Altan T. Metal Forming Handbook

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Example:

You wish to bend a piece of 3 mm thick sheet metal made of St 37-2, material

number 1.0037, in accordance with Fig.4.8.4.The minimum tensile strength

for this material is R

= 360 N/mm

. Using Table 4.8.1 it is accordingly possible

to determine the smallest permissible bending radius at r

i min

= 3 mm. To cal-

culate the blank length of the sheet metal part, we require the sum of the leg

lengths and all the relevant compensation values v, using the appropriate cor-

rection factor k for each case.

Sum of the leg lengths:

371

Bending

abcd mm

rmmsmm

vmm

rmmsmm

+++=+++=

=°= =

=+⋅=

=⋅⋅+⋅

⋅+

(

)

=°= =

=+⋅

30607520185

90 5 3

065

0761

180

0761 253 635

45 3 3

065

for

, , :

. log .

.– –.

, , :

loglog . log . .

.– –.

, , :

. log .

.–

065

10650 065

135

180

065 233 263

135 10 3

065

0911

180

0911 2

=+=+=

=⋅⋅+⋅

⋅+

(

)

=°= =

=+⋅=

=⋅⋅+⋅

⋅

vmm

rmmsmm

for

10103

0251136626 225 184

635263184 1082

1 185108217418

123

(

)

⋅

=⋅ ⋅ ⋅ °=

=++

=++++= =

tan

. . – tan . –.

–.–.–. –.

–. .

π mm

vvvv

vmm

The total extended length adds up to:

abcdv mm

ges

The sum of the compensation values amounts to:

The result is rounded up to the next higher millimeter,

resulting in a final blank dimension of 175 mm.

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

Bending force and bending work during V-die bending

V-die bending is a production technique that is widely used. Here, bend-

ing takes place between the bending punch and die (Fig. 4.8.5). This is

generally followed by the application of pressure on the die, which

serves to relieve residual stresses (reducing springback).

When bending angles in the V-die, the bending force F

[N] required

for forming depends on the die width w [mm], as this determines the

bending moment. In contrast to this, the magnitude of the bending

radius plays only a minor role, provided that the corresponding die

width w has been correctly selected. Generally speaking, the dimension

l[mm] of the die is selected in accordance with the sheet metal thickness

s[mm]:

When using a conventional die in which the sheet metal is positioned

at a distance w on both sides while the punch presses centrally onto the

sheet metal, with a coil stock width of b

[mm] the following bending

force is obtained:

372

Sheet metal forming and blanking

Fig. 4.8.4 Bent workpiece (cf. example)

d=20

5°

a=30

lsmm=⋅

[]

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

These are simplified formulae which, however, are sufficient in prac-

tice for approximate calculations. The bending work W

[Nm] amounts

to:

if the path of the punch from impact with the sheet metal until the end

of the bending process is designated as h [m]. The constant value x

takes into account the uneven force progression along the path of the

punch and generally lies between x = 0.3 and 0.6, depending on bend-

ing requirements and machine setting.

Example:

If a 2 mm thick aluminium sheet AlMg5Mn measuring 50 mm wide with a min-

imum tensile strength of R

=280 N/mm

is bent in a V-die with a width of

w = 15mm using a punch displacement of h = 7mm, the resulting bending force

is:

When bending using a V-die, the process factor x = 1/3. The bending work thus

amounts to:

4.8.2Roll forming and variety of sections

Process

Roll forming is categorized according to DIN 8586 as a method of form-

ing by bending using a rotary die motion (cf.Fig.2.1.3). During roll

forming, the sheet coil or blank is formed in a stepless and continuous

373

Bending

bsR

Nws

bsR

Nws

⋅⋅

[]

≥

⋅

⋅⋅

[]

for

WxFhNm

=⋅⋅

[]

mm mm Nmm

⋅⋅

50 4 280

3733

W NmNmJ

=⋅ ⋅ = =

3733 0007 87 87,...

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

process to create profiles with the aid of rollers (Fig.4.8.6). One pair of

rollers is required per forming stage (Fig.4.8.7). The advantage of the

shaping process, compared to other bending methods, is that sheet strips

of any length can be formed at a relatively high speed.

Basic section geometries

Sections are manufactured in steps using a combination of simple basic

roll forms:

–Rolling:The edge of a profile is shaped by more than 180°. Three dif-

ferent configurations are most commonly used (Fig.4.8.8).

374

Sheet metal forming and blanking

Fig. 4.8.5 Bending in the V-die

die

punch

workpiece

Fig. 4.8.6

Pair of forming rolls:

last forming stage

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

–Folding:step-by-step bending of straight legs around a common point

(Fig.4.8.9).

–Beading: recesses in the profile, e.g. for reinforcement (Fig.4.8.10).

–Doubling:The material is doubled back on itself, for example at the

edges (Fig. 4.8.11).

375

Bending

Fig. 4.8.8 Rolling:aeye: circular shape with only one radius;boval: circular shape, made

up of several radii;cloop: material not entirely bent over

ab c

Fig. 4.8.7 Roll forming: roll bending in several stages

Fig. 4.8.9Folding

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

Application examples

By combining the basic shapes, it is possible to manufacture any option-

al section(Fig. 4.8.12to 4.8.17).Figure 4.8.18illustrates the step-by-step

manufacture of a special section geometry.

Roll bending radius

During the roll bending process, the actual bending radius must be

between the maximum and the minimum admissible bending radius:

with the modulus of elasticity E [N/mm

], the yield strength R

or R

p0,2

[N/mm

] (cf.Sect.4.2.2) and the material coefficient c [–] in accordance

with Table 4.8.3.

376

Sheet metal forming and blanking

Fig. 4.8.10 Beading

Fig. 4.8.11 Doubling

Fig. 4.8.12 U/C and rail sections

rsc

max

min

⋅

[]

=⋅

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

377

Bending

Fig. 4.8.13 Hat sections

Fig. 4.8.14 Angular, L and Z sections

Fig. 4.8.15

T and omega sections

Fig. 4.8.17

Wide strip sections

Fig. 4.8.16 Closed sections

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

Example:

When roll forming with 2 mm thick strain hardened sheet steel in St 37-2

(material number 1.0037) with the modulus of elasticity E = 2.1 · 10

N/mm

for

steel and yield strength R

= 220…240 N/mm

the useful radius range is calcu-

lated as follows:

The actual radius when bending at right angles to the rolling direction should

thus be at least 1 mm in size.

378

Sheet metal forming and blanking

mm N mm

Nmm

mm mm

rmm mm

max

min

⋅⋅

⋅

=⋅=

22110

2 240

210 000

240

875

2051

Fig. 4.8.18

Step-by-step manufacture of

a section using 20 rolls

(flower diagram)

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

Roll forming lines

Roll forming lines are classified according to two different operating

modes. Start-stop lines consist in their simplest form of a decoiler, a coil

loop, a roll forming machine and a parting die. The product length is

determined by the roll forming machine (Fig. 4.8.19).Due to the use of

the start-stop method, blanking and embossing operations are also pos-

sible using the roll forming machine's positioning system.

Continuous running roll forming linesoperate at a constant speed. This

type of line is used to process sheet metal blanks or strips to create long

sections. While on blank processing lines cut-to-length blank metal

379

Bending

Table 4.8.3: Material coefficient c for the calculation of r

i min

Material coefficient c

Material

Bending direction

compared to rolling

direction of sheet

Material soft

annealed

Material strain

hardened

Al transverse 0.01 0.30

longitudinal 0.30 0.80

Cu transverse 0.01 1.00

longitudinal 0.30 2.00

Ms 67, Ms 72,

transverse 0.01 0.40

CuZn 37 longitudinal 0.30 0.80

St 13

transverse 0.01 0.40

longitudinal 0.40 0.80

C 15 – C 25

transverse 0.10 0.50

St 37 – St 42 longitudinal 0.50 1.00

C 35 – C 45

transverse 0.30 0.80

St 50 – St 70 longitudinal 0.80 1.50

Fig. 4.8.19 Start-stop roll forming line

decoiler with coil

infeed unit coil loop

roll forming machinestraightening machine outfeed

roller conveyo

parting device

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

sheets are processed, when using a sheet strip processing line the pro-

duced profile is parted after roll forming using automatic cut-to-length

devices.

The advantage of a blank processing line is that the complex pro-

file parting die can be substituted by a simple cross-cutting shear. This

eliminates the need for costly die inserts, a particularly important con-

sideration where different profile shapes or widths are being processed.

However, a further point of consideration is that this method requires a

certain minimum sheet length, as the feed force of the rollers must

always be greater than the torque necessary for the forming process

which has to be overcome. Furthermore, adequate provision must be

made to ensure efficient blank guidance.

The benefit of a continuously operating line producing off the coil is

its higher output when processing greater profile lengths. This produc-

tion method is essential if continuously running processes such as lon-

gitudinal seam welding, gluing, foaming, etc. have to be integrated dur-

ing profiling, or where with respect to the machine size excessive mass-

es would have to be accelerated.

In practice, the basic forms of this processing technique are fre-

quently upgraded, either due to insufficient output or because addi-

tional punched patterns have to be taken into consideration. In this

case, double loop lines with a separate punching device are used.

To keep set-up times to a minimum, roll forming lines are frequently

equipped with dual swivel-mounted decoilers capable of accommodat-

ing two coils and coil end welding machines. The coil end welding

machines join the end of the previous coil to the start of the next one,

saving not only the time required for feeding in a new coil but also

reducing the amount of scrap generated at the start and end of each coil.

Die changeover techniques with roll stand sets and rolling dies

To allow different section shapes to be manufactured on a single pro-

duction line, various roll changeover techniques are available. The

rolling dies are mounted on rigid stands or on stands with telescopic

roll set or directly mounted on the gear blocks (Fig.4.8.20).The stands

are connected to the gear units of the roll forming machine by means

of universal joint driven cardan shafts.

Width adjustable roll setspermit the manufacture of products with dif-

ferent web widths and leg heights using a common roll set (Fig. 4.8.21).

380

Sheet metal forming and blanking

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