Analysis of Thickness Error Mechanisms in Flexible Vacuum Fixture-Based Milling of Thin Al6061 Skins

The mechanisms of thickness error occurring during the milling of 3.0 mm thin aluminum Al6061 skins were investigated using a flexible vacuum fixture with a 3 × 3 array of vacuum pads. The effects of fixture setting error, cutting-induced deformation, and post-machining thickness distribution were evaluated for three specimens. The correlation analysis between pin height setting error and pocket-milled thickness yielded no meaningful relationship with the resulting thickness error. However, the skin underwent repeated elastic deformation under cutting forces. Owing to lever-type deformation about the supporting pins, the measured displacement at a fixed location gradually increased, reaching approximately 0.06 mm on average. Process improvements, such as reducing tool-path interval and applying separated roughing and finishing, lowered the maximum-to-minimum thickness deviation from 1.49 to 0.83 mm and further to 0.27 mm. This study highlights the necessity of deformation-minimization techniques to achieve high thickness accuracy in vacuum-supported thin-plate milling processes.