An experimental study of optical glass machining

Owing to brittleness and hardness, optical glass is one of the materials that is most difficult to cut. Nevertheless, as the threshold value of the undeformed chip thickness is reached, brittle materials undergo a transition from the brittle to the ductile machining region. Below this threshold, it is believed that the energy required to propagate cracks is larger than the energy required for plastic deformation. Thus, plastic deformation is the predominant mechanism of material removal in machining these materials in this mode. An experimental study is conducted to diamond-cut BK7 glass in ductile mode. As an effective rake angle plays a more important role than a nominal rake angle does, a discussion about this effective angle is carried out in the paper. The investigation presents the feasibility of achieving nanometric surfaces. Power spectral density (PSD) analysis on the machined surfaces shows the difference between the characteristics of the two modes. During the experiments, it is recognised that tool wear is a severe problem. Further study is in process to improve the cutting tool life.     

A study on the effect of tool-edge radius on critical machining characteristics in ultra-precision milling of tungsten carbide

A crack-free surface can be finished on brittle materials by a specialized but traditional machining technique known as ductile-mode machining. In ductile-mode machining of brittle material, crack propagation is suppressed by selecting a suitable combination of tool and machining parameters leading to the removal of material through plastic deformation enabled by dislocation motion. In ductile-mode machining, the tool–workpiece interaction is of critical significance for the capability of the cutting process to finish a crack-free surface on a brittle material. This interaction is largely dictated by the cutting-edge radius of the tool when the undeformed chip thickness is comparable to the edge radius as is the case of ductile-mode machining. This paper presents the experimental results of ductile-mode milling of tungsten carbide to investigate the effect of cutting-edge radius on certain critical machining characteristics associated with the ductile–brittle transition specific to milling process of brittle material. The experimental results have established that an increase in the cutting-edge radius within a certain range increases the critical feed per edge leading to the improvement of material removal rate in ductile-mode milling. An increasingly negative effective rake angle is desired during milling with larger edge-radiused tool to suppress the crack propagation in the cutting zone to achieve ductile-mode machining. The results also identify the effect of the edge radius on certain other parameters such as critical specific cutting energy, plowing effect and subsurface damage depth to comprehend the ductile–brittle transition phenomenon in ductile-mode milling.     

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.     

Research on single-point diamond fly-grooving of brittle materials

The purpose of this paper is to investigate the machining mechanisms that accompany the single-point diamond fly-cutting operation in grooving of brittle materials. Single-point diamond fly-cutting is widely used in precision machining of free-form optics, semiconductor devices, and micro-electromechanical system (MEMS) components among many others. The undeformed chip zone was analyzed and its relation to the critical brittle/ductile transition depth of cut was discussed. Then, a mechanics-based model was proposed to describe the material stress condition under the diamond tool. The machining parameters were incorporated into the model to understand fly-cutting behavior. It was shown that the fly-cutting technique is highly suitable for the ductile removal of brittle materials by generating large compressive pressures in the chip formation zone. This condition can be further enhanced by a small feedrate and a large negative rake angle of the diamond tool used. The theoretical results were substantiated and verified by fly-grooving experiments performed on mono-crystalline silicon.     

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.     

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.     

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.     

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.     

Single-point diamond turning of CaF2 for nanometric surface

Single-crystal CaF₂ is an important optical material. In this work, single-point diamond turning experiments were performed to investigate the nanometric machining characteristics of CaF₂. The effects of tool feed, tool rake angle, workpiece crystal orientation and cutting fluid were examined. It was found that two major types of microfracturing differing in mechanism limited the possibility of ductile regime machining. The critical conditions for microfracturing depend strongly on the tool rake angle and the type of cutting fluid. The results also indicate that one type of the microfractures is caused by thermal effect, and can be completely eliminated by using a sufficiently small undeformed chip thickness and an appropriate negative rake angle under dry cutting conditions. Continuous chips and ductile-cut surfaces with nanometric roughness were generated.     

The upper bound of tool edge radius for nanoscale ductile mode cutting of silicon wafer

For ductile mode cutting of brittle materials, such as silicon wafers, the undeformed chip thickness has to be smaller than the tool edge radius. In practical application, for high production rate, the undeformed chip thickness is expected to be as large as possible. Therefore, the tool edge radius is expected to be as large as possible. In this study, the upper bound of the tool edge radius is investigated through cutting experiments.     

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.     

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.     

On the effect of crystallographic orientation on ductile material removal in silicon

In this work the critical chip thickness for ductile regime machining of monocrystalline, electronic-grade silicon is measured as a function of crystallographic orientation on the (0 0 1) cubic face. A single-point diamond flycutting setup allows sub-micrometer, non-overlapping cuts in any direction while minimizing tool track length and sensitivity to workpiece flatness. Cutting tests are performed using chemically faceted, −45° rake angle diamond tools at cutting speeds of 1400 and 5600 mm/s. Inspection of the machined silicon workpiece using optical microscopy allows calculation of the critical chip thickness as a function of crystallographic orientation for different cutting conditions and workpiece orientations. Results show that the critical chip thickness in silicon for ductile material removal reaches a maximum of 120 nm in the [1 0 0] direction and a minimum of 40 nm in the [1 1 0] direction. These results agree with the more qualitative results of many previous efforts.     

Characteristics of ultrasonic vibration-assisted ductile mode cutting of tungsten carbide

In this study, investigations were carried out to evaluate the characteristics of ultrasonic vibration-assisted cutting of tungsten carbide material using a CNC lathe with CBN tool inserts. The cutting forces were measured using a three-component dynamometer, and the machined workpiece surfaces and chip formation were examined using a SEM. The experimental results showed that the radial force F _x was much larger than the tangential force F _z and axial force F _y . The SEM observations on the machined workpiece surfaces and chip formation indicated that the critical condition for ductile mode cutting of tungsten carbide was mainly the maximum undeformed chip thickness when the tool cutting edge radius was fixed, that is, the ductile mode cutting can be achieved when the maximum undeformed chip thickness was smaller than a critical value. Corresponding to the chip formation mode (ductile or brittle), three types of the machined workpiece surfaces were obtained: fracture free surface, semi-fractured surface and fractured surface. It was also found that the cutting speed has no significant effect on the ductile chip formation mode.     

Optimizing the geometric parameters of chamfered edge for rough machining Fe–Cr–Ni stainless steel

Geometry of cutting edge has great influence on performance and reliability of modern precision cutting tools. In this study, two-dimensional finite element model of orthogonal cutting of Fe–Cr–Ni stainless steel has been built to optimize the geometric parameters of chamfered edge. A method to measure the chip curl radius has been proposed. The effect of cutting edge geometric parameters on tool stress and chip curl radius has been analyzed. Then, the chamfered edge parameters have been optimized based on numerical simulation results. It finds that, keeping the equal material removal rate, the optimal geometric parameters of chamfered edge for rough machining Fe–Cr–Ni stainless steel are that the rake angle is from 16° to 17°, and the chamfer length is from 60 to 70 μm. Small (large) rake angle combined with small (large) chamfer length is more reasonable to reduce the tool stress. When the length of land is approximately equal to undeformed chip thickness and the rake angle is larger than 15°, the chip curl radius is minimal. The groove type with large radio of width to depth should be used in the chip breaking based on the optimization results.     

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.     

Thermoviscoplastic modelling of oblique cutting: forces and chip flow predictions

A modelling of oblique cutting for viscoplastic materials is presented. The thermomechanical properties and the inertia effects are accounted for to describe the material flow in the primary shear zone. At the tool–chip interface, a temperature-dependent friction law is introduced to take account of the extreme conditions of pressure, velocities and temperature encountered during machining. The chip flow angle is calculated by assuming that the friction force is collinear to the chip flow direction on the tool rake face. Due to the temperature dependence of the friction law at the tool–chip interface, the chip flow angle predicted by the model, is affected by the cutting speed, the undeformed chip thickness, the normal rake angle, the edge inclination angle and the thermomechanical behavior of the work material. This dependence and the trends predicted by the present approach are confirmed by experimental observations. Effects of cutting conditions on the cutting forces are also presented and compared to experiments.     

An empirical survey on the influence of machining parameters on tool wear in diamond turning of large single-crystal silicon optics

We conducted a series of screening experiments to survey the influence of machining parameters on tool wear during ductile regime diamond turning of large single-crystal silicon optics. The machining parameters under investigation were depth-of-cut, feed rate, surface cutting speed, tool radius, tool rake angle and side rake angle, and cutting fluid. Using an experimental design technique, we selected twenty-two screening experiments. For each experiment we measured tool wear by tracing the tool edge with an air bearing linear variable differential transformer before and after cutting and recording the amount of tool edge recession. Using statistical tools, we determined the significance of each cutting parameter within the parameter space investigated. We found that track length, chip size, tool rake angle and surface cutting speed significantly affect tool wear, while cutting fluid and side rake angle do not significantly affect tool wear within the ranges tested. The track length, or machining distance, is the single most influential characteristic that causes tool wear. For a fixed part area, a decrease in track length corresponds to an increase in feed rate. Less tool wear occurred on experiments with negative rake angle tools, larger chip sizes and higher surface velocities. The next step in this research is to perform more experiments in this region to develop a predictive model that can be used to select cutting parameters that minimize tool wear.     

横向超声振动对金刚石线锯切割硬脆材料锯切力及临界切削深度的影响

硬脆晶体材料,如SiC、Ge和Si等,由于其临界切削深度极小,常规加工方法很难实现塑性模式加工,研究横向超声振动金刚石线锯对硬脆材料锯切力和临界切削深度的影响有重要意义。在研究线锯受迫振动的基础上,分析金刚石线锯在横向超声波激励下柔性旋转点切割硬脆材料的条件;用特征函数对超声激励下金刚石线锯的振动切割状态进行表征;应用磨削理论建立了单颗金刚石磨粒切割硬脆材料的力学模型;推导出超声振动激励下金刚石线锯锯切硬脆材料临界切削深度的计算公式。以单晶SiC为对象,进行了超声振动线锯切割和普通线锯切割对比试验。结果表明相同条件下,超声振动线锯切割SiC的锯切力比普通线锯的锯切力减少22.4%~64.2%,临界切削深度增加1倍,晶片表面粗糙度有明显的改善。试验结果与理论分析具有良好的一致性。