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.     

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.     

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.     

Influence of size effect on burr formation in micro cutting

Burr is an important character of the surface quality for machined parts, and it is even more severe in micro cutting. Due to the uncut chip thickness and the cutting edge radius at the same range in micro cutting process, the tool extrudes the workpiece with negative rake angle. The workpiece flows along the direction of minimum resistance, and Poisson burr is formed. Based on the deformation analysis and experiment observations of micro cutting process, the factor for Poisson burr formation is analyzed. It is demonstrated that the ratio of the uncut chip thickness to the cutting edge radius plays an important role on the height of Poisson burr. Increasing the uncut chip thickness or decreasing the cutting edge radius makes the height of exit burr reduce. A new model of micro exit burr is established in this paper. Due to the size effect of specific cutting energy, the exit burr height increases. The minimum exit burr height will be obtained when the ratio of uncut the chip thickness to the cutting edge radius reaches 1. It is found that the curled radius of the exit burr plays an important role on the burr height.     

Thermomechanical approach for the modeling of oblique machining with a single cutting edge

This article aims to predict performances of oblique machining with a single cutting edge. A thermomechanical approach for the modeling of oblique cutting with a single cutting edge is proposed. A good agreement was found between predicted and experimental data. New rules were established to determine experimentally the average friction coefficient and chip flow angle at the rake face. The computation algorithm permits to predict all thermomechanical parameters such as cutting forces, cutting temperatures, and chip geometry. Besides, all predicted oblique machining parameters are mainly controlled by the Po-criterion, which is defined as the ratio of tool–chip contact length to uncut chip thickness.     

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.     

A FINITE ELEMENT ANALYSIS OF BURR FORMATION IN FACE MILLING OF A CAST ALUMINUM ALLOY

The research discussed in this article focuses on the effects of tool geometry (i.e., rake angle and cutting edge radius) and flank wear upon burr formation in face milling of a cast aluminum alloy. As to tool edge preparation, the use of a tool with variable cutting edge radius was investigated using FEM, and compared for its cutting performance (i.e., burr reduction and tool life) with a conventional tool with uniform cutting edge radius. In order to evaluate 3D face milling through 2D orthogonal cutting simulations, the cross-sections that consist in the cutting speed direction and chip flow direction were selected at different locations along the tool rounded corner. At these cross-sections, the local value of cutting edge radius and their associated tool rake angles as well as the effective uncut chip thickness were determined for 2D cutting simulations. In addition, 3D face milling simulations were conducted to investigate more realistic chip flow and burr generation. Comparisons were made for burrs produced from 3D simulations with a sharp tool, 3D simulations with a worn tool and face milling experiments. Finally, recommendations for cutting tool design are made to reduce burr formation in face milling.     

Slip-line solution for orthogonal cutting with a chip breaker and flank wear

A slip-line field model for orthogonal cutting with chip breaker and flank wear has been developed. For a worn tool, this slip-line field includes a primary deformation zone with finite thickness; two secondary shear zones, one along the rake face and the other along the flank face; a predeformation zone; a curled chip; and a flank force system. It is shown that the cutting geometry is completely determined by specifying the rake angle, tool-chip interface friction and the chip breaker constraint. The chip radius of curvature, chip thickness, and the stresses and velocities within the plastic region are readily computed. Grid deformation patterns, calculated with the velocity field determined, demonstrate that the predicted effects of changes in frictional conditions at the tool-chip interface and of the rake angle on chip formation are in accord with experimental observations. The calculated normal stress distribution at the tool-chip interface is in general agreement with previously reported experimental measurements. The model proposed predicts a linear relationship between flank wear and cutting force components. The results also show that non-zero strains occur at and below the machined surface when machining with a worn tool. Severity and depth of deformation below the machined surface increases with increasing flank wear. Forces acting on the chip breaker surface are found to be small and suggest that chip control for automated machining may be feasible with other means.     

Development of a capacity analysis and planning simulation model for semiconductor fabrication

Micro-machining has gained increased application to produce miniaturized parts in various industries. However, the uncut chip thickness in micro-machining is comparable to cutting edge radius. The relationship between the cutting edge radius and uncut chip thickness has been a subject matter of increasing interest. The acoustic emission (AE) signal can reflect the stress wave caused by the sudden release of the energy of the deformed materials. To improve the precision of machining system, determination of the minimum uncut chip thickness was investigated in this paper. The AE signal generated during micro-cutting experiments was used to analyze the chip formation in micro-end milling of Inconel 718. The finite element method (FEM) simulation was used to analyze the results of the experiments. The results showed that the cutting tool geometry and material properties affected the minimum uncut chip thickness. The estimation of the minimum uncut chip thickness based on AE signals can produce quite satisfactory results. The research on the minimum uncut chip thickness can provide theoretical basis for analysis of surface quality and optimal choice of cutting parameters.     

Contribution of specific work of fracture to size effect in microcutting

Specific work of fracture (R) has been widely used to quantify the energy consumed in formation of new surfaces during metal cutting. R becomes a significant portion of total cutting energy in microcutting, thereby influencing the phenomenon of “size effect. ” Therefore, this work presents an evaluation of specific work of fracture for sharp and rounded-edged tools, knowing cutting forces and shear angles from LS-DYNA simulations of orthogonal microcutting of low-carbon AISI 1215 steel. The R was also evaluated as a function of process parameters such as cutting speed, rake angle, tool edge radius, and uncut chip thickness, so as to illustrate the effect of these variables on the magnitude of R and contribution of R to the specific cutting energy or “size effect.” It is observed that R increases with an increase in uncut chip thickness, whereas it decreases with an increase in cutting speed, rake angle, and tool edge radius. U_R defines the contribution of fracture to the specific cutting energy (U) due to specific work of fracture R. At all the parametric conditions, the contribution of fracture is higher at lower uncut chip thickness and it is of 8–36% in microcutting of AISI 1215 steel.     

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.     

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 New Slip-Line Field Modeling of Orthogonal Machining with a Rounded-Edge Worn Cutting Tool

In this study, a new slip-line field model and its associated hodograph for orthogonal cutting with a rounded-edge worn cutting tool are developed using Dewhurst and Collins's matrix technique. The new model considers the existence of dead metal zone in front of the rounded-edge worn cutting tool. The ploughing force and friction force occurred due to flank wear land, chip up-curl radius, chip thickness, primary shear zone thickness and length of bottom side of the dead metal zone are obtained by solving the model depending on the experimental resultant force data. The effects of flank wear rate, cutting edge radius, uncut chip thickness, cutting speed and rake angle on these outputs are specified.     

Delamination-free drilling of carbon fiber reinforced plastic with variable feed rate

A large number of drilling have been performed to assemble aircraft parts of carbon fiber reinforced plastic (CFRP). Although high quality is required in machining the holes with high productivity in terms of reliability of parts, delamination often occurs around the holes in drilling. This paper presents a novel drilling method with variable feed rate to machine the delamination-free holes at a high machining rate. In the drilling, the holes are machined at the standard feed rates when the chisel moves in material; and are finished with the negative thrust at higher feed rates after the chisel exits from the workpiece. Orthogonal cutting tests were conducted to measure the cutting forces and the friction angles for the uncut chip thicknesses and the rake angles. The negative thrusts were measured in large uncut chip thicknesses at large rake angles of the lips. Then, the drilling tests were conducted to verify the change in the cutting force in the variable feed rate drilling up to 100 holes. Negative thrust component appears consistently to raise the workpiece up in the exit process even though the tool wear progresses with repeating drillings. As a result, the variable feed rate drilling remarkably controls delamination compared to the constant feed rate drilling in the 100th drilling. The cutting process in the variable feed rate drilling is compared with the constant feed rate drilling in a cutting force model based on the minimum cutting energy. The negative thrust is verified when the friction angle becomes smaller than the effective rake angle with increasing the feed rate.     

Modelling the cutting forces in micro-end-milling using a hybrid approach

This paper presents the development of a cutting force model for the micro-end-milling processes under various cutting conditions using a hybrid approach. Firstly, a finite element (FE) model of orthogonal micro-cutting with a round cutting edge is developed for medium-carbon steel. A number of finite element analyses (FEA) are performed at different uncut chip thicknesses and velocities. Based on the FEA results, the cutting force coefficients are extracted through a nonlinear algorithm to establish a relationship with the uncut chip thickness and cutting speed. Then, the cutting force coefficients are integrated into a mechanistic cutting force model, which can predict cutting forces under different cutting conditions. In order to account for the cutting edge effect, an effective rake angle is employed for the determination of the cutting force. A comparison of the prediction and experimental measured cutting forces has shown that the developed method provides accurate results.     

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.     

An improved cutting force model for micro milling considering machining dynamics

Micro milling, as a versatile micro machining process, is kinematically similar to conventional milling; however, it is significantly different from conventional milling with respect to chip formation mechanisms and uncut chip thickness modelling, due to the comparable size of the edge radius to the chip thickness, and the small per-tooth feeding. Considering tool runout and dynamic displacement between the tool and the workpiece, the contour of the workpiece left by previous tool paths is typically in a wavy form, and the wavy surface provides a feedback mechanism to cutting force generation because the instantaneous uncut chip thickness changes with both the vibration during the current tool path and the surface left by the previous tool paths. In this study, a more accurate uncut chip thickness model was established including the precise trochoidal trajectory of the cutting edge, tool runout and dynamic modulation caused by the machine tool system vibration. The dynamic regenerative effect is taken into account by considering the influence of all the previous cutting trajectories using numerical iteration; thus, the multiple time delays (MTD) are considered in this model. It is found that transient separation of the tool-workpiece occurring at a low feed per tooth, caused by MTD and the existing cutting force models, is no longer applicable when transient tool-workpiece separation occurs. Based on the proposed uncut chip thickness model, an improved cutting force model of micro milling is developed by full consideration of the ploughing effect and elastic recovery of the workpiece material. The proposed cutting force model is verified by micro end milling experiments, and the results show that the proposed model is capable of producing more accurate cutting force prediction than other existing models, particularly at small feed per tooth.     

Hybrid analytical–numerical solution for the shear angle in orthogonal metal cutting — Part II: experimental verification

The hybrid analytical–finite element model described in Part I is applied to predict the shear angle for a range of cutting velocity, uncut chip thickness, and two tool orthogonal rake angles. Experimental results and an empirical equation are also presented for the influence of the cutting conditions and cutting tool geometry on the chip–tool contact length. It is shown that there is a linear dependence between the chip–tool contact length/uncut chip thickness ratio and chip thickness/uncut chip thickness ratio over the range of cutting conditions assumed. The increase of the shear angle with the tool orthogonal rake is mostly due to the reduction of the specific shear energy in the primary shear zone and the specific friction energy in the secondary shear zone accompanied by a reduction of the chip–tool contact zone. The uncut chip thickness and cutting velocity influence the shear angle through their effect on the interface temperature and hence on the material flow stress in the secondary shear zone. The change in both parameters does not change significantly the specific shear energy in the primary shear zone. The model results are compared with the experimental results for a work material 0.18% C steel. The agreement between the predicted and experimental results is seen to be exceptionally good.     

EFFECT OF CUTTING SPEED AND SURFACE CHEMISTRY OF CUTTING TOOLS ON THE FORMATION OF BUL OR BUE AND SURFACE QUALITY OF THE GENERATED SURFACE IN DRY TURNING OF AA6005 ALUMINIUM ALLOY

In the present investigation, AA6005 (ISO: AlSiMg) alloy was machined in turning operation with different cutting tools, such as uncoated cemented carbide insert, PVD TiN coated, CVD diamond coated and PCD insert, under dry environment. Effect of cutting speed was studied for each of the cutting tools with regard to the formation of built-up layer (BUL) or built-up edge (BUE). The rake surface of the tools was characterized by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopic microanalysis. Particular emphasis was given on wear mechanism of PVD TiN coated insert, conventionally used in machining ferrous alloys, during dry turning of AA6005 alloy. It has been observed that increase of cutting speed from 200 m/min to as high as 1000 m/min could not substantially reduce formation of BUL over tool rake surface during dry machining of AA6005 alloy with uncoated or PVD TiN coated cemented carbide inserts. The potential of diamond-based tools in dry machining of aluminium alloy was also studied. Finally, the effect of cutting speed on surface finish of the workpiece machined with different cutting tools was studied during dry turning of AA6005 alloy.     

基于黏结-滑移摩擦模型的304不锈钢切削力仿真研究*

通过预测加工304不锈钢时产生的切削力,从而对切削参数和刀具几何参数进行优化,是提高304不锈钢的加工精度、切屑控制及保障刀具寿命的基础。建立304不锈钢切削仿真模型,为提高模型的精确性,选择Johnson-Cook本构方程和黏结-滑移摩擦模型。结果表明：采用黏结-滑移摩擦模型的切削力预测结果更为准确,表明相对于纯剪切摩擦与库仑摩擦模型,黏结-滑移摩擦模型能更准确地描述刀-屑摩擦特性。展开不同参数下的切削力研究,研究发现：切削力随着刀具前角、后角和切削速度的增大而减小,随切削刃钝圆半径和切削厚度、宽度的增大而增大,其中切削宽度、厚度及前角对切削力大小影响较大。研究结果为304不锈钢切削效率的提高和切削机制的研究提供了理论依据。