Enhancing Ductile-mode Cutting of Calcium Fluoride Single Crystals with Solidified Coating
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Enhancing Ductile‑mode Cutting of Calcium Fluoride Single Crystals with Solidified Coating Yan Jin Lee1 · Jing Yi Chong1 · Akshay Chaudhari1 · Hao Wang1 Received: 10 January 2019 / Revised: 7 May 2019 / Accepted: 23 May 2019 © Korean Society for Precision Engineering 2019
Abstract Positive improvements have been observed during machining brittle materials under high hydrostatic pressure, but the techniques to achieve such desirable effects often utilize complex and expensive equipment or tools. This work presents a cost-efficient method to achieve ductile-mode machining of brittle materials at higher uncut chip thicknesses, by the application of a solidified coating on workpiece surface before a machining process. Orthogonal microcutting experiments were conducted on calcium fluoride single crystals oriented with the (111) plane and an increase in critical uncut chip thickness was observed with the solidified coating. The primary cause has been resolved to be mechanical-related and results in a stabilized microcutting process. Transmission electron microscopy provided evidence of slip deformation occurring in the machined subsurface regions and a layer thickness of subsurface damages reduced by ~ 45% under the influence of the solidified coating. In addition, erratic fluctuations in direction of the resultant machining force were subdued with the applied coating, which is proved to be caused by the compressive stresses induced from the sandwiching of the C aF2 material between the tool and the solidified coating. The proposed technique successfully reduces the cost and pollution in the fabrication process of optical components from the use of coolant in an ultraprecision machining process to the time consumed by eliminating the subsurface damage with abrasive slurries in post-machining polishing. Keywords Calcium fluoride · Ductile-mode cutting · Microcutting · Surface integrity · Ultraprecision machining List of Symbols r Tool nose radius w Width of groove ap Uncut chip thickness Fc Cutting force Ft Thrust force FR Resultant force Fs Shear force ζ Resultant force inclination angle
1 Introduction Over the recent years, the development of materials in the semiconductor and optics industries is extremely popular in view of the exclusive electronic and physical * Hao Wang [email protected] 1
Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
properties. Calcium fluoride (CaF2) is commonly used in advanced optical systems due to its wide transmission range (0.125–12 µm) and chemical stability [1]. However, this material presents a great tendency to exhibit brittle failure through the formation of microcracks on the machined surface. With the aid of an ultraprecision machining center equipped with single crystal diamond cutting tool, ductilemode machining can be realized with a deterministic uncut chip thickness to produce crack-free freeform surfa
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