Back-Pressure Forging Using a Servo Die Cushion

Abstract

The application of a servo die cushion to the back-pressure forging process improves the shape accuracy of forged parts. Servo die cushions have excellent performance in precise motion control and high responsiveness to set loads. To use a servo die cushion to obtain these features, back pressure is applied to the bottom outer punch during forward extrusion-type forging. Without back pressure, material flow delay around the central counter punch corner results in an unfilled corner at the bottom outer punch. Applying back pressure to the outer punch reduces the area of the unfilled corner. However, extensive back pressure at the beginning of the forming process causes burrs at the bottom because of the clearance between the counter punch and the outer punch; variable back-pressure settings along the punch stroke effectively remove burrs while also providing a smaller unfilled area by allowing for low back pressure at the beginning of the forming process and high back pressure during the forming process. Furthermore, using the flexible slide motion of the servo press to vary the punch motion leads to even further reduction in the unfilled area.