The cylindricity error of the blank's diameter before rolling is carefully controlled by grinding the basic dimensions according to the recommended formula. The length of the blank must not exceed the specified limit, and the surface roughness of the outer diameter should not exceed the support height H, as illustrated in Figure 1. Figure 1 presents a schematic diagram of the thread rolling process based on the suggested formula, where D represents the major diameter of the thread, h is the distance between the bearing and roller axis, and a refers to the height difference between the workpiece axis and the roller axis, which is set to an optimal value.
The a value significantly influences the quality of the final thread, and it is essential to achieve the ideal condition after processing. To enhance the wear resistance and strength of the support contact surface, a strip-shaped hard alloy is embedded at the top of the support piece. Additionally, the central parallelism of the support surface relative to the rolling wheel is strictly controlled within a certain range, and the support height tolerance is also maintained within limits.
Before the rolling operation, the position of the rolling wheel is adjusted using the test touch method, and the half-pitch processing method of the rolling wheel is determined to ensure that the thread surface roughness meets the required standards. This helps reduce or eliminate burrs and prevents thread disorder during the rolling process.
Rolling force and rolling time selection are critical factors in the cold working process. During rolling, metal undergoes plastic deformation to form threads, and cold hardening occurs, which increases the hardness and strength of the thread surface, especially the base material. When test bolts are heat-treated and reach the desired hardness, the hardness profile (HB) of the precision bolt thread surface can be observed, as shown in Figure 2. For example, with chrome vanadium steel and an M8 thread size, the thread root hardness reaches approximately HB600. The high heat generated during rolling increases wear on the rolling wheel. To minimize heat generation and extend the life of the rolling wheel, it is crucial to choose an appropriate rolling time based on the thread diameter.
In initial production, rolling pressure was often selected manually or calculated theoretically, which led to longer rolling times, excessive heat, and severe damage to the rolling wheel. To address this issue, experiments showed that increasing the rolling pressure could significantly reduce rolling time—by up to 2.5 times the theoretical value. This resulted in less heat generation, a threefold increase in rolling wheel life, and improved thread surface roughness.
Rolling speed (V) plays a key role in determining the degree and rate of material deformation, directly affecting the quality of the thread. It is advisable to determine the optimal rolling speed through experimental testing. Similarly, the feed amount (S) must be carefully selected. A large feed amount may increase machine tool productivity but can cause excessive heating and affect the roundness and dimensional accuracy of the thread. Therefore, the feed rate should be chosen with both efficiency and machining accuracy in mind. Through experimentation, it was found that Sr is the most suitable feed rate.
After applying these optimized process parameters, the thread precision and surface roughness achieved fully meet the technical requirements. Based on actual production over recent years, the pass rate for thread processing has reached 90%, and the performance in assembly and use for locomotive components has been highly satisfactory.
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