Simulation of workpiece forming and centre displacement in plunge centreless grinding

By reducing the set-up time of centreless grinding system, higher process flexibility and productivity can be obtained. This goal is approached by developing a simulation model that assists in efficient centreless grinding system set-up. In this paper, an analytical modelling of plunge centreless grinding is described. The developed model incorporates the workpiece forming mechanism along with workpiece dynamics and facilitates the simulation of workpiece form and its centre displacement. The prototype simulation model has been embedded within a spreadsheet environment and experimentally verified.     

Limit charts for high removal rate centrelessgrinding

An experimental investigation of the high removal-rate centreless grinding process applied to steel and cast-iron has allowed the investigators to present “limit-charts”. These limit charts illustrate the characteristic diagram for boundaries of operation where the grinding variables are infeed rate, workpiece speed and grinding wheel speed. The boundaries determined in this investigation were formed by burn, chatter, or available power (75 kW). It was not possible to achieve an excessive wheel wear condition although this might have been possible if more power had been available. The shape of the diagram means there is an optimum point of operation within the region enclosed by the boundaries. This forms the basis of a control strategy.Kinematic characterisation of the grinding process is developed by attention to the concept of a ‘speedeffect”, a “size effect” and a “shape effect”. Results for the speed effect are distinguished and presented separately from results for the size effect and shape effect to avoid mixed causes for the differing physical effects.A two-dimensional surface is presented showing the variation of grinding energy with variations in infeedrate, workpiece speed and grinding wheel speed. Because speed, shape and size effects are separated on the surface it is possible to read the optimum grinding wheel speed for minimum energy. For these experimental conditions, the optimum grinding wheel speed was found to be approximately 50 m/s. In the region of the optimum speed, specific energy is relatively insensitive to variations in operating speeds and feeds which is a useful feature when devising an automatic control strategy.     

Modelling of grinding gap macro geometry and workpiece kinematics in throughfeed centreless grinding

The paper discusses the simulation of a throughfeed centreless grinding process in a virtual environment (VE). The developed simulations are based on an analytical grinding gap model describing the grinding gap macro geometry and workpiece kinematics. First of all, the model is embedded in a desktop application (Cegris), which facilitates regulating wheel truing and the determination of set-up variables, both of which yield an optimal grinding gap macro geometry in a reduced set-up time. Finally, the Cegris is ported to a CAVE (CAVE Automatic Virtual Environment) for an interactive visualisation of the process, an application used to train machine tool operators.     

A study on the effect of regulating wheel error on the roundness of workpiece in centreless grinding by computer simulation

This paper deals with a simulated study of the centreless grinding operation to understand the role played by the regulating wheel errors on the roundness of the workpiece. Regulating wheels with a flat and specific number of lobes as errors were used for this study. A flat on the regulating wheel increases the roundness error of the work and a projection is formed on the workpiece whenever the flat contacts the workpiece. A lobed regulating wheel produces components with high roundness error and this error shoots up to higher values whenever the number of lobes on the regulating wheel equals a full multiple of regulating wheel-work diameter ratio.     

纯钛和镍包埋渗铝的热力学研究(英文)

分别在敞开和密闭系统中进行纯钛和纯镍的气相渗铝实验。渗铝实验在1100℃下进行3h,包埋粉末含10%Al、5%NH4Cl和85%Al2O3。在敞开条件下,渗铝后纯钛的表面会生成α(Al)固溶体,样品的质量损失达到32.37%。然而,在密闭条件下,纯钛样品的表面生成TiAl3层和α(Al)固溶体,没有质量损失。热力学分析表明,在敞开条件下对纯钛渗铝时,会生成钛的氯化物,导致其质量减少。而在密闭条件下,由于生成的氯化物会达到平衡状态而使生成反应停止。相反,无论是在敞开或密闭条件下,纯镍样品都可以进行表面渗铝,不会导致质量损失。在两种状态下,镍的表面都生成了AlNi和Ni3Al相。     

基于多克隆粒子群的无心磨削工艺参数优化

为了选择无心磨削的最佳工艺参数,建立了磨削工艺参数优化模型,并提出了一种多克隆的粒子群优化算法。将克隆复制、克隆交叉、高频变异和克隆选择引入粒子群优化算法中,从而改善基本粒子群算法容易陷入局部极小值、收敛速度慢的缺点。与其他相关算法相比,新算法在进行磨削工艺参数优化时,搜索速度更快、搜索能力更强。     

Effects of abrasive type cooling mode and peripheral grinding wheel speed on the AISI D2 steel ground surface integrity

In this work the AISI D2 steel ground surface integrity was evaluated at different grinding conditions. Two sets of experiments were conducted. The first and the second sets are referred to as conventional grinding tests (CGTs) and high-speed grinding tests (HSGTs), respectively. For the CGTs two different types of abrasives (aluminium oxide and sol–gel alumina) and two cooling modes (oil-based grinding fluid and cryogenic cooling) were tested at a peripheral wheel speed v_s=22 m/s. For the HSGTs, an electroplated cBN grinding wheel was used at a peripheral wheel speed ranging from 60 to 260 m/s. Experimental results show that the grindability and the surface integrity of the AISI D2 steel could be substantially improved by using the sol–gel grinding wheel and cooling by liquid nitrogen comparatively with conditions using aluminium oxide and cooling with oil-based grinding fluid. These conditions reduce the grinding force components, lower the level of tensile residual stresses and expand the range of stock removals rate for which compressive residual stresses can be obtained. In this case the stock removal rate could be increased 7 times and still having compressive residual stresses. These experimental results were further explored and it was possible to establish linear relationships between the specific energy and the amplitudes of the surface residual stresses when grinding under the CGTs conditions. This relationship is very useful for process control and optimization. Results of the HSGTs show that the highest levels of the compressive surface residual stresses were obtained at a peripheral wheel speed v_s=120 m/s (σ_∥=?520 MPa, σ_⊥=?770 MPa). These values are much higher than those measured at the ground surface generated using the sol–gel grinding wheel and cooling by liquid nitrogen (σ_∥=?295 MPa, σ_⊥=?250 MPa). Further, the CGTs and HSGTs results were used to establish the burn-free and burn conditions in relation with the specific grinding energy. It was shown that condition using the sol–gel grinding wheel and cooling with liquid nitrogen and condition using cBN grinding wheel at peripheral wheel speed up to 180 m/s generate specific grinding energy that are significantly lower than the burning threshold energy.     

On Stability and Dynamics of Milling at Small Radial Immersion

Stability and dynamics of milling at small radial immersion are investigated. Stability charts are predicted by the Semi Discretization method. Two types of instability are predicted corresponding to quasiperiodic and periodic chatter. The quasiperiodic chatter lobes are open and distributed along the spindle speed axis only, while the periodic chatter lobes are closed curves distributed in the plane of spindle speed and depth of cut. Experiments confirm the stability predictions, revealing the two principal types of chatter, the bounded periodic chatter lobes, and some special chatter cases. The recorded tool deflections in these cutting regimes are studied. The experiments also show that the modal properties of a slender tool may depend on spindle speed.     

A new method for the identification of stability lobes in machining

This paper introduces a new method for identifying the stability lobes in milling. The method depends on ramping the spindle speed while monitoring the behaviour of a chatter indicator. Based on the pattern of this indicator, the stability lobes are located accurately. The lobes are identified on-line without stopping the machine. It is not necessary to calculate the frequency spectrum of any vibration signal. The method was tested successfully in immersion down-milling and was shown to be applicable to slotting. Experimental results showed that the frequency characteristics of the stability lobes identified using the developed method are the same as those of the lobes established using constant speed cutting.     

Optimization of Set-up Conditions for Stability of The Centerless Grinding Process

The stability of the centerless grinding process is very sensitive to the set-up conditions due to the uniqueness of the work-holding system. Centerless grinding produces precision components with high productivity only when the set-up condition is optimally chosen. This paper describes the effect of set-up conditions on three stability criteria of the centerless grinding system. It also presents guidelines for determining proper set-up conditions to avoid spinners, chatter vibration and roundness problems. Finally, an algorithm for providing the optimum set-up condition based on process aims is proposed and the simulation results are discussed.     

A comparative study of the cutting forces in high speed machining of Ti–6Al–4V and Inconel 718 with a round cutting edge tool

Titanium alloy Ti–6Al–4V and nickel-based superalloy Inconel 718 have been widely employed in modern manufacturing. The published literature on high speed machining (HSM) of the two materials often involves different machining set-up, which makes it difficult to directly apply the research findings from one material to the other to select the most appropriate tool geometry and cutting conditions. A comparative experimental study of HSM of Ti–6Al–4V and Inconel 718 is conducted in this paper using the same machining set-up. The scope of this study is limited in high speed finish machining, where the tool edge geometry plays a significant role. The experimental set-up and the methods of measuring the cutting forces and the tool edge radius are introduced. A total of 40 orthogonal high speed tube-cutting tests were performed, involving five levels of cutting speeds and four levels of feed rates. Based on extensive experimental data, the similarities and differences between HSM of Ti–6Al–4V and Inconel 718 are quantitatively compared and qualitatively explained in terms of four quantities: (1) the cutting force F_c, (2) the thrust force F_t, (3) the resultant force R, and (4) the force ratio F_c/F_t. A total of 12 empirical regression relationships are obtained.     

An investigation on wear mechanism of resin-bonded diamond wheel in Elliptical Ultrasonic Assisted Grinding (EUAG) of monocrystal sapphire

An investigation is carried out to analyze and quantify the wear mechanisms of resin-bonded diamond wheel in Elliptical Ultrasonic Assisted Grinding (EUAG) of monocrystal sapphire. The EUAG is a new grinding method proposed by the present authors in which an elliptical ultrasonic vibration is imposed on the workpiece by using an elliptical ultrasonic vibrator. In this paper, a series of grinding experiments under the presence/absence of ultrasonic vibration assistance are performed. The grinding forces and work-surface roughness are measured, and the wheel surface is examined too. The experimental results indicate that during grinding, the steady process region performed in EUAG is longer than that in Conventional Grinding (CG) by 20%, meaning that the grinding wheel has a longer sharp cutting period in EUAG. It is validated that the main wear mechanisms in EUAG is micro-fracture and cleavage of abrasive grains, which has a positive effect on the better grinding performance, such as lower grinding forces, force ratio F_n/F_t, wheel loading, and smoother work-surface. This study demonstrates that the improved grinding performance of diamond wheel can be realized by using EUAG method.     

Model-based chatter stability prediction for high-speed spindles

The prediction of stable cutting regions is a critical requirement for high-speed milling operations. These predictions are generally made using frequency response measurements of the tool/holder/spindle set, obtained from a non-rotating spindle. However, significant changes in system dynamics occur during high-speed rotation. In this paper, a dynamic model of a high-speed spindle-bearing system is elaborated on the basis of rotor dynamics predictions, readjusted with respect to experimental modal identification. Variations in dynamic behaviour according to speed range are then investigated and determined with accuracy. Dedicated experiments are carried out in order to confirm model results. By integrating the proposed speed-dependant transfer function into the chatter vibration stability approach of Budak–Altintas [S. Tobias, W. Fishwick, Theory of regenerative machine tool chatter, The Engineer February (1958)] a dynamic stability lobes diagram is predicted. The proposed method enables a new stability lobes diagram to be established that takes into account the effect of spindle speed on dynamic behaviour. Significant variations are observed and allow the accurate prediction of cutting conditions. Finally, experiments are performed in order to validate chatter boundary predictions in practice. The proposed modelling approach can also be used to qualify a spindle design in a given machining process and can easily be extended to other types of spindle.     

On the dynamics of ball end milling: modeling of cutting forces and stability analysis

This paper presents a dynamic force model and a stability analysis for ball end milling. The concept of the equivalent orthogonal cutting conditions, applied to modeling of the mechanics of ball end milling, is extended to include the dynamics of cutting forces. The tool is divided into very thin slices and the cutting force applied to each slice is calculated and summed for all the teeth engaged. To calculate the instantaneous chip thickness of each tooth slice, the method of regenerative chip load calculation which accounts for the effects of both the surface undulations and the instantaneous deflection is used. To include the effect of the interference of the flank face of the tool with the finished surface of the work, the plowing force is also considered in the developed model. Experimental cutting forces are obtained using a five-axis milling machine with a rotary dynamometer. The developed dynamic model is capable of generating force and torque patterns with very good agreement with the experimental data. Stability of the ball end milling in the semi-finishing operation of die cavities is also studied in this paper. The tangential and radial forces predicted by the method of equivalent orthogonal condition are fitted by the equations F_t = K_t(Z)bh_av and F_r = K_r(Z)F_t, where b is the depth of cut and h_av is the average chip thickness along the cutting edge and Z is the tool axis coordinate. The polynomial functions K_t(Z) and K_r(Z) are the cutting force constants. The interdependency of the axial and radial depths of cut in ball end milling results in an iterative solution of the characteristic equation for the critical width of cut and spindle speed. In addition, due to different cutting characteristics of the cutting edge at different heights of the ball nose, stability lobes are represented by surfaces. Comparison of the time domain simulation for the shoulder removal process in die cavity machining with the analytical predictions shows that the proposed method is capable of accurate prediction of the stability lobes.     

无心磨床精磨监控系统的研制

无心磨床采用微米级数显监控装置,可较精确地实现对硬质合金模压棒材外圆磨削过程的跟踪,研制的专用夹具固定接触传感器,解决了可回转导轨架不能装配线性尺检测位置的难题。该传感器组件不仅能将砂轮进给值精确的传送到数显器上,同时可监视进给过程中机床的稳定性,重要的是提高了精磨产品的合格率。     

Creating Stability Lobe Diagrams during Milling

Motorized spindles are in common use in high-speed milling. The dynamic behavior of the spindle depends on the actual number of revolutions and the temperature. The performance of the spindle during milling, particularly, the behavior concerning chatter, is crucially affected by the speed dependent dynamic behavior of the mechanical system. This paper presents the reasons for the speed-sensitive stiffness and the shifting stability lobes. A new method of measuring and calculating the dynamic behavior during milling by means of sub-space-state-space-identification methods is introduced. Finally, the computed stability lobe diagrams are compared with experimentally determined stability lobe diagrams.     

Tilt angle effects in surface grinding with mounted points

Modern 5-axis machining centers enable the development and use of new machining kinematics and grinding strategies to grind workpieces with superabrasives mounted points. One of these technologies is the oscillating surface grinding (OSG), which can improve the surface characteristics and the tribological properties of machined surfaces. In the current contribution could be observed that the resultant surface roughness values by OSG are not constant within a wavelength of oscillation of the grinding tool. This leads to further studies of the kinematics involved in this process, resulting in the development of a new grinding strategy: the tilt surface grinding (TSG). By using TSG, workpiece surfaces with similar characteristics as OSG could be obtained even without aid of oscillation. In this study, experimental tests are carried out to investigate the influence of parameters involved in these strategies. A significant parameter is the tilt angle α _t, which can be simplified as an angle between the directions of the feed rate in tangential direction v _ft and the feed rate in axial direction v _fa of the grinding tool. The tilt angle α _t is responsible for changes occurred on the OSG when compared to conventional surface grinding. Increasing the tilt angle α _t, a reduction up to 50 % of the roughness values could be achieved. This paper aims to complement and expand the knowledge of the OSG, explaining how the aid of the oscillation tool changes the grinding kinematic, in order to optimize the grinding process.     

Surface generation mechanism of WC/Co and RB-SiC/Si composites under high spindle speed grinding (HSSG)

The surface generation mechanisms of WC/Co and Reaction-bonded SiC/Si (RB-SiC/Si) composites under high spindle speed grinding (HSSG) were investigated in the present work, compared with quasi-static indentation test. The results showed that surface generation mechanism for WC/Co and RB-SiC/Si varied under both quasi-static indentation and dynamic grinding. Only plastic deformation occurred for WC/Co indicating its higher toughness, while pop-out effect induced by phase transformation in RB-SiC/Si would prompt the chipping at phase boundaries under indentation. Under dynamic grinding, WC/Co underwent plastic deformation, grain dislodgement and WC particles crush, while ductile removal, phase boundaries crack (along the grinding direction) and chipping fracture occurred for RB-SiC/Si with the increase of cutting depth. It was found that the binder in the bulk WC/Co and RB-SiC/Si played a decisive role on the material removal mode, and the mechanics of grain dislodgement for WC/Co and RB-SiC/Si were analyzed based on a geometrical model. Besides, three types of grinding wheel wear appeared, including grit dislodgement, flattening and splintering, which bear an obvious influence on the surface generation.     

Self-similar growth of a compound layer in thin-film binary diffusion couples

The diffusion controlled growth of a compound phase A_nB between two thin films of material A and B is studied with the nonlinear Kirkendall effect included. This growth process is important in electronic materials processing and in synthesis of high-temperature materials using multilayer films. Previous models of the growth rate do not solve the diffusion equation, and thus do not fully utilize the predictive capability. This paper describes a self-similar transformation that reduces the nonlinear, time-dependent diffusion equation with two free boundaries into a nonlinear ordinary differential equation, which is solved numerically by a shooting method. It is found that the intrinsic diffusion coefficients of A and B in A_nB can be determined from the positions of the interfaces without using the concentration profile. This provides a simpler method for measuring intrinsic diffusion coefficients. An asymptotic solution valid for small concentration gradients is derived and agrees with the numerical results.     

Engineered wheels for grinding of optical glass

This paper focuses on the evaluation of topographical parameters characterizing the influence on the behaviour and finally the process results by applying engineered diamond wheels in machining hard and brittle materials. Here, coarse-grained, single-layered diamond grinding wheels with electroplated abrasive layers and active-brazed, defined grain pattern have been dressed by a special conditioning process and used in precision grinding experiments on optical glass. The characterization of the abrasive layer topography was done by two topographical parameters, the specific total grain plateau area A′_G,total and the average grain cutting edge width b_G,a, determined by 3D-profilometry of replicated abrasive layers after each dressing step.