Altan T. Metal Forming Handbook

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The gib geometry comprises a high-precision eight-track sliding gib.

This construction is able to largely eliminate slide tilting even under

major eccentric loads.

Efficient hydraulic fine blanking presses with a total force ranging

between 2,500 and 14,000 kN are equipped with a pressure accumulator

drive. The counterpressure piston is integrated in the slide, and the vee-

ring piston is mounted in the press crown. Both pistons are equipped

with support bolts, in order to provide support over the entire surface of

the dies. Hydraulic fine blanking presses are characterized by:

–application of force independent of slide position,

–processing of material thicknesses up to 12mm,

–integration of forming operations into compound progressive and

transfer dies,

–a high degree of operational reliability in the work process.

361

Fine blanking

Fig. 4.7.27 Layout of a mechanical fine blanking press

vee-ring

piston

dieheight

adjustment

lubricationsystem

infeedroll

counterforce

piston

centralsupport

punchtightening

rod(formoving

punchsystem)

double

knuckle-

joint

wormgear

drive

slide

outfeedrollwithscrap

webshear

croppingshear

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Due to the design principle of the fine blanking dies, not only the blank

itself but also all trimmings and inner form slugs are ejected or stripped

of the die into the die area. The workpieces and scrap are then removed

from the die mounting area by means of air jets or removal arms. If

there are parts still remaining in the die area before the next blanking

cycle, the slide movement must be halted in time to avoid damage to

the die. For this purpose, fine blanking presses are equipped with tool

breakage safety systems.Systems used include die stroke-dependent sen-

sors, die-independent sensors in mechanical presses and die-indepen-

dent pressure sensing in hydraulic presses.

Die-independent pressure sensing offers the greatest degree of safety,

but can only be used in hydraulic presses (Fig.4.7.29).The system works

362

Sheet metal forming and blanking

Fig. 4.7.28Layout of a hydraulic fine blanking press

vee-ringpiston

diechange

clampingtable

centralsupport

cropping

shear

outfeed

roll

counterforcepiston

slidegib

actuatingmotor

fixedstop

mainworkingpiston

slidegib

feedheightadjustment

infeedroll

infeedrollbasket

lubricationsystem

rapidtraverseclosingpiston

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

measuring the pressure in the rapid traverse closing cylinders. Pressure is

applied to the rapid traverse closing cylinders from the low pressure

system in such a way that the weight of the slide and the bottom die

can just be raised at tracing speed (Fig.4.7.4).If the operation is unin-

terrupted, the slide switch activates the high-pressure accumulator, and

the main piston initiates the blanking movement.

If there are foreign objects located in the die, the contact between the

foreign object and sheet metal or upper die initiates an increase of pres-

sure in the rapid traverse closing cylinder before the slide switch is

reached. The pressure increase – a maximum of 2 bar – is detected by a

pressure switch, causing the press control system to interrupt the clos-

ing movement immediately. This safety system offers the following

advantages over other systems:

–structurally cost-effective solution,

–monitoring of the entire die space,

–no moving parts in the die space,

–tracing speed and pressure can be set depending on part sensitivity.

363

Fine blanking

Fig. 4.7.29 Die-independent pressure tracing

)(

foreign object

bottom die

slide switch resp.

position encoder in

numerically controlled

presses

pressure switch

tracing speed controller

tracing pressure

controller

rapid traverse closing piston

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Examples of production lines

Figure 4.7.30shows a CNC-controlled hydraulic fine blanking press with

a total press force of 4,000 kN. Its maximum part-dependent stroking

rate is 50/min.

Figure 4.7.31illustrates an automatic production line for the manu-

facture of synchronous tapered plates in three stages. The nerve center

of the line, which produces some 4,500 parts per shift, is a CNC-con-

trolled hydraulic fine blanking press with a total force of 14,000 kN.

The complete line can be operated by a single person.

The blanks produced on a blanking press are stacked on pallets and

then deposited on the height-adjustable pallet platform. From here, the

364

Sheet metal forming and blanking

Fig. 4.7.30 Fine blanking press with a nominal press force of 4,000 kN

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

pallets reach the stacking feeder. An accumulating roller conveyor trans-

ports the pre-perforated blanks to the indexed press transfer unit which

transports them through three die stations. On the outfeed side of the

press, the finished synchronous tapered plates are fed via a buffer con-

veyor towards the stacking unit and the scrap is disposed of separately.

The loaded pallets then travel via an accumulating roller conveyor

towards the height-adjustable removal point.

Outside the press, the dies are prepared on a die change cart (cf.

Fig. 3.4.2).The three individual die modules are mounted on a common

die change plate. This system permits automatic die change to be com-

pleted in a matter of minutes.

365

Fine blanking

Fig. 4.7.31 Overall line for the manufacture of synchronous tapered cups with a 14,000 kN

CNC controlled fine blanking press

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

4Sheet metal forming and blanking

4.8Bending

4.8.1Bending process

Classification of bending processes

Bending by forming as a production process is subdivided according to

DIN 8586 into two groups: bending by forming using linear and rotat-

ing die motions (cf. Fig. 2.1.3).Sections 4.8.2 and 4.8.3 deal with bend-

ing using rotary die motion, i.e. roll forming and roll straightening.

These fall under the category of roll bending.

Bending radius and bending angle

Bending dies should be designed so as to avoid sharp bent edges. The

inside bending radius r

[mm] depends on the sheet metal thickness

s [mm] and should be selected to be as large as possible, because sharp

bent edges may lead to material failure. On principle, the bending

radius should assume the values recommended by DIN 6935, i.e. they

should be selected from the following series (preferably using the values

in bold type):

11,2 1,62 2,53 45 68 1012 1620 2528 3236 4045

5063 80100 etc.

When bending sheet metal, particular attention should also be paid to

the rolling direction. The most suitable direction for bending is trans-

verse to the direction of rolling. If the bending axis is positioned paral-

lel to the rolling direction of the sheet metal, r

imin

must be selected

higher than when bending at right angles to the direction of rolling.

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Table 4.8.1 gives the smallest admissible bending radii for bending

angle aup to a maximum of 120°. For bending angles a> 120°, the next

highest value applies: When bending Q

42-2 sheet steel materials with

a thickness s = 6 mm at right angles to the rolling direction, for exam-

ple, the smallest admissible bending radius r = 10 mm for a#120°and

r = 12 mm for a> 120°.

Springback

When designing a bending die, it is necessary to consider springback that

occurs after unloading. Springback characteristics differ depending on

the material type. Springback occurs with all types of forming by bend-

ing, when bending in presses, folding, roll forming and roll bending.

As a result of springback, the bending die angle adoes not corre-

spond precisely to the angle desired at the workpiece a

(Fig. 4.8.1). The

angle ratio is the so-called springback factor k

, which depends on the

material characteristics and the ratio between the bending radius and

sheet metal thickness (r/s):

With a

: angle at the die (required bending angle) [°],

: desired angle at the workpiece (after springback) [°],

s: sheet metal thickness [mm],

: inside radius at the die [mm],

: inside radius at the workpiece [mm].

The springback factor k

for various materials is given in Table 4.8.2.

367

Bending

Fig. 4.8.1

Elastic recovery after bending:

s sheet metal thickness, a

required

bending angle, a

desired angle,

inside radius of die, r

inside

radius of workpiece

+⋅

[]

–,

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

368

Sheet metal forming and blanking

Table 4.8.1: Minimum bending radius r

i min

for bending angles

under 120°

Bending

direction

compared to

roller

direction of

sheet

> 1

to 1,5

> 2,5

to 3

> 3 to 4 > 4 to 5 > 5 to 6 > 6 to 7

to 390

transverse 1 1.6 2.5 3 5 6 8

longitudinal 1 1.6 2.5 3 6 8 10

over 390

transverse 1.2 2 3 4 5 8 10

to 490

longitudinal 1.2 2 3 4 6 10 12

over 400

transverse 1.6 2.5 4 5 6 8 10

to 640

longitudinal 1.6 2.5 4 5 8 10 12

> 7 to 8 > 8 to 10

> 10

> 12 to 14 > 14 to 16 > 16 to 18 >18 to 20

to 390

transverse 12 16 20 25 28 36 40

longitudinal 16 20 25 28 32 40 45

over 390

transverse 16 20 25 28 32 40 45

to 490

longitudinal 20 25 32 36 40 45 50

over 400

transverse 16 20 25 32 36 45 50

to 640

longitudinal 20 25 32 36 40 50 63

Smallest admissible bending radius r

i min

for sheet metal thickness s [mm]

Smallest admissible bending radius r

i min

for sheet metal thickness s [mm]

> 1,5

to 2,5

Steel sorts with a

minimum tensile

strength [N/mm

]

Steel sorts with a

minimum tensile

strength [N/mm

]

Bending direc-

tion compared

to roller direc-

tion of sheet

Table 4.8.2: Springback factor k

Material

Springback factor k

St 0-24, St 1-24 0.99

St 2-24, St 12 0.99

St 3-24, St 13 0.985

St 4-24, St 14 0.985

stainless austenitic steels 0.96

high temperature ferritic steels 0.99

high temperature austenitic steels 0.982

nickel w 0.99

Al 99 5 F 7 0.99

Al Mg 1 F 13 0.98

Al Mg Mn F 18 0.985

Al Cu Mg 2 F 43 0.91

Al Zn Mg Cu 1.5 F 49 0.935

/s = 1

0.97

0.96

0.92

0.97

0.955

0.96

0.98

0.90

0.935

0.65

0.85

/s = 10

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Accordingly, the necessary angle on the die is

The required inside radius at the die can thus be calculated as

with tensile strength R

[N/mm

] and elasticity module E [N/mm

Bending causes residual stresses in the workpiece. The smaller the

bending radius relative to the sheet metal thickness, the greater these

stresses are. When working with materials which are sensitive to stress-

corrosion, workpiece failure is therefore possible within a relatively short

period of time after forming. Coining following the bending process

helps to reduce residual stresses. When a subsequent heat treatment is

used to reduce residual stresses in the workpiece, it is important to

remember that heat treatment alters the workpiece radii and the angles.

Unwanted deformation during bending operations

If a sheet metal strip has a rectangular cross section, the sides of the rec-

tangle are determined by the coil width and the material thickness. In

the case of thick coil stock and sharp bends, i.e. small inside curvature

radius, this rectangle assumes a trapezoidal shape (Fig.4.8.2).

If several operations are executed on a single workpiece, i.e. several

folding processes are carried out simultaneously, steps must be taken to

ensure that sufficient material flow is available to replace the material

displaced during the forming process. Otherwise, under certain circum-

stances, it is possible that significant weakening or fracturing takes place

at the workpiece corners. In addition, the force required to achieve the

final shape increases.

369

Bending

⋅

[]

Fig. 4.8.2

Deformation of the cross section during bending

=°

[]

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Determining the blank length for bent workpieces

The blank length of the part to be bent is not equal to the fiber length

located at the center of the cross section after bending. The extended

length of bent components 1 [mm] is calculated as

whereby a [mm] and b [mm] stand for the lengths of the two bent legs,

and v [mm] is a compensation factor which can be either positive or

negative (Fig.4.8.3).

It should be noted that ais the bending and bthe opening angle of

the bent part. Values for v are contained in DIN 6935 or can be calcu-

lated for every required angle:

– for b= 0°to 90°

– for b> 90°to 165°

with correction factor k [–]:

The calculated extended length dimensions should be rounded up to

the next higher full millimeter.

370

Sheet metal forming and blanking

– for/s5and

– f r/st5

or o

=+⋅

065

. log

Fig. 4.8.3

Geometry of bent legs

l a bv oropeningangles o nd

l ab oropeningangles o

=++

[]

°°

[]

>° °

f 0t165a

f 165t 180

krsmm

=⋅













⋅+⋅ ⋅+

()

[]

β180

180 2

–













krs mm

=⋅

























⋅+⋅ ⋅+

()

⋅

[]

ββ180

180 2

180

–

– tan

–

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998