Synchronous Optimization of Clamping Force and Cutting Parameters for Thin-Walled Parts

Recently, highly porous metal foams have been used to replace the traditional open-flow channels to improve gas transport and distribution in the cells. Deformation of flow plate, gas diffusion layer (GDL), and metal foam may occur during assembling. When the cell size is small, the deformation may not be significant. For large area cells, the deformation may become significant to affect the cell performance. In this study, an assembling device that is capable of applying uniform clamping force is built to facilitate fuel cell assembling and alleviate the deformation. A compressing plate that is the same size of the active area is used to apply uniform clamping force before surrounding bolts are fastened. Therefore, bending of the flow plate and deformation of GDL and metal foam can be minimized. Effects of the clamping force on the microstructures of GDL and metal foam, various resistances, pressure drops, and cell performance are investigated. Distribution of the contact pressure between metal foam and GDL is measured by using pressure sensitive films. Field-emission scanning electron microscope is used to observe the microstructures. Electrochemical impedance spectroscopy analysis is used measure resistances. The fuel cell performance is measured by using a fuel cell test system. For the cell design used in this study, the optimum clamping force is found to be 200 kgf. Using this optimum clamping force, the cell performance can be enhanced by 50%, as compared with that of the cell assembled without using clamping plates. With appropriate clamping force, the compression force distribution across the entire cell area can approach uniform. This enables uniform flow distribution and reduces mass transfer resistance. Good contact between GDL and metal foam also lowers the interface resistance. All these factors contribute to the enhanced cell performance.

The effects of frictional damping on the dynamic response of a beam structure have been investigated. The frictional damping was contributed from the slip mechanisms in a ‘pseudo’ joint, that is a joint that did not take any static load. Using pseudo joints for vibration control has significant advantages over using joints that are designed both to take static load and to introduce frictional damping; because there is almost no restriction on the configuration of the pseudo joint, the undesirable side effects related to the frictional damping such as fretting corrosion of contact surfaces and loss of static stiffness are zero if the frictional damping from a pseudo joint is utilized. An ‘optimum’ clamping force was found which minimized the magnitude of the frequency response function (FRF) of the structure under excitation. A reduction of about 20 dB in the amplitudes of the FRFs was achieved by adjusting the clamping force at the joint. Frictional damping was found to be significant over the measured frequency range of 20-2000 Hz, and it was particularly effective over the range of 20-1000 Hz. The magnitudes of the FRFs of the structure were determined by the ratio of clamping force to excitation force. They were insensitive to the change of clamping force near its optimum value; also the optimum clamping force was not sensitive to small modifications to the structure. Therefore, utilizing frictional damping in pseudo joints is an effective technique for vibration control.

This paper describes the effects of the clamping force and the pre-clamping force on the fatigue strength of bolt-connection. The results obtained are as follows:(1) The types of load-deflection diagrams of bolt-connection with the clamping force were classified into cross-type and un-cross-type.(2) And also, theoretical expressions of the optimum clamping force and the pre-clamping force were inducted to the both types of the diagrams.(3) The theoretical values of the optimum clamping force nearly fitted to the experimental values.(4) The experimental values of the fatigue limits on the optimum clamping force for the M 5 bolt of bolt-connection increased about 40% against the conventional value, at mean stress σm = 30 kg/mm2.(5) The effects of the pre-clamping force on the bolt-connection were inducted with the improvements on the compressive residual stress in bolt-connection and the optimum clamping force.

Fixture design is a critical step in machining. An important aspect of fixture design is the optimization of the fixture, the primary objective being the minimization of workpiece deflection by suitably varying the layout of fixture elements and the clamping forces. Previous methods for fixture design optimization have treated fixture layout and clamping force optimization independently and/or used nonlinear programming methods that yield sub-optimal solutions. This paper deals with application of the genetic algorithm (GA) for fixture layout and clamping force optimization for a compliant workpiece. An iterative algorithm that minimizes the workpiece elastic deformation for the entire cutting process by alternatively varying the fixture layout and clamping force is proposed. It is shown via an example of milling fixture design that this algorithm yields a design that is superior to the result obtained from either fixture layout or clamping force optimization alone.

Fixture Layout Design (FLD) determines the specific position of locators and clamps to orient and holds the workpiece with respect to a machine tool. The FLD approaches that use Finite Element Analysis (FEA) have been widely used in previous works and have become computationally expensive and specific to a particular problem. Further, the FLD and clamping force optimization were often performed separately by ignoring their interdependence. In the present work, the locators' contact forces are uniformly distributed by suitably varying the fixture layout and clamping force to maximize the part dimensional and form quality. The parametric rigid body model is used to depict the behaviour of the workpiece-fixture system, and it is incorporated with the genetic algorithm to optimize the design variables. A prismatic workpiece with pocket milling operation is considered to validate the proposed methodology. Stability criterion and tool-fixture interference are considered constraints. Subsequently, FEA is used to verify the integrity of the proposed approach. The results infer that the uniform distribution of maximum elastic deformation is achieved due to the uniform distribution of contact forces. The suggested approach is proven effective for designing a milling fixture to manufacture components with high dimensional and form precision.

Thin-walled frame parts are widely used in aerospace and other fields. However, part deformation caused by cutting load and residual stress poses significant challenges to manufacturing accuracy and parts performance. To enhance the machining accuracy and part performance, this research proposes a novel clamping layout optimization method on the basis of deconvolution neural network (DCNN) and particle swarm optimization algorithm (PSO). The method integrates simulation model to simulate deformation and uses DCNN to predict the deformation field efficiently. The PSO is then applied to optimize the clamping layout while meeting physical and interference constraints. The case study shows that this method effectively reduces the deformation in the web area of thin-walled frame parts. Compared with the uniform layout, the optimization objective function value is reduced by 21.4%, and the maximal deformation and average deformation are separately decreased by 16.0% and 39.8%, which verifies the effectiveness of the presented method. This research not only addresses the critical issue of machining deformation in thin-walled parts but also demonstrates a novel combination of artificial intelligence based modeling and simulation techniques, which can be expanded to other advanced manufacturing challenges.

Thin-walled aluminum alloy parts are widely used in the aerospace field because of their favorable characteristics that cater to various applications. However, they are easily deformed during milling, leading to a low pass rate of workpieces. On the basis of on-machine measurement (OMM) and surrogate stiffness models (SSMs), we developed an iterative optimization compensation method in this study to overcome the machining deformation of thin-walled parts. In the error compensation process, the time-varying factors of workpiece stiffness and the impact of prediction model errors were considered. First, we performed machining deformation simulation and information extraction on the key nodes of the machined surface, and an SSM containing the stiffness information of discrete nodes of each cutting layer was established. Subsequently, the machining errors were monitored through intermittent OMM to suppress the adverse impact of prediction model errors. Further, an interlayer correction coefficient was introduced in the compensation process to iteratively correct the prediction model error of each node in the SSM along the depth direction, and a correction coefficient between parts was introduced to realize the iterative correction of the prediction model for the same node position between different parts. Finally, the feasibility of the proposed method was verified through multiple sets of actual machining experiments on thin-walled parts with added pads.

Off-line modelling and planning of optimal clamping forces for an intelligent fixturing system

The monolithic thin-walled parts are widely used in the aeronautic and astronautic field because of its excellent mechanical performance and light weight, but the thin-walled parts are vulnerable to the machining deformation due to its low stiffness and high material removal rate. According to the relative basic theory, the stiffness and internal residual stress of the part are the critical factors affecting the dimensional stability. In this work, the influences of equivalent bending stiffness and residual stress on the dimensional stability of thin-walled parts are studied. Nine typical thin-walled parts in three groups with two materials (7075 aluminum alloy for A1 ~ A3 and B1 ~ B3, and Ti6Al4V titanium alloy for B4 ~ B6) are machined and treated with different processes. Topology optimization technique is used to optimize the structure of parts to enhance the bending stiffness. Corresponding finite element method (FEM) simulations are carried out to further investigate the generation mechanism. The deformations in 312 h after machining are measured using coordinate measuring machine, and the deformation changes of the parts are obtained and analyzed. Finally, based on topological optimization and stress relief technology, a machining deformation control method for the monolithic thin-walled parts is proposed. Results show that the maximum and average deformations of thin-walled are evidently decreased using the proposed method.

In order to assure the dimensional quaLity of compLiant sheet metal assembly, it is quite necessary to use the numerical method to simulate and analyze the assembly process before real assembly. In this paper, the finite element analysis of the whole compLiant sheet metal assembly process is reaLized in two steps: clamping force calculation and joining and springing back simulation. Clamping force is optimized so that the initial gap between parts can be closed. The joints of the two parts are simulated by DOF coupLing in ANSYS. Spring-back force, which is assumed to be equal to the optimum clamping force, is appLied on the joint points. Large displacement static analysis is run in ANSYS to calculate the displacement distribution of both clamped part and compLiant sheet metal assembly. An example of compLiant sheet metal assembly by lap joints is studied to demonstrate the finite element analysis process in this paper.

Deformations of thin-walled plate due to static end milling force

The simultaneous improvement of injection molding process efficiency and product quality, as required by Industry 4.0, is a complex, non-trivial task that requires a comprehensive approach, which involves a combination of sensoring and information techniques. In this study, we investigated the suitability of in-mold pressure sensors to control the injection molding process in multi-cavity molds. We have conducted several experiments to show how to optimize the clamping force, switchover, or holding time by measuring only pressure in a multi-cavity mold. The results show that the pressure curves and the pressure integral are suitable for determining optimal clamping force. We also proved that in-channel sensors could be effectively used for a pressure-controlled SWOP. In the volume-controlled method, only the sensors in the cavity were capable of correctly detecting the end of the filling. We proposed a method to optimize the holding phase. In this method, we first determined the integration time of the area under the pressure curve and then performed a model fit using the relationship between the pressure integral and product mass. The saturation curve fitted to the pressure data can easily determine the gate freeze-off time from pressure measurements.

An adaptive clamp system for deformation control of aerospace thin-walled parts

In order to solve the engineering application problems of the traditional variable pump pressure system driven by valve control or asynchronous motor associated with the rising temperature, complex structure etc, which could results in oil pollution resulted from leaking, low efficiency, and poor pressure retaining accuracy due to time-varying, nonlinear and hysteresis of the hydraulic power chuck. The unstable clamping force would cause machining error of parts. On the basis of analyzing the mathematical model of motor, power mechanism, execution mechanism and pressure sensor, the transfer function of direct drive pump controlled electric hydraulic pressure servo control system is derived, and a clamping force controller based on adaptive fuzzy PID control principle is proposed. The fuzzy rules and inference methods are used to tune the PID parameters online, and thus the flow rate of system is adjusted in real time to keep the clamping force constant. The effectiveness and feasibility of the proposed control method are verified with simulation and experiment. The results show that the fuzzy PID controller has a better dynamic performance and robustness than traditional PID controller, at the same time, the system has the fast response and no overshoot. This kind of system can completely meet the requirements on accuracy and stability of the clamping force in the machining process, especially for machining thin-walled parts.

Despite recent advances and improvements in modeling and prediction of the dynamics of the machining process, an efficient machining process is limited due to chatter and instability of machining system. In fact, the machining system contains various kinds of joints, which cause difficulties in dynamics modeling, simulation and prediction. Moreover, the flexible support system results in large deformation and violent vibration of the workpiece when machining, and the thin-walled workpiece easily gives rise to the chatter of the machining system. Therefore, the dynamics of the flexible support system was considered to calculate stability lobe diagram in the modeling of milling process. The whole machining system was regarded as a closed loop composed by the machine tool structures, support, workpiece and machining process. In this paper, the receptance coupling (RC) method was introduced to predict the dynamics of the closed machining system. A milling process was taken for example to predict the chatter limitations using the dynamics of closed model. The mathematical model of the machining system (machine tool structures, spindle, holder and tool), together with the details of joint contacts, was given based on the RC method. The RC model was used to obtain the dynamics of the system, while receptance of the tool point was coupled. Based on the coupling model of the machining system, the depth limitations under different speeds were estimated for the technology parameter optimization in milling process. The response was considered to be the sum of the cutting point and the support system. The flexibility of the support system was considered to be the feedback of the cutting stiffness. By this means, the traditional model was modified to calculate the stability lobe diagram based on the dynamics of the spindle and support system. Furthermore, the milling experiment was carried out to verify the prediction results, and the dominant natural frequencies of receptance at tool point were obtained by modal testing to define the stability lobe diagram. It was found that the chatter results matched well with the stability lobes. It was concluded that the support system with poor stiffness might cause violent chatter especially when the workpiece was thin-walled. The cutting depth limitations of the flexible support system were lower than that of the rigid one. Moreover, this closed model of the machining system is appropriate for the chatter prediction of the flexible support system or thin-walled workpiece, so it is helpful for a better parameter optimization.