Fabrication of high-precision freeform surface on die steel by ultrasonic-assisted slow tool servo.

The mold core fabrication of a freeform surface on die steel by ultra-precision machining can make the optical elements of freeform be mass-produced by plastic injection and glass mold pressing. However, because steel is a typical difficult-to-cut material, the technical limitations of existing machining methods hardly meet the current requirements of design. In this paper, a novel machining method, one-dimension ultrasonic-assisted slow tool servo (UASTS) turning, is proposed to manufacture the freeform surface with high-precision and large-steepness on the die steel. Aiming at the characteristics of UASTS turning, the tool trajectory is generated by analyzing the compensation of tool radius and confirming the position of ultrasonic displacement. The 2D surface model contours of residual tool marks are established for predicting the 3D surface topography based on considering the effects of kinematics, material elastic recovery and plastic side flow. In the experiments, the large-amplitude bidirectional sinusoidal wave grid (BSWG) surface is successfully fabricated by UASTS technology on the material, Polmax steel, for which the value Rt of surface roughness is less than 70nm and the value Ra of surface roughness can achieve to 3.298nm. The results show that the freeform surface with high-precision and large-steepness can be machined by UASTS turning technology on mold steel.