Study on helical milling of small holes in zirconia ceramics with PCD tools

Fully sintered zirconia ceramic material (ZrO₂) is widely used because of its excellent properties of high strength, high hardness, and excessive temperature resistance. However, machining high quality small holes in ZrO₂ workpiece is an ongoing challenge. In this study, the helical milling process using PCD tools is investigated to fabricate high quality small holes in ZrO₂. A three-factor and three-level response surface method experiment is designed taking the crack area at the hole entrance (Δ_w), inner hole surface roughness (R_a) and crack area at the hole exit (Δ_e) as response values. The influences of spindle speed (n), feed per tooth (f_z), and axial depth of cut (a_p) on each response value and their interacting relationships are investigated. Then, the regression models of each response value are established. The optimal sets of parameters for helical milling have been obtained by optimizing each independent variable. Furthermore, the accuracy of the constructed regression models has been evaluated. The results demonstrate that Δ_w, R_a and Δ_e values are reduced by 73.94%, 82.18%, and 59.08% respectively using the optimized helical milling process and parameters compared with the results before optimization. Two strategies are proposed to further reduce Δ_e, and the final results demonstrate that Δ_e can be further decreased by up to 49.56%. Analysis of the cutting edge by SEM shows that the main wear mechanism is stripping wear. The EDS analysis of the cutting edge shows that there is no significant change in the cutting edge material.