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While improving the machining accuracy of parts, it is particularly important to study the assembly process, especially the assembly of interference fit. Usually, in the precision assembly of interference fit, the pressure assembly process and the heating containment method are often used. For the pressure assembly process, it is generally used in applications where the interference is small; for the heating package method, it should be carefully selected according to the thermal sensitivity of the components.
Therefore, for the assembly of high-precision spindles (such as electric spindles), it is very unsuitable to use the above process. Therefore, using the heat-sensitive and mechanical properties of the parts, combined with the high-precision spindle structure, a cold shrink assembly process was developed.
1. Cold shrinking process test and application
On the basis of technical analysis, the following experimental research was carried out: cooling temperature and cold shrinkage test of parts with different materials; cooling temperature and cold shrinkage test of parts with different materials, and freezing time were 2h. The test data is shown in Table 1 and Table 2.
It can be seen from the process test data that the parts after freezing for 2 hours at -30~-40°C are transformed from the assembly interference amount of 0.02-0.05mm into the assembly process of the clearance fit amount of 0.1-0.13mm, which greatly reduces the assembly strength and error. To ensure the quality of the assembly.
Therefore, the relevant data is consulted, and the theoretical calculation formula of the cold shrinking process is obtained. T=[-(l+a)/dα+t ] K where “-†is used for cold fitting; T is total cold shrinking (or Heating) temperature (°C); a is the actual required gap (0.05~0.09mm); α is the coefficient of linear expansion 1/°C (copper = 17.6×10-6, steel = 12×10-6); l is the actual interference Quantity (mm); t is the assembly ambient temperature (°C); d is the diameter or shaft diameter (mm); K is the compensation part before the fit, due to the temperature generated by various conditions and the required clearance for the free installation of the parts The coefficient is generally taken from 1.1 to 1.4; the value of K is generally a large value on a hot day and a small value on a cold day.
Example 1: Copper sleeve, material ZQSn6-6-3, d = 105 mm, I = 0.015 mm (measured), α = 17.6 × 10-6, t = 30 ° C, K = 1.4.
2. Application verification
According to the above theory, verification in the company's electric spindle assembly proved to be feasible. When the cold shrinking process is adopted, the cold box shrinkage is more suitable. It is not affected by any external factors, and it is easy to use and shortens the production cycle. According to the external parts, the low-temperature box volume is selected. The lowest temperature of the low-temperature box can reach -30~-40°C, which is suitable for the cold shrinking process requirements of general parts.
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