Cold heading rollers are a method of obtaining roller blanks by using molds to force plastic flow in metal segments at room temperature, filling the space formed by the concave die and punch. The mechanical performance of rollers has been improved, and the plastic deformation generated causes the material to undergo cold work hardening. The metal streamline is not cut off, and the original defects of the metal will be compacted.
This production method can save raw materials; High productivity, generally with a high degree of automation for cold heading machines, ranging from 70 to 100 pieces per minute; The roller size and shape are accurate, and the surface roughness value is low. The accuracy of the mold and machine tool can ensure the size and shape accuracy of the roller. In cold heading, the metal surface is smoothed by the smooth surface of the mold under high pressure, and the surface roughness value is low.
When cold heading tapered rollers, before forming, except for the chamfer area at the small end of the roller and the column center, most of the substrate is subjected to uniaxial compressive stress and biaxial tensile stress. The closer it is to the large end of the roller, the greater the tensile stress, which can cause deformation between metal grains and reduce metal plasticity. Therefore, when the shape and size design of the blank or mold is unreasonable, the material is poor, and the process is improper, the cold heading roller often cracks at the large end chamfer.
In addition, the friction between the blank and the mold in the cold heading roller, the uneven internal structure of the material, and the unreasonable size and shape of the mold can also cause additional stress inside the roller, reduce the plasticity of the metal, increase the deformation resistance of the metal, and generate residual stress inside the roller.
This additional stress can cause changes in the size and shape of the rollers, as well as a decrease in process performance. Especially the additional stress caused by external friction is detrimental to the quality and processing technology of the roller. It mainly occurs in the axial area of the small end chamfer of the roller, reducing the internal and external quality of the chamfer, increasing mold wear, and reducing mold life. The main factors affecting friction include material properties, mold structure shape, surface quality, and lubrication effect.
Due to the fact that cold heading rollers are carried out at room temperature, they have great deformation resistance, especially when the material section is filled with mold space to form the roller blank. The entire roller body is basically subjected to triaxial compressive stress, and the deformation resistance is extremely high. The larger the roller, the greater the deformation resistance. In addition, when the deformation degree of the roller during cold heading exceeds the allowable deformation degree of the material itself, cracks will form on the circumferential surface of the roller.
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