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Abstract: A reconfigurable flexible poles die was developed. The die can be used to implement the process of“the multi-point pressing and forming sheet”. Sheet metal is restricted by the elastic pressing forces putting onthe upper and lower surfaces of sheet in stamping process. The method is an effective way to enhance the bucklingcritical stresses and reduce wrinkling of sheet. The results of tests indicate that the die can achieve “one diebrings multi-purpose” and suppress the wrinkle of sheet. The process of the multi-point pressing and formingsheet provides a practical and effective way for the curved sheet metal forming.41373
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Key words: stamping process, wrinkle, reconfigurable, dieCLC number: TG 386 Document code: A 1
IntroductionWrinkling of sheet metal seriously influences theforming quality and the life of die. It may make theforming process unable to carry on. So effectively sup-pressing wrinkle seems to be highly important[1].Inrecent years, in order to meet the demand of light andtough sheet metal, thin sheet metal parts are widely ap-plied in various fields. The study of Wrinkling becomesone of the hottest research topics[2].Since the near century, many scholars have broughtmassive studies and discussions into the instability ofsheet. These researches covered from the experimentalstudy and the theoretical analysis to the numerical sim-ulation, from the sheet drawing and the bend formingto the hydroforming, from the wrinkle forecast to thebuckling control and so on[1-3].Recently, numerical simulation of a forming processis applied to study on wrinkle. Depending on the mod-ern advanced research methods, numerical simulationbased on the finite element method has become themain tool of study wrinkling in decades[4].However, majority studies encircle wrinkling bifurca-tion of sheet metal and the situations of wrinkling atthe present time. But these cannot bring any feasiblemethod for controlling the wrinkles in stamping.Currently, the representative research and applica-tion of the reconfigurable die technology is the multi-point forming technology. The basic principle is using a series of regular and adjustable the basic bodies (poles)to construct the structure of a die. Thus, the recon-struction of die can be achieved, but the wrinkling inthe sheet metal has not been suppressed[5].The paper emphasizes how to control wrinkle of sheetforming. Main ideas in the paper are two: One ideais to improve the supporting condition of sheet metaland increase its buckling critical stresses. Thus, “themulti-point pressing and forming sheet” technique willbe proposed. Another idea is to explore a reconfigurabledie which is combining the technique of “the multi-pointpressing and forming sheet”.2 Poles Flexible DieIn order to reduce the wrinkling of sheet, a method ofusing elastic pressing forces putting on surfaces of sheetin stamping process was presented, it can improve thestability of sheet metal.A new reconfigurable die was developed. The die hashigh efficiency in reconfiguration so that it can achievemulti-purpose use. Simultaneously the die can be car-ried out the process of “the multi-point pressing andforming sheet”.Curved sheet metal parts are used in many productssuch as aerospace products, military products, shipsand engineering machines, the batch of parts is small.Different shape requires different die for forming sheetmetal. It leads to high cost and long cycle[6-7]. To meetthis situation, the reconfigurable stamping die has beenexplored. The die can be applied to forming a varietyof sheet parts by reconfiguring.Thenoveldieisnamedas “poles flexible die”.Poles flexible die is a kind of reconfigurable stamping die. Figure 1 shows the structure. The die is com-bined with some discrete adjustable poles which areassembled in an array. The length of pole can be ad-justed.
After analyzing 3D model’s data of sheet part,the length of each pole can be adjusted accurately[8].1—Frame, 2—Screw rod, 3—Adjustable sleeve, 4—Baffle5—Jointer, 6—Punch head, 7—Blank-holder, 8—Sheet metalFig. 1 The structure of poles flexible diePunch head located in the end of pole can be replacedwith relevant shape head. All heads are constituted thesurface of die so that it is conformed to the shape ofsheet part.The die is composed with flexible poles. In orderto realize the multi-point pressing and forming sheetprocess, the adjustable pole is designed as a flexiblepole with the function of “pressing sheet” and “formingsheet”. Figure 2 shows the scheme of the flexible pole.12 34 5 687AAA– –AB– –BBB1—Stepping motor, 2—Frame, 3—Screw rod4—Pressure spring, 5—Adjustable nut, 6—Adjustable sleeve7—Punch head, 8—Pressing distanceFig. 2 The schematic of the flexible poleIt can be seen from Fig. 2 that position of sleeve isrestricted by the adjustable nut in flexible pole. Rotat-ing screw rod can control the position of sleeve. Sleeveis tightly held the adjusted nut under the effort of pres-sure spring. Figure 3 shows the materialization of theflexible pole.1—Screw rod, 2—Pressure spring, 3—Adjustable nut4—Pressing-distance slot, 5—Adjustable sleeve6—Punching headFig. 3 The flexible poleIn the process of sheet metal forming, the correspond-ing flexible poles of the upper and lower dies firstly pro-ceed to “press sheet” by the pressure spring. With thedescending of the upper die, punching heads and sleevesare retracted inward. Pressing force of punching headis gradually increased under the effort of spring. Afterall punching heads and sleeves are retracted a definitedistance (pressing distance), the adjusted nuts will re-sist sleeves and locate the sleeves. Consequently, theheads and sleeves cannot retract inward and all punch-ing heads will be combined to form the die surface,so that a sheet metal can be forced to forming. Thelength of pressing-distance slot has various sizes. Thelong pressing-distance slot may be used for construct-ing the blank-holder. Pressing force on sheet metal isproduced by pressure spring inside die. Changing thecorresponding coefficient of elasticity of spring can gen-erate desired pressing force. Pressing force on sheetmetal is far smaller than forming force, and the press-ing force applied on a sheet metal is from 50 to 600 N.Figure 4 shows the upper die fixed on the upperworktable of the press machine. The surface of thedie is spherical face composed by some punching headsof poles. Poles in the periphery of die are blank-holder poles.1—Blank-holder, 2—Surface of dieFig. 4 The upper die with spherical surface3 The Stamping Process of the Multi-point Pressing and Forming SheetPoles flexible die can be realized the stamping pro-cess of “the multi-point pressing and forming sheet”.The stamping process with discrete flexible poles canbe seen in Fig. 5. It must be gone through four stepsthat are “holding blank”, “pressing sheet”, “pressingand forming sheet” and “shaping sheet”.At the first stage of the forming, blank holders ofthe die achieve “holding blank” as shown in Fig. 5(a).Then both upper and lower punching heads of dies arepressing sheet, and the process is called “multi-pointpressing sheet”, as shown in Fig. 5(b). In the processof stamping, some sleeves reach the designated pointfirstly and other sleeves and punching heads are press-ing sheet surfaces, as shown in Fig. 5(c). Finally, all sleeves and punching heads of poles reach the desig-nated point, surfaces of punching heads are combinedthe surface of die, the sheet metal is shaped by the die,as shown in Fig. 5(d). During the process, the sheetmetal is always confined by punching heads.In the process of forming sheet metal, poles flexibledie composed by “flexible poles” proceeds to “pressingsheet” and “forming sheet” at the same time. The pro-cess of “the multi-point pressing and forming sheet” canprevent buckling and wrinkling of sheet. In the stamp-ing process the force of pressing sheet is far smaller thanthe force of forming sheet, and the force of pressingsheet is produced by springs and the force of formingsheet is produced by press machine.Experiment results indicate that poles flexible die hasgood forming characteristics and can suppress wrinkleof sheet.