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B. GRIZELJ et al.: EFFECT OF SPRING-BACK IN V-TOOL BENDING OF HIGH-STRENGTH STEEL SHEET METAL PLATES Figure 9 Relative bending error for punch radius r st =2 mm observe the minimal bending error at the end of punch displacement. These cases are shown in Figure 8 (with relative bending error zero value at the end of punch travel for 90° tool geometry). Furthermore, it can be seen that the relative bending error is achieved even before the end of punch travel for 75° tool which was expected. It can be also seen that the cases with 75° punch tool geometry have the negative relative bending error which means that the angle after the punch unloading is even lower than before unloading. This effect is called spring-forward, as the “spring-back” effect in these cases is continued in the same direction of bending. Figure 9 shows the relative bending errors for the different cases shown in Figure 3. Also, as in the former case, it can be seen that for the same punch radius, the same material and the material thickness but different tool geometries, the bending errors follow the same curve up to a point in which the full contact of the sheet with the lower tool happens. But as opposed to the former case, it can be seen that at the punch travel end, the relative bending errors are more grouped than in the case with punch radius r st = 0,4 mm. Also looking at the amount of relative bending error from Figure 9, it can be concluded that with the larger punch radius, the relative bending error is lower. CONCLUSION The high-strength steel sheet metal plates are used for the production of light-weight high-strength. For the FEM simulations in this paper, two types of tools, two types of materials, two types of punch radius, and two sheet thicknesses were chosen. The relative bending error after the tool release was observed for all planned cases. The results were grouped in two groups by the amount of punch radius and shown in diagrams in Figures 8 and 9. REFERENCES [1] K. Lange et al., Handbook of metal forming, English language edition published by Society of manufacturing engineers, USA, (1985), 747-760. [2] B. Grizelj, Oblikovanje metala deformiranjem, Strojarskifakultet u Slavonskom Brodu, Slavonski Brod, (2004), 282-306. [3] W. M. Chan; H. I. Chew; H. P. Lee; B. T. Cheok, Finite element analysis of spring-back of V bending sheet metal forming process, Journal of materials processing technology 48 (2004), 15-24. [4] Z. T. Zhang; S. J. Hu, Stress and residual stress distributions in plane strain bending, Int. J. Mech. Sci, 40 (1998), 6, 533-543. [5] W. L. Xu; C. H. Ma; C.H. Li; W.J. Feng, Sensitive factors in springback simulation for sheet metal forming, Journal of Materials Processing technology 151 (2004), 217-222. [6] S. Thippraksmas; S. Rojananan, Investigation of spring-go phenomenon using fi nite element method, Materials and design 29 (2008), 1526-1532. [7] Roymech, Strength of steels, URL: http://www.roymech. co.uk/Useful_Tables/Matter/Steel_Europe.html (25.11.2009.) [8] M. S. Ragab and H. Z. Orban, Effect of ironing on the residual stresses in deep drawn cups, Journal of Materials Processing Technology, vol. 99, issues 1-3, March 2000, 54-61. (doi: 10.1016/S0924-0136(99)00360-X). [9] AK Steel, Cold rolled steels, product data bulletin, AK Steel, 2012, 1-8. [10] ThyssenKrupp Steel, Bake hardening steels, Thyssen- Krupp, 2009, 1-11. [11] Auto/Steel Partnership, High strength steel stamping design manual, Southfi eld USA, 2000, 2-20 [12] MSC.MARC, Volume A: Theory and user information, MSC.Software, 2007, 653. [13] MSC.MARC, Volume B: Element library, MSC. Software, 2007, 153-228. Note: English language: Rosandić Željka, Mechanical Engineering Faculty in Slavonski Brod, Croatia 102 METALURGIJA 52 (2013) 1, 99-102
Z. PATER, A. TOFIL, J. TOMCZAK STEEL BALLS FORMING BY CROSS ROLLING WITH UPSETTING Z. Pater, A. Tofi l, J. Tomczak, Lublin University of Technology, Lublin, Poland METALURGIJA 52 (2013) 1, 103-106 ISSN 0543-5846 METABK 52(1) 103-106 (2013) UDC – UDK 621.73, 77, 669.1=111 Received – Prispjelo: 2012-02-12 Accepted – Prihvaćeno: 2012-07-30 Preliminary Note – Prethodno priopćenje The paper describes a process of forming four balls with a diameter of 22 mm by means of cross rolling with upsetting. The paper also presents the tool used to form semi-fi nished balls. Owing to the application of the fi nite element method (FEM), the course of the rolling process as well as temperature and strain distributions in the obtained balls could be presented. The rolling tests conducted in laboratory conditions at the Lublin University of Technology have proved that the balls produced with the developed rolling method meet the demands for grinding media used in ball mills. Keywords: forming, steel balls, cross rolling, FEM INTRODUCTION Steel balls are employed on a mass scale as grinding media in ball mills which are then used for grinding metal ores, coal, used-up moulding sands, and other materials used in the economy. Owing to their spherical shape, grinding media have the most favourable (minimal) surface-to-weight ratio, which, in turn, results in decreased abrasive wear. At present, grinding media balls are mainly produced by casting, die forging, and highly effi cient skew rolling which is characterized by a good quality of obtained semi-products. In the process of skew rolling, balls are formed by means of two rolls positioned in a skew manner, which rotate in the same direction and which have helical grooves (roll passes) on their perimeter [1 - 3]. Despite its numerous advantages, this forming method is not however widely applied in industrial conditions due to the complex shape of the tools (helical rolls). Research into a modern technology of cross-wedge rolling (CWR) has been done at the Lublin University of Technology for about twenty years [4]; and this technology may also be employed to form balls. This process is not as effi cient as skew rolling, yet for its realization much simpler (less expensive) tools in the form of wedges are needed. Recently, an ever simpler method for ball forming based on the CWR technology has been developed, which is described in the present paper. NATURE OF CROSS ROLLING WITH UPSETTING The developed method for simultaneous forming of a semi-fi nished ball by means of cross rolling with fl at tools is shown in Figure 1. Figure 1 Scheme of cross rolling with upset ting of balls (see in the text) According to this method, the billet (1) in the form of a bar section whose diameter is smaller than the diameter D of the ball being formed (2) is put in between two fl at tools (3) and (4), which have the prongs (5) and (6) distanced from each other at the distance L bigger than the diameter D of the ball being formed (2). Next, the tools (3) and (4) are put into motion at the same velocity v, the prongs (5) and (6), which move in the opposite directions, cut into the billet (1) and make it rotate, reducing at the same time its diameter and cutting it into pieces with a volume equal to the volume of the ball (2). After that, the moving tools (3) and (4) and the concave surfaces of the side prongs (5) and (6) upset the cut-up semi-fi nished product (1), as a result of which the balls (2) whose diameter D is bigger than the diameter of the semi-fi nished product (1) are produced. The rolling process may also be conducted in a system in which only one of the tools (3) or (4) is in plane motion at the velocity v, while the other of the tools (3) or (4) does not move. FEM NUMERICAL MODELLING The modelling work began with designing wedge tools for the rolling process. It was assumed that balls of 22 mm in diameter would be formed by two identical fl at 103
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