Three-dimensional Sheet Metal Parts Research Articles

Study on Rubber-Pad Formation for Sheet Metal

This article introduces four rubber-pad formation techniques for the pressing and drawing of components from thin sheet metals. The Guerin process is the simplest technique of rubber formation, which enables the production of shallow sheet metal parts with complex contours and bends. Free formation is another technique used to draw near-hemispherical shells using a rubber medium. Different bending processes can also be conducted using a rubber pad as a flexible die. Multipoint formation is applied to form complex three-dimensional sheet metal parts with discrete dies and rubber cushions. The fundamentals of these techniques as well as the influence of process parameters on the results are investigated in this paper

Numerical simulation research on the loading trajectory in stretch forming process based on distributed displacement loading

The stretch forming process based on distributed displacement loading presents a better ability to form three- dimensional surface parts, because the stretch forming load is applied by displacement at a series of discrete points and the loading trajectory can be individually controlled at each dis- crete point. To investigate the influence of the loading trajec- tory on the forming results, four different loading trajectories were designed and the corresponding forming processes were numerical analyzed. It was found that the loading trajectory can greatly change the values and distributions of the strain and the stress on the formed surface, the optimal loading tra- jectory among the four trajectories is the one whose material elongations and rotational angles of clamps vary according to cubic functions, the stress and strain in the forming process with this loading trajectory are the smallest, and the deforma- tion distribution of the sheet metal on the formed surface is most uniform. Numerical results shows that the mean and the range of stress of the spherical part formed with optimal load- ing trajectory can be reduced by 26 and 44 %, respectively, compared with those of the part formed with traditional load- ing mode. Through the numerical analyses on the stretch forming processes with different number of loading points, it was concluded that the more discrete loading points are, the more uniformly the longitudinal strain distributes on the formed surface of sheet metal and the smaller the mean strain value will be. Experiments were carried out, and the results show that the three-dimensional sheet metal parts with good quality can be obtained by the stretch forming process.

Longitudinal bending deformation analysis of sheet metal in continuous roll forming process

Development of flexible forming is crucial for manufacturing three-dimensional sheet metal parts. In this paper, continuous roll forming (CRF) is investigated to improve the forming results, especially for doubly curved parts. The CRF process employs an upper flexible roll and a lower flexible roll as a forming tool, and the different forming shapes are achieved by controlling and adjusting the distribution of the roll gap between two rolls. This paper is focused on analysis of the longitudinal bending deformation in CRF process, the plane assumption for longitudinal bending analysis is discussed, and the formulation to calculate the longitudinal strain distributed over the cross-section is derived. Through a series of numerical simulation on CRF processes, the validity of the plane assumption and the other theoretical models to analyze the deformation in CRF are confirmed; the relationship between longitudinal elongation distribution and longitudinal bending deformation is discussed and the strain distribution in transverse, longitudinal, and thickness directions is presented. In addition, the CRF experiments on saddle-shaped and torus-shaped parts were carried out, and the theoretical analysis and simulation results are verified by experiments.

A novel continuous roll forming process of sheet metal based on bended rolls

To effectively manufacture three-dimensional sheet metal parts with various curvatures produced in small-batch quantity, a continuous roll forming (CRF) process is proposed and investigated. The sheet metal is compressed between pairs of bended rolls and formed into a doubly curved surface. The shapes of forming rolls and the gap between the two rolls are piecewise controlled at a series of points. The two bended rolls and the nonuniformly distributed roll gap between them make the sheet metal bent in longitudinal and transverse directions simultaneously after the sheet metal passes through the roll gap, and a three-dimensional surface is formed continuously with the rotation of forming rolls. The relation between longitudinal elongation distribution and longitudinal bending deformation is discussed, and the lateral spreading in CRF process is analyzed; the method to compute the roll gap and the curved profiles of forming rolls are presented. Forming experiments for torus-shaped surface parts were carried out, and the experiment results demonstrate the feasibility and validity of CRF process and the presented methods.

Surface flexible rolling for three-dimensional sheet metal parts

To realize highly effective and continuous fabrication of three-dimensional (3D) sheet metal parts, a new forming method, surface flexible rolling forming, has been investigated. This method takes only two integral flexible rolls as the forming tool. In the forming process, a non-uniform elongation in the rolling direction and a bending deformation in the thickness direction occur simultaneously, and finally three-dimensional surface parts are formed. In this work, the basic principle and forming mechanism of surface flexible rolling are studied. A method to calculate the transversal curvature radius of the arc-shaped roll is brought forward, while the feasibility is verified by the explicit dynamic finite element analysis. An experimental device has been developed and the forming experiments have been performed. Typical 3D surface parts including the convex and saddle surface parts have been obtained. Finite element model of surface flexible rolling is established and the effect of forming parameters such as reduction, velocity, bending radius and friction on the surface shape is analyzed. The forming effects including shape errors and thickness changes are studied by the deviation analysis. The results indicate that the formed surface is quite close to the criteria one; the thickness of the parts changes gradually and keeps within a narrow range. The experimental formed parts are measured and the forming accuracy is investigated. The results show that the accuracy is high, and are consistent with the simulation.

Forming Stability Analysis of Surface Flexible Rolling

Surface flexible rolling method, using two integral working rolls as the forming tool, can achieve fast, flexible and continuous manufacturing of three-dimensional sheet metal parts. This paper introduces the basic principle of surface flexible rolling and discusses the numerical simulation results when the working rolls are bended as circular arcs. The stability indicates the forming effect to some extent and the flow type of the metal can be deduced from stability analysis. To integrate and analyze the simulation results by means of reverse engineering. The analysis results show that the forming process is stable and the effect of surface flexible rolling is fine. It also indicates that inhomogeneous deformation and accumulation occurs during the process. The numerical simulation and experimental results demonstrate that the surface flexible rolling is a feasible and effective way to form three-dimensional sheet metal parts.

Development of Flexible Rolling Device for 3D Plate

To effectively form three-dimensional sheet metal parts with various curvatures produced in small quantities, flexible rolling forming, a new flexible forming method is being developed. Firstly, concept of the flexible rolling forming is introduced and its characteristics and working principle are analyzed by the combination of conventional rolling and multi-point forming. Secondly, the equipment design scheme of flexible rolling forming is put forward and the relevant device is developed. Thirdly, the feasibility and practicality of flexible rolling forming are verified by rolling several typical three-dimensional surface parts such as spherical and saddle surface parts by self-developed device. Finally, verifies the reasonableness of the device and at the same time possesses the important guiding significance to the research and development of practical flexible rolling forming equipment.

Three-dimensional sheet metal continuous forming process based on flexible roll bending: Principle and experiments

To effectively manufacture three-dimensional sheet metal parts with various curvatures produced in small batch quantities, continuous sheet metal forming, a new flexible forming technology is being developed. This process employs an upper flexible roll and two lower flexible rolls as a forming tool, and the shape of a flexible roll can be changed in vertical direction. With the rotation of flexible rolls, the sheet metal is bent in longitudinal and transverse directions simultaneously and is formed continuously. In the present study, the three-dimensional surface of formed part is described by sweep surface based on the characteristic of the forming process and the arc-length parametric equation for describing continuous forming part is developed. The three-roll bending deformation of sheet metal in longitudinal direction is analyzed and the longitudinal curvature equation of the formed part is derived. The shape of flexible roll axis controlled at a number of points is represented by the cubic spline curve and the transverse curvature of the formed part after springback is then calculated piecewise. Typically experiments for forming concave shape surface and saddle-type surface have been performed, the experiment results are measured and analyzed by a binocular stereo vision measurement system, it is demonstrated that the formed surfaces are in good agreement with the desired shapes and the presented equations are useful for the continuous forming process design.

Numerical investigation of multi-point forming process for sheet metal: wrinkling, dimpling and springback

Multi-point forming (MPF) is a new flexible technique for manufacturing three-dimensional sheet metal parts. In this procedure, a pair of opposed matrices of punch elements substitute for the conventional fixed shape die sets, and the sheet metal can be formed rapidly between the matrices. Extensive numerical simulations of the processes for forming spherical and saddle-shaped parts were carried out by dynamic explicit finite element analysis. The contacting process between sheet metal and punch elements in MPF was investigated, and the variations of forming force with respect to the tool travel were analyzed. The wrinkling processes were simulated, and the MPF limit curves without wrinkles for spherical and saddle-shaped parts were obtained. Dimple is a particular defect in MPF, through the comparison of the thickness strains calculated by solid FE and shell FE, the finite elements appropriate for the numerical analysis of dimpling were detected, and the limit forming force without dimples was determined. Springback processes of MPF were simulated based on explicit-implicit algorithm. The springbacks and their distributions under different conditions were investigated.

Flexible manufacturing of sheet metal parts based on digitized-die

Digitized-die forming (DDF) is a flexible manufacturing technology through which a variety of three-dimensional sheet metal parts can be produced in a DDF system. It eliminates the need to design and produce the conventional die. The central component of DDF system is a pair of matrices of punches, the punches are controlled by computer and the desired shape of die is constructed by changing the heights of punches. Based on the flexibility of DDF, new forming processes are designed that cannot be realized in conventional stamping. In varying deformation path DDF, a sheet part is manufactured along an optimal forming path, and large deformation can be achieved for the material with poor formability. In sectional DDF, a sheet part is formed section by section, and this technique makes it possible to manufacture large-size parts in a small DDF press. A closed-loop forming system was built by combining DDF with rapid 3D-shape measurement system. It is used to compensate for material springback and improve dimensional accuracy of the formed part. And a DDF system with multi-point blankholder control system was developed to control the material flow, thereby to prevent sheet parts from wrinkling and tearing. The DDF integrated system is described, and the detailed forming procedures are explained in the paper. Typical examples are presented showing the applicability of the DDF technology.

Multi-point forming technology for sheet metal

Multi-point forming (MPF) is an advanced manufacturing technology for three-dimensional sheet metal parts. In this paper, an MPF integrated system is described that can form a variety of part shapes without the need for solid dies, and given only geometry and material information about the desired part. The central component of this system is a pair of matrices of punches, and the desired discrete die surface is constructed by changing the positions of the punches though CAD and a control system. Typical examples show the applicability of the MPF technology. Wrinkles and dimples are the major forming defects in the MPF process, but numerical simulation is a feasible way to predict forming defects in MPF. In conventional stamping, the method to form sheet metal with a blankholder is an effective way to suppress wrinkling; and the same is true in MPF. An MPF press with a flexible blankholder was developed, and the forming results indicated the forming stability of this technique. Based on the flexibility of MPF, varying deformation path MPF and sectional MPF were explored that cannot be realized in conventional stamping. By controlling each punch in real-time, a sheet part can be manufactured along a specific forming path. When the path of deformation in MPF is designed properly, forming defects will be avoided completely and large deformation achieved. A workpiece can be formed section by section though the sectional MPF, and this technique makes it possible to manufacture large size parts in a small MPF press. Some critical experiments were performed that confirmed the validity of the two special MPF techniques.

A Finite Element Based Die Design Algorithm for Sheet-Metal Forming on Reconfigurable Tools

Tooling cost is a major contributor to the total cost of small-lot production of sheet metal components. Within the framework of an academic/industrial/government partnership devoted to the development of a reconfigurable tool for stretch forming, we have implemented a Finite Element-based procedure to determine optimal die shape. In the reconfigurable forming tool (Hardt, D. E. et al., 1993, “A CAD Driven Flexible Forming System for Three-Dimensional Sheet Metal Parts,” Sheet Metal and Stamping Symp., Int. Congress and Exp., Detroit, MI, SAE Technical Paper Series 930282, pp. 69–76.), the die surface is created by the ends of an array of square pins, which can be individually repositioned by computer driven servo-mechanisms. An interpolating polymer layer is interposed between the part and the die surface to attain a smooth pressure distribution. The objective of the die design algorithm is to determine optimal positions for the pin array, which will result in the desired part shape. The proposed “spring-forward” method was originally developed for matched-die forming (Karafillis, A. P., and Boyce, M. C., 1992, “Tooling Design in Sheet Metal Forming using Springback Calculations,” Int. J. Mech. Sci., Vol. 34, pp. 113–131.; Karafillis, A. P., and Boyce, M. C., 1996, “Tooling And Binder Design for Sheet Metal Forming Processes Compensating Springback Error,” Int. J. Tools Manufac., Vol. 36, pp. 503–526.) and it is here extended and adapted to the reconfigurable tool geometry and stretch forming loading conditions. An essential prerequisite to the implementation of the die design procedure is the availability of an accurate FE model of the entire forming operation. The particular nature of the discrete die and issues related to the behavior of the interpolating layer introduce additional challenges. We have first simulated the process using a model that reproduces, as closely as possible, the actual geometry of the discrete tool. In order to optimize the delicate balance between model accuracy and computational requirements, we have then used the information gathered from the detailed analyses to develop an equivalent die model. An automated algorithm to construct the equivalent die model based on the discrete tool geometry (pin-positions) is integrated with the spring-forward method, to generate an iterative die design procedure that can be easily interfaced with the reconfiguring tool. The success of the proposed procedure in selecting an optimal die configuration is confirmed by comparison with experimental results.

Real-Time Control of Sheet Stability During Forming

During the stamping of complex three-dimensional sheet metal parts, the in-plane compressive stresses created often lead to failure by buckling. These are typically suppressed by binding the material at the periphery to provide a tensile bias. In practice, these biases are difficult to determine, and must be addressed with a combination of a priori analysis and die-making skill. Even then, in-process variations will cause parts to begin failing by tearing or buckling as friction, material, or geometric changes occur. In this paper two methods are presented for controlling the blankholder force in-process to ensure optimal forming conditions at all times. This is effectively a signature-following method based on replicating either a previously determined optimal forming-punch force trajectory or a normalized average thickness trajectory. The method is implemented using closed-loop control of these quantities, and subjected to experiments where various disturbances are presented. Previously reported results for axisymmetric shapes indicated the ability to eliminate the effect of uncertain initial blankholder force settings, friction variations, and blank placement errors. In this paper, the work is extended to include material property changes and thickness variations, both of which require a scaling of the optimal trajectory based on simple process mechanics. The work is then extended to include nonsymmetric parts, in particular a square dish-shaped part with corners of unequal radii. Results from these experiments are essentially identical to the axisymmetric case, with a virtually complete elimination of common process disturbances on forming stability.

Flexible manufacturing with laser: Concepts, solutions and experiences

Utilising traditional machinery, sheet metal parts are usually produced in large series. Decreasing lot sizes and quickly changing demand calls for highly flexible production systems. In spite of their rather high system cost, high power laser production systems could prove profitable on account of their big advantages in terms of flexibility. A unique flexible manufacturing system, which is provided by laser, was planned for a broad spectrum of three-dimensional sheet metal parts. In this system, medium-sized series are to be produced in three shifts, with the third shift being unattended. For the implementation of the combination of laser cutting and laser welding, a feasibility study has been performed that includes system and operational cost comparisons.