Roller straightening Bending .1 Bending process

st an d s, fo r exam p le t h ree o r fo u r, are m o u n t ed in a fixed set t in g o n p allet s t o get h er wit h t h e ro lls. Th e p allet s are raised u sin g a cran e. Fo r fast en in g an d cen t erin g, m ech an ical o r h yd rau lic syst em s are u sed d ep en d in g o n t h e level o f au t o m at io n . – Exch an ge o f in d ivid u al p allet s o r co m p let e set s o f ro lls an d st an d bearin gs: Fo r fu lly au t o m at ic reset t in g, t h e t o o l p allet s are exch an ged u sin g a rack h an d lin g syst em . Th is m et h o d is m o re eco n o m ical wh ere freq u en t reset t in gs are req u ired .

4.8.3 Roller straightening

St raigh t en in g is req u ired wh erever p last ically d efo rm ed sh eet m et als, bars, wires o r p ip es h ave t o be st raigh t en ed . A t yp ical ap p licat io n is t h e st raigh t en in g o f sh eet m et al in a st raigh t en in g m ach in e fo r fu rt h er p ro - cessin g in a fo rm in g p ress aft er bein g t aken o ff t h e co il cf. Sect . 4.3 . 383 Bending Fig. 4.8.23 Work shaft exchange frame forming roll w ork shaft w idth adjustment Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 In acco rd an ce wit h DIN 8586, ro ller st raigh t en in g is classified as a ben d in g p ro cess u sin g ro t at in g t o o l m o t io n cf. Fig. 2.1.3 . Ben d in g back u n wan t ed cu rvat u re by m ean s o f a p recisely d efin ed ben d in g p ro cess wh ich m u st be exact ly co o rd in at ed wit h t h e st art in g rad iu s h as p ro ven t o be im p o ssible in p ract ice. In st ead , t h e t ech n iq u e u sed st raigh t en s t h e m et al by im p art in g alt ern at in g ben d in g o p erat io n s u sin g grad u ally in creased ben d in g rad ii. Th is t ech n iq u e is kn o wn as ro ller st raigh t en in g Fig. 4.8.25 . Ben d in g an d co u n t er ben d in g m u st each t ake p lace wit h in t h e p last ic ran ge o f t h e m at erial in o rd er t o en su re t h at t h e ben d in g d irect io n is ret ain ed fo llo win g elast ic reco very, i. e. t h e yield st ren gt h o f t h e m at e- rial m u st be exceed ed . O n t h e o t h er h an d , care m u st be t aken t o en su re t h at t h e st raigh t en ed m at erial d o es n o t su st ain d am age. If ben d in g is t o o p ro n o u n ced , brit t le m at erials can d evelo p sligh t cracks at t h e su rface. Th e last ro ller m u st be set in su ch a way t h at it cau ses t h e ben d in g p ro cess wh ich gen erat es a flat sh eet fo llo win g elast ic reco very Fig. 4.8.26 . As a resu lt o f alt ern at e ben d in g, t h e resid u al st ress in t h e sh eet m at e- rial is red u ced as t h e n u m ber o f ben d in g p ro cesses an d t h e ben d in g 384 Sheet metal forming and blanking Fig. 4.8.24 Stand for a complete stand exchange frame shaft forming roll base plate Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 rad ii in crease. Th e red u ct io n o f resid u al st ress is h igh ly ben eficial fo r fu rt h er p ro cessin g Fig. 4.8.27 . Th e larger t h e n u m ber o f ro llers u sed , t h e lo wer is t h e resid u al st ress in t h e sh eet m et al aft er st raigh t en in g. In all roller straigh ten in g m ach in es, th ere are two rows of straigh ten - in g rollers arran ged in offset form ation relative to each oth er. Th ese are called th e u p p er an d lower rollers. Th e d istan ce between roller cen ters is kn own as th e roller p itch t w an d varies in d ifferen t m ach in es, as d oes th e roller rad iu s R w an d th e n u m ber of straigh ten in g rollers. Straigh ten in g m ach in es with a m in im u m of five rollers are u sed . Five straigh ten in g rollers are su fficien t to straigh ten th ick p lates or coils, or wh ere n o sp ecial d em an d s are m ad e on straigh ten in g accu racy. W h ere th in n er m aterials or m ore strin gen t straigh ten in g req u irem en ts are in volved , for exam p le 385 Bending Fig. 4.8.25 Roller straightening w ith 13 rollers R1 R2 R4 R3 R1 R2 R4 R3 Fig. 4.8.26 Alternate bending Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 fo r fin e blan kin g, m ach in es wit h u p t o 21 ro llers are u sed . Th e feed valu e z w fo r t h e ro llers is lim it ed by t h e geo m et ry o f t h e st raigh t en in g m ach in e Fig. 4.8.28 : wit h t h e ro ller rad iu s R w [m m ], sh eet m et al t h ickn ess s [m m ] an d ro ller p it ch t w [m m ]. In ad d it io n t o t h e lim it s im p o sed by t h e ro ller rad iu s an d p it ch , t h e m at erial p ro p ert ies also h ave t o be t aken in t o acco u n t ; wh en p ro cessin g brit t le alu m in iu m , fo r in st an ce, in o rd er t o avo id crackin g t h e m at erial it is im p o rt an t n o t t o select t h e sm allest ben d in g rad iu s. 386 Sheet metal forming and blanking plastic alteration of length COM PRESSION plastic alteration of length COM PRESSION plastic alteration of length TENSION plastic alteration of length TENSION 1. bend 2. bend 3. bend 4. bend neutral fiber e n o z c it s a l e sh ee t m et al th ic kn es s Fig. 4.8.27 Residual stresses in the sheet metal z R s R s t mm w w w w max – – = ⋅ + ⋅ + [ ] 2 2 2 2 Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 Exa m ple: Sh eet steel St 37 m aterial n u m ber 1.0037 with a th ickn ess of 2 m m h as to be straigh ten ed. Th e straigh ten in g roller p itch of th e straigh ten in g m ach in e is t w = 22 m m , th e roller diam eter R w = 20 m m . Th e m axim u m feed valu e is calcu lated as Roller straightening m achines Gen erally sp eakin g, roller straigh ten in g m ach in es for straigh ten in g sh eet m etal com p rise an u p p er an d a lower row of straigh ten in g rollers with th e sam e d iam eter. Th e two rows are gen erally arran ged h orizon tally. Th e rollers in th e top row are gen erally arran ged p recisely above th e gap s between th e rollers of th e bottom row. To allo w m o d ificat io n o f t h e ro llin g gap , t h e u p p er ro llers m u st be ad ju st able in h eigh t . Th e t yp e o f ad ju st m en t ch aract erizes t h e st raigh t - en in g m ach in e t yp e. W h ile in o n e case each u p p er ro ller can be sep a- rat ely h eigh t ad ju st able, in t h e o t h er t yp e o f m ach in e all t h e u p p er ro llers are gro u p ed t o get h er in a blo ck t o fo rm an assem bly, an d are fed t o ward s t h e lo wer ro llers by a t ilt in g m o t io n o f t h e blo ck. Th e t h ickn ess ran ge, t h e yield st ren gt h an d t h e m o d u lu s o f elast icit y o f t h e co il st o ck t o be p ro cessed d et erm in e t h e d iam et er an d p it ch o f t h e st raigh t en in g ro llers. In t h e case o f m at erials wit h p art icu larly sen - sit ive su rfaces, in t erm ed iat e ro llers em bed d ed bet ween t h e st raigh t en - 387 Bending Wma t t s W W W Fig. 4.8.28 Straightening roller geometry z mm w max – – . = ⋅ + ⋅ + = 2 20 2 2 20 2 22 6 2 2 2 Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 in g an d su p p o rt ro llers are u sed in o rd er t o avo id d am agin g t h e sh eet su rface. Wit h t h e h elp o f st raigh t en in g ro llers wit h cro wn in g cap abili- t y, wavin ess at t h e ed ges an d cen t er o f t h e m at erial can be elim in at ed . Th e first p air of rollers m u st cau se p lastic d eform ation of th e m aterial, bu t a certain roller d iam eter m u st n ot be exceed ed . Th e th ickn ess of a straigh ten able sh eet m etal is accord in gly calcu lated from th e m axim u m straigh ten in g force an d th e torq u e, wh ich in tu rn is d eterm in ed by t h e size o f t h e card an sh aft s bet ween t h e gear an d ro llers an d t h e ro ller d iam et er. As n eit h er t h e gear n o r t h e card an sh aft s can be en larged at will fo r sp ace reaso n s, t h e sh eet t h ickn ess ran ge an d acco rd in gly also t h e flexibilit y o f st raigh t en in g m ach in es wit h ro llers o f eq u al size are rest rict ed . Th is ran ge can be ext en d ed by ap p ro xim at ely o n e an d h alf t im es if in d ivid u al ro llers can be d isen gaged . Usin g t h e fu rt h er d evel- o p ed ro ller syst em d ep ict ed in Fig. 4.8.29 wit h d ifferen t ro ller d iam e- t ers, it is p o ssible t o st raigh t en sh eet m et als fro m 0.5 t o 20 m m in t h ick- n ess. 388 Sheet metal forming and blanking Fig. 4.8.29 Roller straightening machine for larger sheet metal thickness range feed rolls height display lift cylinder feed spindle straightening rollers w ith different diameters Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998 4 Sheet metal forming and blanking 4.9 Organization of stamping plants 4.9.1 Design