Abstract. Nowadays the customer requirements are in permanent changing and according with them the tendencies in the modern industry is to implement flexible manufacturing processes. In the last decades, metal forming gained attention of the researchers and considerable changes has occurred. Because for a small number of parts, the conventional metal forming processes are expensive and time-consuming in terms of designing and manufacturing preparation, the manufacturers and researchers became interested in flexible processes. One of the most investigated flexible processes in metal forming is incremental sheet forming (ISF). ISF is an advanced flexible manufacturing process which allows to manufacture complex 3D products without expensive dedicated tools. In most of the cases it is needed for an ISF process the following: a simple tool, a fixing device for sheet metal blank and a universal CNC machine. Using this process it can be manufactured axis-symmetric parts, usually using a CNC lathe but also complex asymmetrical parts using CNC milling machines, robots or dedicated equipment. This paper aim to present the current status of incremental sheet forming technologies in terms of process parameters and their influences, wall thickness distribution, springback effect, formability, surface quality and the current main research directions.
The paper presents a novel solution for improving the accuracy of the wall area of parts manufactured by single point incremental forming. Thus, a forming tool with a special design that works according to the principle of circumferential hammering is deployed, with a direct improving effect of the forming conditions and consequently of the dimensional accuracy of the part. The research is focused on an experimental study of frustum-of-cone shapes manufactured from sheet metal blanks of DC05 deep drawing steel of 1 mm thickness. A typical customary technological setup is used for the single point incremental forming process, without any additional elements, and two forming tools, a hemispherical and a special one, which use the circumferential hammering effect. Several preliminary tests using both tools were performed in order to prove that part accuracy can be significantly improved by using the circumferential hammering tool. The research was further expanded to investigate the influence on part wall dimensional accuracy of three factors: tool spindle speed, tool feed rate and part dimensional configuration. Using a full factorial plan of experiments the results of 32 test runs were processed. All parts were machined adequately, free of any material fracturing. Based on the achieved machining accuracy of the part walls, precision mathematical models were developed for the prediction of part dimensional accuracy in those areas. The mathematical models were validated by practice, as the predicted accuracies were matched by the experimental results.
In the incremental sheet forming simulation, finite element modelling typically was used to anticipate material behaviour, predict deformation forces, thickness reduction or to identify other information. Because the forming tool motions are difficult to be implemented in finite element method (FEM) software systems, and because of the large number of points that describe the tool path, in order to reduce data preparation time, this paper presents the implementation of a new software tool conceived by the authors in the process of the numerical simulation of incremental sheet forming. The software tool uses a CNC file in G-code format to reveal the interpolation point coordinates of the tool motions and the positioning time in a specific ANSYS format. Usually, a tool path for an incremental sheet forming process consists of thousands of interpolation points and is very difficult and time-consuming to implement manually into a FEM software system. The new software tool solves this issue, at least for ANSYS software, being swift and easy to use. The paper also presents how the software tool is integrated and validated in a case study of an incremental sheet forming process simulation concerning different part configurations.
In incremental sheet forming processes, the expensive dedicated tool are avoided and replaced with a cheap and simple fixing device which support the sheet metal blanks. The current paper presents how a fixing device used for single point incremental forming device is designed, FEM simulated and manufactured. The fixing device can be used for parts with a cone frustum and pyramidal frustum made of DC05 deep drawing steel. The forces developed in the process and the device displacements were estimated using FEM simulation. The device components were manufactured using a CNC machines and the physical assembly is also presented in the paper.
The finite element method is one of the most useful virtual tools in industrial engineering, which allows quick and cheap predictions and verifications. As it is expected, the FEM simulations are also integrated for incremental forming processes to anticipate different aspects as forming forces, the material stress and strain, dimensional accuracy of parts, or simply to check if the desired part can be manufactured without material failure, before to start the effective manufacturing. The FEM predictions validity is influenced by many aspects of which the most significant are the sheet metal blank material properties. Each sheet blank material has its own mechanical properties, according with material standards, considered to be the theoretical properties. In most of the situations, those theoretical material properties are not matched with the real material properties. In this paper, it is presented the experimental method of determination of the real material properties for DC05 deep drawing steel, based on the tensile strength of the sheet specimens, and the true stress-strain curve which, in fact, represents the real material characteristic curve. A comparison is made between the FEM results, obtained using both, the theoretical and the determined real material properties, and the experimental trials for parts with frustum of a cone shapes.
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