Cold Extrusion of External Gear Having Borethrough Expansion of Inner Diameter

The shaping of spur gears by cold extrusion through the expansion of the inner diameter of a workpiece is examined experimentally. A workpiece is set with its inner diameter in the front part of a stepped mandrel, which is placed in a container. Then, it is extruded with the expansion of its inner diameter, by passing through the tooth part of the die. The specifications of gear examined are as follows: module m=1.25-2.0 mm, pressure angle 20 deg, number of teeth Z=18-16, and whole depth h=2.25 module. The material used is the low-carbon steel, JIS S15C. The criterion for the shaping of complete teeth is discussed in terms of reduction in area, expansion ratio of the inner diameter of the workpiece and other factors. This method can produce spur gears of much better shape, i.e. having complete teeth, than the former proposed extrusion method, without reducing the outer diameter of the workpiece. Especially, a gear having a relatively medium-to-large sized bore can also be well shaped by the present method. The needed pressure of the punch, mandrel, and die is far below the strength of the punch and die materials.