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.4.1 The Forming of Spherical Sheet PartA spherical sheet metal part is a typical kind ofcurved sheet metal part. If the sheet metal part wasformed by traditional die, wrinkles often occur in thesurrounding area as well as blank-holder area[11].Acommon way for solving the wrinkles is increasing theblank-holder force. The way may decrease wrinkles ofsheet. However, if the value of the blank-holder force istoo large, cracking may occur in sheet metal[12].Howto control wrinkle of sheet is the key of the experiment.Simulation of the forming process is performed withthe software DYNA-form. It is shown from Fig. 7 thatthe material is prone to wrinkle along the section M-N.Fig. 7 Stresses status in the forming of sheetIn the course of reconfiguration, the 3D model ofsheet part should be analyzed, the extension elongationof each pole and the shape of each punching head arecalculated firstly. Subsequently, it is to adjust the posi-tion of the adjustable nuts and to change the punchingheads, and to construct the surface of the die. Nextstep is to constitute the blank-holder structure of thedie. Some surrounding poles of die are used for blank-holder. The sheet metal part with the spherical shapehas its spherical radius of 150 mm. Heads in the polesare customized.The upper die is installed in the upper table of thepress machine, as shown in Fig.4. The lower die is in-stalled in the lower table. In the process of formingsheet, the blank sheet is placed on the working faceof lower die, and then the upper die moves down. Fi-nally the upper and lower dies are matched to finish theforming of sheet metal.The shaped sheet metal part is shown in Fig. 8. Nowrinkle exists in the sheet. This is due to the processof “the multi-point pressing and forming sheet”.Fig. 8 Spherical sheet partsThe material of the sheet is AISI1020, of elastic mod-ulus E =2.06 GPa, of yield strength 0.35 GPa. Whenthe surfaces of sheet aren’t applied with pressure forces,according to Eq. (1) under the condition of the sheetmetal without pressing force, the critical stresses σcrcan be obtained.σcr ≈ 3.55 × 102ETd2=34.3MPa. (3) There, a, b = 80 mm, d = 1 mm, ET ≈ 0.3E.During the forming of the spherical sheet metal, thesheet can be confined by each flexible pole. The surfacesof sheet are applied with pressure forces, according toEq. (2), the critical stresses of sheet metal with pressingforce can be got.σcrq = 359.3MPa. (4)Comparing Eq. (3) with Eq. (4), it is easy to knowthat the value of buckling critical stress σcrq is obviouslybigger than σcr under the condition of no pressing forceson sheet metal. The sheet metal cannot be prone towrinkle with setting bigger coefficient of elasticity ofspring inside poles. Elastic pressing forces putting onsurfaces of sheet can increase the buckling critical stressand decrease wrinkle of the sheet.4.2 The Forming of Hemispherical Sheet PartHemispherical sheet metal part is an unsymmetricalsheet, as shown in Fig. 9. The opening curved faceis inconvenient to set blank-holder. In the forming ofthe sheet metal, it is prone to wrinkle in the unsup-ported area because the sheet metal is applied imbal-ance stamping force. In order to ensure the trouble-freeforming, it is necessary to increase the pressing forceonto the sheet so as to prevent wrinkle of sheet.Fig. 9 3D model of hemispherical sheet metalBy discharging a half of poles of spherical stampingdie, it can be used to form the hemispherical sheet metalpart. Now, it can be observed conveniently about thesteps of the stamping process. The steps can be seenfrom Fig. 10. Figure 10(c) shows the step of pressingsheet and forming sheet. Figure 11 shows the hemi-spherical sheet with the thickness of 1.8 mm.Fig. 10 The steps of the multi-point pressing and forming sheet processFig. 11 Hemispherical sheet metal part4.3 Forming of Saddle Sheet PartAs shown in Fig. 12,
saddle sheet metal is a kind ofcurved sheet metal part with double curvature. If thesheet is formed directly, wrinkle is often occurred[3].The blank-holder is constructed uneasily becausethe peripheral boundary lines of saddle sheet part aredistributed in the different altitude. Currently, it is al-most impossible to form saddle sheet metal by directlyusing the conventional stamping die. The poles flexibledie can be used to form the sheet metal part directlywith the multi-point pressing and forming sheetprocess.Figure 13 shows the flexible die which is finishedby re-constructing. During the forming sheet theperipheral poles are retracted corresponding dimensionmore than other poles. When the upper and lowerdies are matched, surfaces of peripheral poles and thatof middle poles are combined a continuous forming surface. Then the sheet is shaped to the saddle sheetmetal directly. The structure of blank-holder is called“the multi-point blank-holder”.Rapid prototyping and manufacturing system offused deposition modeling (FDM) can rapidly manufac-ture all kinds of punching heads. A series of punchingheads can be produced in two or three days by usingthe FDM system. The punching heads are installedin the end of poles separately as shown in Fig. 13.The material of the punching heads manufactured byFDM is acrylonitrile butadiene styrene (ABS) plastic.Punching heads can also be manufactured by rapidprototyping system of selective laser sintering (SLS).Figure 14 shows the saddle sheet part of thick-ness 1.0 mm with stainless steel. Wrinkling can beeliminated by using this method.Fig.