Tool design for inner race cold forging with skew-type cross ball grooves

• A cold forging mechanism for the inner race with six skew-angled cross ball grooves was proposed.
• To realize the inner race, six-segmentalized skewed ball grooving dies were introduced and applied to the numerical and experimental investigations.
• Structural integrity evaluation was performed to prevent the fracture of the skewed and segmentalized grooving die.
• The simulated and forged inner races were compared with the target geometries.
• The dimensional accuracy of the forged inner race was achieved within the variation of about 2.0%.

The inner race of CV (Constant Velocity) joints with asymmetric six cross ball grooves is an important load-supporting automobile part that transmits torque between the transmission and a driven wheel. This metal component has conventionally been produced by a machining sequence that includes material removal. As an alternative, a cold forging procedure using a semi-closed die is presented. The proposed cold forging is composed of six longitudinally segmentalized cross-ball grooving dies, a die holder, and forging punches, which were chosen with consideration of their operation mechanisms. Process design and detailed tool design have been conducted. To ensure the appropriateness of the suggested process with respect to deformation behavior, a 3-dimensional finite element simulation including forging load prediction is carried out, and experimental investigations are also performed using SCr420H steel as the initial billet material. Unexpected fracture failure of the die occurred due to the asymmetric geometry of the segmentalized cross ball grooving die, in response to which modification of the die geometry was carried out based on the results from the structural integrity evaluation, thereafter, the inner race was appropriately produced using the modified die. The dimensional accuracy at three critical cross sections of the forged inner race is investigated and compared. It is shown that the proposed semi-closed die cold forging could be successfully applied to the production of the inner race with the six skew-type cross ball grooves, and achieved with the dimensional variation under about 2.0%.