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Fundamental studies on the incremental


Journal of Materials Processing Technology 140 (2003) 447–453

Fundamental studies on the incremental sheet metal forming technique
Jong-Jin Park? , Yung-Ho Kim
Department

of Mechanical and System Design Engineering, Hong-Ik University, 72-1 Sangsu-Dong, Mapo-Ku, Seoul 121-791, South Korea

Abstract The idea of incremental forming technique has been investigated for production of sheet metal components. With this technique, the forming limit curve (FLC) appears in a different pattern, revealing an enhanced formability, compared to conventional forming techniques. In the present study, the formability of an aluminum sheet under various forming conditions was assessed and dif?cult-to-form shapes were produced with the technique. By utilizing knowledge and experience obtained during the present study, it became possible to produce some free surfaces. ? 2003 Elsevier B.V. All rights reserved.
Keywords: Incremental forming; Formability; Forming limit curve; Sheet metal; Aluminum

1. Introduction A sheet metal component is usually produced with dies and punches, manufactured in accordance with the shape and dimensions of the component. This conventional method is adequate for mass production because the cost of dies and punches can be shared with a large number of products. Recently, however, new production methods for a small size lot are being developed, since the customer’s demand was so diversi?ed that the lot size has become small. Among various methods, using simple tool, small hammer or laser, the incremental forming method with simple tool has gained a great attention. In addition to many valuable results [1–5], Shim and Park [6] and Kim and Park [7] performed a series of experiments and suggested the straight groove test as a method to assess the formability in the incremental forming. They also investigated the effect of forming parameters, such as tool shape, tool size and feed rate, on the formability. In the present study, further investigations on the method were performed such as development of the positive forming method, application of jigs for complex shapes and comparative studies with stretching and deep drawing methods by ?nite element analysis.

2. Characteristics of incremental forming In the incremental forming of sheet metal, a simple-shaped tool imposes deformation locally on the sheet in a consecutive manner. An example of the incremental forming, called the negative forming, is shown in Fig. 1. In this example, the ball tool moves on the sheet according to a programmed tool path on a CNC milling machine. The sheet is located with the periphery ?xed by bolts on a die, which is hollow and square in cross section. When a triangular cone is to be formed, for an example, the tool movement required is as follows: (1) the tool pushes the sheet by a vertical feed and moves along a triangle, (2) after the tool moves a little bit inside the triangle followed by a vertical feed, it moves along a smaller triangle, and (3) by repeating this process, a triangular cone is gradually formed from the bottom to the top. Several characteristics of deformation are observed in this method. First, the deformation mode transfers from plane-strain stretching to biaxial stretching as the curvature of radius of tool movement increases. Second, as shown in Fig. 2, the forming limit curve (FLC) appears to be a straight line with a negative slope in the positive region of the minor strain and thus the formability can be expressed as the value of εmax + εmin [4] (Fig. 2). It is noted that the formability is greatly enhanced in the case of plane-strain stretching. Third, the formability increases as the size of the tool or the magnitude of the vertical feed decreases.

? Corresponding author. Tel.: +82-2-332-5693; fax: +82-2-322-7003. E-mail address: jjpark@hongik.ac.kr (J.-J. Park).

0924-0136/$ – see front matter ? 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00768-4

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J.-J. Park, Y.-H. Kim / Journal of Materials Processing Technology 140 (2003) 447–453

Fig. 1. Incremental forming of an aluminum sheet on CNC milling machine.

Fig. 2. Comparison of FLCs in both incremental and conventional forming methods.

Fig. 3. Forming of rectangular cones: (a) by negative forming, (b) by positive forming, (c) jig for positive forming, and (d) strain distributions of both methods.

J.-J. Park, Y.-H. Kim / Journal of Materials Processing Technology 140 (2003) 447–453

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3. Forming of complex shapes The materials used in the present study was an aluminum sheet of 0.3 mm in thickness, annealed at 350 ? C for 2 h. From material property tests, the elastic modulus of 70 GPa, the planar anisotropies of R0 = 0.51, R45 = 0.75, and R90 = 0.48 were found with the average ?ow stress σ = 140? 0.25 MPa [6]. The speed of the tool move? ε ment was 25 mm/s, and the horizontal and vertical feeds were 1 and 0.2 mm, respectively. Bearing oil was used as the lubricant at the interface between the tool and the sheet. The shape in Fig. 3(a) was produced by the negative forming method, which is presented in Fig. 1. A drawback of this method is that cracks occur easily at corners and edges. A shape with sharp edges such as the one in Fig. 3(b) can be formed with a help of the jig in Fig. 3(c) [8,9]. The jig consists of two blank holders, four guide posts, a support column, and a base plate. In the process of forming, the blank holders with a sheet inserted in between are bolted and located by the guide posts. At this

point, the sheet is supported by the support column. As the tool moves on the sheet, a desired shape is formed from the top to the bottom. This method is called the positive forming. Strain distributions measured from the shapes in Fig. 3(a) and (b) are compared in Fig. 3(d). It is shown that those by the negative forming are distributed not only at the plane-strain stretching mode but also at the biaxial stretching mode, while those by the positive forming are only at the plane-strain stretching mode. Thus, it is concluded that the positive forming is a better method because it utilizes the formability characteristics in the incremental forming. 3.1. Octagonal cones With the jig in Fig. 3(c), three octagonal cones were formed, as shown in Fig. 4(a)–(c). They are different by the curvature of the surface: ?at, convex, and concave. Strain distributions were measured on both sides of side 1 and side 2 and are presented in Fig. 4(d). They are all around

Fig. 4. Forming of octagonal cones: (a) with ?at surface, (b) with convex surface, (c) with concave surface, and (d) strain distributions with FLCs.

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J.-J. Park, Y.-H. Kim / Journal of Materials Processing Technology 140 (2003) 447–453

Fig. 5. Forming of a bucket shape: (a) formed shape, (b) support tool, and (c) strain distributions with FLCs.

the plane-strain stretching mode below the FLCs. Stretching and deep drawing processes of the shape were simulated by the commercial FEM package, PAM-STAMP. The periphery of the sheet was ?xed in the stretching process, but it was allowed to move as far as no wrinkles occurred in the deep drawing process. The strain distributions for these processes which were obtained from the simulations are compared also in Fig. 4(d). It is revealed that they are almost around the biaxial stretching mode with some points above the FLC. It is revealed that there is a possibility of crack occurrence. 3.2. Bucket shape In order to form the bucket shown in Fig. 5(a), which has a de?nite shape in the bottom, the support tool of the jig in Fig. 3(c) was replaced with the one in

Fig. 5(b). Strain distributions at three locations on the bucket were measured and are presented in Fig. 5(c). They are all around the plane-strain stretching mode below the FLCs. Stretching and deep drawing processes of the shape were simulated by PAM-STAMP. The strain distributions obtained from the simulations are compared in Fig. 5(c). It is noted that they are almost around the biaxial stretching mode, with a possibility of crack occurrence. 3.3. Stepped shape A stepped shape shown in Fig. 6(a) was formed with the jig in which the support column was replaced with the one in Fig. 6(b). In the process of forming, the ?rst column supported the sheet when the upper part of the shape was formed, while the second column supported it when

J.-J. Park, Y.-H. Kim / Journal of Materials Processing Technology 140 (2003) 447–453

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Fig. 6. Forming of a stepped shape: (a) formed shape, (b) support tool, (c) strain distributions with FLCs, (d) FEM simulation of stretching, and (e) FEM simulation of deep drawing.

the lower part of the shape was formed. Strain distributions were measured, as presented in Fig. 6(c). It is shown that they are around the plane-strain stretching mode below the FLCs. Stretching and deep drawing processes of the shape were simulated by PAM-STAMP, as shown in Fig. 6(d) and (e). The maximum of the major strain reached 1.82 in the former, while that in the latter reached 1.66. Strain distributions obtained from the simulations are compared also in Fig. 6(c). It is shown that they are almost around the biaxial stretching mode with a possibility of crack occurrence. 3.4. Forming with a pattern As presented in the examples above, it was veri?ed that the incremental forming method was far better than conventional forming methods, in terms of forming capabilities. However, it was found that the support column should be

carefully prepared as the shape became complicated. Otherwise, the formed shape would not match well with a desired shape. A disk shape shown in Fig. 7(a) was formed with the jig in which the support column was replaced with the pattern in Fig. 7(b). In the process of forming, a sheet was located on the pattern and the tool moved along circular paths with appropriate vertical movements. Stretching process of the shape was simulated by PAM-STAMP, as shown in Fig. 7(c). It was found that the maximum of the major strain reached 0.22. Strain distributions were measured as well as obtained from the simulation, as presented in Fig. 7(d). The strains from the incremental forming are all around the plane-strain stretching mode while those from the stretching process are distributed from the plane-stretching mode to the biaxial stretching mode. They are all below the FLCs. However, it is expected that cracks would take place in the stretching process as the shape of the disk becomes more complicated.

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Fig. 7. Forming of a complex disk: (a) formed shape, (b) support tool (or pattern), (c) FEM simulation of stretching, and (d) strain distributions with FLCs.

4. Conclusion Fundamental investigations of the incremental sheet metal forming technique were performed in the present study. Also, conventional techniques such as stretching and deep drawing processes were simulated by a commercial FEM package for the purpose of comparison in the aspect of forming capabilities. The result of the present study can be summarized as follows: (1) The incremental forming technique, especially with the positive forming method, is better than conventional ones. The forming capability increases as the plane-strain mode of deformation is more introduced. (2) With the negative forming method, it is dif?cult to form sharp corners or edges because cracks easily occur due to the biaxial mode of deformation. (3) With the positive forming method, it is possible to form complicated shapes with sharp corners or edges because the plane-strain mode of deformation becomes quite dominant. (4) In the positive forming method, the support column of the jig should be properly designed, depending upon complexity of the shape to be formed.

(5) It is necessary to develop a forming apparatus in order to apply the incremental forming technique to steels or thick plates.

Acknowledgements The present study was supported by 2003 Hong-Ik University Research Fund. References
[1] H. Iseki, H. Kumon, Forming limit of incremental sheet metal stretch forming using spherical rollers, J. JSTP 35 (1994) 1336. [2] S. Matsubara, Incremental backward bulge forming of a sheet metal with a hemispherical head tool, J. JSTP 35 (1994) 1311. [3] K. Dai, Z.R. Wang, Y. Fang, CNC incremental sheet forming of axially symmetric specimen and the locus of optimization, J. Mater. Process. Technol. 102 (2000) 164. [4] H. Iseki, An approximate deformation analysis and FEM analysis for the incremental bulging of sheet metal using a spherical roller, J. Mater. Process. Technol. 111 (2001) 150. [5] H. Iseki, T. Naganawa, Vertical wall surface forming of rectangular shell using multistage incremental forming with spherical and cylindrical rollers, J. Mater. Process. Technol. 130 (2002) 657.

J.-J. Park, Y.-H. Kim / Journal of Materials Processing Technology 140 (2003) 447–453 [6] M.S. Shim, J.-J. Park, The formability of aluminum sheet in incremental forming, J. Mater. Process. Technol. 113 (2001) 654. [7] Y.-H. Kim, J.-J. Park, Effect of parameters on formability in incremental forming of sheet metal, J. Mater. Process. Technol. 130 (2002) 42.

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[8] A. Murata, F. Matsuda, Pressing and forming device, Japan Patent Of?ce, Publication No. 2000153313A (16 November 1998), 2000. [9] S. Matsubara, H. Amino, S. Aoyama, Y. Lu, Apparatus for dieless forming plate materials, European Patent Of?ce, International Publication No. WO 99/38627 (8 May 1999, Gazette 1999/31), 2000.


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