Microlens crack-generation model on hard-brittle material using slow-tool-servo turning

Abstract

Slow tool servo turning is an advanced ultraprecision machining approach that enables the one-step fabrication of microlens structures on component surfaces. However, when applied to hard-brittle materials, the process is challenged by crack generation, which significantly compromises the functional performance of the final components. This study proposes a theoretical model to quantitatively analyze crack formation during microlens fabrication using slow tool servo turning. Three types of cutting modes, “full cut”, “transformation cut”, and “half cut”, involved in microlens generation are firstly introduced and integrated into the analytical model. Each trajectory point along the tool path is evaluated individually to determine its potential for crack initiation. The calculation of specific cutting energy accounts for tool edge effect and tool path compensation. A log-normal probability distribution function is implemented to generate random crack lengths and assess whether the crack occurs on the microlens surface. Results show that crack position, depth and density on the microlens is highly dependent on the angular position of critical uncut chip thickness. At a small feed, trajectory points on the feed-in side of the microlens are more prone to generating half-cut cracks on the feed-out side. As the feed increases, cracks related to the transformation-cut and full-cut modes tend to occur sequentially, resulting in deeper, denser cracks and a shift of the crack zone toward the microlens center. The proposed model provides a theoretical framework for quantitatively understanding fracture formation in hard-brittle materials during slow tool servo microlens fabrication.