4 Sheet metal forming and blanking
4.5 Blanking processes
In blan kin g o p erat io n s, a d ifferen ce is m ad e bet ween o p en an d clo sed co n t o u rs cf.
Fig. 2.1.29 . O p en co n t o u r blan kin g t ech n iq u es are u sed
m ain ly fo r blan kin g st rip s m ad e o f sh eet p lat es. Th is t ech n iq u e is sim i- lar t o t h e act io n o f a p air o f scisso rs cf.
Fig. 2.1.31 . Th e blan kin g ele-
m en t s u sed fo r o p en blan kin g can be eit h er lo n git u d in al o r circu lar kn ives. In clo sed blan kin g p ro cesses, t h e sh eared co n t o u r is clo sed , fo r
exam p le wh en p iercin g. Here, t h e p ro cess is ap p lied u sin g blan kin g d ies: Th e relat ive m o vem en t o f t h e blan kin g p u n ch t o t h e fem ale
blan kin g d ie sep erat es t h e m et al Fig. 4.5.1
. Th e p u n ch m akes co n t act wit h t h e sh eet m et al, in it ially cau sin g elast ic d efo rm at io n . Th e p last ic
d efo rm at io n st age t h en fo llo ws, leavin g t h e sh eet m et al wit h a p erm a- n en t cam ber. Th e u p p er ed ge o f t h e sh eet m et al t h en ben d s an d d raws
in , fo llo wed by a sh earin g act io n wh ich leaves a visible, sm o o t h area o n t h e cu t su rface. If t h e sh earin g st ren gt h is exceed ed , cracks are fo rm ed .
Th ese gen erally ru n fro m t h e ed ges o f t h e fem ale blan kin g d ie an d lead
Fig. 4.5.1
Phases of the blanking process contact of the
punch elastic and plastic
deformation shearing and
crack formation breakthrough stripping
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
t o co m p let e breakt h ro u gh o f t h e m et al as t h e m o vem en t o f t h e p u n ch p ro gresses. A fo rce-t im e d iagram o f a t yp ical blan kin g p ro cess is p ro vid -
ed in Fig. 4.6.7
. A ro u gh fract u re zo n e t h en fo rm s o n t h e cu t su rface u n d ern eat h t h e sh earin g zo n e cf.
Fig. 4.7.8 , left: Th e sh eet m et al m at e-
rial sp rin gs back aft er t h e blan kin g p ro cess, cau sin g it t o clam p o n t o t h e lat eral su rface o f t h e p u n ch . Th e sh eet m et al h as t o be sep arat ed fro m
t h e p u n ch by m ean s o f a st rip p er d u rin g t h e ret u rn st ro ke. Du e t o t h e wid e variet y o f ap p licat io n s in st am p in g p lan t s, o n ly
p ro cesses u sed in t h e p ro d u ct io n o f sin gle p art s by clo sed d ies will be d escribed h ere.
Positioning of blanks in the strip and m aterial savings Gu id elin es o n t h e eco n o m ical layo u t o f blan ked , p u n ch ed an d d rawn
p art s: Clo se co o p erat io n bet ween co m p o n en t d evelo p ers an d d ie d esign ers
can h elp t o su bst an t ially red u ce m at erial wast e. Workp ieces sh ou ld be d esign ed with th e sm allest p ossible su rface
area; th eir sh ap e sh ou ld be su ch th at th ey can be lin ed u p or n ested in each oth er in th e sh eet m etal strip so as to en su re m in im u m waste. Th e
arran gem en t of p arts illu strated in Fig. 4.5.2 b
, for in stan ce, is m ore favorable th an th at in
4.5.2 a . Th e best p ossible m aterial u tilization is
ach ieved wh ere th e su rface sh ap es are com p letely in terlockin g, i. e. can be blan ked of th e strip with ou t leavin g an y scrap wh atsoever
Fig. 4.5.3 .
Th e d rawback of scrap -free blan kin g is th at bu rr occu r on both sid es of th e workp iece an d th e d esign of th e blan kin g tools n eed ed in th is case is
m ore com p licated . In cases wh ere it is im p ossible to d esign workp iece sh ap es so th at op ti-
m u m in -lin e arran gem en t is p ossible in th e strip , it m ay be p ossible to
269
Blanking processes
Fig. 4.5.2 Arrangement of parts in the strip: b offers greater material savings than a
a b
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
u t ilize t h e rem ain in g scrap fo r o t h er wo rkp ieces Fig. 4.5.4
. If scrap skelet o n s o r webs h ave beco m e d efo rm ed d u rin g t h e in it ial blan kin g
p ro cess, t h ey m u st go t h ro u gh a st raigh t en in g m ach in e befo re fu rt h er p ro cessin g cf.
Sect . 4.8.3 .
T, L an d U-sh ap ed wo rkp ieces are id eally p ro d u ced in alt ern at ively reversed m an n er t o p t o t ail
Fig. 4.5.3 e ,
f an d
Fig. 4.5.5 . It is im p o r-
t an t t o rem em ber wh en blan kin g t o p t o t ail t h at t h e st raigh t st rip ben d s wit h in t h e st rip p lan e if su bject ed t o u n ilat eral st ress an d st ret ch ed d u r-
in g t h e blan kin g p ro cess, o r if u n ilat eral t en sio n s are released . Th is ap p lies in p art icu lar t o so ft , t h ick sh eet m at erial. In t h ese cases, it m ay
be m o re p ract ical t o blan k u sin g a m u lt ip le p u n ch arran gem en t fo r t h e sim u lt an eo u s p ro d u ct io n o f several wo rkp ieces rat h er t h an u sin g t h e
t o p t o t ail arran gem en t Fig. 4.5.4
. O ccasio n ally, it is p o ssible t o im p ro ve m at erial u t ilizat io n by blan k-
in g an o t h er u sable co m p o n en t b fro m t h e m ain p iece a, as d em o n st rat - ed in
Fig. 4.5.6 .
Figure 4.5.6c illu st rat es t h e t wo p art s assem bled . Th e
m u lt ip le arran gem en t o f wo rkp ieces is o ft en eco n o m ical Fig. 4.5.7
. In t h e in t erest s o f o p t im u m m at erial savin gs, web o r skelet o n wid t h s
sh o u ld be kep t as n arro w as p o ssible Table 4.5.1. Th e d esign at io n s o n t h e blan kin g st rip are in d icat ed in
Fig. 4.5.8 . It is im p o rt an t t o rem em -
270
Sheet metal forming and blanking
Fig. 4.5.3
Examples of favorable part arrangement in the sheet metal strip
a a
c b
d e
f
Fig. 4.5.4 Utilization of scrap pieces
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
ber t h at t h e st rip wid t h b [m m ] d ep en d s o n t h e t yp e o f m at erial an d t h e st rip len gt h l
e
[m m ]. In an y even t , it is essen t ial t o assess wh et h er t h e savin gs m ad e in t erm s o f m at erial are n o t n egat ed by excessively h igh
d ie co st s. Th e m at erial u t ilizat io n o f t h e sh eet m et al is:
wit h t h e st rip len gt h L [m m ] an d wid t h b
S
[m m ], t h e n u m ber o f wo rk- p ieces p ro d u ced fro m t h e st rip z [–] an d t h e su rface area o f o n e wo rk-
p iece A [m m
2
] wit h o u t d ed u ct in g t h e blan ked in sid e co n t o u rs. Wit h sizes co rresp o n d in g t o t h o se in
Fig. 4.5.8 , t h e m at erial u t ilizat io n fo r
ro u n d wo rkp ieces arran ged in n ro ws in t h e sh eet m et al st rip is calcu - lat ed u sin g t h e fo llo win g eq u at io n
wh ereby
271
Blanking processes
Fig. 4.5.5
Top-to-tail blanking
Fig. 4.5.6 Utilization of w aste piece b in an assembly c
a b
c
η
A S
z A L b
= ⋅ ⋅
[ ]
–
η π
A S
S
z A V b
n d
d b b
= ⋅ ⋅
= ⋅
+ ⋅
[ ]
– , 4
2
b a
d n
a a
d n
V mm
S R
= ⋅ + ⋅ + ⋅
= ⋅ + + ⋅
⋅
[ ]
2 2
2 1 2 1 0 866
– –
.
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Th e feed st ep
is calcu lat ed fro m t h e h o le d iam et er d [m m ] an d t h e st rip wid t h b [m m ]. In p ercen t age t erm s, t h e great est savin gs are m ad e by t ran sit io n
fro m a sin gle ro w t o d o u ble o r t rip le ro ws, wh ile in gen eral t h e great er t h e n u m ber o f ro ws o f h o les t h e m o re favo rable is t h e o verall m at erial
u t ilizat io n Fig. 4.5.7
. Th e h igh p ercen t age o f savin gs gain ed wh en ch an gin g t o t wo ro ws o f h o les o ft en ju st ifies t h e p ro cu rem en t o f d o u -
272
Sheet metal forming and blanking Table 4.5.1: Web and rim w idths in accordance w ith VDI 3367 7.70
0.1 0.5 1 1.5 2 3 b
a 0.8
1 0.8
0.9 1 1.3 1.6
1.6 1.9
0.9 1.0
1.1 1.4 1.7 51… 100
11… 50 1.8
2.2 1.0
1.2 1.3 1.6 1.9 2.5
over 100 2.0
2.4 1.2
1.5 1.5 1.8 2.1 2.7
1.5 2.2 3 4.5 2.3
0.9 1.2
1.0 1.1
1.1 1.4 1.7 11… 50
1.8 2.2
1.0 1.2
1.3 1.6 1.9 51… 100
2.0 2.4
1.2 1.5
1.5 1.8 2.1 über 100
2.2 2.7
1.4 1.7
1.7 2 2.3 1.5 2.5 3.5 4
2.5 1.9
2 2.2
2.4 3.5
2 2.2
2.4 2.6
4 2.1
2.3
2.5 2.7
2.9 5
up to 10 and round parts
w eb w idth b rim w idth a
strip w idth
b
S
w eb length l
e
resp. rim length l
a
in mm
b a
b a
b a
b a
b a
b a
b a
side cutter scrap i
up to 10 and round parts
side cutter scrap i
over 100 mm
to 200 mm
to 100 mm
Sheet metal thickness s in mm
V d
b mm = +
[ ]
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
273
Blanking processes
Fig. 4.5.7
Strips using single and multiple-row punch arrangement w ith utilization factor h
A
in 56.8
65.0 67.4
73.2
Fig. 4.5.8
Definitions on the strip: d hole diameter; a rim w idth; b w eb w idth; i side cutter w aste; l
e
w eb length; l
a
rim length; b
S
strip w idth; L strip length; V feed step; t pitch; a
R
space betw een the row s
l
e
i b
a l
e
l
a
l
a
b
b b
L 6 °
a = t
a a
d
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
ble d ies o r, wh ere d o u ble d ies d o n o t rep resen t a viable in vest m en t d u e t o sm all bat ch sizes, t h e u se o f a feed syst em fo r o ffset blan kin g.
Blanking force and blanking work Th e n ecessary blanking force F
S
[N] fo r p u n ch es wit h flat gro u n d wo rk- in g su rfaces clo sed blan kin g co n t o u rs is calcu lat ed wit h t h e fo llo win g
eq u at io n : wh ereby A
S
is t h e sh eared su rface in m m
2
an d k
S
t h e resist an ce t o sh ear, sh earin g st ren gt h o r relat ive blan kin g fo rce o f t h e sh eet m et al exp ressed
in N m m
2
. Th e sh eared su rface is d et erm in ed fro m t h e len gt h o f t h e sh eared co n t o u r l
S
[m m ] m u lt ip lied by t h e t h ickn ess o f t h e sh eet m et al s [m m ], wh ereby t h e len gt h o f t h e sh eared co n t o u r is t aken t o m ean t h e
su m o f all sh eared ed ge len gt h s in m m . If t h e rat io o f t h e p u n ch d iam - et er d t o t h e sh eet m et al t h ickn ess s is great er t h an 2, t h e fo llo win g
eq u at io n is su fficien t fo r an ap p ro xim at e calcu lat io n o f t h e sh earin g st ren gt h k
S
:
wh ereby R
m
[N m m
2
] is t h e t en sile st ren gt h o f t h e sh eet m et al. Th e blan kin g fo rce m u st n o t exceed t h e n o m in al p ress fo rce wit h in
t h e n o m in al fo rce-st ro ke cu rve, given in p ress sp ecificat io n s, as o t h er- wise t h e m ach in e will be o verlo ad ed . If t h e sh eet m et al t h ickn ess is
larger t h an t h e n o m in al p ress fo rce st ro ke p o sit io n i. e. d ist an ce befo re BDC, wh ere n o m in al p ress fo rce is given , t h en t h e p erm issible p ress
fo rces are sm aller cf. Sect . 3.2.1
. Th ese can be ascert ain ed fro m t h e lo ad versu s st ro ke d iagram in clu d ed in t h e o p erat in g in st ru ct io n s.
Th e force of th e retu rn stroke for strip p in g th e workp iece off th e p u n ch is arou n d 3 to 5 of th e blan kin g force wh en th e ratio of th e
p u n ch d iam eter d to th e sh eet m etal th ickn ess s is arou n d 10 d s = 10. In th e case of sm aller d s ratios, th e retu rn stroke forces in crease su bstan -
tially, am ou n tin g with d s = 2 to arou n d 10 to 20 of th e blan kin g force, wh ile th is d rop s fu rth er with greater d s ratios. A greater retu rn stroke
force is req u ired for strip p in g tou gh m aterials th an for brittle on es. Th e retu rn stroke force m u st be taken in to accou n t wh en d esign in g th e
p u n ch an d d ies, an d in extrem e cases also wh en d im en sion in g th e p ress. Lat eral fo rces can also o ccu r d u rin g blan kin g o p erat io n s. Part icu lar
at t en t io n m u st be p aid t o t h ese, in p art icu lar wh en t h e st rip layo u t h as
274
Sheet metal forming and blanking
F A
k l
s k N
S S
S S
S
= ⋅
= ⋅ ⋅
[ ]
,
k R
N mm
S m
= ⋅
0 8
2
. ,
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
a co n figu rat io n o f n arro w webs bet ween clo sely p o sit io n ed blan ks an d in cases wh ere t h e blan kin g p ro cess is n o t p erfo rm ed sim u lt an eo u sly,
fo r exam p le d u e t o o ffset p u n ch es Fig. 4.5.9 g
. In t h is case, t h e web is su bject ed t o st ress as a resu lt o f t h e blan kin g an d h o rizo n t al fo rces. Th e
h o rizo n t al fo rce exert ed lies ap p ro xim at ely bet ween 2 an d 10 o f t h e blan kin g fo rce, wh ereby t h e lo wer valu e ap p lies t o t h in , brit t le sh eet
m et al an d t h e h igh er o n e t o t h icker, t o u gh er m at erial t yp es. Th e u se o f blu n t blan kin g ed ges in creases t h e blan kin g an d h o rizo n t al fo rce.
It is p o ssible t o red u ce t h e n ecessary blan kin g fo rce if in st ead o f a flat p u n ch wit h p arallel cu t t in g ed ges, a bevelled p u n ch wit h an o bliq u e
sh earin g act io n is u sed Fig. 4.5.9 a, b
. Th e h eigh t d ifferen ce h [m m ] t o be select ed in t h is case sh o u ld be aro u n d 0.6 t im es fo r brit t le m at erial
t o 0.9 t im es t h e sh eet m et al t h ickn ess s. Th e bevel an gle sh o u ld be n o great er t h an 5°, in o rd er t o p reven t d am age t o t h e cu t ed ge by lat eral
d isp lacem en t o f t h e p u n ch . Un ilat eral d isp lacem en t o f t h e p u n ch an d m at erial can be p reven t ed by u sin g a p u n ch wit h a h o llo w o r p o in t ed
face o r wit h a gro o ve Fig. 4.5.9 c, d
. Ho wever, t h is resu lt s in d efo rm a- t io n o f t h e slu g. If t h e slu g is in t en d ed fo r u se as a flat wo rkp iece, t h e
p u n ch face m u st be flat an d t h e fem ale d ie m u st h ave a h o llo w o r p o in t ed co n figu rat io n
Fig. 4.5.9 e, f .
275
Blanking processes
Fig. 4.5.9
Punch and female die shapes h = difference in height h
h h
h h
a
e
b c
d
f
d
g
flat punch
bevelled punch
punch w ith groove
punch w ith pointed finish
pointed female die
hollow ed female die
offset punches h
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
276
Sheet metal forming and blanking
W h en u sin g a bevel gro u n d p u n ch o r fem ale d ie, t h e blan kin g fo rce is red u ced by at least 30 co m p ared t o p u n ch es o r fem ale d ies wit h a
flat su rface, as o n ly a cert ain p art o f t h e d ie is en gaged at an y o n e t im e Fig. 4.5.10
. Ho wever, o verall t h e blan kin g wo rk rem ain s t h e sam e, as t h e red u ced fo rce act s o ver a lo n ger st ro ke.
If t h ere are several p u n ch es m o u n t ed in a d ie, it is also p o ssible t o red u ce t h e blan kin g fo rce by cau sin g t h e p u n ch es t o act in seq u en ce
Fig. 4.5.9 g . Ho wever, in o rd er t o en su re a sm o o t h p ress o p erat io n an d
red u ce t h e st ress o n t h e d ies, wh en u sin g t h is m et h o d , t h e h eigh t d if- feren ce bet ween each p u n ch fo rce sh o u ld n o t be great er t h an t h e p en -
et rat io n d ep t h o f t h e d ie u n t il t h e fract u re o f t h e sh eet m et al aro u n d 0.3 t o 0.4 · s. Bevel grin d in g o f p u n ch es o r fem ale d ies an d o ffset t in g in
t h e case o f several p u n ch es also h elp t o red u ce n o ise d u rin g t h e blan k- in g p ro cess.
Exa m ple:
Assu m in g a 2 m m t h ick, h ard brass sh eet Ms 63 k
S
= 380 N m m
2
is t o be p erfo rat ed u sin g a flat gro u n d p u n ch o f 113 m m in d iam et er. Th e sh eared su r-
face is:
Acco rd in gly, t h e blan kin g fo rce will be:
Th e blan kin g wo rk is d et erm in ed wit h t h e fo llo win g eq u at io n :
with th e blan kin g force F
S
an d m aterial th ickn ess s. Th e factor x [–] takes in to con sid eration th e actu al p rogression of force wh en blan kin g an d
d ep en d s on th e m aterial. It lies with in th e ap p roxim ate ran ge of 0.4 to 0.7, wh ereby th e lower valu e ap p lies for brittle m aterials, a large blan kin g
clearan ce an d th ick sh eet m etals, an d th e u p p er valu e is u sed p rim arily for tou gh er m aterials, a sm all blan kin g clearan ce an d th in sh eet m etals.
For ap p roxim ate calcu lation , th e followin g eq u ation ap p lies:
A d
s mm
mm mm
S
= ⋅ ⋅ = ⋅ ⋅ = π
π 113 2 710 0
2
. F
A k
N kN
S S
S
= ⋅
= ⋅
= =
710 380 270 000 270 ,
W x F s Nm resp kNm
S S
= ⋅ ⋅
[ ]
.
W F s Nm resp kNm
S S
= ⋅ ⋅
[ ]
2 3
.
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
277
Blanking processes
U
l2 l2
line of engagement of the punch
flat punch
bevelled punch
Fig. 4.5.10 Blanking force reduction w ith inclined
punch shape
W h en calcu latin g blan kin g en ergy an d blan kin g forces, in p articu lar wh ere th in sh eet m etals are in volved , it can h ap p en th at a m ach in e is
d esign ed to h ave ad eq u ate en ergy for blan kin g bu t n ot ad eq u ate blan k- in g force. In th ese cases, it is p ossible to red u ce th e blan kin g force by
u sin g a bevelled or corru gated p u n ch or a fem ale blan kin g d ie.
Exa m ple:
Assu m in g t h at 100 m m d iam et er circu lar d isks cu t co n t o u r circu m feren ce U = 314 m m are t o be p ro d u ced fro m 2.5 m m t h ick sh eet st eel wit h a sh earin g resis-
t an ce o f k
S
= 100 N m m
2
, wit h t h e blan kin g fo rce
t h e blan kin g en ergy is calcu lat ed as fo llo ws:
To en su re t h e co rrect lo cat io n o f t h e blan kin g d ie in t h e p ress, it is essen t ial t o kn o w t h e lo cat io n o f t h e resu lt an t blan kin g fo rce fre-
q u en t ly also called t h e cen t er o f blan kin g fo rces. At t h is lo cat io n , t h e su m o f t h e in d ivid u al fo rces o ccu rrin g aro u n d t h e p erip h ery o f t h e
blan kin g ed ges is co n sid ered t o act as a sin gle fo rce. Th is fo rce sh o u ld be ap p lied as far as p o ssible in t h e cen t er o f t h e slid e, as t h is resu lt s in
o n -cen t er lo ad in g an d sym m et rical elast ic d eflect io n o f t h e p ress. O n - cen t er lo ad in g also en su res t h e best p o ssible d ie life, an d p ress o verlo ad
F A k mm mm N mm kN
S S
S
= ⋅
= ⋅
⋅ =
2 5 314 100 78 5
2
. .
W F s
N m
N m Nm
S S
= ⋅ ⋅ = ⋅ ⋅ = ⋅ ≈ 2
3 2
3 78 500 0 0025 130 8 131
, .
.
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
d u e t o o ff-cen t er lo ad in g is elim in at ed . W h en d ies are p ro vid ed wit h a lo cat in g p in fo r clam p in g in t h e p ress, t h is sh o u ld be p o sit io n ed in t h e
lo cat io n o f t h e resu lt an t fo rce. To d et erm in e t h e lo cat io n o f t h e resu lt in g blan kin g fo rce, t h e sh ap e,
sco p e an d p o sit io n o f sh eared co n t o u rs are o f im p o rt an ce. Th e p o sit io n o f t h e cen t er o f fo rce is gen erally d et erm in ed by calculating the center of
gravity of the blanking edges cut contours o r wh ere sim p le geo m et ries are in vo lved circle, rect an gle, by ascertaining the center of gravity of the
punch perim eters cut contour perim eters. W h ere asym m et rical p u n ch co n figu rat io n s are in vo lved , t h is calcu lat io n m u st be m ad e fo r t h e x
an d t h e y d irect io n : Fo r t h e lo cat io n o f t h e resu lt an t fo rce in t h e x d irect io n d ist an ce
fro m t h e y axis, t h e fo llo win g ap p lies:
wh ere U [m m ] rep resen t s t h e relevan t p erim et ers o f t h e in d ivid u al p u n ch es an d a [m m ] is t h e d ist an ce o f t h eir cen t ers o f gravit y fro m t h e
select ed y axis. If t h e lin e cen t ers o f gravit y are t aken as a basis, t h en
wh ere 1 [m m ] is t aken t o rep resen t t h e len gt h s o f t h e blan kin g ed ges. Th e lo cat io n o f t h e resu lt in g fo rce in t h e y d irect io n d ist an ce fro m t h e
x axis is calcu lat ed fro m t h e co rresp o n d in g eq u at io n fo r y, by su bst i- t u t in g t h e d ist an ces o f t h e p erim et er blan kin g ed ge len gt h s fro m t h e
x axis fo r b [m m ]:
278
Sheet metal forming and blanking
x U
a U
a U
a U
U U
mm
S n
n n
= ⋅ +
⋅ +…+ ⋅
+ +…+
[ ]
1 1
2 2
1 2
,
x l
a a
a mm
S n
n n
= ⋅ + ⋅ +…+ ⋅ + +…+
[ ]
1 1
2 2
1 2
1 1
1 1
1 ,
y U
b U
b U
b U
U U
mm y
l b
b b
mm
S n
n n
S n
n n
= ⋅ +
⋅ +…+
⋅ +
+…+
[ ]
= ⋅ + ⋅ +…+ ⋅
+ +…+
[ ]
1 1
2 2
1 2
1 1
2 2
1 2
1 1
1 1
1
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Usin g t h ese eq u at io n s, it is p o ssible t o d et erm in e t h e p o sit io n o f t h e cen t er o f fo rce S
R
x
S
; y
S
in an y select ed syst em o f co o rd in at es. Th e eq u at io n s can o n ly be u sed fo r d ies wit h sim u lt an eo u sly act in g flat
gro u n d p u n ch es an d flat fem ale d ies. It m akes sen se t o lay t h e co o rd i- n at e axes t h ro u gh as m an y cen t ers o f gravit y as p o ssible, so red u cin g
t h e d ist an ces t o zero an d sim p lifyin g t h e calcu lat io n s. St ep s sh o u ld be t aken t o avo id layin g t h e axes bet ween cen t ers o f gravit y as o t h erwise
at t en t io n m u st be p aid t o + – sign s gen erally ap p licable fo r x: t o t h e righ t p o sit ive valu es, t o t h e left n egat ive valu es; fo r y: u p ward s p o sit ive
valu es, d o wn ward s n egat ive valu es. Th e fo rm u las req u ired t o calcu lat e t h e cen t ers o f gravit y o f t h e in d ivid u al p erim et ers o r lin e elem en t s can
be fo u n d wh ere ap p licable in bo o ks o f t ables. Mo re co m p lex p art sh ap es can o ft en be co m p o sed o f basic figu res q u ad ran t s, rect an gles,
et c.. No wad ays, co m p u t er p ro gram s are freq u en t ly u sed t o d et erm in e t h e su rface cen t er o f gravit y.
W h ere t h e t wo fo rce act io n lin es in t h e x an d y d irect io n in t eract , we o bt ain t h e cen t er o f blan kin g fo rces S
R
x
S
; y
S
, wh ich sh o u ld be id eally in t h e cen t er o f t h e slid e, o r in t h e case o f sm aller t o o ls, at t h e o p t im u m
p o sit io n o f t h e cen t erin g p in .
Exa m p l e:
A wo rkp iece wit h o u t sid e d im en sio n s 80 3 90 m m wit h a sem icircu lar p u n ch ed h o le rad iu s 30 m m an d t wo ro u n d h o les wit h a d iam et er o f 20 m m is t o be p ro -
d u ced in t wo st ep s Fig. 4.5.11 a
. Two blan kin g o p erat io n s t ake p lace in t h e p ress: p iercin g an d blan kin g, as in d icat ed in
Fig. 4.5.11 b an d c . Th e cen t er o f
fo rce S
R
is d et erm in ed in it ially o n t h e basis o f t h e cen t ers o f gravit y o f t h e p u n ch p erim et ers, an d t h en u sin g t h e lin e cen t ers o f gravit y fo r t h e p u rp o se o f
co m p ariso n .
279
Blanking processes
➔
U U mm mm U
mm mm
mm mm mm U
mm mm
mm
1 2
3
4
20 62 83 1
2 60 60
94 25 60 154 25 2 80 2 90 340
= = ⋅
= = ⋅ ⋅ +
= +
= = ⋅ + ⋅ =
π π
.
. .
l l mm mm l
mm mm
l mm l
l mm
l l
mm
5 6 7
8 9
11 10 12
20 62 83 1
2 60 94 25
60 90 80
= = ⋅ = = ⋅ ⋅ =
= =
= =
= π
π .
.
,
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Th e d ist an ces bet ween t h e cen t ers o f gravit y in t h e resp ect ive co o rd in at e syst em in m m are:
a Calcu lat io n o f t h e co o rd in at es o f t h e cen t er o f fo rce o n t h e basis o f t h e p erim et ers
Fig. 4.5.11 b :
b Cen t er o f fo rce calcu lat ed u sin g t h e lin e cen t ers o f gravit y Fig. 4.5.11 c
:
280
Sheet metal forming and blanking
b b
b b
a a
a a
1 2
4 3
9 10 12
11
25 38 33
65 105
145 =
= =
= =
= =
= .
a a
a a
b b
b b
5 6
7 8
9 11
10 12
40 20
45 90
= =
= =
= =
= =
b b
b b
5 6 7
8
20 50 9
70 =
= =
= .
C : y d
P :
d Position i :
enter of gravity of the semi circular arc r
rawn from the straight line osition in the current coordinate system b
y rawn from the straight line n the current coordinate system b
S
S
– = ⋅ =
=
= =
= ⋅ + ⋅
+ =
= =
2 60
19 10
70 19 10 50 9 60 0
94 25 19 10 60
94 25 11 67
50 11 67 38 33
7
3
π π
.
– .
. .
. .
. –
. .
x l a
l a l a
l a l a
l a
l a
l a
l l
l l
l l
l l
x
S S
= ⋅ + ⋅ + ⋅ + ⋅ + ⋅ + ⋅ + ⋅
+ ⋅ + + + + +
+ + =
⋅ + ⋅ + ⋅ +
⋅ + ⋅ + ⋅
+ ⋅
+ ⋅
5 5
6 6
7 7 8 8 9 9 10 10 11 11 12 12 5 6 7
8 9
10 11
12
62 83 0 62 83 40
94 25 20 60 20
90 65 80 105 90 145 80 105 62
. .
. .83
83 62 83 94 25 60 90 80
90 80 +
+ +
+ +
+ +
. .
a a
a a
1 2
3 4
40 20
105 =
= =
=
x U a
U a
U a U
a U
U U
U x
y U b
U b
U b
U b
U U
U U
y
S S
S
= ⋅ +
⋅ + ⋅ +
⋅ +
+ +
= ⋅ + ⋅
+ ⋅ + ⋅
+ +
+ =
= ⋅ +
⋅ + ⋅ +
⋅ +
+ +
1 1
2 2
3 3
4 4
1 2
3 4
1 1
2 2
3 3
4 4
1 2
3 4
62 83 0 62 83 40 154 25 20
340 105 62 83 62 83 154 25 340
66 6 .
. .
. .
. .
S S
R
enter of gravity S n the coordinate system =
⋅ + ⋅ + ⋅ + ⋅
= ⇒
62 83 0 62 83 0 154 25 38 33 340 25
619 9 23 25
66 6 23 25 .
. .
. .
, . ;
. .
C i
b
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Th is p o in t co rresp o n d s t o t h e p o in t calcu lat ed u sin g t h e p erim et ers.
Blanking clearance between punch and fem ale die Th e blan kin g fo rces ascert ain ed o n t h e basis o f t h e eq u at io n s p ro vid ed
h ere assu m e t h at t h e co rrect blan kin g clearan ce h as been select ed bet ween t h e p u n ch an d t h e fem ale d ie. Th e p recise size o f t h e blan kin g
clearan ce u [m m ] d ep en d s o n t h e t h ickn ess o f t h e sh eet m et al an d it s t en sile an d sh earin g st ren gt h , as well as o n t h e blan kin g sp eed , t h e t yp e
o f blan kin g p lat e p erfo rat io n wit h o r wit h o u t clearan ce an gle a [°] an d t h e req u ired q u alit y o f t h e cu t su rface
Fig. 4.5.12 . A larger blan kin g
281
Blanking processes
66 6
5 5
6 6
7 7 8 8 9 9 10 10 11 11 12 12 5 6 7
8 9
10 11
12
= −
= ⋅ + ⋅ + ⋅ + ⋅ + ⋅ + ⋅ + ⋅
+ ⋅ + + + + +
+ + .
x ame value as with s the same xis y
l b
l b
l b
l b l
b l
b l
b l
b l
l l
l l
l l
l y
S S
s a, a y a
S S
S R
y enter of gravity S the coordinate system
= ⋅ + ⋅ + ⋅
+ ⋅
+ ⋅ + ⋅ +
⋅ + ⋅ +
+ +
+ +
+ +
= ⇒
62 83 20 62 83 20
94 25 50 9 60 70 90 45 80 90
90 45 80 0 62 83 62 83 94 25 60
90 80 90 80
43 25 66 6 43 25
. .
. .
. .
. .
. ; .
C in
c.
Fig. 4.5.11
Example to determine the surface center of gravity S
R
:
a
part to be produced
b
determination using lengths of the perimeters
c
determination using line centers of gravity
y y
3
3
a
3
a
b
3
a
7 7
=
a
a
6
b
l =l
6
b
l
b
l
l l
b
b
l b
7
l
7
a b
a
6
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
clearan ce gen erally red u ces th e n ecessary force an d work req u irem en t, an d th u s also tool wear; with a sm aller clearan ce, in con trast, a q u alita-
tively im p roved cu t su rface an d greater p art accu racy are freq u en tly ach ieved . In con ven tion al blan kin g with blan kin g sp eed s in th e region
of 0.1 to 0.2 m s, th e op tim u m blan kin g clearan ce is between 2 an d 10 of th e sh eet m etal th ickn ess cf.
Fig. 4.7.6 , wh ereby th e lower valu e
ap p lies to th in n er or softer sh eet m etals. W h en p iercin g sh eet m et al, t h e d im en sio n o f t h e p ierced h o le is
d et erm in ed by t h e blan kin g d ie; t h e o p en in g in t h e blan kin g d ie m u st acco rd in gly be select ed t o be t wice t h e blan kin g clearan ce great er t h an
t h e blan kin g p u n ch . W h en blan kin g ext ern al co n t o u rs, by co n t rast , t h e fem ale d ie d et erm in es t h e size o f t h e blan ks. Th e p u n ch m u st acco rd -
in gly be co n figu red sm aller by t wice t h e blan kin g clearan ce.
Exa m ple:
A 3 m m t h ick m ed iu m -h ard st eel sh eet is t o be p ro cessed in a p ro gressive t o o l cf
. Sect . 4.1.1 . In it ially a h o le wit h a d iam et er o f 10 m m is t o be p ierced , aft er
wh ich t h e circu lar blan k wit h a d iam et er o f 28 m m ext ern al co n t o u r is t o be blan ked o u t o f t h e m et al st ri
p Fig. 4.5.13
: 1st st ro ke – p iercin g:
Th e p iercin g h o le p u n ch is given t h e n o m in al d im en sio n 10 m m . Th e fem ale blan kin g d ie h as t h e d iam et er
2n d st ro ke – blan kin g: Th e fem ale blan kin g d ie h as t h e wo rkp iece d im en sio n 28 m m d iam et er. Th e
blan kin g p u n ch h as t h e sm aller d iam et er:
282
Sheet metal forming and blanking
a a
1
u
Fig. 4.5.12 Dimensions for piercing and blanking on the punch and the
female die
d mm
mm mm mm mm
= + ⋅
⋅ =
+ =
10 2 3
6 100
10 0 36 10 36 .
.
d mm
mm mm
= =
28 0 36 27 64 – . .
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
283
Blanking processes
Fig. 4.5.13
Workpiece and die dimensions used in the manufacture of a pierced disk 28
10 10.36
10 0.18
0.18 27.64
28
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
4 Sheet metal forming and blanking
4.6 Shearing lines 4.6.1 Slitting lines
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