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Fundamental studies on the incremental sheet metal forming technique

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Fundamental studies on the incremental sheet metal forming technique
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  Journal of Materials Processing Technology 140 (2003) 447–453 Fundamental studies on the incrementalsheet 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, theforming 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 difficult-to-form shapeswere produced with the technique. By utilizing knowledge and experience obtained during the present study, it became possible to producesome 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 diesandpunches,manufacturedinaccordancewiththeshapeanddimensions of the component. This conventional method isadequate for mass production because the cost of dies andpunches can be shared with a large number of products.Recently, however, new production methods for a small sizelot are being developed, since the customer’s demand wasso diversified that the lot size has become small. Amongvarious methods, using simple tool, small hammer or laser,the incremental forming method with simple tool has gaineda 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 assessthe formability in the incremental forming. They also inves-tigated 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 methodwere performed such as development of the positive formingmethod, application of jigs for complex shapes and compar-ative studies with stretching and deep drawing methods byfinite element analysis. ∗ Corresponding author. Tel.:  + 82-2-332-5693; fax:  + 82-2-322-7003.  E-mail address:  jjpark@hongik.ac.kr (J.-J. Park). 2. Characteristics of incremental forming Intheincrementalformingofsheetmetal,asimple-shapedtool imposes deformation locally on the sheet in a consecu-tive manner. An example of the incremental forming, calledthe negative forming, is shown in Fig. 1. In this example, the ball tool moves on the sheet according to a programmedtool path on a CNC milling machine. The sheet is locatedwith the periphery fixed by bolts on a die, which is hollowand 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 pushesthe sheet by a vertical feed and moves along a triangle, (2)after the tool moves a little bit inside the triangle followedby a vertical feed, it moves along a smaller triangle, and(3) by repeating this process, a triangular cone is graduallyformed from the bottom to the top.Several characteristics of deformation are observed inthis method. First, the deformation mode transfers fromplane-strain stretching to biaxial stretching as the curvatureof radius of tool movement increases. Second, as shown inFig. 2, the forming limit curve (FLC) appears to be a straightline with a negative slope in the positive region of the minorstrain and thus the formability can be expressed as the valueof   ε max  +  ε min  [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 themagnitude of the vertical feed decreases. 0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0924-0136(03)00768-4  448  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 millingmachine.Fig. 3. Forming of rectangular cones: (a) by negative forming, (b) by positive forming, (c) jig for positive forming, and (d) strain distributions of bothmethods.Fig. 2. Comparison of FLCs in both incremental and conventional formingmethods.   J.-J. Park, Y.-H. Kim/Journal of Materials Processing Technology 140 (2003) 447–453  449 3. Forming of complex shapes The materials used in the present study was an alu-minum sheet of 0.3mm in thickness, annealed at 350 ◦ Cfor 2h. From material property tests, the elastic modulusof 70GPa, the planar anisotropies of   R 0  =  0 . 51,  R 45  = 0 . 75, and  R 90  =  0 . 48 were found with the average flowstress  ¯ σ   =  140 ¯ ε 0 . 25 MPa [6]. The speed of the tool move- ment was 25mm/s, and the horizontal and vertical feedswere 1 and 0.2mm, respectively. Bearing oil was usedas the lubricant at the interface between the tool and thesheet.The shape in Fig. 3(a) was produced by the negativeforming method, which is presented in Fig. 1. A draw-back of this method is that cracks occur easily at cornersand edges. A shape with sharp edges such as the one inFig. 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 guideposts, a support column, and a base plate. In the processof forming, the blank holders with a sheet inserted in be-tween are bolted and located by the guide posts. At this Fig. 4. Forming of octagonal cones: (a) with flat surface, (b) with convex surface, (c) with concave surface, and (d) strain distributions with FLCs. point, the sheet is supported by the support column. As thetool moves on the sheet, a desired shape is formed fromthe top to the bottom. This method is called the positiveforming.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 theplane-strain stretching mode but also at the biaxial stretch-ing mode, while those by the positive forming are onlyat the plane-strain stretching mode. Thus, it is concludedthat the positive forming is a better method because itutilizes the formability characteristics in the incrementalforming. 3.1. Octagonal cones With the jig in Fig. 3(c), three octagonal cones wereformed, as shown in Fig. 4(a)–(c). They are different by thecurvature of the surface: flat, convex, and concave. Straindistributions were measured on both sides of side 1 andside 2 and are presented in Fig. 4(d). They are all around  450  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. Stretchingand deep drawing processes of the shape were simulated bythe commercial FEM package, PAM-STAMP. The periph-ery of the sheet was fixed in the stretching process, but itwas allowed to move as far as no wrinkles occurred in thedeep drawing process. The strain distributions for these pro-cesses which were obtained from the simulations are com-pared also in Fig. 4(d). It is revealed that they are almostaround the biaxial stretching mode with some points abovethe 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), whichhas a definite shape in the bottom, the support toolof the jig in Fig. 3(c) was replaced with the one inFig. 5(b). Strain distributions at three locations on thebucket were measured and are presented in Fig. 5(c).They are all around the plane-strain stretching mode be-low the FLCs. Stretching and deep drawing processes of the shape were simulated by PAM-STAMP. The straindistributions obtained from the simulations are comparedin Fig. 5(c). It is noted that they are almost aroundthe 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 theone in Fig. 6(b). In the process of forming, the first col-umn supported the sheet when the upper part of the shapewas formed, while the second column supported it when   J.-J. Park, Y.-H. Kim/Journal of Materials Processing Technology 140 (2003) 447–453  451Fig. 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 distributionswere measured, as presented in Fig. 6(c). It is shown thatthey are around the plane-strain stretching mode below theFLCs.Stretching and deep drawing processes of the shapewere simulated by PAM-STAMP, as shown in Fig. 6(d)and (e). The maximum of the major strain reached 1.82in the former, while that in the latter reached 1.66. Straindistributions obtained from the simulations are comparedalso 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 verified thatthe incremental forming method was far better than conven-tional forming methods, in terms of forming capabilities.However, it was found that the support column should becarefully prepared as the shape became complicated. Other-wise, the formed shape would not match well with a desiredshape.A disk shape shown in Fig. 7(a) was formed with the jig in which the support column was replaced with the pat-tern in Fig. 7(b). In the process of forming, a sheet waslocated on the pattern and the tool moved along circularpaths with appropriate vertical movements. Stretching pro-cess of the shape was simulated by PAM-STAMP, as shownin Fig. 7(c). It was found that the maximum of the ma-  jor strain reached 0.22. Strain distributions were measuredas well as obtained from the simulation, as presented inFig. 7(d). The strains from the incremental forming are allaround the plane-strain stretching mode while those from thestretching process are distributed from the plane-stretchingmode to the biaxial stretching mode. They are all below theFLCs.However,itisexpectedthatcrackswouldtakeplaceinthe stretching process as the shape of the disk becomes morecomplicated.
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