Analytical model to determine the critical conditions for the modes of material removal in the milling process of brittle material

Brittle materials are prone to cleavage-based fracture during machining. In conventional scale machining of brittle material, crack-propagation is the dominant mechanism of material removal which results in a degraded machined surface. The challenge is to perform machining of brittle material such that the material removal occurs predominantly by chip formation rather than the characteristic brittle fracture. In this case, a high quality finish is achieved on the machined surface. Ductile-mode machining has emerged as a promising technique to finish a crack-free machined surface on macroscopically brittle materials. In the past, ductile-mode machining has mostly been performed by single-edge cutting process. This paper outlines an analytical model to determine the critical conditions for finishing a crack-free surface on brittle material by milling process. Four distinct modes of machining have been identified in the milling process of brittle material. In this model, the critical conditions for different modes of machining have been determined with respect to the relationship between the radial depth of cut and the depth of subsurface damage caused by the brittle fracture during machining. Verification tests were performed on tungsten carbide workpiece and the experimental results have validated the proposed machining model. It has been established that if the radial depth of cut is greater than the subsurface-damage depth in the milling process of brittle material, it is possible to finish a crack-free machined surface by removal of material through a combination of plastic deformation and brittle fracture. However, if the radial depth of cut is less than the subsurface damage depth, brittle fracture must be prevented in ductile-mode milling to finish a crack-free machined surface.     

Effect of tilt angle on cutting regime transition in glass micromilling

Recent development in mechanical micromachining technology has increased the realization of micromachining as a feasible manufacturing process of micro-scale components including glass-based devices. It has been found that glass can be machined in a ductile regime under certain controlled cutting configurations. However, favorable ductile regime machining instead of brittle regime machining in micromilling of brittle glass is still not fully understood as a function of cutting configuration. In this study, the effect of tilt angle along the feed direction on cutting regime transition has been studied in micromilling crown glass with a micro-ball end mill. Straight glass grooves were machined in water bath by varying the tool tilt angle and the feed rate, and the resulting surface was characterized using the scanning electron microscope and the profilometer to investigate the glass cutting regime transition. In characterizing the cutting regimes in glass micromilling, rubbing, ductile machining, and brittle machining regimes are hypothesized according to the undeformed chip thickness. It is found that a crack-free glass surface can be better machined in the ductile mode using a 45° tilt angle and feed rates up to 0.32 mm/min. During each milling pass, surface roughness was found to decrease from the entry zone to the groove bottom and then increase to the exit zone regardless of the cutting regime.     

Micro-end-milling of single-crystal silicon

Ductile-regime machining of silicon using micro-end-mill is almost impossible because of the brittle properties of silicon, crystal orientation effects, edge radius of the cutter and the hardness of tool materials. Micro-end-milling can potentially be used to create desired three dimensional (3D) free form surface features using the ductile machining technology for single-crystal silicon. There is still a lack of fundamental understanding of micro-end-milling of single-crystal silicon using diamond-coated tool, specifically basic understanding of material removal mechanism, cutting forces and machined surface integrity in micro-scale machining of silicon. In this paper, further research to understand the chip formation mechanism was conducted. An analysis was performed to discover how the chips are removed during the milling process. Brittle and ductile cutting regimes corresponding to machined surfaces and chips are discussed. Experiments have shown that single-crystal silicon can be ductile machined using micro-end-milling process. Forces generated when micro-end-milling single-crystal silicon are used to determine the performance of the milling process. Experimental results show that the dependence of the cutting force on the uncut chip thickness can be well described by a polynomial function order n. As cutting regime becomes more brittle, the cutting force has more complex function.     

A novel method for determination of the subsurface damage depth in diamond turning of brittle materials

Micro-structured surfaces on brittle materials, e.g. ceramic and glass, are gaining increasing industrial applications such as optics, semiconductor and biomedical. However, these materials tend to be damaged with brittle fracture in machining. To generate crack-free surfaces, ductile-regime machining should be maintained for the entire micro-structured surface. In ductile-regime machining the material is removed by both plastic deformation and brittle fracture, but the cracks produced are prevented from extending into the finished surface. In this paper, a machining model has been developed for fast tool servo (FTS) diamond turning of micro-structured surfaces on brittle materials. Based on the model, a damaged region analysis method (DRAM) is proposed to determine the subsurface damage depth (C_m) by analyzing the surface damaged region of a machined micro-structured surface with sinusoidal wave along radial direction. Only one micro-structured surface is required to be machined to obtain C_m, which greatly reduces the effort for determination of C_m. With C_m, the maximum feedrate for machining a crack-free micro-structured surface can be determined. Machining experiments have verified the validity of DRAM.     

Cutting process in glass peripheral milling

Peripheral glass milling for trimmings of several devices and touch panels is studied with measuring cutting forces and observing surface damages. Peripheral millings were performed to cut the end faces of 1 mm thick glass plates. In order to discuss the typical cutting force in glass milling, the cutting forces were compared with those of 0.45% carbon steel (AISI 1045) at high feed rates in a large radial depth of cut. The differences of the cutting force in glass milling from that of metal milling are: (1) the change in the cutting force does not correspond to the uncut chip thickness; and (2) the maximum cutting force does not change with the feed rate. A model is proposed to predict the cutting forces in glass millings, which are performed in ductile, ductile/brittle complex and brittle modes. The cutting force depends on the uncut chip thickness in a ductile mode. In a brittle mode, the mean value of the cutting force does not change though the vibration component becomes large. Because the uncut chip thickness changes with the dynamic displacement of the cutting edge, the cutting process is performed in a ductile/brittle complex mode when the cutting mode changes in ductile–brittle transition. The critical uncut chip thickness at the transition from a ductile to a ductile/brittle complex mode and that of the transition from a ductile/brittle complex to brittle mode are determined in the rate of the cutting force change. The force model is verified by the cutting forces in up- and down-cutting milling operations. Then, the surface finishing and crack propagation in up- and down-cutting millings were analyzed to define the cutter path in glass trimming. Cracks propagate to the surface to be finally finished in down-cutting; while cracks propagate to the chip to be removed in up-cutting. The cutter path in up-cutting milling should be selected to finish the fine surfaces.     

Investigations of yield stress,fracture toughness,and energy distribution in high speed orthogonal cutting

This paper presents a novel prediction method of the yield stress and fracture toughness for ductile metal materials through the metal cutting process based on Williams' Model [38]. The fracture toughness of the separation between the segments in serrated chips in high speed machining is then deduced. In addition, an energy conservation equation for high speed machining process, which considers the energy of new created workpiece surfaces, is established. The fracture energy of serrated chips is taken into the developed energy conservation equation. Five groups of experiments are carried out under the cutting speeds of 100, 200, 400, 800 and 1500 m/min. The cutting forces are measured using three-dimensional dynamometer and the relevant geometrical parameters of chips are measured with the aid of optical microscope. The experiment results show that the yield stress of machined ductile metal material presents an obviously increasing trend with the cutting speed increasing from 100 to 800 m/min while it decreases when the cutting speed increases to 1500 m/min further. Meanwhile, the fracture toughness between the chip and bulk material displays a slightly increasing tendency. In high speed machining, the fracture toughness of the separation between the segments in serrated chips also presents increasing trend with the increasing cutting speed, whose value is much greater than that between the chip and bulk material. In the end, the distribution of energy spent in cutting process is analyzed which mainly includes such four portions as plastic deformation, friction on the tool–chip interface, new generated surface and chip fracture. The results show that the proportion of plastic deformation is the largest one while it decreases with the cutting speed increasing. However, the proportions of energy spent on new created surface and chip fracture increase due to the increasing of both the chip's fracture area and the fracture toughness.     

A study of the surface scallop generating mechanism in the ball-end milling process

This paper presents the model, simulation and experimental verification of the scallop formation on the machined surface in the ball-end milling process. In the milling process, the cutting edges of the cutter are in a motion of combined translation and rotation. The periodical variation of the cutting edge orientation during spindle rotation results in two kinds of scallops generated on the machined surface: the pick-interval scallop and the feed-interval scallop. Because of the low feed and comparably large pick used in the conventional ball-end milling process, the emphasis of previous works has been placed on studying the geometric generating mechanism of the pick-interval scallop while the feed-interval scallop has been largely ignored. Trend of the high-speed and high efficiency machining, however, has pushed the feed reaching the same level of the pick. For the high-speed machining where the high feed/pick ratio is used, the feed-interval scallop must be taken into account. This paper presents a new model that describes the path-interval and feed-interval scallops generating mechanism in the ball-end milling processes. Parameters such as the tool radius, feed/pick ratio, initial cutting edge entrance angle, and tool-axis inclination angles have been studied and experimental verified. It was found that the feed-interval scallop height was 3–4 times large than the path-interval scallop height at the high-speed machining case. The scallop height was continuously reduced by increasing the tool-axis inclination angle. An inclination angle up to 10° is, however, good enough for most tool diameters from the surface roughness viewpoint.     

Experimental study on abrasive waterjet polishing for hard–brittle materials

The rapid growth of hard–brittle materials necessitates the development of compatible machining techniques, especially for the precision machining. The abrasive waterjet (AWJ) machining is a powerful tool in processing hard–brittle materials. In the last decades, some of AWJ machining technologies, such as AWJ cutting, AWJ milling and AWJ drilling have gradually become mature and steady. However, a few investigations on precision surface machining for hard–brittle materials by AWJ had been carried out. In this research, the ductile erosion mechanism of hard–brittle materials by AWJ in small erosion angle has been analyzed. In theory, the ductile erosion can achieve micromaterial removal and the surface eroded is smooth and without any fracture. Based on the ductile erosion mechanism, the feasibility of polishing for hard–brittle materials by the AWJ has been investigated. A group of polishing experiments is performed. The polished surfaces of workpieces were observed with scanning electron microscope (SEM) and measured by atomic force microscopy (AFM). The results of these polishing experiments indicate that AWJ has a great potential to be used as a precision surface machining technology.     

A study of materials swelling and recovery in single-point diamond turning of ductile materials

This paper presents an investigation of the effect of materials swelling in ultra-precision machining of ductile materials. The combined influence of materials swelling and recovery was found to affect the surface roughness in single-point diamond turning. It is interesting to note that the effect of materials swelling for ductile materials would be overwhelmed by the impact of recovery when the depth of cut is extremely small and the front clearance is small. In addition, radically different surface roughness profiles were found for different materials even though they are machined under the same cutting conditions. The difference in the machining behaviour could not be accounted by the elastic recovery alone but by the plastic deformation induced in the machined layer. The findings in the present study provide an important means for improving the surface roughness in ultra-precision machining.     

车削和铣削加工表面塑性变形对TC4试件疲劳性能的影响

目的 探究车削和铣削加工表面塑性变形层中纤维组织方向对钛合金TC4试件抗疲劳性能的影响规律和机理,完善表面完整性研究体系,为实际航空结构件的抗疲劳制造提供参考。方法 基于钛合金TC4进行车削和铣削加工的表面完整性试验和试件疲劳性能测试试验,通过合理地选用加工参数以控制试件的表面完整性指标,从而更加直观地分析塑性变形层中纤维组织方向与试件疲劳寿命之间的关系。结果 实验中车削疲劳试件的表面粗糙度、表面显微硬度和表面残余应力3个指标均优于铣削试件,两者的表面形貌也十分类似,但后者的抗疲劳性能达到前者的12~48倍,塑性变形层中纤维组织方向的不同是铣削试件抗疲劳性能远大于车削试件的主要因素。结论 不同加工方式形成的加工表面机制不同,表层纤维组织的塑性变形方向和变形程度也存在较大差异。车削疲劳试件的纤维组织沿试件的周向,铣削疲劳试件的纤维组织沿试件的轴向,在承受轴向的交变载荷时,沿着轴向的纤维组织可以抑制疲劳裂纹的萌生和扩展,大幅提升钛合金TC4试件的抗疲劳性能。     

纵扭复合超声端面铣削表面微结构建模与试验研究

目的 对纵扭复合超声端面铣削加工表面微结构进行预测,以优化加工参数。方法 对纵扭复合超声端面铣削进行运动学分析,并在其基础上建立三维运动轨迹方程。对刀尖轨迹仿真,且研究该运动方式下的加工特性。通过对切削刃和工件离散化建立纵扭复合超声端面铣削表面微结构理论模型,并利用MATLAB进行三维表面仿真。对TC4钛合金进行超声振动切削试验。结果 理论仿真和切削试验结果均表明超声纵扭端面铣削时,随振幅的增加,由振动引起的表面微观结构特征愈加明显。扭纵幅值比增大时,加工表面微观结构凹坑效应弱化,At/Al=0.55时,加工表面呈条形片状微观结构。振动频率和主轴转速会影响表面微观结构单元的疏密程度。结论 加工表面微结构的生成与振动频率、振幅、扭纵复制比、切削速度等加工参数相关,铣削实验得到的加工表面变化趋势与表面理论模型吻合,该表面模型能够优化超声加工参数。     

钛合金侧铣加工残余应力与变形研究北大核心

为研究加工残余应力对钛合金薄壁结构变形的影响,利用有限元软件建立2D切削模型,分析每齿进给量与刀具刃口半径对加工表面残余应力的影响。利用切削仿真得到的残余应力对薄壁件进行变形仿真分析;分别以每齿进给量0.06、0.08 mm进行闭腔侧铣试验。结果表明:与实测值相比,仿真表面残余应力误差约为20%、最大变形量误差约为12%。所提方法具有一定的可靠性与可行性。     

Machining characteristics and fracture morphologies in a copper-beryllium (Cu-2Be) alloy

The technology of materials removal is improved greatly by the introduction of advanced cutting tools like cubic boron nitride, ceramics, polycrystalline diamond and the more recent whisker-reinforced materials. In this paper, the influence of cutting temperature on machinability, mechanical properties, microstructure, and fracture morphology of Cu-2Be alloy using a polycrystalline diamond cutter is investigated. The information on machining, microstructure, and fracture morphology of Cu-2Be alloy are very useful to understand their fabrication characteristics and the basic mechanisms of its deformation and fracture. The machinability (in terms of surface finish) of Cu-2Be alloy is evaluated as a function of cutting temperature, resulting from wet and dry cutting. Machining is carried out on a Hardinge Cobra 42 CNC machine (Hardinge Inc., Elmira, NY), and the machining parameters used—cutting speed, depth of cut, and feed rate—are kept constant during both wet and dry cutting. The machined surface finish on Cu-2Be alloy is measured using a surface finish analyzer (Surftest 401, series 178) technique. The machined specimens are examined for their strength and hardness properties using a standard Universal Testing Machine and Rockwell hardness tester, respectively. Wet cutting (using coolants) produced a smooth surface finish when compared with dry cutting of the Cu-2Be alloy. The machined specimens are examined for their microstructural features using a Nikon optical microscope. The specimens are etched using a suitable etchant solution for revealing such microstructure constituents as grain size, phase proportions, and the possible overheated areas (especially in dry cutting). The fractured surfaces from the tensile and impact toughness tests are investigated for their fracture morphologies (dry and wet cutting) using a microprocessor-controlled scanning electron microscope (Jeol Model JSM 5910 LV). A detailed analysis is also made to understand and interpret the basic fracture mechanisms responsible for crack initiation and crack propagation. The Cu-2Be alloy showed relatively higher mechanical properties in wet cutting in comparison to dry cutting operations. Fracture studies demonstrated intergranular and ductile fractures as dominant modes of fracture mechanisms in Cu-2Be alloy.     

Influence of tool inclination on brittle fracture in glass cutting with ball end mills

Glass milling is discussed with influences of tool inclination on brittle fracture. Cutting tests are performed to observe surfaces in the up-cut and the down-cut processes with a ball end mill inclined in the feed direction of the cutter. Brittle fracture occurs in the down-cut process at high feed rates. Then the machined surfaces in cutting with the ball end mills tilted in the vertical plane with respect to the feed direction are associated with those of the up-cut and the down-cut processes. The cutting forces are also measured to discuss brittle fracture with the change of the undeformed chip thickness. The scratches on the surface finished with the tilted ball end mill are shown in an analytical model with a notched edge shape. The maximum feed rates at which brittle fracture does not occur are shown with the tool inclination in the cutting tests.     

Study on surface damages caused by turning NiCr20TiAl nickel-based alloy

The machined surface damages were investigated after NiCr20TiAl nickel-based alloy was turned in the various cutting conditions. It is found that the plastic deformation and carbide particles of NiCr20TiAl alloy are responsible for the machined surface damages during turning. When the carbide particles are removed from the machined surface during machining, the residual cracks inside surfaces are linked together due to a steep rise in the shear stress, which results in the surface cavities and plucking. The slip zones are formed inside the machined surfaces because the dislocation movement is inhibited by many grain boundaries of NiCr20TiAl alloy during machining, and a work-hardening layer with a non-uniform depth is produced. At the cutting speed of 100 m/min, feed of 0.15 mm/r and depth of cut of 1 mm, a relatively smooth surface with the thinner work-hardening layer is generated owning to the reduced cutting force.     

Characteristics of high speed micro-cutting of tungsten carbide

In this study, experiments are carried out to evaluate the characteristics of high speed cutting of tungsten carbide material using a Makino V55 high speed machine tool with cubic boron nitride (CBN) tool inserts. The cutting forces were measured using a three-component dynamometer, the surface roughness of the machined workpiece was measured using a Mitutoyo SURFTEST 301, and the machined workpiece surfaces and the chip formation were examined using a scanning electron microscope (SEM). Experimental results indicate that the radial force F_x is much larger than the tangential force F_z and the axial force F_y. Two types of surfaces of the machined workpiece are achieved: ductile cutting surface and fracture surface. Continuous chips and discontinuous chips are formed under different cutting conditions. Depth of cut and feed rate almost have no significant effect on the surface roughness of the machined workpiece. The SEM observations on the machined workpiece surfaces and chip formation indicate that the ductile mode cutting is mainly determined by the undeformed chip thickness when the tool cutting edge radius is fixed. Ductile cutting can be achieved when the undeformed chip thickness is less than a critical value.     

氧化锆陶瓷旋转超声加工脆塑转变特性研究

目的揭示旋转超声振动对硬脆材料脆塑转变特性及工艺参数对材料脆塑转变临界切削深度的影响规律。方法以氧化锆陶瓷为研究对象,在硬脆材料压痕断裂试验基础上,从理论上分析了超声振动加工硬脆材料脆塑转变的临界条件,并进行了纵向振动及纵扭共振形式的旋转超声振动划痕与普通划痕对比试验。结果在相同试验条件下,超声振动划痕较普通划痕有较高的材料脆塑转变临界切削深度。适当地增大超声能量,纵扭共振比纵向振动具有更大的脆塑转变临界切削深度值;而随着进给速度的增大,纵向振动比纵扭共振具有更大的材料脆塑转变临界切削深度。结论通过不同划痕条件的对比,超声振动能有效提高氧化锆陶瓷的脆塑转变临界切削深度,增大塑性域加工范围,提高材料表面加工质量,验证了理论分析的正确性。     

预测数控加工表面粗糙度的解析模型研究

解析模型是基于刀具切削刃包络面形成的原理来研究零件表面形貌的形成.在解析模型的基础上研究球头刀铣削过程的零件表面生成机理、分析影响加工表面粗糙度大小的因素以及表面粗糙度的趋势,进而预测表面粗糙度,有助于数控加工条件的最优化.本文利用计算机图形学算法进行建模,该模型能够仿真已加工表面轮廓的形成和表面形貌的可视化、预测表面粗糙度和评估加工过程参数的合理性.     

Research on full-aperture ductile cutting of KDP crystals using spiral turning technique

Single point diamond face flycutting technique is widely applied to fabricate potassium dihydrogen phosphate (KDP) crystals because of their characteristics of softness, brittleness, deliquescence and anisotropy. This paper studies the feasibility of machining KDP crystals through spiral turning method based on the ductile cutting mechanism of brittle materials. The spiral scratch method is employed to analyze various conditions of achieving brittle–ductile transition in different cutting directions where the critical depth of cut is obtained. The cutting forces and surface formation processes within different cutting parameters are discussed and the parameters ensuring full-aperture ductile turning are acquired. White light interferometer and Atomic Force Microscope (AFM) are used to evaluate the coherence of machined surface qualities in different cutting directions. The results indicate that the critical depth of cut varies from cutting directions significantly and fluctuates from about 200 nm to 1000 nm under the experimental condition. Cutting forces display periodical variation with the change of cutting direction and the periodical trend blurs, even ultimately disappears, with the decrease of the cutting parameters. Root-Mean-Square (RMS) value of the fabricated surface roughness fluctuates from 1.3 nm to 1.7 nm on the whole surface with no distinct anisotropic distribution of the surface morphology according to the results observed by AFM. Compared with face flycutting method, spiral turning method has capability to generate complex surface profiles such as aspheric or freeform surfaces and can also be used to compensate the deformation error of KDP crystals caused by vacuum suction during fabrication.     

TB6钛合金高速铣削三维表面形貌及疲劳行为(英文)

钛合金高速铣削以高效率、高质量的优点而被广泛应用于航空航天工业。为了优化高速铣削中控制表面粗糙度的工艺参数,通过高速侧面铣削试验及疲劳试验,研究高速铣削参数对三维表面形貌和疲劳寿命的影响。通过疲劳试样断口观测,揭示表面形貌对疲劳裂纹产生的作用机制。研究表明:铣削速度优选范围为100～140m/min,每齿进给量优选范围为0.02～0.06mm/z,可保障表面粗糙度在0.8μm内。基于表面应力集中系数建立的疲劳寿命模型与实验结果的平均误差为6.25%。疲劳裂纹起源于试件已加工表面的相交棱处。