Weak Rigidity Research Articles

Deformation prediction in flank milling of thin-walled parts based on cutter-workpiece engagement

Due to the weak rigidity of thin-walled parts, deformation inevitably occurs due to the influence of cutting forces during the machining process, making it challenging to achieve high machining accuracy. To address the above problem, it is necessary to conduct advanced predictions to determine the machining errors in thin-walled parts. This paper proposes a force-induced deformation prediction model based on cutter-workpiece engagement (CWE). The thin-walled structure is divided into two substructures, removal volume and workpiece. The finite element (FE) model of removal volume between any two discrete feed positions can be constructed based on CWE. At each feed position, the birth-death element method is employed to simulate actual material removal, updating the workpiece's stiffness. In the simulation process, utilizing node offset techniques to modify the cutter-workpiece contact zone, ensuring a high level of geometric consistency between the workpiece's FE model and its actual machining shape. Finally, the proposed method is proven via flank milling experiments. The machining errors predicted using the method proposed in this paper exhibit higher accuracy.

Boosting Circularly Polarized Luminescence from Alkyl-Locked Axial Chirality Scaffold by Restriction of Molecular Motions.

Boosting the circularly polarized luminescence of small organic molecules has been a stubborn challenge because of weak structure rigidity and dynamic molecular motions. To investigate and eliminate these factors, here, we carried out the structure-property relationship studies on a newly-developed axial chiral scaffold of bidibenzo[b,d]furan. The molecular rigidity was finely tuned by gradually reducing the alkyl-chain length. The environmental factors were considered in solution, crystal, and polymer matrix at different temperatures. As a result, a significant amplification of the dissymmetry factor glum from 10-4 to 10-1 was achieved, corresponding to the situation from (R)-4C in solution to (R)-1C in polymer film at room temperature. A synergistic strategy of increasing the intramolecular rigidity and enhancing the intermolecular interaction to restrict the molecular motions was thus proposed to improve circularly polarized luminescence. The though-out demonstrated relationship will be of great importance for the development of high-performance small organic chiroptical systems in the future.

Feature fusion and distillation embedded sparse Bayesian learning model for in-situ foreknowledge of robotic machining errors

In recent years, robotic machining of complex surface has become a research hotspot for intelligent manufacture. Considering the weak rigidity characteristics of industrial robot structure, the machining errors will be generated during the robotic machining process, which will further induce the undesirable impact on machining quality and production cycles. Therefore, this work proposes an in-situ foreknowledge model for robotic machining errors of complex curved parts. The static and dynamic features during the robotic manufacture process are obtained, where the robots’ stiffness concerning end effect operator is extracted as the static features, as well as the dynamic features of the monitored dynamic cutting forces. Furthermore, the structured fusion features are utilized as the dataset to train the foreknowledge model based on sparse Bayesian learning method. In order to adapt to the rapid response in the industrial field, the principal component analysis strategy is adopted to reduce feature dimension, by which the complexity of fusion features is reduced by 50.51 %. In addition, based on the adjustment of sparse weights, the complexity of the sparse Bayesian foreknowledge model is reduced by 74.31 %. According to the validation experiment on the blade shaped workpiece of marine propeller, the results indicate that the prediction accuracy of the proposed in-situ foreknowledge model is in the range of 60.03–87.07 µm, meeting the engineering requirements of robotic machining errors. The proposed in-situ foreknowledge sparse model displays the potential prospects for robotic machining errors prediction and control in the industrial application field.

The machinability of titanium alloy thin-wall parts in cooling minimum quantity lubrication (CMQL) environments

The machining of thin-wall components made of titanium alloys is challenging because the poor machinability of the material leads to severe problems such as accelerated tool wear and poor surface quality, while the weak rigidity of the thin-wall structure results in unavoidable vibration and surface form errors. To address these issues, this paper investigated the mechanisms and performance of cooling minimum quantity lubrication (CMQL) in milling titanium thin-wall parts. To verify the efficiency of CMQL, different cooling/lubrication strategies, including conventional flood cooling, minimum quantity lubrication (MQL) and CMQL with different temperature levels, were investigated. The cutting force, tool wear state, chip formation, surface integrity, and surface form errors were compared and analysed in detail. The experiment results show that MQL is inadequate at higher spindle speeds due to its ineffective cooling capacity and weakened lubrication ability. In contrast, CMQL has demonstrated its feasibility and superiority in milling titanium thin-wall parts under all conditions. The outcomes indicate that a lower temperature level of CMQL is advantageous to producing better wear resistance and lower thermomechanical loads, and the CMQL (− 15 ºC) machining environment can remarkably improve the overall machining performance and control the surface form errors of the machined thin-wall parts. At the spindle speed of 3000 rpm, the surface roughness measured under CMQL (− 15 °C) condition is reduced by 16.53% and 23.46%, the deflection value is decreased by 54.74% and 36.99%, while the maximum thickness error is about 53.51% and 20.56% smaller in comparison to flood cooling and MQL machining. In addition, CMQL is an economical and sustainable cooling/lubrication strategy; the outcomes of this work can provide the industry with useful guidance for high-quality machining of thin-wall components.

Manifestations of the weak shear rigidity of marine sediments in amplitudes and travel times of acoustic normal modes and lateral waves

Compressional-to-shear wave conversion at interfaces within unconsolidated marine sediments and interference of shear waves within thin sediment layers have been found to make significant contributions to sound attenuation [O. A. Godin, J. Acoust. Soc. Am. 149, 3586–3598 (2021)] and produce unexpectedly strong perturbations in the normal mode group speed even when the shear speed is small compared to the compressional wave speed. This paper extends the previous analysis to the lateral waves and their applications to characterize the compressional wave speed and attenuation in the seabed. Shear wave-induced contributions to horizontal refraction of normal modes over horizontally inhomogeneous seabed are modeled to characterize the bearing and travel time errors that arise in the fluid-bottom approximation. The acoustic travel time and amplitude effects of the shear waves are found to accumulate at different rates with the propagation range depending on the stability of the shear wave interference within the seabed. The ways to distinguish between the manifestations in acoustic observables of the weak shear rigidity in the seabed and the other environmental uncertainties are discussed for range-independent, range-dependent, and horizontally inhomogeneous shallow-water waveguides. [Work supported by ONR.]

3D ice-based fixturing system for thin-walled parts machining

Fixtures have an important influence on the machining process of thin-walled parts (TWP) and directly determine their final quality. Aiming at the problem of machining deformation caused by weak rigidity of TWP, an ice-based fixturing (IBF) is designed by taking advantage of the rigidity and viscosity of ice. The feasibility of IBF is then demonstrated based on the published papers of the authors of this paper. Afterwards, the structure and refrigeration process of IBF are described in detail, and the refrigeration capacity of the IBF system is analyzed, and the experimental results show that the IBF can achieve uniform and reliable icing. Finally, the process steps are formulated. This paper can provide a valuable reference for the development of advanced fixtures.

A path planning method for compliant jointing of weakly rigid helicopter fuselage

PurposeThe purpose of this paper is to propose a strategy for the final assembly of helicopter fuselage with weak rigidity parts and mismatched jointing butt ends.Design/methodology/approachThe strategy is based on path planning methods. Compared with traditional path planning methods, the configuration-space and collision detection in the method are different. The obstacles in the configuration-space are weakly rigid and allow continuous contact with the robot. The collision detection is based on interference magnitudes, and the result is divided into no collision, weak collision and strong collision. Only strong collision is unacceptable. Then a compliant jointing path planning algorithm based on RRT is designed, combined with some improvements in search efficiency.FindingsA series of planning results show that the efficiency of this method is higher than original RRT under the same conditions. The effectiveness of the method is verified by a series of simulations and experiments on two sets of systems.Originality/valueThere are few reports on the automation technology of helicopter fuselage assembly. This paper analyzes the problem and provides a solution from the perspective of path planning. This method contains a new configuration-space and collision detection method adapted to this problem and could be intuitive for the jointing of other weakly rigid parts.

Study on milling flatness control method for TC4 titanium alloy thin-walled parts under ice fixation

Thin-walled parts are characterized by weak rigidity and are very prone to deformation during machining, which directly affects the machining accuracy and performance of the parts, so controlling the machining deformation of thin-walled parts is an urgent process problem to be solved. To address such problems, this paper proposes a flatness control machining method based on ice holding of workpieces. The method uses frozen suction cups to solidify liquid water to achieve stress-free clamping of workpieces. We conducted a low-temperature tensile test to investigate the low-temperature mechanical properties of the material. Comparative tests of ice-free and low-temperature ice-fixed milling were conducted to compare and analyze the changes of flatness and milling force in the two working conditions, to investigate the influence of machining parameters on the flatness of thin-walled parts, and to reveal the mechanism of low-temperature milling of ice-fixed workpieces. In addition, the low-temperature milling performance of titanium/aluminum alloy based on ice-fixation was compared. The results show that TC4 has good plasticity, high flexural strength ratio and strong resistance to deformation at low temperatures. Compared with no ice-holding, ice-holding machining effectively improves the flatness of the workpiece, and the order of the machining parameters affecting flatness is: feed rate > milling depth > spindle speed. The milling forces under ice-holding conditions were all greater than those without ice holding. The stiffness and hardness, resistance to damage and deformation of titanium alloy at low temperature are greater than those of aluminum alloy. This method provides a new method for high-precision machining of thin-walled parts.

The Residual Stress Control of Thin-walled Sleeve Parts with Weak Rigidity

Photoelectric products in the military industry tend to be lightweight in design, requiring the part to be lightweight and have high specific strength. As one of the structural parts of photoelectric products, the thin-walled sleeve part has the peculiarity of lightweight and thin wall thickness. Nevertheless, the residual stress during machining processing poses an adverse effect on the accuracy of the parts. In this paper, the influence of residual stress on the accuracy of the parts during the processing was studied. Before fine machining, the residual stress was reduced by artificial aging techniques and ultrasonic beam control methods. Finally, the effect of the two stress relief methods was evaluated according to the qualified rate of the processing accuracy of the parts. The research results show that the axial residual stress of the part is larger than the circumferential residual stress. Both artificial aging techniques and ultrasonic beam control can eliminate residual stress, and the ultrasonic beam control method is more targeted and uniform.

The Experimental Study on the Mechanical Properties of Fiber-Reinforced Metal Laminates Using an Innovative Heat-Solid Integrated Forming Technology

In the forming process of fiber-reinforced metal laminates (FMLs) product, the exploratory compound forming technology, including hot stamping of aluminum alloy and laying process of fiber prepreg which is named HFQ-FMLs was proposed to solve the puzzles such as weak rigidity, low strength and integration deformation with large difficulty, and the feasibility and mechanical properties of the innovative forming process were studied. Firstly, based on the modified metal volume fraction formula, the theoretical values of the mechanical properties of the HFQ-FMLs plates were calculated. Compared with the experimental results, the minimum error is 1%, proving that the HFQ-FMLs technology scheme is feasible. Secondly, three kinds of metal sheets with different heat treatments and specimens by HFQ-FMLs were carried out for the tensile tests, the mechanical properties distributions were demonstrated, and the influence regularity of the strain rate and rolling direction on the stress analysis was considered at the same time. As can be seen from the distribution of yield strength and tensile strength, the yield stress of metal sheets obtained by HFQ-FMLs technology along the 45° is superior to the raw material and can increase by 46% under strain rate = 0.01 s−1. While, because the vacuum thermal curing treatment makes the aluminum alloy happen double aging, the metal sheet strength dropped, and the jointing strength between the metal and fiber prepreg became weak too, which made the strength limit of the new material improve weakly. Thirdly, the fractured style of the FMLs under different conditions was studied qualitatively. It is helpful to achieve the development rule of defects, optimize the craft route, and avoid deformation failure.

Cutting Chatter in Ultrasonic Elliptical Vibration Cutting and Its Influence on Surface Roughness and Tool Wear

Ultrasonic elliptical vibration cutting has a wide range of applications in the field of precision cutting of difficult-to-machine metal materials. However, due to its intermittent cutting characteristics and the weak rigidity of the horn, cutting chatter is prone to occur during its cutting process, which has an important impact on cutting surface quality and tool wear. In this paper, the rigid/viscoplastic rod model is used to simulate the horn in the ultrasonic elliptical vibration cutting device, and the influence factors of the amplitude-frequency response of the horn are analyzed. The influence of cutting speed and cutting depth on cutting chatter was studied by ultrasonic elliptical vibration cutting experiment of tungsten heavy alloy, and the influence of cutting chatter on cutting surface morphology and diamond tool wear was studied. The research shows that cutting speed will change the excitation frequency of the horn, and reasonable cutting speed can inhibit the occurrence of cutting chatter and avoid resonance of the horn. The cutting depth will affect the excitation amplitude and amplify the vibration amplitude when chatter or resonance occurs. The experimental results show that in ultrasonic elliptical vibration cutting of heavy tungsten alloy, chatter suppression can significantly improve the quality of the cutting surface and reduce the wear of diamond tools.

Vibration and deformation suppression in mirror milling of thin-walled workpiece through a magnetic follow-up support fixture

Large thin-walled workpieces have large deformation and vibration during end milling due to their weak rigidity, which can lead to degradation of the surface quality of workpiece. To this end, this paper proposes a new method to suppress vibration and deformation during mirror milling of thin-walled workpieces by using a magnetic follow-up support fixture. The main idea is to design a fixture that can clamp around the milling area of the workpiece in real time, follow the tool motion and continuously provide a controlled air pressure force to the opposite side of the milling area without providing the mirroring support fixture that occupies a large space to the opposite side of workpiece. First, to accurately characterize the motion of the fixture, a mathematical model of the axial magnetic force considering the lateral offset is developed. On this basis, to efficiently investigate the capability of the fixture in improving the dynamic characteristics and dynamic response of workpiece, the finite element method (FEM) is used to analyze the frequency response functions (FRF) and the mode shapes of workpiece under the action of the fixture. Then, the vibration amplitude and deformation of workpiece on the milling path are simulated and studied by considering the variation of the air pressure and the position of the fixture. Finally, the prototype of the fixture is developed and then the effectiveness and feasibility of the proposed method are verified under the experimental tests with different machining parameters.

Surface defect and chatter monitoring in robotic drilling CFRP composites using acoustic emission technique

Robotic machining is feasible for cutting composites in the field of aviation manufacturing with the benefit of high flexibility and applicability. Riveting is widely used as an important joining technique of aircraft structures requiring hole making on CFRP composites. However, due to the inhomogeneous and anisotropy characteristics of CFRP composites and the weak rigidity of robot, surface damage, and chatter are prone to occur during robotic drilling process, which significantly affect geometrical accuracy of assembled structure. In this work, robotic drilling trials on CFRP composites with different parameters (spindle speed: 3000–15000 rpm, feed rate: 0.01–0.10 mm/rev) were conducted to understand the formation mechanisms of surface damage, as well as the relationship between acoustic emission (AE) signals and defect characteristics. The root mean square (RMS), fast Fourier transform and short-time Fourier transform were successfully used for processing AE signals. The occurrence of entry burr and exit burr were investigated based on different fiber cutting angle and corresponding fiber fracture mechanisms. Drilling status and detailed mechanisms of indentation during robotic drilling were identified/monitored by processing AE signals in the time domain, frequency domain, and time-frequency domain analysis. Matrix cracking, delamination and fiber failure during robotic drilling process was highly related with AE frequency of 0–129, 133–203 and 221–346 kHz, respectively.

A State-of-the-Art Review on Chatter Stability in Machining Thin−Walled Parts

Thin−walled parts are widely used in many important fields because of performance and structural lightweight requirements. They are critical parts because they usually carry the core functions of high−end equipment. However, their high−performance machining has been facing severe challenges, among which the dynamics problem is one of the most important obstacles. The machining system is easily subjected to chatter due to the weak rigidity of the thin−walled structure and slender cutting tool, which significantly deteriorates the surface quality and reduces the machining efficiency. Extensive studies aiming at eliminating machining chatter have been carried out in the recent decades. This paper systematically reviews previous studies on the identification of system dynamic characteristics, modeling and prediction of chatter stability, and chatter elimination/suppression methods and devices. Finally, existing problems are summarized, and future research is concluded.

Effects of weak shear rigidity of the seabed on sound propagation in a range-dependent ocean

The shear wave speed is often small compared to the compressional wave speed in unconsolidated marine sediments. Sediment stratification and especially mass density variation in the seabed enhance the effects of weak shear on acoustic normal modes in range-independent ocean and produce significant shear wave contributions to mode attenuation in shallow water [O. A. Godin, J. Acoust. Soc. Am., 149, 3586–3598 (2021)]. A distinctive feature of the shear-induced perturbations in the mode phase speed and attenuation is their rapid variation with sound frequency due to shear wave interference within a sediment layer. This paper investigates theoretically the shear wave-induced perturbations in normal mode phase, travel time, and attenuation when water depth and/or sediment layer thickness vary gradually with range. Frequency dependencies of the adiabatic normal mode travel time and attenuation are found to be highly sensitive to range dependence of the seabed layering. Gradual changes in the sediment layer thickness have an effect similar to frequency averaging or artificially increased shear wave attenuation. Implications of these findings for geoacoustic inversions will be discussed. Contributions of shear rigidity and cross-range gradients of the sediment layer thickness to horizontal refraction of normal modes will be also examined. [Work supported by ONR.]

Identifying the event horizons of parametrically deformed black-hole metrics

Recent advancements in observational techniques have led to new tests of the general relativistic predictions for black-hole spacetimes in the strong-field regime. One of the key ingredients for several tests is a metric that allows for deviations from the Kerr solution but remains free of pathologies outside its event horizon. Existing metrics that have been used in the literature often do not satisfy the null convergence condition that is necessary to apply the strong rigidity theorem and would have allowed us to calculate the location of the event horizon by identifying it with an appropriate Killing horizon. This has led earlier calculations of event horizons of parametrically deformed metrics to either follow numerical techniques or simply search heuristically for coordinate singularities. We show that several of these metrics, almost by construction, are circular. We can, therefore, use the weak rigidity and Carter's rotosurface theorem and calculate algebraically the locations of their event horizons, without relying on expansions or numerical techniques. We apply this approach to a number of parametrically deformed metrics, calculate the locations of their event horizons, and place constraints on the deviation parameters such that the metrics remain regular outside their horizons. We show that calculating the angular velocity of the horizon and the effective gravity there offers new insights into the observational signatures of deformed metrics, such as the sizes and shapes of the predicted black-hole shadows.

Online measurement method for roundness of large aluminum alloy ring based on laser ranging principle

Aluminum alloy rings are widely used in aerospace, energy, petrochemical, and equipment applications. The loss of roundness often occurs because of the weak rigidity and unstable plasticity of aluminum alloy rings. Therefore, an online measurement technique for the roundness of aluminum alloy rings is critical for ensuring the processing quality of aluminum alloy rings. In this paper, an online roundness measurement method based on the laser ranging principle and improved particle swarm optimization (IPSO) is proposed with the aid of the rotation of the ring during the rolling process. First, the distance from the outer surface of the ring to the laser rangefinder was measured using a triangular laser rangefinder with the help of the rotation of the ring in the rolling process. Second, the distance data were denoized using an improved Kalman filter algorithm to reduce the influence of random noise on the measurement results. Finally, the roundness and geometric center were evaluated using the IPSO algorithm combined with the minimum zone circle (MZC) method and compared with the results obtained using the standard particle swarm optimization (PSO) algorithm, genetic algorithm (GA), and least square (LS) algorithm. The experimental results show that the roundness measured by the IPSO algorithm is less than 0.06 mm, demonstrating better performance in the roundness evaluation. The measurement method for roundness proposed in this paper is suitable for the online evaluation of large aluminum alloy rings, avoiding the influence of ring size on roundness measurement.

Extended Cosmic Ray Decreases with Strong Anisotropy after Passage of Interplanetary Shocks

The passage of an interplanetary shock and/or interplanetary coronal mass ejection often causes a rapid decrease in the Galactic cosmic-ray (GCR) flux, known as a Forbush decrease, followed by a recovery of the flux over some days. These local effects are of short duration and strongly rigidity dependent, with higher-rigidity particles exhibiting much weaker effects. In contrast, we present data for two events in which the cosmic-ray flux gradually decreased for about 1 week after shock passage, then recovering over the following week, with the highest anisotropy levels observed throughout Solar Cycle 24. These extended decreases have a weak rigidity dependence and are much more prominent in observations at higher cutoff rigidity, where the initial Forbush decrease is not clearly detected and other variations are generally weak, as we demonstrate using data from the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand with a cutoff rigidity of about 17 GV. We propose that these extended decrease events were initiated upon the passage of an interplanetary shock that inhibited the inflow of GCRs along the interplanetary magnetic field, possibly due to magnetic mirroring at the shock. We also discuss the general behavior of GCR anisotropy as observed at this high cutoff rigidity.

Research on the influence of robot structural mode on regenerative chatter in milling and analysis of stability boundary improvement domain

Regenerative chatter is an important factor affecting the efficiency and quality of robot milling of large and complex curved parts, especially when the weak rigid posture is forced to be selected under geometric interference. And the regenerative chatter in robotic milling is not only related to the tool-spindle mode but also affected by the tool point low-frequency vibration caused by robot structure mode. At some specific spindle speed, the tool point low-frequency vibration interrupts the occurrence process of chatter, increases the critical cutting depth, and generates a stability boundary improvement domain (SBID). This phenomenon makes it possible to avoid the adverse effects of the weak rigidity of the robot structure, and ensure and even improve machining efficiency. In order to reveal the mechanism of low-frequency vibration improving the stability boundary, a tool-workpiece engagement state considering the tool point low-frequency vibration caused by robot structure mode is analyzed, and radial and tangential tool-workpiece separation models, time-varying process damping model, and axial cutting depth modulation model are established to describe the mechanism of tool-workpiece engagement state. And then the time delay coefficient dependent on the cutting state is proposed. Finally, the stability prediction model considering the tool point low-frequency vibration caused by robot structure mode is established and verified by experiments with different postures. Through simulation and experiment with different postures, a set of evaluation indexes is proposed to evaluate the affection of the low-frequency vibration, and it is found that different robotic postures will lead to different SBIDs, thus proving the effectiveness of posture optimization on stability improvement. The results show that the proposed robotic milling stability model can effectively characterize the effect of low-frequency vibration on the stability boundary, and provide a theoretical basis for accurate positioning of SBID and efficient robotic milling.

On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium

We report on the energy dependence of Galactic cosmic rays (GCRs) in the very local interstellar medium (VLISM) as measured by the Low Energy Charged Particle (LECP) instrument on the Voyager 1 spacecraft. The LECP instrument includes a dual-ended solid-state detector particle telescope mechanically scanning through 360° across eight equally spaced angular sectors. As reported previously, LECP measurements showed a dramatic increase in GCR intensities for all sectors of the ≥211 MeV count rate (CH31) at the Voyager 1 heliopause (HP) crossing in 2012; however, since then the count rate data have demonstrated systematic episodes of intensity decrease for particles around 90° pitch angle. To shed light on the energy dependence of these GCR anisotropies over a wide range of energies, we use Voyager 1 LECP count rate and pulse height analyzer (PHA) data from ≥211 MeV channel together with lower-energy LECP channels. Our analysis shows that, while GCR anisotropies are present over a wide range of energies, there is a decreasing trend in the amplitude of second-order anisotropy with increasing energy during anisotropy episodes. A stronger pitch angle scattering at higher velocities is argued as a potential cause for this energy dependence. A possible cause for this velocity dependence arising from weak rigidity dependence of the scattering mean free path and resulting velocity-dominated scattering rate is discussed. This interpretation is consistent with a recently reported lack of corresponding GCR electron anisotropies.