The cylindricity error of the blank's diameter before rolling is carefully controlled by grinding the base data according to the previously mentioned formula. The length of the blank must not exceed the designated limit, and the surface roughness of the outer diameter should not exceed the support height H (as shown in Figure 1). Figure 1 presents a schematic representation of the thread rolling process based on the recommended formula, where D represents the major diameter of the thread, h denotes the distance between the bearing and roller axes, and a refers to the height difference between the workpiece axis and the roller axis, which is suggested to be within an optimal range.
The value of 'a' directly impacts the quality of the final thread and should ideally be set after processing. To enhance the wear resistance and strength of the contact area, a strip of hard alloy is inserted at the top of the support piece. Additionally, the central parallelism of the support surface relative to the rolling wheel is strictly controlled, along with the tolerance of the support height. Before rolling begins, 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’s surface roughness meets the required standards. This helps reduce or eliminate burrs and prevents misalignment during the rolling process.
Rolling force and rolling time are critical factors in the cold working process, where metal is plastically deformed to create the thread. This process leads to cold hardening, thereby increasing the hardness and strength of the thread surface—especially the base material. When test bolts undergo heat treatment and reach the desired hardness, the resulting hardness profile (HB) of the precision bolt thread is illustrated in Figure 2. Using chrome vanadium steel for M8 threads, it can be observed that the hardness at the thread root reaches approximately HB600. Due to the high heat generated during rolling, the rolling wheel experiences increased wear. To minimize this heat and extend the tool life, it is essential to select an appropriate rolling time based on the thread diameter, as suggested by the hardness profile in Figure 2.
Moreover, since rolling time is closely related to the rolling pressure, initial production often relied on calculated values or manual settings. However, using conventional rolling pressure led to longer rolling times, excessive heat generation, and severe damage to the rolling wheel. To address this issue, the rolling pressure was increased through testing, reducing the rolling time by up to 23 times and eventually reaching 2.5 times the theoretical value. This significantly reduced heat generation, extended the rolling wheel's life by threefold, and improved the thread surface finish.
Rolling speed (V) plays a crucial role in determining the degree and rate of material deformation, directly affecting thread quality. It is advisable to determine the optimal rolling speed through experimentation. Similarly, the feed amount (S) must be chosen carefully. While a larger feed rate increases machine productivity, it can cause overheating and affect the roundness, outer diameter, and inner diameter of the thread. Therefore, selecting the feed rate requires a balance between efficiency and machining accuracy. Through extensive testing, an ideal feed rate of Sr was identified.
After implementing these optimized process parameters, the thread precision and surface roughness achieved fully meet the technical specifications. Based on real-world production over recent years, the thread processing pass rate has reached 90%, demonstrating excellent performance in assembly and use within locomotives.
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