An experimental study on the machining characteristics in ductile-mode milling of BK-7 glass

Glass is considered as one of the most challenging materials to machine because of its high hardness coupled with high brittleness. The challenge, in machining such a brittle material, lies in achieving the material removal through plastic deformation rather than characteristic brittle fracture. It has already been established that every brittle material, no matter how brittle it is, can be machined in ductile mode under certain critical conditions. The critical conditions are material specific, and hence, every material tends to show unique behavior in terms of critical conditions during machining process. This paper outlines the results of an experimental study to determine the critical chip thickness for ductile–brittle transition, chip morphology, and the effect of cutting speed on the critical conditions in peripheral milling process of BK-7 glass. It is established experimentally that the cutting speed affects the chip morphology, machined surface quality, and critical conditions due to possible thermal effects in such a way that ductile–brittle transition phenomenon is facilitated at high cutting speeds.     

An experimental study of edge radius effect on cutting single crystal silicon

According to the hypothesis of ductile machining, brittle materials undergo a transition from brittle to ductile mode once a critical undeformed chip thickness is reached. Below this threshold, the energy required to propagate cracks is believed to be larger than the energy required for plastic deformation, so that plastic deformation is the predominant mechanism of material removal in machining these materials in this mode. An experimental study is conducted using diamond cutting for machining single crystal silicon. Analysis of the machined surfaces under a scanning electron microscope (SEM) and an atomic force microscope (AFM) identifies the brittle region and the ductile region. The study shows that the effect of the cutting edge radius possesses a critical importance in the cutting operation. Experimental results of taper cutting show a substantial difference in surface topography with diamond cutting tools of 0° rake angle and an extreme negative rake angle. Cutting with a diamond cutting tool of 0° rake angle could be in a ductile mode if the undeformed chip thickness is less than a critical value, while a ductile mode cutting using the latter tool could not be found in various undeformed chip thicknesses.     

Brittle–ductile mode cutting of glass based on controlling cracks initiation and propagation

Owing to brittleness and hardness, functional glass is one of the most difficult to cut materials. This paper proposes a new machining method—brittle–ductile mode machining combining both properties of brittle breakage and plastic flow of glass. Edge-indention experiments are first conducted in order to deduce the laws of crack initiation and propagation in the process of glass cutting, then a single-straight tool with big inclination angle is designed for glass cutting based on the laws of crack initiation and propagation and properties of plastic flow. With this new tool, the lateral and subsurface cracks initiation can be suppressed, and media cracks propagate away from machined surface. At the same time, the requirements for machining glass in ductile manner can be fulfilled. Validation experiments show that highly efficient and precise glass cutting can be achieved at the cutting depth of sub-millimeter level, and an integral and crack-free surface with good finish can be obtained. This method overcomes the process restriction on critical cutting depth and tool feed for ductile regime turning technology and can be transferred to mass production.     

A study of the effect of tool cutting edge radius on ductile cutting of silicon wafers

Ductile mode cutting of silicon wafers can be achieved under certain cutting conditions and tool geometry. An experimental investigation of the critical undeformed chip thickness in relation to the tool cutting edge radius for the brittle-ductile transition of chip formation in cutting of silicon wafers is presented in this paper. Experimental tests for cutting of silicon wafers using diamond tools of different cutting edge radii for a range of undeformed chip thickness are conducted on an ultra-precision lathe. Both ductile and brittle mode of chip formation processes are observed in the cutting tests. The results indicate that ductile cutting of silicon can be achieved at certain values of the undeformed chip thickness, which depends on the tool cutting edge radius. It is found that in cutting of silicon wafers with a certain tool cutting edge radius there is a critical value of undeformed chip thickness beyond which the chip formation changes from ductile mode to brittle mode. The ductile-brittle transition of chip formation varies with the tool cutting edge radius. Within the range of cutting conditions in the present study, it has also been found that the larger the cutting edge radius, the larger the critical undeformed chip thickness for the ductile-brittle transition in the chip formation.     

An experimental approach to study the capability of end-milling for microcutting of glass

Glass is a hard and brittle material. It is finding mounting quantum of applications in semiconductor, opto-electronics, and mold manufacturing sectors. However, glass is not amenable to machining because of its low fracture toughness. If machined with conventional approach, the mechanism of material removal in machining of glass is fracture based that results into poor quality of the machined surface and imparts subsurface damage. In order to achieve superior surface finish, glass must be machined in ductile mode. Ductile-mode machining is now a well-established technique but most of the work has been performed with single-point cutting processes. To assess the capability of ductile-mode machining with multipoint cutting process, fundamental studies are highly desired. This paper reports the results of an experimental investigation into ductile-mode machining of glass by milling process. Side-milling tests have been performed on the glass workpiece to identify the key parameters governing the ductile-brittle transition mechanism. Experimental results demonstrate that fracture-free surface can be machined on glass by milling process. Cutting forces were analyzed to comprehend the dynamic behavior of the cutting process in ductile mode.     

Study of the upper bound of tool edge radius in nanoscale ductile mode cutting of silicon wafer

In cutting of brittle materials, experimentally it was observed that there is an upper bound of tool cutting edge radius, beyond which, although the undeformed chip thickness is smaller than the tool cutting edge radius, the ductile mode cutting cannot be achieved. However, why there is an upper bound of tool cutting edge radius in nanoscale ductile mode cutting of brittle materials has not been fully understood. In this study, based on the tensile stress distribution and the characteristics of the distribution obtained from molecular dynamics simulation of nanoscale ductile cutting of silicon, an approximation for the tensile stress distribution was obtained. Using this tensile stress distribution with the principles of geometrical similarity and fracture mechanics, the critical conditions for the crack initiation have been determined. The result showed that there is a critical tool cutting edge radius, beyond which crack initiation can occur in the nanoscale cutting of silicon, and the chip formation mode is transferred from ductile to brittle. That is, this critical tool cutting edge radius is the upper bound of the tool cutting edge radius for ductile mode cutting of silicon.     

A review on ductile mode cutting of brittle materials

Brittle materials have been widely employed for industrial applications due to their excellent mechanical, optical, physical and chemical properties. But obtaining smooth and damage-free surface on brittle materials by traditional machining methods like grinding, lapping and polishing is very costly and extremely time consuming. Ductile mode cutting is a very promising way to achieve high quality and crack-free surfaces of brittle materials. Thus the study of ductile mode cutting of brittle materials has been attracting more and more efforts. This paper provides an overview of ductile mode cutting of brittle materials including ductile nature and plasticity of brittle materials, cutting mechanism, cutting characteristics, molecular dynamic simulation, critical undeformed chip thickness, brittle-ductile transition, subsurface damage, as well as a detailed discussion of ductile mode cutting enhancement. It is believed that ductile mode cutting of brittle materials could be achieved when both crack-free and no subsurface damage are obtained simultaneously.     

Micromechanical machining of soda lime glass

Micromechanical machining, which is the mechanical removal of materials using miniature cutting tools, is one of the fabrication methods in the microrealm that has recently attracted a great deal of attention because it has the advantage of being able to machine complex shapes from brittle materials. The most challenging problem in the mechanical machining of brittle material is the fabrication of fracture-free surfaces. To avoid brittle fractures, a thorough investigation is required to find the machining parameters in the ductile cutting regime, which is characterized by plastic deformation of the material when the chip thickness is smaller than the critical value. In this study, cutting forces and surface characteristics of soda lime glass are examined in detail. Conical scratch tests are performed to identify the critical chip thickness, and the cutting forces in the ductile regime are modeled. In addition, coated ball end mill cutters were used to perform machining on inclined soda lime glass to investigate the feed rate effects, up and down milling, and depth of cuts on the surface finish and to examine tool wear.     

AN EXPERIMENTAL STUDY OF ORTHOGONAL MACHINING OF GLASS

An experimental study of machining glass with a geometrically defined cutting tool is presented. Orthogonal cutting conditions are employed to permit a focus on the fundamental modes of chip and surface formation. Analysis of the machined surfaces under an optical microscope identifies four regimes that are distinctly different with respect to either chip formation or surface formation. For a very small target uncut chip thickness, one on the order of the cutting edge radius, pure rubbing of the edge with no chip formation is observed. Edge rubbing imparts light scuffmarks on the machined surface giving it a frosted appearance. At a larger uncut chip thickness, ductile-mode chip formation occurs ahead of the cutting edge and a scuffed surface remains after the subsequent rubbing of the edge across the freshly machined surface. A further increase in uncut chip thickness maintains a ductile-mode of chip formation, but surface damage initiates in the form of surface cracks that grow down into the machined surface and ahead of the tool. The transition to this machining mode is highly dependent on rake angle. Increasing the uncut chip thickness further causes brittle spalling of chips leaving half-clamshell shaped divots on the surface. This experimental identification of the machining modes and their dependence on uncut chip thickness and rake angle supports the use of geometrically defined cutting tools to machine glass in a rough-semi-finish-finish machining strategy as is traditionally employed for machining metals.     

AN EXPERIMENTAL STUDY OF ORTHOGONAL MACHINING OF GLASS

Abstract

An experimental study of machining glass with a geometrically defined cutting tool is presented. Orthogonal cutting conditions are employed to permit a focus on the fundamental modes of chip and surface formation. Analysis of the machined surfaces under an optical microscope identifies four regimes that are distinctly different with respect to either chip formation or surface formation. For a very small target uncut chip thickness, one on the order of the cutting edge radius, pure rubbing of the edge with no chip formation is observed. Edge rubbing imparts light scuffmarks on the machined surface giving it a frosted appearance. At a larger uncut chip thickness, ductile-mode chip formation occurs ahead of the cutting edge and a scuffed surface remains after the subsequent rubbing of the edge across the freshly machined surface. A further increase in uncut chip thickness maintains a ductile-mode of chip formation, but surface damage initiates in the form of surface cracks that grow down into the machined surface and ahead of the tool. The transition to this machining mode is highly dependent on rake angle. Increasing the uncut chip thickness further causes brittle spalling of chips leaving half-clamshell shaped divots on the surface. This experimental identification of the machining modes and their dependence on uncut chip thickness and rake angle supports the use of geometrically defined cutting tools to machine glass in a rough-semi-finish-finish machining strategy as is traditionally employed for machining metals.     

石英玻璃的单颗磨粒划擦应力场解析模型及损伤可控磨削机理研究

构建了单颗磨粒划擦各向同性硬脆材料的弹性应力场解析模型,并以此为基础提出单颗磨粒划擦各向同性硬脆材料表面的裂纹失稳扩展临界函数,临界函数包含原始表面应变速率、磨削液等因素对裂纹扩展造成的影响。将石英玻璃作为研究对象,深入分析了表面微裂纹损伤的可控磨削机理。在进行石英玻璃的磨削试验中,材料的磨削机理随单颗磨粒磨削深度的增加而变化,依次是塑性域去除、低载半脆性域去除、全脆性域去除和高载半脆性域去除。在1 mm/min的工件进给速度下,可以对石英玻璃进行塑性域磨削,从而获得无裂纹损伤的光滑磨削表面,然而其磨削效率较低,在实际生产中不能发挥理想的作用。对石英玻璃开展全脆性域磨削时,材料去除率较高、加工表面表面质量好、微裂纹损伤深度较小,砂轮自锐性良好,是一种优良的精密磨削工艺。     

Research into the transition of material removal mechanism for C/SiC in rotary ultrasonic face machining

Ceramic matrix composites of type C/SiC with superior properties have got increasing importance in many fields of industry, especially in the aerospace area. Rotary ultrasonic machining is a high-efficiency processing technology for these advanced materials. However, due to the inhomogeneity and anisotropy of these composites, the machining process is still challenging to achieve desired result due to the lack of understanding and control of material removal mechanism. In this paper, the maximum depth of penetration by diamond abrasives in workpiece material is proposed to quantify the material removal modes. A model of maximum depth of penetration for rotary ultrasonic face machining (RUFM) was developed based on the indentation theory. An experimental RUFM of C/SiC was carried out, and it revealed that the material removal mechanism transited from ductile mode to brittle fracture mode with the decrease of cutting speed. Similar transition was observed with the increase of feed rate and cutting depth. By comparing the measured cutting force with simulation, a critical depth of penetration for the cutting mechanism transition was defined at about 4 μm. The processed surface topography was studied, and the transition of material removal modes was identified by the sudden change of the 3D surface roughness map at the critical penetration depth. Thus, the maximum depth of penetration model developed in this paper can be applied to identify the ductile or brittle fracture removal mode in RUFM of C/SiC using the cutting parameters. This allows controlling the material removal mechanism to achieve desired machining efficiency and quality.     

Self-tuned ultrasonic elliptical vibration cutting for high-efficient machining of micro-optics arrays on brittle materials

Ultrasonic elliptical vibration cutting is a very promising technique for the machining of brittle materials. However, its machining performance is currently limited by the ductile machining model and the machining strategy with a constant feed rate, leading to low machining efficiency. To overcome this defect, this paper presents a novel self-tuned ultrasonic elliptical vibration cutting (SUEVC) technique to achieve high-efficient ductile-regime machining of the micro-optics array on brittle materials. The proposed SUEVC includes a ductile-regime machining model and a tool path generation method. In SUEVC, the feed rate adaptively changes with respect to the local shape variation of the desired surface along the feeding direction to ensure both crack-free surface and high machining efficiency. Finally, two 1 × 3 spherical micro-optics arrays were successfully fabricated on single-crystal MgF₂ by SUEVC and the traditional machining strategy respectively. Results demonstrated that the SUEVC could enhance the machining efficiency by 30% relative to the traditional machining strategy, while maintaining similar surface roughness and a crack-free surface.     

AN ANALYSIS OF SURFACE CRACKING DURING ORTHOGONAL MACHINING OF GLASS

It has been proposed that a rough-semi-finish-finish strategy may be possible in the machining of glass and other brittle materials to achieve higher productivity than is realized through either grinding or ductile-mode machining, both of which have been studied extensively to date. A previously presented experimental study of orthogonal glass cutting exhibited clear transitions in machining modes as the uncut chip thickness is increased. One of those modes involves ductile-mode chip formation combined with surface damage in the form of surface cracks that protrude down into the machined surface and ahead of the cutting edge. Here, a model is formulated and exercised to better understand this surface-cracking damage. The finite element method is used with a custom written re-meshing subroutine employed under a commercial software package. The analysis focuses on the crack depth and lead (ahead of the tool) as a function of the normalized process force and the fracture toughness of the work material. It is found that load ratio, the ratio of the cutting (surface-tangential) force to the thrust (surface-normal) force, plays a significant role in the crack growth problem, as does the manner in which the thrust load is distributed relative to the cutting load. It is shown that point-wise application of the loads produces results far off from the experimental results, whereas distributed loads can produce results well aligned with the experiments. Given that ductile-mode chip formation occurs during surface cracking, the load distributions found to work well exhibit qualitatively the same characteristics that one would expect based on extending the well-known mechanics of metal cutting.     

Investigation of the effect of cutting tool edge radius on material separation due to ductile fracture in machining

This paper investigates the interaction between cutting tool edge radius and material separation due to ductile fracture based on Atkins’ model of machining. Atkins’ machining model considers the energy needed for material separation in addition to energies required for shearing at the primary shear zone and friction at the secondary shear zone. However, the effect of cutting tool edge radius, which becomes significant at microcutting conditions, was omitted. In this study, the effect of cutting tool edge radius is included in the model and its influence on material separation is investigated. A modification to the solution methodology of Atkins’ machining model is proposed and it is shown that the shear yield stress and the fracture toughness of the work material can be calculated as a function of uncut chip thickness.     

Theoretical modelling of cutting forces in helical end milling with cutter runout

A theoretical cutting force model for helical end milling with cutter runout is developed using a predictive machining theory, which predicts cutting forces from the input data of workpiece material properties, tool geometry and cutting conditions. In the model, a helical end milling cutter is discretized into a number of slices along the cutter axis to account for the helix angle effect. The cutting action for a tooth segment in the first slice is modelled as oblique cutting with end cutting edge effect and tool nose radius effect, whereas the cutting actions of other slices are modelled as oblique cutting without end cutting edge effect and tool nose radius effect. The influence of cutter runout on chip load is considered based on the true tooth trajectories. The total cutting force is the sum of the forces at all the cutting slices of the cutter. The model is verified with experimental milling tests.     

NANOSCALE CUTTING OF MONOCRYSTALLINE SILICON USING MOLECULAR DYNAMICS SIMULATION

It has been found that the brittle material, monocrystalline silicon, can be machined in ductile mode in nanoscale cutting when the tool cutting edge radius is reduced to nanoscale and the undeformed chip thickness is smaller than the tool edge radius. In order to better understand the mechanism of ductile mode cutting of silicon, the molecular dynamics (MD) method is employed to simulate the nanoscale cutting of monocrystalline silicon. The simulated variation of the cutting forces with the tool cutting edge radius is compared with the cutting force results from experimental cutting tests and they show a good agreement. The results also indicate that there is silicon phase transformation from monocrystalline to amorphous in the chip formation zone that can be used to explain the cause of ductile mode cutting. Moreover, from the simulated stress results, the two necessary conditions of ductile mode cutting, the tool cutting edge radius are reduced to nanoscale and the undeformed chip thickness should be smaller than the tool cutting edge radius, have been explained.     

Nanometric cutting in a scanning electron microscope

A nanometric cutting device under high vacuum conditions in a scanning electron microscope (SEM) was developed. The performance, tool-sample positioning, and processing capacity of the nanometric cutting platform were studied. The proposed device can be used to realize a displacement of 7 μm, with a closed-loop resolution of 0.6 nm in both the cutting direction and the depth direction. Using a diamond cutting tool with an edge radius of 43 nm formed by focused ion beam (FIB) processing, nanometric cutting experiments on monocrystalline silicon were performed on the developed cutting device under SEM online observation. Chips and machining results of different depths of cut were studied during the cutting process, and cutting depths of less than 10 nm could be obtained with high repeatability. Moreover, the cutting speed was found to exhibit a strong relationship with the brittle–ductile transition depth on brittle material. The experimental results of taper cutting and sinusoidal cutting indicated that the developed device has the ability to perform multiple degrees of freedom (DOFs) cutting and to study nanoscale material removal behaviour.     

高硅铝合金的金刚石涂层刀具铣削损伤机理研究

高硅铝合金由于硅含量很高,故切削加工性较差,切削刀具极易磨损且已加工表面存在大量缺陷.为进一步研究材料加工损伤,采用化学气相沉积法制备了金刚石涂层铣刀,开展70％Si/Al(70％指质量分数)合金材料铣削试验.试验研究了铣削力、刀具磨损及加工损伤机理,并与常用TiN涂层铣刀进行了对比.结果 表明:铣削过程中由于初晶硅硬...     

DETECTION OF DUCTILE TO BRITTLE TRANSITION IN MICROINDENTATION AND MICROSCRATCHING OF SINGLE CRYSTAL SILICON USING ACOUSTIC EMISSION

Machining of brittle materials entails two modes of material removal: pure plastic deformation and brittle fracture. The mode of material removal is generally identified by surface quality observations in a scanning electron microscope (SEM) or an atomic force microscope (AFM) after machining. Hence, there is a need for the development of in-process monitoring technology in order to detect whether the mode of material removal is ductile or brittle, and thereby predict surface quality. In the present paper, acoustic emission (AE) is proposed as a means of monitoring the ductile to brittle transition. Microindentation and microscratching tests of single crystal silicon were conducted using an ultrafine-motion table with very small motion error. The obtained AE signals were correlated with crack initiation and the ductile to brittle transition. The critical force f_c defined as the force at which AE was induced during the microindentation and microscratching tests was measured to be 40 ∼ 50 mN. AFM observations revealed the critical depth of cut d_c to be 0.20 μm in the microscratching test.