Adjusting the Contact Surface of Forming Tools in Order to Compensate for Elastic Deformations during the Process

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1 Adjustng the Contact Suface of Fomng Tools n Ode to Compensate fo Elastc Defomatons dung the Pocess Knut Goßmann, Hajo Weme, Andè Hadtmann, Las Pente, Sebastan Kechenbaue Insttute fo Machne Tools and Contol Engneeng, TU Desden Desden, Gemany Summay: Nowadays, despte poweful smulaton pogams, the tool desgn pocess stll contans manual and not epoducble wok. In specfc, the manual de spottng s mostly dependent on the wokes expeence and consumes a lot of tme. A lage potental to educe tme and costs s seen by deceasng the de matung. The pape ntoduces an appoach to obtan deep dawng tools fom FE smulaton wth LS-DYNA, whch need less addtonal manual matung untl good pats can be manufactued. Theefoe, the cuent tool desgn pocess was analyzed and t was found out, that not popely assessng elastc tool and pess popetes n FE smulatons n one of a the causes that lead to addtonal de spottng effot. Hence, a methodology was developed to compensate fo the effects of those elastc popetes. Dependng on the ntensty, afoe mentoned machne and tool popetes ae ncluded n the FE model. Based on fome eseach wok at the IWM the effects of elastc defomatons and dslocatons of the de suface on the fnal shape of the pat ae calculated. Deved fom the calculated defomatons, a tansfomatons matx s calculated and a new de suface s obtaned afte a few teatons. The new de suface has the same shape unde load lke the ntal de suface wthout load. The new method was tested though an expemental set-up, whch allowed an excessve defomaton of the de unde load. Ths expement does not eflect the ealty but seves fo geneal demonstaton puposes of the compensaton appoach. As expected the smulaton and expement show a massve mpact of the de deflecton on the daw-n of the manufactued pat. The de defomaton affects the dstbuton of the blankholdefoce on the pat. It was found a hghe pessue on the de cones and lowe pessue n the cente. By means of the compensaton method, the de suface was adjusted to acheve that the de suface unde load s the same as the ntal suface wthout defomatons. The expements show that the fnal shape of the pat, whch was dawn wth the compensated de, s vey close to the shape, whch was pedcted wthout calculatng the de defomaton.

2 Ths wok shows the basc feasblty of the compensaton fo effects of elastc tool and pess popetes. Snce t s unsue how much of an mpact the compensaton of the de has on the spngback behavou of the pat, futue eseach wll take ths effect nto account. Keywods: Sheet metal fomng, Tool defomaton, Pocess smulaton, Compensaton 1 Intoducton Ove the past yeas the desgn of deep dawng tools has developed emakably fom pue tal-andeo based desgn, to a desgn pocess based on fnte element smulaton. Today, even n the ealy stages of the desgn pocess, FEA s employed to obtan nfomaton about manufactuablty and tool desgn [4]. The smulaton-based compensaton fo spngback s state-of-the-at technology n ndustal tool manufactung. The softwae ndusty s stll wokng on developng dffeent appoaches to compensate fo spngback and to econstuct the sufaces, whch ae obtaned va FE smulaton. In patcula, spngback smulaton of hgh stength and ulta hgh stength steel demands bette accuacy n fomng smulaton [4]. Accodng to [6], tool ty-outs epesent appoxmately 30% of the oveall cost of a tool. The adjustment of the tool suface n ode to elease o to estan the mateal flow nto the cavty s stll dependent on the expeence of the opeato, and s nethe pat of the tool desgn no of the FE smulatons. Sgnfcant amounts of tme and money can be saved by pe-calculatng the de spottng dung the tool desgn pocess. Pactcal expeence [3, 5] shows, that the elastc tool and pess defomatons unde load consdeably affect the deep dawng esults. Snce the tool desgn s based on the use of gd tools, the pevously mentoned elastc defomatons can be assumed to be a man eason fo the need fo addtonal de spottng. In ode to compensate fo those defomatons, the elastc popetes of the pess and tool must be consdeed dung the desgn pocess, and theefoe, be mplemented nto the FE pocess smulaton.

Vaous studes at the IWM and othe eseach nsttutons show the feasblty of smulatng the effects of elastc tool and pess popetes on the fnal shape of the dawn pat.

3 Vaous studes at the IWM and othe eseach nsttutons show the feasblty of smulatng the effects of elastc tool and pess popetes on the fnal shape of the dawn pat. The followng pape ntoduces a methodology, whch compensates fo elastc defomatons aleady n the ealy stages of the tool desgn pocess. It s expected to futhe educe the de spottng tme. 2 Cuent Tool Desgn Pocess Fgue 1 shows the common ndustal tool desgn pocess. At the pesent tme, afte a pat s desgned, the tool geomety s then deved fom the shape of the pat. Subsequently, a fomablty analyss s conducted by means of an FE smulaton. If wnkles, excessve thnnng o cacks ae obseved, the pat and tool geomety ae teatvely modfed untl these defects ae no longe pesent. The esultng tool geomety seves as the bass fo the spngback smulaton, whch wll follow. Fgue 1: Cuent tool desgn pocess [2] Smulaton-based spngback compensaton s state-of-the-at technology. A complete spngback smulaton s geneally mplct and nvolves a tmmng smulaton, followed by mesh coasenng and the actual calculaton of the spngback. Wth the excepton of the node coodnates, the meshes befoe and afte the spngback smulaton ae dentcal, whch enables the tansfomaton matx and the coecton of the tool suface to be calculated [2]. The esults ae then checked fo effectveness by means of a fomng- and a spngback smulaton. If the smulatons delve satsfactoy esults, no futhe compensaton s equed, and the tool suface s econstucted fom the FE mesh befoe beng sent to be manufactued. Though manual de spottng, the tool suface s mpoved to meet the demanded shape accuacy and to contol the daw-n. Ths step n the pocess s, howeve, dependent on the expeence of the opeato, s not epoducble and s vey tme consumng.

3 Tool Suface Compensaton fo Elastc Defomatons Falue to popely assess the elastc tool and pess popetes n the tool desgn pocess s one of the easons why addtonal manual de spottng s needed.

4 3 Tool Suface Compensaton fo Elastc Defomatons Falue to popely assess the elastc tool and pess popetes n the tool desgn pocess s one of the easons why addtonal manual de spottng s needed. Theefoe, the followng methodology was developed to compensate fo the negatve effects esultng fom elastc tool and pess behavou. Cuently used deep dawng smulatons utlze gd tool sufaces and neglect elastc pess popetes. It s clea, that to be able to compensate fo these effects, the FE model has to be capable of calculatng them. 3.1 Effects of Elastc Tool and Pess Popetes It s necessay to make a dstncton between local and global elastc defomatons, and to be awae of the causes. Fgue 2 gves an ovevew of the nfluences that local defomatons, global deflexons and global dslocatons can have on the fnal shape of the pat. Fgue 2: left: FE-smulaton wth solely elastc tool popetes; mddle: FE-smulaton wth elastc tool and am; ght: elastc nfluence of am tltng on fnal pat shape. Fom the nfomaton above, t can be concluded, that the vaous factos taken nto account poduce the same esult; the dstbuton of the blankholde foce changes and thus nfluences the mateal flow. Ths eques that all factos be evaluated. If found to have a lage enough ntensty to sgnfcantly mpact the esult of the smulaton, these factos then need to be consde n the smulaton. Systematc consdeaton of elastc popetes s now an addtonal and necessay step n the tool desgn pocess. A smulaton usng elastc tools enables the use to pnpont local tool defomatons, whch usually ognate fom sheet thckenng and/o hgh nomal suface pessue at the de adus (see Fgue 2, left). Local tool defomatons caused by sheet thckenng ae benefcal fo the daw-n, wheeas defomatons at the de adus ae undesed. Local defomatons at the de adus ae athe small and, n most cases, neglgble. Theefoe, t appeas dffcult and unlkely to be able to conduct compensaton solely fo local defomatons. In ode to compensate fo global tool deflexons and tool dslocatons (see Fgue 2, mddle and ght), the FE model has to be honed wth the capablty of consdeng elastc pess popetes. Fome eseach wok at the IWM and vaous othe nsttutes has shown that t s possble to demonstate the

5 pevously mentoned effects on the dawn pat by means of FE smulaton [5]. The lnea and nonlnea elastc popetes of the pess can be mplemented n FEA by usng dscete elements, such as spngs and dampes, makng the computaton slghtly moe tme consumng. Calculatng the tools elastc defomaton n FE smulatons entals a sgnfcant ncease n computatonal tme and effot. In [1], vaous appoaches ae ntoduced to decease the numbe of degees of feedom and hence educe computaton tme. Snce all educton methods stll have to contan the suface nodes n ode to appopately epesent the contact condtons between the sheet and the tool, the compensaton method ntoduced n ths pape can be appled wth all of educton methods. Futhemoe, due to the avalablty of mult-coe pocessos and the ablty of LS-DYNA fo paallel computaton wth up to 8 pocessos, consdeably lage FE models can be handled. Hence, the authos n [1] conclude that a full 3D dscetzaton also shows geat potental. 3.2 Advanced Tool Desgn Pocess Afte spngback compensaton, the tool suface epesents the best shape fo achevng the spngback of the dawn pat back to the desed pat shape. Theefoe, t s useful to consde the elastc effects at ths step n the tool desgn pocess. At ths pont, the elastc popetes of the tool and pess have not yet been assessed. Theefoe, the tool suface dung the actual deep dawng pocess wll dffe fom the gd tool suface that was used fo the pevous fomng and spngback smulaton. In the followng appoach shown n Fgue 3, the spngback compensaton s succeeded by the compensaton fo effects, whch ae esults of elastc tool and pess popetes. The compensaton pocess s teated untl the de suface nodes deved fom spngback smulaton and the de suface nodes unde pocess load match one anothe. The new suface s then econstucted fom the compensated node set and manufactued. Due to othe nfluences on the deep dawng pocess whch ae not mplemented n the FE smulatons, e.g. fcton and mateal popetes, the de spottng tme can not be elmnated, but can be educed. Fgue 3: Tool desgn and stages of compensaton Fgue 4 shows the fou man stages of the compensaton method. Usng the de suface 1 (DS1), whch s defned though spngback compensaton wth gd tool and pess popetes, the elastc defomatons and/o dslocatons of the de ae calculated by means of FE smulaton. DS2 shows dffeent defomatons, whch esult fom consdeng am tltng, global deflecton, and/o local

6 defomatons, o any combnaton of these popetes, n the FE model. As explaned n chapte 3.3, DS2 seves as the nput paamete fo the suface compensaton. The new de suface (DS3), a esult of the compensaton, s then mplemented n an addtonal FE smulaton, whch delves both the new de suface unde pocess load and the nfluence on the fnal pat shape. The new contact suface of the de unde pocess load (DS4) now equals the desed de suface fom the spngback compensaton wth no pocess load placed on the de. Fgue 4: Types of elastc nfluences and the compensaton 3.3 Automatc Tool Suface Compensaton The fst step of the automatc suface compensaton method s to calculate the tools elastc defomatons, whch ae caused by pocess load. Fo ths pupose, LS-DYNA s used. The FE model has to be able to contan the popetes mentoned n chapte 3.1, whch vay n domnance dependng on the machne and tool. Dung the fnal step of deep dawng, the most pessue s placed on the tool, theefoe causng the lagest defomatons of the ente pocess to occu. At ths pont the nodes and the new coodnates ae extacted fom the FE-model. The tansfomaton vecto s then calculated by subtactng the new node coodnates fom the ntal node coodnates. Δ = p s 0 x = y z p p p x y z s s s Δx = Δy Δ z

Subsequently, ths vecto s negated, then multpled wth a scale facto and ultmately added to the poston vecto of the ntal nodes. a = 0.9 Δ s = s 1 + a x = y z s 1 s 1 s 1 0.9 Δx 0.9 Δy 0.

7 Subsequently, ths vecto s negated, then multpled wth a scale facto and ultmately added to the poston vecto of the ntal nodes. a = 0.9 Δ s = s 1 + a x = y z s 1 s 1 s Δx 0.9 Δy 0.9 Δz The esult s a new set of node coodnates, whch defnes the new de suface and seves as the nput fo the next teaton. Fgue 5: Automatc suface compensaton Afte evey teaton the dffeences Δs ae summed up fo all de suface nodes and dvded by the numbe of suface nodes. Ths numbe seves as the stop cteon. If the compensated suface nodes meet the stop cteon, 1 n n 2 Δ C, 1 the compensaton s aboted. 4 Applcaton of the Compensaton Method to S-Ral Tool 4.1 Expemental Set-up Snce the avalable hydaulc pess has a small clampng aea and the foces ae compaatvely small, no sgnfcant global deflectons ae expected. Hence, the fou cones of the de st on washes and enable an excessve defomaton of the de unde load to occu (see Fgue 6). In addton, snce the deflecton of the de s by fa the most domnant facto, the expemental set-up enables a well-defned compensaton solely fo ths facto. It s clea, that ths expement does not eflect the ealty and can only seve fo geneal demonstaton puposes.

Fgue 6: Tool set-up fo expemental evaluaton of de suface compensaton 4.2 FE model Snce the des elastcty s the most domnant nfluence on the fnal pat, elastc pess popetes ae neglected n the FE model.

Theefoe, the de and blankholde ae meshed wth sold elements type 2, and possess an elastc mateal model MAT001.

8 Fgue 6: Tool set-up fo expemental evaluaton of de suface compensaton 4.2 FE model Snce the des elastcty s the most domnant nfluence on the fnal pat, elastc pess popetes ae neglected n the FE model. Howeve, due to the blankholde plate s heght of just 37 mm, the blankholde elastcty can not be dsegaded. Theefoe, the de and blankholde ae meshed wth sold elements type 2, and possess an elastc mateal model MAT001. Fgue 7: FE-model set-up fo LS-DYNA wth elastc blankholde and elastc de The blankholde foce s placed n +z decton though the use of gd pn sufaces. The gd punch dsplacement s defned by a velocty cuve whch eaches a dawng depth of 40 mm. The de mountng on the washes s epesented by lockng all degees of feedom at the appopate contact suface, see Fgue 7. The contacts ae defned usng contact segments on the de and blankholde suface. The FE model contans elements. In ode to decease computaton tme, the FE smulaton uns smultaneously on fou pocessos and takes 3 h 30 mn.

4.3 Suface Compensaton The suface compensaton method was appled to the expemental tool set-up. As pevously mentoned, the man elastc nfluence s the global deflexon of the de.

Fgue 8: De compensaton: left: pogess of stop cteon; ght: offset of compensated de on ntal de suface In Fgue 8, left, the pogess of the stop cteon wth each teaton s dsplayed.

9 4.3 Suface Compensaton The suface compensaton method was appled to the expemental tool set-up. As pevously mentoned, the man elastc nfluence s the global deflexon of the de. Snce ths defomaton ndcates values up to 0.6 mm, local defomatons ae of almost no mpact. Fgue 8: De compensaton: left: pogess of stop cteon; ght: offset of compensated de on ntal de suface In Fgue 8, left, the pogess of the stop cteon wth each teaton s dsplayed. Afte 7 teatons the stop cteon s met and the compensated contact suface s extacted and econstucted to get appopate data fo mllng. Fgue 9 shows the dffeence between both des n FE smulaton. Despte a lage defomaton of the compensated de, the de cleaance s mpoved, tansfomed back nto the desed value of 1.4 mm and the de suface s a plane. Fgue 9: Defomaton at the end of the deep dawng pocess and ts nfluence on dawng cleaance. a) uncompensated S-Ral de, b) compensated S-Ral de 4.4 Expemental Results Fo the de compensaton a efeence pat was defned by means of FE smulaton. It was assumed, that the de would st flat on the am and no global deflectons would occu. Theefoe, the de nodes that wee actually n contact wth the am wee locked n all degees of feedom. By conductng the compensaton pocess, the FE smulaton esults fo the uncompensated and compensated de, whch both sat on the washes, wee automatcally obtaned (see Fgue 10).

Fgue 10 smulaton compaed wth expemental esults As expected, the esults fom the smulaton and expement wth the soft de, showed a sgnfcant nfluence of the de deflexon on the mateal flow.

10 Fgue 10 smulaton compaed wth expemental esults As expected, the esults fom the smulaton and expement wth the soft de, showed a sgnfcant nfluence of the de deflexon on the mateal flow. The global deflecton caused the dstbuton of the blankholde pessue on the pat to be affected and the mateal flow to be estaned on the oute cones of the pat (see Fgue 10 n the mddle and Fgue 11 a). The fomng smulaton and expement wth the compensated de show a consdeable mpovement of the mateal daw-n. The unfom textue of the flange suface (Fgue 11 b) ndcates a homogeneous dstbuton of the blankholde pessue. It can be concluded that the de suface s now back to the desed suface shape. Fgue 11: a) dawng wth uncompensated de suface: the shnng aea eflects the hgh local pessue, whch s placed on the pat and holds the mateal fom flowng; b) dawng wth compensated de: unfom pessue dstbuton, mateal flow unestaned 5 Concluson Ths pape llustates a methodology that compensates fo the effects of elastc tool and pess popetes on the fnal shape of the pat. The new method was tested though an expement, whch nduced a global deflecton of the de and showed that usng the conventonal de leads to a massve mpact on the daw-n. By applyng the compensated daw, the daw-n was coected.

11 6 Outlook of Futue Reseach Wok Snce t s unpedctable how much of an mpact the compensaton of the de wll have on the spngback behavou of the pat, futue effot must take ths effect nto account. Addtonal eseach wok needs to be done on local defomatons. It s mpotant to be able to dstngush between benefcal and hamful local defomatons. Concevably, lghte, and theefoe less stff, am and pess table constucton could be used, whch could esult n mpoved dynamc behavou, e.g. n sevo pesses. 7 Lteatue [1] Haufe, A.: Sheet metal fomng smulaton wth elastc tools n Ls-Dyna, Numsheet, 2008, pages [2] Menhadt, J.: "Smulaton- und Messdatenbasete Rückfedeungskompensaton", EFB Sevopessen und Wekzeugsysteme zu Blechveabetung, 2009, pages [3] Clake, M.: Sheet metal fomng smulaton and eal wold toolng, 10th Intenatonal LS- DYNA Use s confeence, 2008, pages [4] Roll, K.: Smulaton of sheet metal fomng Necessay developments n the futue, 7. LS- DYNA Anwendefoum, 2008, pages A-I-59 A-I-68 [5] Goßmann, K.; Weme, H.; Hadtmann, A.; Pente, L.: Faste to Sound Pats by Advanced Fomng Pocess Smulaton Advanced Fomng Pocess Model Includng the Elastc Effects of the Fomng Pess and Tool, Steel Reseach Int. 78, 2007, page [6] Stalmann, A.; Weget, P.: Wekzeugtechnk de Zukunft Anfodeungen und Möglchketen, EFB Sevopessen und Wekzeugsysteme zu Blechveabetung, 2009, pages

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