Draw cushions Press types and press construction

bu lky, cost-in ten sive tu rn in g d evice to be in stalled d own stream from th e d ou ble-action p ress. Th ere are ad van tages in form in g a p art in th e reverse p osition , i. e. th e bottom of th e p art facin g u p ward s. In th is case p art ejec- tion is sim p ler th rou gh a slid e cu sh ion in su bseq u en t form in g station s. Th e altern ative is to work with a d raw cu sh ion or n itrogen cylin d ers. Fu rth erm ore, th e cen terin g in su bseq u en t station s is im p roved , sin ce th e p arts can be p osition ed over th eir en tire in n er sh ap e by th e bottom d ie. In ad d ition , th e sen sitive su rfaces of th e m aterial – for exam p le wh en workin g with au tom otive bod y p an els – are less likely to su stain d am age d u rin g tran sp ortation in th e p ress an d wh en cen terin g in th e d ie. It is also p ossible to in stall th e blan kin g p u n ch es at a lower level for su bse- q u en t op eration s if th e p art h as p reviou sly been tu rn ed over. In t h e case o f single-acting deep drawing presses, in co n t rast , t h e blan k h o ld er fo rce is t ran sm it t ed fro m u n d ern eat h by a d raw cu sh io n Fig. 3.1.9 an d cf. Fig. 4.2.3 . Bo t h t h e fo rm in g an d t h e blan k h o ld er fo rce are ap p lied by t h e slid e, as t h e slid e fo rces t h e d raw cu sh io n d o wn - ward s. Aft er t h e d rawin g, wit h o u t “t u rn in g o ver”, t h e p art is id eally p o sit io n ed fo r t h e su bseq u en t o p erat io n s. W h en u sed t o get h er wit h a h yd rau lically d riven slid e m o vem en t , t h e d raw cu sh io n can also be u sed fo r active counter-drawing Fig. 3.1.10 an d cf. Fig. 4.2.4 . Fro m t h e p o in t o f view o f en ergy savin gs, it is ben eficial t h at t h e slid e h o ld s t h e blan k wh ile t h e d raw cu sh io n ap p lies t h e fo rm - in g fo rce. In o rd er t o ach ieve as great a d egree o f flexibilit y as p o ssible, d o u ble- act io n p resses can also be co n figu red wit h a d raw cu sh io n .

3.1.4 Draw cushions

Th e fu n ct io n o f t h e d raw cu sh io n is t o h o ld t h e blan k d u rin g d eep d rawin g o p erat io n s in m ech an ical an d h yd rau lic p resses cf. Sect . 2.1.4 . Th e cu sh io n t h u s p reven t s t h e fo rm at io n o f wrin kles an d , as a resu lt o f t h e eject io n fu n ct io n , raises t h e p art s t o t h e t ran sp o rt level d u rin g t h e ret u rn m o t io n o f t h e slid e. Mo d ern d raw cu sh io n s can be eit h er h yd rau lically o r p n eu m at ically act u at ed . A m ajo r d ifferen ce bet ween t h e t wo is t h e larger fo rce o f t h e h yd rau lic d raw cu sh io n . Th e cu sh io n fo rce is in it iat ed d u rin g d o wn ward t ravel o f t h e slid e by, fo r exam p le, fo u r h yd rau lic cylin d ers via t h e lift in g brid ges an d co m - 44 Fundamentals of press design Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 Fig. 3.1.10 Single-action hydraulic press w ith active draw cushion for counter draw ing slide cylinder slide ejector counter draw ing die active draw cushion draw cushion cylinder slide draw punch p ression colu m n s in to th e corn er areas of th e blan k h old er on th e d ie Fig. 3.1.11 an d cf. Fig. 4.4.39 . W h en th e slid e reach es th e bottom d ead cen ter, th e d raw cu sh ion is m oved back to th e u p p er p osition eith er in an u n con trolled , i. e. togeth er with th e retu rn m otion of th e slid e, or a con - trolled m ovem en t, i. e. after a p rescribed stan d still p eriod , by th e lift cylin d er actin g in th e cen ter of th e liftin g brid ge. Im p act d am p in g is ach ieved by m ean s of op tim ized p rop ortion al valve tech n ology, en su rin g th at th e d raw cu sh ion reach es its u p p er en d p osition with ou t vibration s. Th e ben efit h ere is th at th e d raw cu sh ion force can be con trolled on a p ath -d ep en d en t an d a tim e-d ep en d en t basis d u rin g th e d rawin g p rocess. It is p ossible, for in stan ce, n ot on ly to red u ce an d in crease th e overall blan k h old er force, bu t th e p ressu re levels of th e fou r cylin d ers can also be m od ified in d ep en d en tly of each oth er d u e to th e sh ort p ressu re resp on se tim e with in a ran ge of 25 to 100 of th e rated p ressu re. As a resu lt, th e blan k h old er p ressu re can be in d ivid u ally con trolled over th e en tire d eep d rawin g p rocess at each of th e fou r corn er p oin ts of th e d ie. Th e p recision con trol of th e blan k h old er follows a p rescribed force ver- su s tim e p rofile Fig. 3.1.12 . 45 Press types and press construction Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 Th is o ffers t h e d iem aker an d t h e m ach in e o p erat o r t h e cap abilit ies fo r o p t im izat io n o f t h e d rawin g p ro cess an d fo r t h e red u ct io n o f d ie st art -u p t im es. Th e st ret ch fo rm in g p ro cess, fo r exam p le, can be in it iat - ed by briefly raisin g t h e blan k h o ld er fo rce, o r t h e m at erial flo w co rre- sp o n d in gly facilit at ed by red u cin g t h e p ressu re in cert ain areas o f t h e blan k h o ld er cf. Sect . 2.1.4 . To im p ro ve p art q u alit y, o u t p u t an d t o o l life as well as red u cin g n o ise, t h e d raw cu sh io n can be p re-accelerat ed . Th is m ean s t h at t h e d raw cu sh io n is alread y set in m o t io n befo re t h e slid e im p act s t h e blan k. Th is red u ces t h e relat ive sp eed bet ween t h e slid e an d blan k h o ld er co n sid erably as co m p ared t o an u n co n t ro lled d raw cu sh io n . Du e t o t h e red u ced im p act o f t h e blan k h o ld er, t h e n u m - 46 Fundamentals of press design Fig. 3.1.11 Design of a hydraulic draw cushion female die draw n part blank holder pressure column lifting bridge punch moving bolster foundation displacement cylinder lifting cylinder Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 ber o f p ress st ro kes can be in creased wit h o u t co m p ro m isin g p art q u a- lit y, wh ile at t h e sam e t im e t h e n o ise level is red u ced by as m u ch as 8 d BA. An im p o rt an t ch aract erist ic o f t h e h yd rau lic d raw cu sh io n wit h d is- p lacem en t cylin d ers is t h at t h e blan kh o ld in g fo rce, in co n t rast t o t h e p n eu m at ic d raw cu sh io n , is n o t fu lly ap p lied in st an t an o u sly o n ap p li- cat io n o f t h e slid e, an d is o n ly bu ilt u p co m p let ely wit h t h e st art o f t h e p ist o n m o vem en t t h ro u gh d isp lacem en t o f t h e o il co lu m n in t h e cylin - d ers. Th is allo ws t h e vibrat io n s co m m o n ly en co u n t ered o n im p act o f t h e slid e, as in d icat ed in Fig. 3.1.13 , t o be avo id ed . Th e n egat ive in flu - en ce o f vibrat io n s an d n o ise, an d also o n t h e d rawin g p ro cess it self, are elim in at ed . Th is is o f p art icu lar sign ifican ce wh en p ro d u cin g large sh eet m et al co m p o n en t s wit h h igh st ro kin g rat es. It is p o ssible t o large- ly avo id su p erim p o sed vibrat io n s affect in g t h e d rawin g p ro cess at slid e im p act sp eed s o f, fo r exam p le, 40 wit h a relat ively large lead o f t h e 47 Press types and press construction Fig. 3.1.12 Progression of blank holder force in a hydraulic draw cushion w ith a constant displacement pressure of 100 back right, 80 back left and front right and pressure changeover from 80 to 25 front left 1 s displacement pressure draw cushion travel slide travel time pressure travel front left back right back left front right damping phase 100 25 80 80 80 Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 p n eu m at ic d raw cu sh io n Fig. 3.1.13 , top. Ho wever, it is o n ly m argin al- ly p o ssible t o ach ieve a co n st an t blan k h o ld er fo rce at h igh im p act sp eed s Fig. 3.1.13 , bottom . Th e co n d it io n s fo r t h e h yd rau lic d raw cu sh - io n wit h d isp lacem en t cylin d ers Fig. 3.1.11 are sign ifican t ly m o re favo rable. Sin ce t h e p ressu re bu ild -u p d o es n o t t ake p lace u n t il t h e begin n in g o f t h e p ist o n m o vem en t , even at m axim u m im p act sp eed t h e blan k h o ld er fo rce p eaks o n ly m in im ally an d is q u ickly d issip at ed Fig. 3.1.12 . Acco rd in gly, in gen eral t erm s a lead o f t h e blan k h o ld er in t h e d ie o f so m e 30 m m is su fficien t t o en su re t h at t h e en t ire d rawin g p ro cess t akes p lace at a co n st an t blan k h o ld er fo rce. 48 Fundamentals of press design impact speed 40 time blank holder force rated blank holder force lead travel point of impact w ith draw cushion start of draw ing delayed impact speed 100 time blank holder force rated blank holder force start of draw ing delayed point of impact w ith draw cushion lead travel Fig. 3.1.13 Progression of blank holder force of a pneumatic draw cushion depending on impact speed Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 3 Fundamentals of press design 3.2 M echanical presses 3.2.1 Determination of characteristic data