Mild steel, semi-hard steel, nickel-chromium steel, normal-Fire tool steel, brass, light alloy and other materials can be used as blank to make screws by rolling and forming. It is difficult to get smooth and smooth surface when cutting some materials with low hardness, and the lower the hardness of the blank when rolling, the smoother the surface of the workpiece. In contrast, readily cut steel is suitable for cutting, but it is prone to surface peeling when rolled. According to whether the billet surface is smooth after rolling, the deformation resistance of the material and the fullness degree of the thread, it is judged whether the material is suitable for rolling processing. In addition, the head of the screw is upsetting forming, so the upsetting property of the material is also one of the conditions to judge whether the material is suitable for rolling thread.
During roll rolling, the blank surface contacts with the die in turn, and the contact part is gradually extruded to form the concave part of thread. The extruded metal gradually fills the die, forming a threaded tooth. Therefore, the billet flows only in the longitudinal plane passing through the screw axis, not along the circumferential direction. Correspondingly, the area near the bottom of the thread showed obvious work hardening, and the central part of the screw did not work hardening.
The blank diameter will be the main factor to determine the precision of the product when the die position has sufficient precision and other rolling conditions are relatively reasonable. When determining the blank diameter, it is usually designed and calculated according to the principle of volume invariability, that is, the amount of metal compression on the blank surface is equal to the amount of uplift forming the thread teeth. Alloy tool steels Cr12, Cr12MoV and CrWMn are often used in making die for rolling. Other materials with similar properties can also be used, such as steel with a relatively low alloy content when the blank is hard.