Theoretical model for cutting force in rotary ultrasonic milling of dental zirconia ceramics

Rotary ultrasonic machining or ultrasonic vibration assisted grinding has superior performance in machining hard and brittle materials, such as dental zirconia ceramics. However, there are few reports about cutting force modeling of rotary ultrasonic milling (RUM) for dental ceramics, especially for cutting force model in feed direction. In this study, the theoretical model of cutting force both in axial direction and feed direction is proposed under the assumption that brittle fracture is the primary mechanism of material removal in RUM of dental ceramics. The effective cutting time and material removal volume have been analyzed to develop the cutting force model. Besides, the number of active abrasive particles has been calculated for the first time during the modeling. The effect of overlapping and intersection of fracture zone in peripheral direction on material removal volume has also been considered via the parameters K ₁ and K ₂. In addition, the relationships between the cutting force and input variables are revealed through the theoretical model. Finally, pilot experiments of RUM on dental zirconia ceramics are conducted to verify the theoretical model. The experimental results are consistent well with the model predictions. Therefore, the theoretical model can be applied to evaluate the cutting force in RUM of dental ceramics.     

Mathematical model for cutting force in rotary ultrasonic face milling of brittle materials

Rotary ultrasonic machining of brittle materials, such as glass, ceramics, silicon, and sapphire, has been explored in a large number of experimental and theoretical investigations. Mechanistic models have been developed to predict the material removal rate or cutting force in the rotary ultrasonic machining of brittle materials. However, most merely describe the rotary ultrasonic machining process of drilling holes in brittle materials. There are no reports on the development of a cutting force model for flat surface rotary ultrasonic machining, i.e., rotary ultrasonic face milling. This paper presents a mathematical model for the cutting force in the rotary ultrasonic face milling of brittle materials under the assumption that brittle fracture removal is the primary mode of material removal. Verification experiments are conducted for the developed cutting force model and show that the trends of input variables for the cutting force agree well with the trends of the developed cutting force model. The developed cutting force model can be applied to evaluate the cutting force in the rotary ultrasonic face milling of brittle materials.     

Nd-Fe-B烧结永磁材料的车削加工研究

对Nd-Fe-B烧结永磁材料的车削加工进行了试验研究。用线性断裂力学的方法建立了硬脆材料车削加工时的材料去除模型。探讨了车削过程中背吃刀量、进给量和车削速度对车削力及加工表面质量的影响，采用多元回归分析方法得出了主车削力的经验公式，并给出了用于检验回归结果与试验结果符合程度的误差评判参数。分析了Nd-Fe-B烧结永磁材料车削过程中刀具的磨损状况，以及刀具几何参数对加工质量的影响。     

金刚石线锯横向超声振动切割SiC单晶表面 粗糙度预测*

把横向超声振动应用到金刚石线锯切割硬脆材料加工中,基于冲量原理分析了线锯横截面上不同位置处金刚石磨粒对工件的法向锯切力。应用压痕断裂力学理论,定量分析了在法向和切向载荷共同作用下磨粒下方中位/横向裂纹扩展的长度和深度。研究了振动磨粒在工件上间歇加载和卸载使横向裂纹优先扩展并抑制中位裂纹扩展的屏蔽效应。建立了横向振动线锯切割硬脆材料时线锯横截面不同位置处磨粒的材料去除模式模型,得到了横向振动线锯切割硬脆材料晶片表面粗糙度的预测公式。以SiC单晶为切割对象,进行普通线锯和横向超声振动线锯切割对比试验,测定线锯的锯切力和晶片表面粗糙度,并对表面形貌进行观察。结果表明,横向超声振动线锯切割SiC是以脆性去除为主塑性去除为辅的混合材料去除模式;同等试验条件下,超声振动线锯切割能使晶片表面粗糙度降低25.7%。表面粗糙度测试结果与理论预测具有较好的一致性。     

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.     

Si_3N_4陶瓷材料的磨削试验研究

通过用树脂结合剂金刚石砂轮对Si3 N4陶瓷进行磨削试验 ,从磨削过程中的磨削力、磨削比能、磨削温度及磨削表面的微观形貌变化等方面 ,综合探讨Si3 N4材料的磨削加工机理。研究结果表明 :Si3 N4在磨削加工中在磨粒切深较大时主要以脆性断裂方式去除 ,其磨削温度与材料去除机理有着密切联系。     

旋转超声钻削的切削力数学模型及试验研究

通过分析硬脆材料脆性断裂去除机理和旋转超声加工特点,确定旋转超声加工时单颗磨粒的切削时间、切削深度、切削速度及切削轨迹长度,建立旋转超声恒进给率钻削硬脆材料的切削力数学预测模型。光学玻璃加工试验研究表明,切削力随进给速度的增大而增大,随主轴转速的提高而减小;在高进给速度条件下,切削力对主轴转速的变化更为敏感,在低主轴转速条件下,切削力对进给速度的变化更为敏感;从而很好地验证了已建立的切削力数学预测模型。旋转超声加工和普通加工的对比试验表明,旋转超声钻削加工可以有效降低切削力,一定程度上减小出孔崩边尺寸,从而提高加工效率、降低加工成本。根据旋转超声加工的表面粗糙度值略高于普通加工,提出硬脆材料脆性断裂去除时磨粒实际切削深度决定加工表面粗糙度的判断。     

高速锯切单晶硅的锯切力和锯缝崩边研究

探讨在单晶硅的高速精密锯切中,锯切用量与锯切崩边幅度大小之间的关系。通过使用金刚石薄锯片对单晶硅进行高速锯切,测量和分析不同参数下的锯切力,并结合锯切力比来分析金刚石锯片对单晶硅的锯切中力与崩边相互联系的特征。结果表明:在高速锯切单晶硅过程中,锯切深度、进给速度增大都能引起锯切力与力比的增大,也造成了单晶硅崩边情况更加严重。但是转速的提高则可以使锯切力大幅降低,并有效抑制加工过程中沟槽侧面的崩边问题。锯切深度与进给速度的增加引起锯切力增大时使单晶硅材料更加倾向于脆性断裂而被去除,但是提高转速降低锯切力后可使单晶硅渐转化为塑性去除,有效提高了加工产品质量。     

氮化硅陶瓷球研磨去除机制试验与仿真研究

为研究研磨过程中氮化硅陶瓷球的材料去除形式及磨损行为,结合陶瓷材料动态压痕断裂力学理论,进行陶瓷球研磨加工试验,采用超景深三维显微镜和扫描电镜对研磨后陶瓷球表面进行观察,同时建立单颗金刚石磨粒冲击作用有限元模型并进行仿真研究。试验结果表明:氮化硅陶瓷球表面材料去除以脆性断裂去除和粉末化去除为主,陶瓷球表面残留有大量贝壳状缺陷和呈簇状随机分布的粉末化材料区域;研磨过程中,陶瓷球表面存在擦伤、划伤和凹坑等缺陷;磨粒冲击作用时,表面材料会受微切削作用产生破碎去除,同时也会受挤压作用产生脆性断裂去除,当磨粒以滚动方式作用在陶瓷球表面时,陶瓷球表面更容易形成粉末化去除,且材料去除率更高。仿真结果表明:各磨粒冲击作用方式产生的最大等效应力由大到小的顺序为滚动磨粒变切深、滚动磨粒定切深、磨粒挤压、滑动磨粒定切深,其中,滚动磨粒变切深产生的亚表面裂纹最深。     

Applying an On-line Crack Detection Technique For Laser Cutting by Controlled Fracture

A new laser cutting system, which uses a laser to induce controlled fracture and combines an image processing technique for crack detection and compensation of the laser path to control crack direction, is proposed in this study. The laser cutting is based on the controlled fracture technique, the laser beam strikes the surface of a thin brittle plate which results in a stable fracture. The cutting of the material results from crack propagation along the cutting path. Because the crack propagation at the crack tip is slower than the moving laser beam, the actual fracture trajectory will deviate from the desired trajectory, for example, when cutting a curve or a right angle, especially at high cutting speed. In order to eliminate the deviation, the actual position of the crack tip must be detected on-line and the path of the laser beam must be modified. The compensation is based on the lagging distance between the crack tip and the laser spot. The laser cutting system comprises a CO2 laser, a digital image-processing system, and an XYZ positioning table. The cutting of right angles for silicon wafers and the cutting of circular arcs for alumina ceramics are carried out to validate the system. The image processing speed of the crack detection system is 0.1 s per detection, which is fast enough for continuous on-line crack detection, so that the method can be applied for any complicated curve cutting.     

单颗金刚石磨粒切削氮化硅陶瓷仿真与试验研究

为探索氮化硅陶瓷单颗磨粒切削加工的机理,进行单颗金刚石磨粒切削氮化硅陶瓷的仿真与试验。选用截角八面体模拟金刚石磨粒,基于Johnson-Holmquist ceramic硬脆材料本构模型,采用有限元网格法进行单颗磨粒直线切削仿真,分析工件材料的切屑去除、划痕形貌、应力动态变化与分布、切削力变化等现象,以及工艺参数对切削力的影响。制备单颗金刚石磨粒工具,在平面磨床上进行单颗磨粒切削氮化硅陶瓷的试验,进一步分析划痕形貌、切削力的变化,并验证有限元仿真的正确性。研究表明,划痕光直平整,塑性隆起很少,边缘存在较大尺寸的破碎,划痕内有局部小尺寸的破碎;划痕的深度和宽度比磨粒的切削深度和宽度尺寸略大。应力与切削力存在动态波动。随着砂轮速度的增加,切向力和法向力减小;随着切削深度的增加,切向力和法向力增大。切削力比在4～6之间变化。     

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.     

玻璃和45钢的切削力特性与刀具磨损的研究

对比研究了切削玻璃和45钢时切削力与刀具磨损之间的关系,描述了玻璃和45钢在切削过程中塑性变形和脆性断裂的不同,优化了玻璃等脆性材料的切削加工参数。其研究结果对加工玻璃、陶瓷等难加工的脆性材料具有一定的实际意义。     

Simulation research on the anisotropic cutting mechanism of KDP crystal using a new constitutive model

Potassium dihydrogen phosphate (KDP) exhibits anisotropic and hydrostatic pressure-dependent mechanical characteristics during processing. However, none of the existed material models is capable of describing the mechanical properties of the crystal. Thus, a new constitutive model, which combines the anisotropic elastic and pressure-dependent plastic model has been proposed in this paper. In addition, the tensile stress failure criterion is adopted as the fracture criterion of KDP crystal. Subroutine of the new material model is programmed and integrated into the commercial finite element software LS-DYNA. On the basis of that, the unknown material parameters of KDP are successfully identified with the aid of the nanoindentation/scratch technique and finite-element simulation. Finally, 2-D and 3-D cutting simulations applying the new model are performed to investigate the influence of cutting parameters on the brittle ductile transition depth and cutting force. The simulation results show good agreement with the KDP cutting experiment results, which confirm the validity and capability of the proposed constitutive model.     

工程陶瓷切削有限元仿真

工程陶瓷已广泛应用到工业领域。作为典型的高硬度脆性材料，其切削复杂，切削过程是一个不连续的过程，而内部微裂纹扩展是导致断裂和产生切屑原因。这里以脆性断裂理论为基础建立适合工程陶瓷车削过程分析的有限元模型：断裂分析模型，采用不连续的模拟方式，对工程陶瓷的不连续切削过程，以及加工表面的破坏状况进行分析。     

Investigation on grinding temperature in ultrasonic vibration-assisted grinding of zirconia ceramics

The feasibility of ultrasonic vibration-assisted grinding (UVAG) in machining brittle materials, such as zirconia ceramics, has been preliminarily proved. The high temperature generated in grinding processes is a main factor responsible for thermal and surface/subsurface damage. However, there are few reports about grinding temperature for zirconia ceramics by UVAG. In this study, a grinding force model is used for the analysis of grinding temperature in UVAG based on kinematic principle of ultrasonic vibration and brittle material fracture removal mechanism. Then, the heat fluxes equation during grinding process is analyzed. And the grinding temperature model is developed based on theoretical grinding force model and heat fluxes. Finally, pilot experiments are carried out to analyze influence of vibration parameters and process parameters on UVAG temperature and verify the mathematical model. The comparison results show that ultrasonic vibration has an important influence on grinding temperature with the reduction of 10.6%. In addition, there is a good consistency between mathematical model and the experimental results. The average relative error is within 10%. Therefore, the mathematical model could be used to predict the UVAG temperature.     

岩石切削过程中破坏机制的离散元分析

利用离散单元方法(DEM)对两种不同属性的花岗石样本模型进行了切割模拟,通过调整不同的切削参数计算并分析了岩石切削过程中的切削力、切削深度、切削比能及切削力倾角的变化规律,从细观上探讨了临界切深对岩石脆性破坏模式和塑性破坏模式及其转变的影响,指出了当切削深度逐渐增大时,破坏模式依次从塑性破坏模式到过渡破坏模式最后转变为脆性破坏模式,同时切削力倾角并不随切削深度变化。将离散单元法分析的结果与实际实验进行了比较,表明该方法是合理的。     

高速切削过程绝热剪切局部化断裂预测

基于高速切削过程绝热剪切饱和极限理论,结合锯齿形切屑绝热剪切带的变形和受力条件,以及材料的动态塑性本构关系,建立以切削速度、切削厚度和刀具前角为预测变量的高速切削过程绝热剪切局部化断裂的预测模型,并以淬硬45钢和FV520(B)不锈钢为例,预测其发生绝热剪切局部化断裂的临界切削条件。通过高速切削试验和金相试验,讨论了切削条件对绝热剪切局部化断裂过程的影响规律和敏感程度,验证了绝热剪切局部化断裂的预测结果。结果表明:较大切削厚度和较小刀具前角会降低绝热剪切局部化断裂的临界切削速度,建立的绝热剪切局部化断裂预测模型能有效预测切屑发生绝热剪切局部化断裂的临界切削条件。     

Adiabatic Shear Mechanisms for the Hard Cutting Process

The most important consequence of adiabatic shear phenomenon is formation of sawtooth chip. Lots of scholars focused on the formation mechanism of sawtooth, and the research often depended on experimen...     

Finite element simulation of high-speed machining of titanium alloy (Ti�C6Al�C4V) based on ductile failure model

A Johnson?CCook material model with an energy-based ductile failure criterion is developed in titanium alloy (Ti?C6Al?C4V) high-speed machining finite element analysis (FEA). Furthermore, a simulation procedure is proposed to simulate different high-speed cutting processes with the same failure parameter (i.e., density of failure energy). With this finite element (FE) model, a series of FEAs for titanium alloy in extremely high-speed machining (HSM) is carried out to compare with experimental results, including chip morphology and cutting force. In addition, the chip morphology and cutting force variation trends under different cutting conditions are also analyzed. Using this FE model, the ductile failure parameter is modified for one time, afterword, the same failure parameter is applied to other conditions with a key modification. The predicted chip morphologies and cutting forces show good agreement with experimental results, proving that this ductile failure criterion is appropriate for titanium alloy in extremely HSM. Moreover, a series of relatively low cutting speed experiments (within the range of HSM) were carried out to further validate the FE model. The predicted chip morphology and cutting forces agree well with the experimental results. Moreover, the plastic flow trend along an adiabatic shear band is also analyzed.