneven and asymmetric thermal deformation.
For example, in a lathe, heat is generated due to the friction of the bearings and gears in the headstock. After working for a period of time, due to the large force at the front end of the main shaft, the temperature of the front end of the headstock rises higher than that of the rear end, causing it to deviate in the vertical and horizontal planes. Move or tilt; the heat of the spindle box is transferred to the bed, so that the temperature of the upper part of the bed is higher than the bottom, and the guide rail is arched upward, which intensifies the inclination of the main shaft.
The uneven thermal deformation of each part of the machine tool not only destroys the geometric accuracy of the machine tool, but also changes the relationship between various moving positions and transmission ratios, thereby reducing the machining accuracy of the machine tool and causing machining errors. Only when the thermal equilibrium state is reached, can the machining accuracy be stabilized. However, due to the large size and heat capacity of the machine tool, this process takes a long time, so it is difficult to control the machining dimensions in a short time.
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