Research on the bottom hole size of internal thread extrusion

The tooth height rate of the thread directly determines the connection quality of the thread, and the diameter of the preformed bottom hole of cold extruded thread workpiece determines the size of the tooth height rate. In this paper, according to the theory of equal volume in plastic deformation of metal and combining the crest defects that occur during the extrusion test, a formula for calculating the diameter of the preformed bottom hole of the workpiece in the process of internal thread cold extrusion is deduced. Further, the formula is modified and improved by taking into account the size of the internal thread, the structural parameters of the extrusion tap, and the plastic performance parameters of the workpiece itself. Three metal materials of 40Cr, 45# steel, and Al-6061 aluminum alloy are selected for the workpiece, and the bottom holes of their specimens are preformed according to the formula. And the thread numerical simulation and experimental verification are carried out. It is found that the results of numerical simulation and experimental verification of tooth height rate of the internal thread are almost the same, with the error less than 5%. The tooth height rate of the thread can be guaranteed to be 60 ~ 96% when the thread is extruded within reasonable bottom hole diameter range. It shows that the calculation formula of preformed bottom hole diameter deduced in this paper based on the plastic deformation theory is reliable and can be popularized. It provides a theoretical reference for the determination of the bottom hole diameter in process of internal thread cold extrusion.