Analysis of error motions of ultra-high-speed (UHS) micromachining spindles

This paper presents an experimental approach to analyze radial and axial error motions of miniature ultra-high-speed (UHS) spindles. The present work focuses on identifying the sources of error motions and quantifying them, specifically for the UHS spindles with hybrid ceramic bearings. Since effective application of micromachining processes, which commonly utilize miniature UHS spindles, require a high level of dimensional accuracy, form accuracy, and surface finish, the (unwanted) motions of the UHS spindles (and the associated tool-tip runout) must be well-understood. In this work, a laser Doppler vibrometer (LDV)-based measurement technique is used to measure radial and axial motions of the spindle from a sphere-on-stem precision artifact. The influence of temperature fluctuations, dynamically-induced effects, contact-bearing defects, and tool-attachment errors are analyzed. The spindle speeds are varied from 40 krpm to 160 krpm, and the over-hang lengths of 15 mm and 7.5 mm are considered. The variations arising from tool attachment to the collet are also studied. It is seen that (1) the thermal state of the spindle exhibits a cyclic behavior that results in significant changes to the spindle motions, (2) spindle speed and over-hang length significantly affect the spindle motions, and (3) the variations arising from the tool attachment to the collet can be described using a normal distribution, and may cause more than ±50% amplitude variations to the spindle motions.     

Modeling of dynamic characteristic of the aerostatic bearing spindle in an ultra-precision fly cutting machine

Axis orientation stability of aerostatic bearing spindles has great influence on machining precision of ultra-precision fly cutting machines used for processing ultra-precision optical components of large diameter. Mid-spatial frequency errors (amplitude<0.1 μm, wavelength about 100 nm) always existed on the machined surfaces along feeding direction. Generally, the waviness errors on processed surfaces will impact the performance of workpiece used as optical components greatly, and the tilting motions of spindles were believed to be the main source which produced the waviness errors. In this paper, to study the tilting motions of spindles, the Euler dynamic equations of angular displacements of spindles were proposed, and analytic solutions of the equations were also presented. At the same time, the 3D surface profile simulations of workpieces based on analytic solutions of Euler equations were achieved. The simulation results have been verified by lots of experiments on an ultra-precision fly cutting machine. At last, the inertia tensor criterion which can decrease the waviness errors of machining surface was represented, and it can be applied to instruct the structure design of aerostatic bearing spindles.     

Analysis of bearing configuration effects on high speed spindles using an integrated dynamic thermo-mechanical spindle model

This work presents the effects of bearing configuration on the thermo-dynamic behavior of high speed spindles using the comprehensive dynamic thermo-mechanical model. The dynamic thermo-mechanical model consists of a comprehensive bearing dynamic model, a shaft dynamic model and a thermal model. The thermal model is coupled with the spindle dynamic model through bearing heat generation and thermal expansion of the whole system based on the bearing configuration. Thus the entire model becomes a comprehensive dynamic thermo-mechanical model. The new thermo-mechanical model also considers a pertinent mapping between bearing stiffness and shaft stiffness matrices based on bearing configurations, so that more general cases of bearing configurations can be modeled. Based on this model, the effects of bearing orientation on the spindle dynamics are systematically described and experimentally validated. It is shown that bearing orientation has a significant effect on spindle stiffness. Finally, the effects of various bearing configurations on spindle thermal and dynamic behavior are illustrated through numerical analysis with three different spindles.     

Wear,cutting forces and chip characteristics when dry turning ASTM Grade 2 austempered ductile iron with PcBN cutting tools under finishing conditions

Experimental studies of wear, cutting forces and chip characteristics when dry turning ASTM Grade 2 austempered ductile iron (ADI) with polycrystalline cubic boron nitride (PcBN) cutting tools under finishing conditions were carried out. A depth of cut of 0.2 mm, a feed of 0.05 mm/rev and cutting speeds ranging from 50 to 800 m/min were used. Flank wear and crater wear were the main wear modes within this range of cutting speeds. Abrasion wear and thermally activated wear were the main wear mechanisms. At cutting speeds greater than 150 m/min, shear localization within the primary and secondary shear zones of chips appeared to be the key-phenomenon that controlled the wear rate, the static cutting forces as well as the dynamic cutting forces. Cutting speeds between 150 and 500 m/min were found to be optimum for the production of workpieces with acceptable cutting tool life, flank wear rate and lower dynamic cutting forces.     

Atomic motion during the migration of general [0 0 1] tilt grain boundaries in Ni

We generalize a previous study of the atomic motions governing grain boundary migration to consider arbitrary misorientations of [0 0 1] tilt boundaries. Our examination of the nature of atomic motions employed three statistical measures of atomic motion: the non-Gaussian parameter, the “dynamic entropy” and the van Hove correlation function. These metrics were previously shown to provide a useful characterization of atomic motions both in glass-forming liquids and strained polycrystalline materials. As before, we find highly cooperative, string-like motion of atoms, but the grain boundary migration itself is a longer timescale process in which atoms move across the grain boundary. These observations are consistent with our previous results for Σ5 [0 0 1] tilt boundaries. It is evident from our work that the grain boundary structure and misorientation have a significant influence on the rate of grain boundary migration.     

An integrated thermo-mechanical-dynamic model to characterize motorized machine tool spindles during very high speed rotation

High speed machining (HSM) is a promising technology for drastically increasing productivity and reducing production costs. Development of high-speed spindle technology is strategically critical to the implementation of HSM. Compared to conventional spindles, motorized spindles are equipped with built-in motors for better power transmission and balancing to achieve high-speed operation. However, the built-in motor introduces a great amount of heat into the spindle system as well as additional mass to the spindle shaft, thus complicating its thermo-mechanical-dynamic behaviors. This paper presents an integrated model with experimental validation and sensitivity analysis for studying various thermo-mechanical-dynamic spindle behaviors at high speeds. Specifically, the following effects are investigated: the bearing preload effects on bearing stiffness, and subsequently on overall spindle dynamics; high-speed rotational effects, including centrifugal forces and gyroscopic moments on the spindle shaft and, subsequently, on overall spindle dynamics; and the spindle dynamics on the cutting point receptance. The proposed integrated model is a useful tool for differentiating quantitatively different effects on the spindle behaviors. The results show that a motorized spindle softens at high speeds mainly due to the centrifugal effect on the spindle shaft.     

Finite size effect on the piezoelectric properties of ZnO nanobelts: A molecular dynamics approach

The size scale effect on the piezoelectric response of bulk ZnO and ZnO nanobelts has been studied using molecular dynamics simulation. Six molecular dynamics models of ZnO nanobelts are constructed and simulated with lengths of 150.97 Å and lateral dimensions ranging between 8.13 and 37.37 Å. A molecular dynamics model of bulk ZnO has also been constructed and simulated using periodic boundary conditions. The piezoelectric constants of the bulk ZnO and each of the ZnO nanobelts are predicted. The predicted piezoelectric coefficient of bulk ZnO is 1.4 C m^?2, while the piezoelectric coefficient of ZnO nanobelts increases from 1.639 to 2.322 C m^?2 when the lateral dimension of the ZnO NBs is reduced from 37.37 to 8.13 Å. The changes in the piezoelectric constants are explained in the context of surface charge redistribution. The results give a key insight into the field of nanopiezotronics and energy scavenging because the piezoelectric response and voltage output scale with the piezoelectric coefficient.     

High contrast solid-state electrochromic devices from substituted 3,4-propylenedioxythiophenes using the dual conjugated polymer approach

Solid-state electrochromic windows from 3,4-propylenedioxythiophene (ProDOT) derivatives using dual polymer electrochromic architecture were fabricated and their electro-optical characteristics were recorded. DibenzylProDOT (DiBz-ProDOT) and biphenylmethyloxymethyl ProDOT (BPMOM-ProDOT) were used as cathodically coloring polymers, whereas bis(2-(3,4-ethylenedioxy)thienyl)-N-methyl carbazole (BEDOT-NMCz) was used as their complementary anodically coloring polymer. Straightforward assembly of the devices was designed by means of using a UV photo-curable gel. These devices exhibited response speeds of approximately 1 s with photopic contrasts as high as 52% for the complete device, and were found to exhibit good stability under open circuit conditions. Convenience of reporting photopic values to characterize electrochromic devices compared to reporting contrasts at a single wavelength is remarked.     

Radial error measuring device based on auto-collimation for miniature ultra-high-speed spindles

This study has developed a radial error measuring device for miniature ultra-high-speed spindles because it is very difficult to measure the radial error motion of miniature ultra-high-speed spindles by the conventional measurement method using capacitive-type displacement sensors. The authors have proposed an optical measurement method based on auto-collimation, which evaluates the radial error motion according to the movements of a laser beam reflected from a target sphere attached to the spindle end. This optical measurement method is suitable for radial error measurements of miniature ultra-high-speed spindles because of its applicability to a small target sphere, high-speed response and minor susceptibility to electric noise. In this paper, the measurement principle, and basic characteristics of the optical measurement method in addition to an approximate analysis are shown. The radial error motion of a miniature ultra-high-speed spindle with a steel ball 1 mm in diameter are measured by an optical measuring device designed and manufactured to implement the proposed method. The measurement results show that the optical measuring device is able to measure the radial error motion of ultra-high-speed spindles with a maximum rotational speed of 200 krpm.     

Low-frequency internal friction of hydrogen-free and hydrogen-doped NiTi alloys

The internal friction (IF) and Young’s modulus of the Ni_50.8Ti_49.2 shape memory alloy have been measured as a function of temperature (130 K < T < 335 K) by a dynamic mechanical analyser at various strain amplitudes and frequencies. Besides the one associated with the austenite/martensite transformation, several other IF peaks have been observed both in the hydrogen-free and in the hydrogen-doped states of the material. Some of these peaks are non-thermally activated processes caused by stress-assisted hysteretic motions of twin boundaries and dislocations; some others represent thermally activated relaxations caused by reorientation of hydrogen elastic dipoles or by stress-induced motions of twin boundaries interacting with hydrogen. The present low-frequency measurements provide new information concerning the amplitude and frequency dependences of the damping processes, thus throwing new light on their structural mechanisms.     

A shape memory alloy based tool clamping device

The traditional collet-chuck mechanism for tool clamping is a significant source of errors in spindles due to stack-up tolerances. This, in turn, adversely affects the tool's error motions particularly in demanding micro-cutting operations performed with ultra-high-speed miniaturized spindles. Hence, novel thought for miniature tool clamping is needed to minimize tool run-out and error motions in order to meet the necessary cutting speeds and accuracy requirements. In this paper a couple of Shape Memory Alloy (SMA) based solutions for the clamping of miniature tools will be explored. For clamp actuation the so-called Two-Way Shape Memory Effect (TWSME) property of NiTi SMAs will be exploited. The basic principles, design requirements, analysis and physical realization of these devices will be discussed. It will be shown through experimental verification tests that clamping forces in excess of tens of Newtons are possible, confirming thus the feasibility of the proposed solutions.     

Magnetorheological fluid-controlled boring bar for chatter suppression

An innovative chatter suppression method based on a magnetorheological (MR) fluid-controlled boring bar for chatter suppression is developed. The MR fluid, which changes stiffness and undergoes a phase transformation when subjected to an external magnetic field, is applied to adjust the stiffness of the boring bar and suppress chatter. The stiffness and energy dissipation properties of the MR fluid-controlled boring bar can be adjusted by varying the strength of the applied magnetic field. A dynamic model of a MR fluid-controlled boring bar is established based on an Euler–Bernoulli beam model. The stability of the MR fluid-controlled boring system is analyzed, and the simulation results show that regenerative chatter can be suppressed effectively by adjusting the natural frequency of the system. Experiments in different spindle speeds utilizing a MR fluid-controlled boring bar are conducted. Under a 1 Hz square wave current, chatter can be suppressed, as evidenced by the elimination of chatter marks on the machined surfaces and the reduction in the vibration acceleration at the tip of the boring bar.     

Grinding performance and workpiece integrity when superabrasive edge routing carbon fibre reinforced plastic (CFRP) composites

Data is presented for wheel wear, cutting forces and workpiece integrity when high speed routing 10 mm thick CFRP laminates using single layer electroplated diamond and CBN grinding points as opposed to standard end milling tools. A 60,000 rpm retrofit spindle was utilised to accommodate the 10 mm diameter wheels having grit sizes of 76, 151 and 252 μm employed under either roughing or finishing parameters. Wear of CBN points exhibited a near two-fold increase over diamond with a similar ratio for cutting forces. Despite use of flood cooling, point geometry when roughing compromised life and integrity due to excessive clogging.     

Electrochemical investigation of PEDOT films deposited via CVD for electrochromic applications

A patterned solid-state electrochromic device on an ITO-coated plastic substrate was demonstrated that incorporates poly-3,4-ethylenedioxythiophene (PEDOT) deposited via a solventless oxidative chemical vapor deposition (oCVD) technique. In this paper, we present a thin-film electrochemical and optical analysis of oCVD PEDOT. oCVD PEDOT films about 100 nm thick on ITO/glass had optical switching speeds of 13 and 8.5 s, for light-to-dark and dark-to-light transitions, respectively. The color contrast was 45% at 566 nm and is 85% stable over 150 redox cycles. An Anson plot indicates that oCVD PEDOT color transition speeds are limited by ion diffusion rates, rather than electron or hole conductivity. Dimensionless analysis predicts gains of up to in oCVD PEDOT redox switching speeds by reducing the film thickness an order of magnitude to 10 nm. oCVD is a temperature-controlled process capable of conformal conductive polymer depositions onto a range of substrates from the vapor phase. Compatible substrates include plastic, paper and fabric. Non-conductive dispersion additives are not needed with oCVD, eliminating a potential source of defect-causing corrosion. oCVD offers powerful capabilities that may overlap with key challenges for the designers and fabricators of organic thin-film electronics, including OLED lighting and displays, electrochromics, photovoltaics, and semiconductors.     

Velocity effect analysis of dynamic magnetization in high speed magnetic flux leakage inspection

The investigation described in this paper focuses on the velocity effect of dynamic magnetization and magnetic hysteresis due to rapid relative motion between magnetizer and measured specimens in high-speed magnetic flux leakage (MFL) inspection. Magnetization intensity and permeability of ferromagnetic materials along with the duration of dynamic magnetization process were analyzed. Alteration of the intensity and distribution of magnetic field leakage caused by permeability of specimen were investigated via theoretical analysis and finite-element method (FEM) combined with the actual high-speed MFL test. Following this, a specially designed experimental platform, in which motion velocity is within the range of 5 m/s–55 m/s, was employed to verify the velocity effect and probability of a high-speed MFL test. Preliminary results indicate that the MFL technique can achieve effective defect inspection at high speeds with the maximum inspection speed of about 200 km/h being verified under laboratory conditions.     

Dynamics of the formation of the self-trapped exciton in the MX complex PtBr(en)

We have directly time-resolved the coupled electronic and vibrational dynamics of the self-trapping process in a quasi-one-dimensional system, the halide-bridged mixed-valence transition metal linear chain (MX) complex [Pt(en)₂][Pt(en)₂Br₂]·(PF₆)₄, (en: ethylenediamine, C₂H₈N₂) using femtosecond spectroscopic techniques in the vibrationally impulsive limit. In these experiments, we impulsively excite the optical intervalence charge-transfer transition with light pulses 35 fs in duration, short compared to the period of the characteristic chain-axis vibrational motion. The red-shifted absorbance of the self-trapped exciton state, which forms on a time scale of ∼200 fs, is modulated by vibrational wavepacket oscillations that correspond to lattice motions induced by the optical excitation. In addition to detecting an oscillatory response consistent with impulsive stimulated Raman excitation of the ground-state symmetric chain-axis stretching mode at ∼175 cm⁻¹, and its harmonics, we find that the self-trapped exciton absorbance is strongly modulated by a heavily damped, low frequency wavepacket component at ∼110 cm⁻¹. The coherence time of this new frequency component closely parallels the induction of the self-trapped exciton absorbance, consistent with a wavepacket corresponding to the lattice motion that carries the excited system to the self-trapped state. The spectral evolution of the low-frequency wavepacket oscillation provides a detailed picture of the coupled electron-lattice dynamics of the photo-excited state.     

Machinability improvement of titanium alloy (Ti–6Al–4V) via LAM and hybrid machining

Titanium alloy (Ti–6Al–4V) is one of the materials extensively used in the aerospace industry due to its excellent properties of high specific strength and corrosion resistance, but it also presents problems wherein it is an extremely difficult material to machine. The cost associated with titanium machining is also high due to lower cutting speeds (<60 m/min) and shorter tool life. Laser-assisted machining (LAM) and consequently hybrid machining is utilized to improve the tool life and the material removal rate. The effectiveness of the two processes is studied by varying the tool material and material removal temperature while measuring the cutting forces, specific cutting energy, surface roughness, microstructure and tool wear. Laser-assisted machining improved the machinability of titanium from low (60 m/min) to medium-high (107 m/min) cutting speeds; while hybrid machining improved the machinability from low to high (150–200 m/min) cutting speeds. The optimum material removal temperature was established as 250 °C. Two to three fold tool life improvement over conventional machining is achieved for hybrid machining up to cutting speeds of 200 m/min with a TiAlN coated carbide cutting tool. Tool wear predictions based on 3-D FEM simulation show good agreement with experimental tool wear measurements. Post-machining microstructure and microhardness profiles showed no change from pre-machining conditions. An economic analysis, based on estimated tooling and labor costs, shows that LAM and the hybrid machining process with a TiAlN coated tool can yield an overall cost savings of ～30% and ～40%, respectively.     

A thermal model for high speed motorized spindles

Lack of a more complete understanding of the system characteristics, particularly thermal effects, severely limits the reliability of high speed spindles to support manufacturing. High speed spindles are notorious for their sudden catastrophic failures without alarming signs at high speeds due to thermal problems. In this paper, a finite difference thermal model is developed to characterize the power distribution of a high speed motorized spindle, in particular the characterization of heat transfer and heat sinks. Without loss of generality, this model is built upon and verified by a custom-built high performance motorized milling spindle of 32 KW and maximum speed of 25 000 rpm (1.5 million DN).     

Development of a reversible machining method for fabrication of microstructures by using micro-EDM

A new micromachining method for the fabrication of micro-metal structures by using micro-reversible electrical discharge machining (EDM) was investigated. The reversible machining combines the micro-EDM deposition process with the selective removal process, which provides the ability of depositing or removing metal material using the same micro-EDM machining system. From the discharge mechanism of micro-EDM, the process conditions of micro-EDM deposition were analyzed firstly. Using the brass and steel materials as a tool electrode, the micro-cylinders with 200 μm in diameter and height-to-diameter ratio of more than 5 were deposited on a high-speed steel surface. Then the machining procedure was transformed easily from deposition to selective removal process by switching the process conditions. Different removal strategies including micro-EDM drilling and micro-EDM milling were used in the machining. Micro-holes with 80 μm in diameter are drilled successfully in the radial direction of the deposited micro-steel cylinder. Also, a brass square column with 70 μm in side length and 750 μm in height, and a micro-cylinder with 135 μm in diameter and 1445 μm in height are obtained by using micro-EDM milling. Finally, the characteristics of the deposited material were analyzed. The results show that the material components of a deposited micro-cylinder are almost the same as those of the tool electrode, and the metallurgical bonding has been formed on the interface. In addition, the Vickers-hardness of 454Hv of the steel deposited material is higher when compared to the hardness of 200Hv of the raw steel electrode.     

Dynamic characteristics of nanoindentation using atomistic simulation

Atomistic simulations are used to investigate how the nanoindentation mechanism influences dislocation nucleation under molecular dynamic behavior on the aluminum (0 0 1) surface. The characteristics of molecular dynamics in terms of various nucleation criteria are explored, including various molecular models, a multi-step load/unload cycle, deformation mechanism of atoms, tilt angle of the indenter, and slip vectors. Simulation results show that both the plastic energy and the adhesive force increase with increasing nanoindentation depths. The maximum forces for all indentation depths decrease with increasing multi-step load/unload cycle time. Dislocation nucleation, gliding, and interaction occur along Shockley partials on (1 1 1) slip planes. The indentation force applied along the normal direction, a tilt angle of 0°, is smaller than the force component that acts on the surface atoms. The corresponding slip vector of the atoms in the (1 1 1) plane has low-energy sessile stair-rod dislocations in the pyramid of intrinsic stacking faults.