Altan T. Metal Forming Handbook

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The roll stand for profile roll changeoveris used for the manufacture of

compact sections with fixed dimensions. However, if spacers are manu-

ally inserted into each stand, it is also possible to manufacture sections

with different widths (Fig. 4.8.22). Both stand types – rigid and with

telescopic roll set – permit the rolling dies to be exchanged by detach-

ing the front stand upright. However, this involves long set-up times.

Drive shaft changeoveroffers a compromise between exchanging indi-

vidual rollers and exchange of a complete stand. At a special work sta-

tion equipped with a suitable crane harness, the upper and lower drive

shafts are exchanged together (Fig. 4.8.23).This helps to avoid set-up

errors and saves the need for a second stand set. The time required for

this changeover process lies somewhere between that required for

exchanging rollers and that required when using pallets for the stands.

381

Bending

Fig. 4.8.20

Roll stands

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

It is also possible to use a separate set of stands for each different sec-

tion. This permits not only fast resetting but also a high degree of process

reliability and repeat accuracy(Fig.4.8.24).For stand changeoverthere are

two different changeover techniques possible:

–Exchange of individual pallets using a crane: Pallet changeover is

advisable where short set-up times are required. The roll forming

382

Sheet metal forming and blanking

Fig. 4.8.21 Width adjustable telescopic roll set

gearbox cardanshaft telescopicshaft

widthadjustment

formingroll

frame

Fig. 4.8.22 Roll stand for exchange of section rolls

gear box cardan shaft forming roll shaft

width adjustmentframe

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

stands, for example three or four, are mounted in a fixed setting on

pallets together with the rolls. The pallets are raised using a crane. For

fastening and centering, mechanical or hydraulic systems are used

depending on the level of automation.

–Exchange of individual pallets or complete sets of rolls and stand

bearings: For fully automatic resetting, the tool pallets are exchanged

using a rack handling system. This method is more economical

where frequent resettings are required.

4.8.3Roller straightening

Straightening is required wherever plastically deformed sheet metals,

bars, wires or pipes have to be straightened. A typical application is the

straightening of sheet metal in a straightening machine for further pro-

cessing in a forming press after being taken off the coil (cf. Sect. 4.3).

383

Bending

Fig. 4.8.23 Work shaft exchange

frame

forming roll work shaft

width adjustment

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

In accordance with DIN 8586, roller straightening is classified as a

bending process using rotating tool motion (cf. Fig.2.1.3).Bending

back unwanted curvature by means of a precisely defined bending

process which must be exactly coordinated with the starting radius has

proven to be impossible in practice. Instead, the technique used

straightens the metal by imparting alternating bending operations

using gradually increased bending radii. This technique is known as

roller straightening (Fig. 4.8.25).

Bending and counter bending must each take place within the plastic

range of the material in order to ensure that the bending direction

is retained following elastic recovery, i.e. the yield strength of the mate-

rial must be exceeded. On the other hand, care must be taken to ensure

that the straightened material does not sustain damage. If bending

is too pronounced, brittle materials can develop slight cracks at the

surface. The last roller must be set in such a way that it causes the

bending process which generates a flat sheet following elastic recovery

(Fig. 4.8.26).

As a result of alternate bending, the residual stress in the sheet mate-

rial is reduced as the number of bending processes and the bending

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

radii increase. The reduction of residual stress is highly beneficial for

further processing (Fig.4.8.27).The larger the number of rollers used,

the lower is the residual stress in the sheet metal after straightening.

In all roller straightening machines, there are two rows of straighten-

ing rollers arranged in offset formation relative to each other. These are

called the upper and lower rollers. The distance between roller centers is

known as the roller pitch t

and varies in different machines, as does the

roller radius R

and the number of straightening rollers. Straightening

machines with a minimum of five rollers are used. Five straightening

rollers are sufficient to straighten thick plates or coils, or where no special

demands are made on straightening accuracy. Where thinner materials

or more stringent straightening requirements are involved, for example

385

Bending

Fig. 4.8.25 Roller straightening with 13 rollers

<<<

Fig. 4.8.26

Alternate

bending

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

for fine blanking, machines with up to 21 rollers are used. The feed

value z

for the rollers is limited by the geometry of the straightening

machine(Fig. 4.8.28):

with the roller radius R

[mm], sheet metal thickness s [mm] and roller

pitch t

[mm].

In addition to the limits imposed by the roller radius and pitch, the

material properties also have to be taken into account; when processing

brittle aluminium, for instance, in order to avoid cracking the material

it is important not to select the smallest bending radius.

386

Sheet metal forming and blanking

plastic alteration of length

COMPRESSION

plastic alteration of length

COMPRESSION

plastic alteration of length

TENSION

plastic alteration of length

TENSION

1. bend

2. bend

3. bend

4. bend

sheet metal

thickness

Fig. 4.8.27

Residual

stresses in

the sheet

metal

zRsRstmm

ww wwmax

––=⋅+ ⋅+

()

[]

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

Example:

Sheet steel St 37 (material number 1.0037) with a thickness of 2 mm has to be

straightened. The straightening roller pitch of the straightening machine is t

22 mm, the roller diameter R

= 20 mm. The maximum feed value is calculated as

Roller straightening machines

Generally speaking, roller straightening machines for straightening sheet

metal comprise an upper and a lower row of straightening rollers with

the same diameter. The two rows are generally arranged horizontally. The

rollers in the top row are generally arranged precisely above the gaps

between the rollers of the bottom row.

To allow modification of the rolling gap, the upper rollers must be

adjustable in height. The type of adjustment characterizes the straight-

ening machine type. While in one case each upper roller can be sepa-

rately height adjustable, in the other type of machine all the upper

rollers are grouped together in a block to form an assembly, and are fed

towards the lower rollers by a tilting motion of the block.

The thickness range, the yield strength and the modulus of elasticity

of the coil stock to be processed determine the diameter and pitch of

the straightening rollers. In the case of materials with particularly sen-

sitive surfaces, intermediate rollers embedded between the straighten-

387

Bending

Wmax

Fig. 4.8.28 Straightening roller geometry

zmm

w max

––.=⋅ + ⋅ +

(

)

=220 2 220 2 22 62

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

ing and support rollers are used in order to avoid damaging the sheet

surface. With the help of straightening rollers with crowning capabili-

ty, waviness at the edges and center of the material can be eliminated.

The first pair of rollers must cause plastic deformation of the material,

but a certain roller diameter must not be exceeded. The thickness of a

straightenable sheet metal is accordingly calculated from the maximum

straightening force and the torque, which in turn is determined by the

size of the cardan shafts between the gear and rollers and the roller

diameter. As neither the gear nor the cardan shafts can be enlarged at

will for space reasons, the sheet thickness range and accordingly also

the flexibility of straightening machines with rollers of equal size are

restricted. This range can be extended by approximately one and half

times if individual rollers can be disengaged. Using the further devel-

oped roller system depicted in Fig. 4.8.29with different roller diame-

ters, it is possible to straighten sheet metals from 0.5 to 20 mm in thick-

ness.

388

Sheet metal forming and blanking

Fig. 4.8.29 Roller straightening machine for larger sheet metal thickness range

feedrolls

heightdisplay

liftcylinder

feedspindle

straightening

rollerswith

different

diameters

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

4Sheet metal forming and blanking

4.9Organization of stamping plants

4.9.1Design

Sheet metal stamping plants are among the most capital-intensive of all

manufacturing installations in industry. The high degree of investment

necessary and long-term commitment to selected manufacturing meth-

ods mean that careful investment and production planning are essen-

tial. In the production of sheet metal parts using deep drawing, stretch

drawing and blanking processes, major criteria include part accuracy,

die life, line output and its availability. The implementation of mea-

sures for continuous in-process quality assurance using suitable sensor

technology allows objective assessment of part quality (cf. Sect. 4.9.3).

Stamping plants generally comprise different sheet metal process-

ing equipment, each of which manufactures a certain range of parts

(Fig. 4.9.1).The aim of efficient stamping plant layout is to ensure the

most economical operation possible. The selection of the individual

equipment depends primarily on the size, shape and metal forming

requirements imposed on the components, as well as the planned

batch sizes.

The sheet metal is supplied from the rolling mill in the form of coil.

When producing medium-sized to large parts such as bathtubs, sinks,

car doors or roofs, a separate blanking line cuts the blanks from the coil

stock and stacks them (cf. Fig.4.6.5).The blanks are destacked by the

blankloader (cf. Fig. 4.4.15and 4.4.16)and fed individually and auto-

matically into the press where they are formed. Due to its high capacity,

a blanking line is able to service several press lines (cf. Fig. 4.4.19)or

large-panel transfer presses (cf. Fig. 4.4.27and 4.4.38).

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

390

Sheet metal forming and blanking

crossbar transfer press

tri-axis transfer press

press line

try-out center

storage of dies

storage of

finished parts

transfer

simulators

transfer press

for small and

medium-sized

parts

coils

blanking line

blank stack

turnover device

Fig. 4.9.1 Stamping plant for the manufacture of large sheet metal parts

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