Ultraprecision ductile mode machining of glass by micromilling process

Abstract

Glass is considered as a difficult-to-machine material because of its susceptibility to brittle fracture during machining. To avoid brittle fracture and achieve superior surface finish, glass must be machined in ductile mode. Mostly, ductile mode machining is performed by a single point cutting tool. This paper presents the results of an experimental study to achieve ductile mode machining of glass by micromilling process. Fracture-free slots have been cut in soda-lime glass workpiece by micro-endmilling. Experimental results have established that fracture-free slots can be machined in glass by micromilling process within the controlled set of cutting conditions. These cutting conditions are conducive to highly compressive hydrostatic stresses developed in the cutting zone to suppress the crack propagation during machining. The feed per edge and the axial depth of the cut have been identified as the critical factors for the ductile–brittle transition in microcutting of glass by milling process.

Introduction

Glass is considered as a representative functional material for optics, electronics and fluidics. It is an amorphous solid material. Glass is supercooled from its molten state to a rigid state at below transition temperature. The resultant crystal structure is amorphous, which is responsible for its high brittleness. Under loading, a crack is initiated from a flaw in the glass structure that propagates under continuous loading and causes brittle fracture after reaching the critical value. The critical size of crack for brittle fracture of glass is very small because of its low fracture toughness. High cutting stresses developed in the cutting zone during machining process can easily lead to brittle fracture of glass. Besides this, glass has high hardness. The low fracture toughness and high hardness account for poor machinability of glass. Notwithstanding, glass has some superior physical, mechanical, optical or electronic properties [1]. These properties include high hardness, homogeneity, optical transparency, isotropy and various refractive indices [2]. Due to these properties, glass has extensive utility in optical and electro-mechanical industries. It is widely used in opto-electronics, wafers, biological instrumentation, telescopes lenses, microscopic slides, biomedical parts and plastic injection molds. Glasses are of many types depending upon its chemical composition. Soda-lime glass is the most prevalent and inexpensive commercially available type of glass. As the use of glass is expanding in precision product manufacturing, an improvement in mechanical micromachining processes is highly desired for machining complex shapes in glass work-material more efficiently. If machined in conventional way, the material removal in machining of glass takes place by brittle fracture resulting in deteriorated machined surface. Glass must be machined in ductile mode to achieve superior surface finish and to maintain high surface integrity.

The technological justification of ductile mode machining is based upon micro-indentation results of brittle materials. Marshall and Lawn [3] established that if the indenter load and depth of penetration are below a critical value during the indentation process, even extremely brittle materials like glass can undergo plastic deformation without showing any signs of fracture. Machining is essentially an indentation like process. Blackley and Scattergood [4] achieved similar results in machining of brittle materials. They reported that if the undeformed chip thickness is less than a critical value, the material can be removed by plastic deformation in machining of brittle materials. They achieved fracture-free machining of germanium and silicon by maintaining feed rate and undeformed chip thickness below a critical value. Ductile mode machining of various brittle materials with a single point cutting tool has been reported extensively in the past two decades. Nakasuji et al. [5] examined the machinability of several brittle materials in connection with indentation results. He stated that plastic deformation is the predominant mechanism of the material removal when the resolved shear stress in the easy slip direction exceeds a certain critical value inherent to the work-material before cleavage takes place. Fang [6] established through diamond turning of a single crystal silicon that surface roughness in ductile mode machining is dictated by feed per revolution and that a further reduction in average roughness value is possible by selecting finer feed per revolution. Liu et al. [7] reported that ductile mode cutting of brittle materials is possible when the cutting edge radius is at submicron level and is mainly determined by undeformed chip thickness, which is mainly influenced by the feed rate and the depth of the cut. Patten et al. [8] performed ductile machining of silicon carbide and suggested that ductility of many semiconductors and ceramics during machining is due to the formation of a high pressure phase at the cutting edge leading to plastic response rather than brittle fracture at small size scales.

Single point diamond turning has been applied successfully in machining glass. Initial studies used the increased plasticity of glass at elevated temperature. Brehm et al. [9] reported single point turning of glass enabled by thermal softening at elevated temperature to achieve transparent surfaces. Schinker and Doll [10] achieved transparent surfaces on a variety of inorganic glasses by diamond turning due to adiabatic melting and instantly annealing at higher cutting speeds. They also reported that cutting conditions to obtain transparent machining were strongly dependent on the type of glass. Puttick et al. [11] performed grooving test on soda-lime glass by a diamond tool and reported that below a critical depth of the cut predicted in order of magnitude by a fracture mechanics analysis, the material is removed by the action of plastic flow, leaving crack-free surfaces. Fang and Zhang [12] reported that below a certain threshold defined by undeformed chip thickness, it is believed that the energy required to propagate cracks is larger than the energy required for plastic deformation and hence the plastic deformation is the predominant mechanism of material removal below this threshold.

Whilst ductile mode machining by a single point cutting tool is a well established technology, ductile mode machining with multipoint cutting tool is still an emerging area of research. Single point cutting process has limited applications for machining complex features in work-material. On the other hand, micro-endmilling is a versatile process for machining three-dimensional shapes and free forms. However, milling process has not been applied more frequently for machining brittle materials such as glass and ceramics.

Due to mounting interest in using hard and brittle materials for mold and die manufacturing in the recent years, it is highly desired to extend the application of micromilling for machining brittle materials. Typically, glass is press molded to desired shape for non-imaging application. For optical application, a shape on glass is typically made by grinding, followed by a series of polishing class of processes to remove the damage caused by grinding [13]. Some non-conventional machining processes like electric discharge machining (EDM), ultrasonic machining and laser ablation have also been applied for ultraprecision machining of glass. However, machining of three-dimensional features in brittle work-material with superior surface finish is still extremely tedious and time consuming process. Processes like photolithography, chemical etching, chemical–mechanical polishing (CMP) and electrolytic in-process dressing (ELID) grinding have been applied to achieve nanometric surface finish on glass work-material. The slow nature of the machining process and the dependence of material removal on the chemical reaction limit the material removal rate. Furthermore, abrasive based processes impart subsurface damage to the machined surface that adversely affects the fatigue characteristics of the machined part. Contour control of the feature to be machined is not easily achievable due to non-deterministic nature of the loose abrasive processes. This causes flatness error on the machined surface. Compared to this scenario, ductile mode micromilling can machine complex contours in brittle material in a cost efficient way. The contour of the machined features can be controlled more precisely with a fixed mechanical tool through numerical control.

Takeuchi et al. [14] performed ultraprecision three-dimensional micromachining of glass workpiece by a diamond ball endmill. They obtained a glass mask of 1 mm in diameter and 30 μm in height with a surface roughness of 50 nm. Matsumura et al. [15] performed analysis of cutting force in the milling process of glass. He reported that ductile mode was characterized by smooth signal of cutting force recorded by dynamometer while brittle mode machining was characterized by sharp fluctuations in the cutting force signal due to the occurrence of repeated fracture above the critical value of chip thickness. Matsumura and Ono [16] established that much deeper fracture-free grooves can be machined on glass by using ball endmill tilted at an appropriate angle towards the workpiece surface. They also demonstrated that ductile mode machining of glass is possible with carbide endmill if the edge roundness is at submicron level. It was further established that optimum cutting conditions for milling process to achieve ductile machining of glass are conducive to the development of a highly compressive hydrostatic force in the cutting zone necessary for suppression of crack propagation during machining [17].

The fundamental studies are still needed to better understand the mechanism of ductile mode machining of brittle materials by milling process. Furthermore, slot milling of glass has not been performed by using square endmill. The literature review presented here has identified the need to machine three-dimensional square shaped slots in glass with improved productivity by applying mechanical micromachining process.

This study attempts to perform ductile regime machining of glass by micromilling process to achieve fracture-free slots. Ductile machining of glass has mostly been performed with diamond cutting tool. Hence, one of the most critical issues in ductile mode machining has been the high cost of diamond tool due to severe tool wear [12]. This study has utilized superfine grain carbide endmill for machining of glass. According to author’s best knowledge, this is the first study that has used a square end micromilling cutter made of carbide to perform slot milling of glass work-material. The research work is expected to make a significant contribution for machining three-dimensional sharp edged features in hard and brittle materials by micromilling process. Furthermore, a study on tool wear has been carried out to identify the wear pattern of micro-endmill in cutting process of glass to open a window of opportunity for future studies to focus on diminishing the tool wear by applying appropriate strategies.