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.     

钛合金高速旋转超声椭圆振动侧铣削切屑特征和刀具磨损研究

难加工材料钛合金在采用传统铣削方式时,随着切削速度的增加,切削力和切削温度都迅速增加,使得切削条件恶化并加速刀具磨损,从而导致刀具过早失效。将超声椭圆振动加工技术引入到高速铣削中,进行了钛合金高速旋转超声椭圆振动侧铣削试验。从切屑特征以及刀具后刀面磨损两个方面研究了高速超声椭圆振动铣削参数匹配对钛合金加工的影响。首先基于高速超声椭圆振动铣削过程中刀具-工件的运动学特点推导出高速超声椭圆振动铣削加工参数与振动参数间的匹配关系,然后利用本实验室自行研制的超声椭圆振动铣削装置进行了不同参数匹配关系下的验证性切削试验。试验结果表明：合理的参数匹配使得超声椭圆振动铣削在高速条件下依然能够实现分离型断续切削加工。相比普通铣削加工,分离型的高速超声椭圆振动铣削能够获得更加微细的切屑,切削热能够被及时地带走;良好的切削条件使得刀具的后刀面磨损均匀而缓慢,从而延长刀具的使用寿命;高速超声椭圆振动铣削能够有效地提高生产效率。     

Experimental investigation on the effects of thermomechanical loading on the vibrational stability during rotary ultrasonic machining

Ultrasonic vibration is assumed to be stable or unchangeable during the process of rotary ultrasonic machining (RUM) on brittle materials, neglecting the effects of different processing parameters. However, no experimental evidence has been reported to validate this assumption. In this study, the effect of thermomechanical load on the stability of ultrasonic amplitude during RUM was investigated by theoretical analysis and experimental procedures on quartz glass and sapphire. It was shown that the instability of ultrasonic amplitude during the machining process is mainly attributed to variation of resonant frequency under the implementation of thermomechanical load. The thermal effects of ultrasonic vibration decrease the resonant frequency of the ultrasonic machine, while mechanical loading during the machining process increases the resonant frequency. Furthermore, a higher feed rate or a harder material leads to a higher resonant frequency change. The variation of ultrasonic power can be used to review the validity of difference-neglected assumption when different values of processing variables, materials, or even machine tools are used during modeling. The results of this study should be well considered for future references when designing an ultrasonic machine.     

EXPERIMENTAL INVESTIGATIONS ON SUBSURFACE DAMAGE IN ROTARY ULTRASONIC MACHINING OF GLASS BK7

The present study aimed to establish an exact comprehension of the subsurface damage patterns generated during rotary ultrasonic machining (RUM) of glass BK7 process. First, the subsurface damage of the specimens produced with and without ultrasonic using three different diamond tools was compared and characterized by means of the bonded interface sectioning technique. Then, the RUM scratching tests were conducted on the polished specimen surfaces with two diamond tools to investigate the subsurface damage formation mechanisms involved in formal RUM process. The damage characteristics of the RUM scratches were investigated with respect to the kinematics principles of the abrasives and the dynamic fracture theory of the brittle material. As a result, three types of RUM-induced subsurface damage were identified: pulverization, chipping and cracking. The pulverization layer characterized by finer debris was just populated near the top surface of the RUM specimen without extending into the interior material. The finer debris should emerge simultaneously, resulting from the increased strain rate and the decreased dynamic fracture toughness of the material, both of which were aroused by the inertia loading of the abrasive when reaching the vertex of its sinusoidal trajectory.     

Influences of vibration on surface formation in rotary ultrasonic machining of glass BK7

This paper presented a fundamental investigation of the surface formation mechanisms involved in rotary ultrasonic machining (RUM) of glass BK7 process. Comparative observations of the scratches, generated in the scratching tests with and without ultrasonic, were performed using optical microscopy, white-light interferometer, and scanning electron microscopy (SEM). Giving consideration to the scratch morphologies and the abrasive process kinematics, the mechanisms of surface formation provoked by the ultrasonic superposition were investigated. Additionally, the formal machining tests with and without ultrasonic were also conducted to validate these surface formation mechanisms. As a result, a nondimensional parameter K was proposed to quantitatively describe the ultrasonic effects of the abrasives as well as to correlate these effects with the machining conditions. Due to the periodic variation in the effective work angle of the abrasive, the material accumulated slightly at the RUM groove entrance, whereas serious material accumulation appeared at the exit. The stress imbalance on the specimen surface induced by the dramatic fluctuation of the abrasive inertia load caused plenty of tortuous cracks in 0.2 μm-sized length emerge on the RUM grooves generated in the ductile material removal stage. A novel theoretical model of the surface formation mechanisms involved in formal RUM process was established by incorporating the ultrasonic effects, such as the lower dynamic fracture toughness of material, cyclical variation in the effective work angle of the abrasive, and the larger abrasive inertia force. Experimental results obtained in formal machining tests revealed that superimposing an ultrasonic vibration could distinctly reduce the cutting force of the diamond tool without seriously worsening the surface quality of the specimens.     

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.     

Experimental investigation of tool wear and machining rate in rotary ultrasonic machining of nickel alloy

Nickel alloys possess the excellent potential at high temperature and resistance to oxidation/corrosion owing to its high nickel content. These materials necessitate non-traditional machining methods. The rotary ultrasonic machining (RUM) process comes into existence as a superior alternative to the conventional machining of nickel alloys. The processing of these alloys using RUM needs attention. This article details the multi-response optimization in RUM of nickel alloy using the desirability concept. The present work is carried out with two shapes of the tool: (i) Plain tool and (ii) lateral slotted tool. During RUM, the process parameters—power rating tool rotation, abrasive diamond grit size and feed rate are varied. Compared with the plain tool, the lateral slotted tool shows the more efficient machining rate (MR) with less tool wear (TW). The micro-graphs disclose the mechanism of MR and TW during RUM.     

Rotary ultrasonic machining: effects of tool natural frequency on ultrasonic vibration amplitude

Rotary ultrasonic machining (RUM) is a hybrid machining process that combines the material removal mechanisms of grinding and ultrasonic machining. RUM has been applied to hole-making for a wide range of materials. It is known that ultrasonic vibration amplitude has significant effects on cutting force, torque, and surface finish in RUM. One experimental observation that has been reported in the literature multiple times states that different tools show different vibration amplitudes on the same ultrasonic power level. However, no analyses can be found in the literature to explain this observation. The existence of this knowledge gap makes it difficult to explain some experimentally obtained trends or to conduct more realistic physics-based modeling work. The objectives of this research are to understand the effects of tool natural frequency on ultrasonic vibration amplitude in RUM, to provide an explanation to the observation and verification of measurement methods, and also to guide tool design and selection in RUM. Ultrasonic vibration amplitudes of tools are measured by three methods and compared. It is found that tool natural frequency significantly affects ultrasonic vibration amplitude. The tool with its natural frequency closest to that of the ultrasonic power supply (20?kHz) generates the highest ultrasonic vibration amplitude on every ultrasonic power level tested.     

硬脆材料旋转超声加工技术的研究现状及展望

旋转超声加工是一种复合特种加工技术,它复合了传统超声加工和普通磨削加工的材料去除方式,在提高硬脆材料去除效率、减小切削力、提高加工精度和表面完整性等方面具有显著优势。自旋转超声加工技术发明至今,国内外学者开展了大量的有关旋转超声加工装备及工艺的研究工作,并且已在几乎所有主要的硬脆难加工材料中得到实际应用。本研究在简要概述旋转超声加工技术的基本原理和发展过程基础上,总结国内外学者在材料去除机理、工艺特性、加工新形式以及装备研发等几方面的主要研究成果,并对旋转超声加工技术的发展趋势及值得关注的问题进行展望。     

Chipping minimization in drilling ceramic materials with rotary ultrasonic machining

Ultrasonic machining (USM) has been considered as a new cutting technology that does not rely on the conductance of the workpiece. USM presents no heating or electrochemical effects, with low surface damage and small residual stresses on workpiece material, such as glass, ceramics, and others; therefore, it is used to drill microholes in brittle materials. However, this process is very slow and tool wear dependent, so the entire process has low efficiency. Therefore, to increase microhole drilling productivity or hole quality, rotary ultrasonic machining (RUM) is considered as a strong alternative to USM. RUM, which presents ultrasonic axial vibration with tool rotation, is an effective solution for improving cutting speed, precision, tool wear, and other machining responses beyond those of the USM. This study aims to reduce the microchipping or cracking at the exit of the hole, which inevitably occurs when brittle materials are drilled, with consideration of tool wear. To this end, response surface analysis and desirability functions are used for experimental optimization. The experimental results showed that the proposed RUM scheme is suitable for microhole drilling.     

高体积分数SiCp/Al复合材料旋转超声铣磨加工的试验研究

由于大量高硬度增强相SiC颗粒的存在,高体积分数铝基碳化硅（SiCp/Al）复合材料的机械加工十分困难。旋转超声加工被认为是加工这种材料的有效方法。通过超声辅助划痕试验,分析高体积分数SiCp/Al复合材料旋转超声铣磨加工的材料去除机理。在超声振动的作用下,材料中铝基体发生塑性变形,其表面得到夯实;SiC增强相被锤击成细小的颗粒而发生脱落,形成较大的空洞。由于材料加工的缺陷大多产生于SiC颗粒的去除过程中,SiC颗粒的去除方式对加工表面的质量起着决定性的作用,选择合适的工艺参数可以有效提高加工表面质量。旋转超声加工工艺特征试验表明,超声振动可有效降低切削力;主轴转速对轴向切削力的影响最大,其次是进给速度,切削深度对轴向切削力的影响较小;另外主轴转速对表面质量的影响效果也最大,并随主轴转速的增大表面粗糙度增大。因此在加工过程中,可以适当加大切削深度,在保证加工质量的基础上,选择较大的进给速度,在保证刀具寿命的前提下,选择合适的主轴转速,以获得较优的加工表面质量和加工效率。     

ROTARY ULTRASONIC MACHINING OF CARBON FIBER-REINFORCED POLYMER: FEASIBILITY STUDY

Carbon fiber-reinforced polymer (CFRP) has been widely used in aircraft components, automotive parts, and sporting goods. Hole machining is the most frequently employed operation of secondary machining for fiber-reinforced composites. However, challenges (delamination, splintering, burr, short tool life, low machining precision, and low surface quality) still remain for their widespread applications. Rotary ultrasonic machining (RUM) is a non-conventional machining process that has been used to drill holes in composite materials. However, it has not been used to drill this type of CFRP. In this article, RUM is introduced into drilling holes in this type of CFRP for the first time. The feasibility to machine carbon fiber-reinforced epoxy using RUM is investigated experimentally. Chips, edge chipping, surface roughness, tool wear, and thrust force were measured. Effects of RUM process variables (rotation speed, vibration amplitude, and feedrate) on thrust force and surface roughness were studied. Results showed that RUM could be used to drill holes in CFRP with high productivity and low tool wear. A better surface was produced by higher rotation speed and lower feed rate.     

超声波铣削加工材料去除率的理论模型

超声波铣削加工适于硬脆材料复杂型腔的加工，加工中影响材料去除率的因素很多，为了优化加工参数以及更好地控制加工过程，研究了各种加工参数对材料去除率的影响规律。分析了工具的高频振动、旋转以及机床进给三种运动综合作用下超声波铣削加工的材料去除机理，在传统超声波加工机理以及材料去除率模型的基础上，基于压痕断裂理论，建立了超声波铣削加工材料去除率理论模型。该模型可用于仿真实验研究及预测超声波铣削加工中的材料去除率。     

变幅杆连接工艺对旋转超声加工质量影响的研究

超声旋转加工是一种适用于脆硬材料加工的新技术,它强化了原加工过程,其加工效率随着材料脆性的增大而提高,实现了低耗能的目标。在超声旋转加工中,超声加工工具与变幅杆的连接对超声振动共振频率和超声波加工性能有很大影响。本文在深入研究旋转超声加工基本原理的基础上,分析了旋转超声加工时能量传递系统的关键制件-变幅杆和工具杆对超声加工精度的影响,并对变幅杆和工具杆制造工艺及连接的方法进行了初步研究。     

超声纵扭辅助铣削高强铝合金表面润湿性能研究

针对超声辅助加工在工件表面形成微刻划表面可以提高高强铝合金表面的微结构性能的现象,进行了单激励旋转超声纵扭复合铣削表面微观结构的试验,基于水接触角理论和纵扭铣削运动学理论分析了加工参数对水接触角的影响;搭建了单激励超声纵扭铣削试验平台,采用正交试验法研究了不同加工参数对表面粗糙度、铣削力以及表面润湿性能的影响。结果表明：超声振幅为4μm时表面质量最佳,切削速度和进给量与表面粗糙度和水接触角呈正相关的关系;超声加工方式下的表面水接触角较普通方式更大,而在超声加工时低振幅加工比高振幅加工的表面水接触角大,当转速达到一定值时,高振幅和低振幅所加工的表面水接触角差别不大。合适的加工参数条件下超声纵扭加工方式可以降低加工表面的粗糙度,改变表面的润湿性。     

高温合金蜂窝芯高速铣削材料去除机理与损伤行为

高温合金蜂窝芯材料具有高比刚度、轻质和能量吸收特性好等优异性能,被视为下一代高超声速飞行器热防护结构极具潜力的材料。高速铣削是高温合金蜂窝芯零件成型过程中重要的减材制造工艺,在蜂窝芯材料高速铣削时,蜂窝芯材料面内刚度低且高温合金塑性好,较小的切削力就会使蜂窝壁产生较大的塑性变形,导致蜂窝芯加工精度较低、加工损伤难以控制,对后续焊接、装配等工序产生不利影响。基于有限元仿真对蜂窝壁切削材料去除机理进行了深入研究,探索了铣削参数、刀具类型和铣削方式对铣削过程中铣削力和加工损伤的影响。研究结果表明,蜂窝壁切入角是影响蜂窝芯材料切削加工过程中瞬时应力分布和成屑机理的关键性因素。得到了铣削参数、刀具类型和铣削方式对高温合金蜂窝芯加工过程中加工损伤的影响规律。对于铣削参数,过大的进给量会导致芯格变形等加工损伤,降低切削速度会提高微小毛刺等加工损伤发生的频率;本文采用的三种刀具的对比结果表明,立式铣刀加工质量最好。插铣方式会产生明显的轴向冲击,而侧铣方式可以有效避免轴向冲击。研究成果为高温合金蜂窝芯低损伤高性能加工提供了理论依据和工艺技术储备。     

Analysis of rotary electrical discharge machining characteristics in reversal magnetic field for copper-en8 steel system

The material removal rate (MRR), along with the electrode wear rate (EWR), plays an important role in analysing machine output during electrical discharge machining. This work focuses on the improvement of machine output by introducing an induced magnetic field on the workpiece during rotary electrical discharge machining (REDM) of EN-8 steel with a rotary copper electrode. The workpiece was placed inside the induced magnetic field, wherein polarity of the magnetic field gets reversed periodically. Using Taguchi’s recommended design of experiments, we initially conducted experiments with eight input parameters at different levels . Significant parameters were identified with the help of the signal-to-noise ratio and ANOVA. Finally, another set of experiments was conducted for analysis of the process and development of empirical expressions for MRR and EWR. Experimental results established that rotary electrical discharge machining with a polarity reversal magnetic field delivers better machining output than machining in a non-magnetic field. Thus, this work benefits the EDM process by reducing the machining costs and by producing better geometrical trueness on workpieces, as MRR increased and EWR decreased.     

Relationship between subsurface damage and surface roughness of glass BK7 in rotary ultrasonic machining and conventional grinding processes

Subsurface damage (SSD) induced during the abrasive machining process considerably influences the technological application of the optical components. However, to date, there is no rapid and effective method to inspect the depth of SSD. For the purpose of precise and nondestructive evaluation of the SSD depth generated in rotary ultrasonic machining (RUM) and conventional grinding (CG) processes, a theoretical model, ground on indentation fracture mechanics of brittle material, was proposed by analyzing the correlation between the median and lateral crack systems aroused by a sharp indenter. It was found that the SSD depth was nonlinear monotone increasing with square of surface roughness (SR), namely, SSD?=?χSR²?+?l. Utilizing this model, the SSD depth could be quickly and precisely predicted through the SR (p–v value.) of the machined surface, geometrical features of the abrasive, and the material mechanical properties. To validate the feasibility of this method, both RUM and CG tests were conducted on the BK7 glass specimens with a Sauer Ultrasonic 20. Subsequently, the SSD of these specimens was exposed with the polishing–etching technique. The measurement results of SSD depth were consistent with the prediction values of this model, which reflected the feasibility of using this model to rapidly and accurately predict the SSD depth.     

Prediction of cutting force, tool wear and surface roughness of Al6061/SiC composite for end milling operations using RSM

The results of mathematical modeling and the experimental investigation on the machinability of aluminium (Al6061) silicon carbide particulate (SiCp) metal matrix composite (MMC) during end milling process is analyzed. The machining was difficult to cut the material because of its hardness and wear resistance due to its abrasive nature of reinforcement element. The influence of machining parameters such as spindle speed, feed rate, depth of cut and nose radius on the cutting force has been investigated. The influence of the length of machining on the tool wear and the machining parameters on the surface finish criteria have been determined through the response surface methodology (RSM) prediction model. The prediction model is also used to determine the combined effect of machining parameters on the cutting force, tool wear and surface roughness. The results of the model were compared with the experimental results and found to be good agreement with them. The results of prediction model help in the selection of process parameters to reduce the cutting force, tool wear and surface roughness, which ensures quality of milling processes.     

旋转超声电解复合加工小孔的仿真加工

为研究旋转超声电解复合加工小孔的成型过程,进行了旋转超声电解复合加工小孔试验,得到了不同加工时间孔的截面,并根据试验参数,进行了基于ANSYS的二维仿真加工和三维仿真加工。对小孔的入口直径、底面直径和加工深度进行了对比分析,结果表明由于三维仿真加工中采用了管电极,并考虑了电解加工中阴极超声高频振动对电解液电导率的影响,故其仿真结果更加接近试验值,间接证明了旋转超声电解复合加工小孔三维仿真加工的可靠性,展示了不同时刻的三维加工型腔,为旋转超声电解复合加工的成型过程和成型规律的研究提供了参考。