Fixture System Research Articles

Digital Twin-Driven Reconfigurable Fixturing Optimization for Trimming Operation of Aircraft Skins

The incomplete constraint induced by multipoint reconfigurable fixturing and the inherently weak rigidity of thin shell parts significantly hinder the stability of flexible fixturing systems. In particular, during the trimming operation, the number of effective locators may change with the progressive separation of the desired shape from that of the blank part, which easily produces the cliff effect (instantaneous dramatic reduction) of the system stiffness. As a result, the location layout becomes a main crux in reality. Regarding this issue, the author herein presents a digital twin-based decision-making methodology to generate reconfigurable fixturing schemes through integrating virtual and physical information. Considering the intrinsic features of the trimming process, such as the time-varying propagation of the system stiffness and the coupling effects of multiattribute process parameters, the hidden Markov model was introduced to cope with reconfigurable fixturing optimization. To achieve fast convergence and seek a feasible solution, local information (where low system rigidity occurs) was extracted and shared to guide the optimization process in a front-running simulation. To demonstrate the presented method, trimming experiments were performed on a large-size compliant workpiece held by a reconfigurable fixturing system that was developed independently by our research group. The experimental results indicate that the proposed method could adaptively iterate out the optimal locating schema and process control reference from the virtual fixturing and trimming simulation to guarantee the time-varying stability of the trimming process in the real world. Clearly, the digital twin-based reconfigurable fixturing planning approach generated a high possibility of building a context-specific, closed-loop decision-making paradigm and allowing the reconfigurable fixturing system to behave in a more adaptable and flexible manner.

Accuracy Analysis for a Flow Line Process using a Mobile Holding Fixture for Machining CFRP Components

<div class="section abstract"><div class="htmlview paragraph">The aerospace sector is challenged to produce airplanes more efficiently and resiliently in the future. This leads to an increasing demand for improving productivity and flexibility as well as providing solutions for sustainable developments. A bottleneck in production is the machining of large-scale components. Apart from the machining tasks, non-productive operations like fixture adjustment, component handling, referencing and localization are performed within the machining station and can constitute up to 50% of the overall workload. In the UniFix project, Fraunhofer IFAM is participating in the development of a mobile fixture system for large-scale aircraft components, like vertical tail plane and landing flap components of the single aisle aircrafts. By installing components into a mobile holding fixture with an according referencing scheme, a flow line can be established that is composed of specialized workstations discharging the machining station from non-productive processes, leading to higher productivity.</div><div class="htmlview paragraph">This paper focusses on the component localization and referencing strategies for this mobile holding fixture. Here, specific challenges are caused by the transport in between the referencing and the machining process. Therefore, influences that contribute to the overall geometric uncertainty of the referencing process for machining are evaluated in detail. In this work, an automated laser scanner based localization of the components will be presented. Furthermore, estimations on the uncertainty in the involved systems will be conducted and propagated through the localization process.</div></div>

Preparation of Papers for IFAC Conferences & Symposia: Adaptive fixture system for reducing machining distortion caused by residual stresses in milling

Machining distortion caused by residual stresses is one of the major challenges in the production of thin-walled monolithic parts, which are widely used in the aerospace industry. The relevant influencing factors include the selected process parameters, the machining sequence as well as the used fixture system. This paper presents an adaptive fixture system for reducing machining distortion caused by residual stresses in milling. To validate the advantage of this system, machining distortion experiments are conducted on monolithic parts made of Ti-6A1-4V using a conventional and the adaptive fixture system.

Clamping deformation of thin circular workpiece with complex boundary in vacuum fixture system

With the increasing requirements on the parallelism and flatness of thin-walled flat parts, vacuum fixtures have been widely used in precision manufacturing. While several research studies focus on using the finite element method to predict the clamping deformation in the vacuum clamping system, it is very time-consuming and often unable to obtain the convergent solution with complex boundary conditions. In this paper, based on Kirchhoff plate theory, a new method was proposed to solve the clamping deformation of the thin circular workpiece with various complex boundary conditions subjected to vacuum negative pressure. The improved Rayleigh–Ritz method was applied to solve the spatial partial derivatives. The admissible function was composed of polynomial and trigonometric functions, which explained the unconstrained state of the workpiece. Furthermore, Courant’s penalty method was used to handle the constraint conditions. Simulations and experiments were conducted to verify the convergence, feasibility, accuracy, and applicability of the proposed method. The results showed that the proposed method is well-suited for arbitrary boundary conditions with vacuum fixture system, and its convergence was good compared to the finite element method. Finally, the influence factors of clamping deformation were discussed comprehensively and were optimized to reduce the clamping deformation. The proposed method can quickly calculate the clamping deformation of the thin circular workpiece under different vacuum clamping conditions, which provides the guidance for fixture design and the selection of clamping parameters for researchers in the processing field.

Extended PKM Fixturing System for Micro-Positioning and Vibration Rejection in Machining Application.

The paper aims to present a mechatronic device able to micro-position the workpiece and to reject disturbances due to machining operation. A decoupling method is proposed for a parallel kinematic machine (PKM) fixturing platform composed by a 3-DoF flexure-based piezo-actuated mechanism. The parallel platform, with a vertical motion and two rotations, is described and its kinematics and dynamics are studied. The coupling undesirable effect is investigated based on a set of poses. To improve the quasi-static regulator model for a set-point following system, a bump less switching controller and a fine-tuning procedure, to estimate the parameter uncertainty and enable the external disturbance containment in an extended broadband frequency range, are presented. The platform and the piezo-actuator controllers are modelled based on a gain scheduling, standard ISA form method, to guarantee the stability. The accuracy is demonstrated through a set of simulations and experimental comparisons. A sensitivity analysis that evaluates the tracking performance and the disturbance rejection based on the number of signal amplitudes, frequencies, and phases is discussed. A validation phase has shown that the developed architecture presents a steady state error lower than 1.2 µm, a vibration reduction of 96% at 1130 Hz with a maximum resolving time of 6.60 ms.

Optimization of flexible fixture layout to improve form quality using parametric finite element model and mixed discrete-integer genetic algorithm

Dimensional and form accuracy of the workpiece can be improved by effective fixture layout design which shows minimum deformation of the workpiece during machining. Flexible fixtures are inevitable in industries owing to high product variety and shortened production time. Hence, an integrated approach is presented to select the optimum position of locating and clamping elements in a flexible fixture that provide good form accuracy. In this approach, a Parametric Finite Element Model (PFEM) is developed using the information about the workpiece, fixture plan and machining condition. PFEM is used to predict the elastic deformation of the workpiece for the fixture layouts generated using a discrete Genetic Algorithm (GA) with mixed integer-discrete variables. The objective is to minimize the maximum deformation of the workpiece by optimizing fixture layouts. The stability of the workpiece and fixture system is ensured by implementing non-negative reaction force constraints in GA. The proposed approach is applied for a prismatic workpiece to carry out pocket milling operation. The significance of this work is to express the flexibility and computational effectiveness of PFEM to accommodate variation in the workpiece, machining condition and fixture plan while designing flexible fixtures. Further, it highlights a significant reduction in search space due to the use of discrete GA and stability constraint as it takes less objective function calculations. An experimental analysis is performed to study the effectiveness of the proposed approach. Therefore, the proposed approach provides a viable solution to the optimization problem in flexible fixtures.

Optimal layout and reconfiguration of a fixturing system constructed from passive Stewart platforms

The distinguishing property of Reconfigurable Manufacturing Systems (RMS) is that they can rapidly and efficiently adapt to new production requirements, both in terms of their capacity and functionalities. For this type of systems to achieve the desired efficiency, it should be possible to easily and quickly setup and reconfigure all of their components. This includes fixturing jigs that are used to hold workpieces firmly in place to enable a robot to carry out the desired production processes.In this paper, we formulate a constrained nonlinear optimization problem that must be solved to determine an optimal layout of reconfigurable fixtures for a given set of workpieces. The optimization problem takes into account the kinematic limitations of the fixtures, which are built in shape of Sterwart platforms, and the characteristics of the workpieces that need to be fastened into the fixturing system. Experimental results are presented that demonstrate that the automatically computed fixturing system layouts satisfy different constraints typically imposed in production environments.

Locator Placement Optimization for Minimum Part Positioning Error During Machining Operation Using Genetic Algorithm

Fixture design is one of the main factors which affect the final product quality. Proper design of fixture plays an important role in ensuring the required tolerance of the product. Proper placement of locators is one of the prominent factors in fixture design. Locators are elastic: they deform under clamping and machining forces causing rigid body displacement of the workpiece which in turn affects the part quality. In this article, a 3-2-1 type of fixturing system having elastic locators around a considerably rigid rectangular workpiece is considered. A genetic algorithm is proposed, which uses a fitness function that evaluates the positioning error of the workpiece under external forces and torque. Among several variables, 12 variables, which define the placement of locators, are chosen to be optimized while minimizing the positioning error of the workpiece at the point of action of machining force. The proposed algorithm optimizes the 12 interlinked variables, within the specified region, for machining force and torque at a single point. However, when the cutting tool moves to any other point on the workpiece, it is observed that either the workpiece loses its contact with any one of the locators or the positioning error increases by a large value. To overcome this issue, the proposed algorithm is further modified for placement optimization to cater for multi-point machining, and the isostatism of the workpiece is ensured by checking the magnitude and direction of displacement (of what?) at each point of workpiece-locator contact. Finally, the original and modified GA algorithms are explained through a case study where the single point optimized placement shows loss of contact when machining force is applied at other points. The placement optimized from the modified algorithm shows that the isostatism of the workpiece remains intact while all four positioning errors are converged towards the same value. The results obtained from the proposed and modified algorithm are verified using ANSYS simulation.

Analysis and optimization of the machining fixture system stiffness for near-net-shaped aero-engine blade

Low stiffness of the near-net-shaped blade machining fixture of aero-engine creates challenges for the CNC machining accuracy and efficiency of near-net-shaped blade. This study aims to optimize the fixture’s material and structure to improve the stiffness of the blade-fixture system. First, the main influencing factors of the near-net-shaped blade tenon root CNC machining process were analyzed, and the influences of the positioning and clamping component materials and structures on the stiffness of the machining fixture were analyzed based on finite element analysis (FEA). Second, the mechanical properties of the positioning and clamping components of different materials and structures were analyzed based on cross-scale analysis from microscopic and macroscopic perspectives. Finally, the positioning and clamping components of the machining fixture were designed based on material selection and structure optimization, and the optimized machining fixture with new material and structural was verified by CNC machining process. The results showed that the material and structure of the positioning and clamping components were the main influencing factors of the machining fixture stiffness. The positioning and clamping components of the PEEK-GF30 material of the new splicing structure could effectively improve the blade-fixture stiffness.

Alternative System Design for High Temperature Solid Particle Erosion Wear Problem

In order to simulate the wear behavior of materials subjected to solid particle erosion due to high temperature as well as velocity effect in operating conditions such as power plants and jet engines, ASTM G76 standard is not sufficient and test rig compliant with ASTM G211-14 standard must be put forward. In this context, a test rig, alternative to existing systems where solid particle erosion wear experiments can be performed in compliance with high temperature (~ 700 °C) and particle impact velocity (~ 150 m/s) conditions, has been designed and produced. In order to ensure the widespread use of this test rig, a resistance wire system was used to reach high temperatures of the heating system by achieving a different design solution. In the setup of this design, the perfect gas equation, heating power and ohm laws are also used. In addition, the designs of specimen fixture and particle feeding system, which are the determining parameters in erosion wear tests, have been updated. With the produced test rig; solid particle erosion experiments were carried out using Inconel 718 test specimens at 600 °C temperature, ~ 97 m/s erosive particle impact velocity and three different erosive particle impact angles (30°, 60° and 90°). In the experiments, especially the change of temperature over time was confirmed by the data obtained from both the test rig and the specimens.

Assembly Part Positioning on Transformable Pin Array Fixture by Active Pin Maximization and Joining Point Alignment

Designing and installing jigs and fixtures for new products are very costly tasks. In order to rapidly respond to frequent product changes in a manufacturing system, we developed a transformable pin array fixture system including reconfiguration planning software that finds the optimal position for mounting an assembly part and automatically generates a pin height control code. We first classified main fixturing quality factors such as type of contact, distance between a pin and a joining point, and part shape. We then formulate the part positioning problem as an alignment problem between the joining point set and the pin point set in the xy plane in consideration of the fixturing quality factors. A car door trim assembly module was used to verify the proposed method. Finite-element analysis results showed that in a car door trim assembly process such as ultrasonic welding and screwing, relocating an assembly part at the optimal position reduces its deformation.

电子束蒸发球面夹具系统膜厚均匀性的研究

电子束蒸发球面夹具系统膜厚均匀性的研究

Modeling of jamming phenomenon in fixture design application: an analytical, numerical, and experimental study

Jamming may occur during the loading of the workpiece in the fixture and cause an improper locating process or damage to the workpiece, fixture, and loading system. Consideration of the jamming phenomenon is a necessary task in the planning of the workpiece loading strategy. In the present paper, the minimum norm principle is used to present an analytical model for predicting the jamming occurrence conditions through calculating reaction forces at the contact points. The implementation of this model results in an optimization problem that is solved using the augmented Lagrange multiplier method. The proposed model is applied to two well-known jamming problems, namely the peg-in-hole and block and palm configurations. Numerical analysis is also conducted in Adams software to verify the theoretical predictions. Finally, the experimental setups are fabricated for these configurations to validate the theoretical predictions and numerical results. The results show that the maximum errors between the theoretical predictions and experimental results are 14.4% and 3.8% for the jamming-out angle in the peg-in-hole study and jamming-in travel in the block and palm problem, respectively. Moreover, the worst-case error of 6.9% is obtained for the theoretical prediction of the jamming-in travel of block compared to the numerical result. Agreement between the theoretical, numerical, and experimental results proves the capabilities of the suggested model in the accurate prediction of the jamming occurrence conditions and its potentials for use in the three-dimensional jamming-prone fixturing systems.

An approach for optimising the fixturing configuration in flexible machining fixtures

Optimal fixture configuration is critical to minimise the machining distortion of the workpiece. This study presented an approach for optimisation of fixturing configuration in flexible machining fixtures. Herein, a flexible fixture system composed of adaptive clamping unit, locating unit, and auxiliary locating unit was investigated. Response surface methodology (RSM) was employed to describe the relationship between fixturing parameters (distance between two zero-point fixtures m 1, the clamping force of float fixture m 2, and quantity of float fixture m 3) and machining distortion. Furthermore, sensitivity analysis was implemented to evaluate the effects of fixturing parameters on machining distortion. Adaptive penalty function (APF) was constructed to search optimal settings of fixturing parameters. Comparative analysis of different optimal fixturing parameters obtained by RSM and APF was conducted. Single-sided component was adopted to validate the effectiveness of the proposed approach. Machining distortion was found to first decrease and then increase with the augment of m 1 and it reduced with the increment of m 2 and m 3. Besides, m 3 exhibited the greatest influence on machining distortion, 61.78%. The interaction between m 2 and m 3 resulted in the lowest influence, 0.019003%. Furthermore, the maximum machining distortion through optimised flexible clamping was only 0.312 mm, which reduced by 57.6% compared to traditional clamping.

Transfer Learning via Deep Neural Networks for Implant Fixture System Classification Using Periapical Radiographs.

In the absence of accurate medical records, it is critical to correctly classify implant fixture systems using periapical radiographs to provide accurate diagnoses and treatments to patients or to respond to complications. The purpose of this study was to evaluate whether deep neural networks can identify four different types of implants on intraoral radiographs. In this study, images of 801 patients who underwent periapical radiographs between 2005 and 2019 at Yonsei University Dental Hospital were used. Images containing the following four types of implants were selected: Brånemark Mk TiUnite, Dentium Implantium, Straumann Bone Level, and Straumann Tissue Level. SqueezeNet, GoogLeNet, ResNet-18, MobileNet-v2, and ResNet-50 were tested to determine the optimal pre-trained network architecture. The accuracy, precision, recall, and F1 score were calculated for each network using a confusion matrix. All five models showed a test accuracy exceeding 90%. SqueezeNet and MobileNet-v2, which are small networks with less than four million parameters, showed an accuracy of approximately 96% and 97%, respectively. The results of this study confirmed that convolutional neural networks can classify the four implant fixtures with high accuracy even with a relatively small network and a small number of images. This may solve the inconveniences associated with unnecessary treatments and medical expenses caused by lack of knowledge about the exact type of implant.

Integrating the setup planning with fixture design practice by concurrent consideration of machining and fixture design principles

In the conventional setup planning methods, the required fixtures are planned in a sequential order relative to other activities of the setup planning. In the present study, a new approach has been developed and a concurrent method has been proposed for integrating the setup planning and fixture design practice, which paves the way for considering the fixture design principles in parallel to setup planning activities. This eliminates the need to revise the setup plans to remedy the possible conflicts which are common to conventional procedures. The fixture principles have been cast into mathematical relations as a means to developing the proposed method. In the present study, the locating surfaces are determined as the most important step of the fixture system design. The reference faces which cannot be used as locating surfaces are also distinguished, in order for the designer to revise the design of the workpiece.

Conformable fixture systems with flexure pins for improved workpiece damping

Holding low stiffness freeform components during machining is challenging, due to clamping deformation and vibration issues. This paper presents a novel fixture concept that conforms to the surfaces of freeform components by filling the space between locators with arrays of deformable flexure pins. The introduction of flexures has minimal effect on workpiece-fixture dynamic response, meaning no changes to the tooling or machining parameters of an existing process are necessary. The flexure concept is shown to reduce clamping forces for a freeform component compared to a conventional fixture, while simultaneously reducing workpiece deformation. A mathematical model is used to define the flexure dimensions required to achieve the desired dynamic response, and a Finite Element (FE) model is used to predict workpiece-fixture clamping interactions, showing good agreement with experimental values for workpiece deformation. A flexure-based fixture is evaluated in a milling experiment, and found to reduce workpiece vibration when machining excitation causes flexures to resonate. Models reliably predict the relative machining responses of conventional and flexure-based fixtures, and allow observation of flexure resonant behaviour. The concept was shown to reduce the clamping force required to hold a workpiece by up to 60 % (with deformation reduced by up to 47 % for a given clamp force), and reduce workpiece vibration in milling by 30 % compared to a conventional fixture.

Approach to increase flexibility in automobile body shops through component-integrated fixture functions

Due to shorter product life-cycles, increasing product customization and the co-existence of electric and combustion engine vehicles, variant flexibility is gaining importance in the automobile production. The automobile body shop is characterized by inflexible, rigid fixture systems dedicated to meet the geometrical requirements of specific body parts. Changes in part geometry or dimension require the development of new fixture systems, thus increasing product variety results in higher fixture costs. This paper presents an approach for a fixtureless body shop based on component-integrated fixture-functions, increasing variant flexibility and reducing fixture costs. The approach is implemented using a body part assembly of an electric vehicle.

Novel Minimal Access Bone Anchored Hearing Implant Surgery and a New Surface Modified Titanium Implant, the Birmingham Experience.

Bone anchored hearing implants (BAHI) are widely used and highly successful, accompanied with a high level of patient satisfaction across most techniques. A large UK teaching hospital switched from the previously used wide diameter titanium fixture and drilling system to the novel minimally invasive technique and laser ablated titanium implant. Before this change the rates of fixture failure and skin problems necessitating abutment change were 1% each. Retrospective case note review of consecutive BAHI patients drawn from an electronic database between January 2015 and October 2016. Data from a total of 118 procedures were reviewed, with different combinations of surgical techniques and implant types. Sixty procedures were performed via the novel minimally invasive technique with 21 failures (35%). Fourty-eight modified minimally invasive technique procedures were performed with seven failures. In 64 of the procedures, laser ablated titanium fixtures were placed with 21 failures (32.8%). In 54 procedures wide diameter titanium fixtures were placed with eight failures (5%). Initial experience with the novel minimally invasive technique and laser ablated titanium fixture showed significantly higher failure rates than expected. This prompted a change to an open technique and subsequent abandonment of the laser ablated titanium fixture and custom drilling solution in our institution.

Prediction of PCBN tool life in hard turning process based on the three-dimensional tool wear parameter

The ideal scenario for the implementation of a machining process is to be able to predict tool performance without the need to conduct practical experiments. However, in an industrial environment, each set of machining conditions is unique, since the machine-tool conditions, machined material, cutting tool, and fixture system can vary. This can lead to differences between the predicted values and practical results. In this context, the aim of this research was to show and discuss a tool performance test methodology and a tool-life prediction model using the three-dimensional (volumetric) wear parameter WRM (volume of material removed from the tool) applied to hard turning with PCBN tools. The wear parameter WRM is measured at the beginning of the tool life (up to 25%) by focus variation microscopy (FVM). The tool wear rate (WRRM) is then calculated based on the ordinary least squares (OLS) method, and the tool life is estimated (TWRM) adopting the volume of material removed from the tool (WRMmax) as the criteria for the end of tool life. The tool-life model developed was capable of predicting the tool life with errors below 4% at the higher values of cutting speed adopted (vc = 150–187.5 m/min), that is, the cutting speeds applied industrially. The methodology adopted and the model developed represent a significant time reduction in the experimental machining tests, streamlining the research and development of the cutting tool grades, as well as the machining process optimization.