Altan T. Metal Forming Handbook

ver’s cab. Both operations are generally executed in a single line using

two die sets.

When manufacturing side members, two different methods are used

in conjunction with different types of production lines. These are dis-

tinguished from each other already in the blank feed stage.

Side members with a length of up to 7 m are generally manufactured

from plates, which have a maximum thickness of 4mm. The plates are

fed into the press on an intermittent basis transversely to the longitu-

dinal axis of the press. With each stroke the blanking die in the press

punches a contoured blank from the plate. The blank is then bent in

the next station to become a side member.

To produce side members with a length ranging from 4 to 13m,

blankswith a thickness of 5 to 10 mm are available. The blanks are

generally cross-cut directly off the coil to the contoured blank width

(cf.Sect.4.6.1) in order to ensure optimum material utilization. The

camber shaped blanks produced using this method have to be correct-

ed in a separate unit by straightening. The straightened blanks are fed

individually into the longitudinal axis of the press and processed to cre-

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Sheet metal forming lines

Fig. 4.4.12Side members used in truck production

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

ate a contoured blank. This is done either by an all round blanking

stroke or by local edge blanking – generally at both ends of the side

member.

The advantage of processing sheet plates is that the blanks do not

assume a camber shape due to the all round blanking cut used in their

manufacture. Thus, the straightening process can be omitted. However,

in this case a lower degree of material utilization is achieved. In the case

of complex side member geometries, involving different flange and

web heights, however, it is possible to save on material input by nesting

the contoured blanks (cf. Fig.4.5.2to 4.5.5).The blanking cut leaves

behind scrap webs which are broken down into scrap segments.

Production line concepts

There are two basic line types used to produce side members. These dif-

fer in principle, based on the infeed and outfeed direction of the sheet

metal: Whereas sheet plates below 7m in length can still be fed into the

press through the press uprights transversely to the longitudinal press

axis, longer untreated blanks have to be fed parallel to the longitudinal

press axis.

In lines wheretransport takes place parallel to the longitudinal press axis,

the blanks are fed by means of an infeed conveyor belt inside the press

uprights (Fig.4.4.13). Cross-feed units simultaneously feed the uncut

blank into the die and the previously contoured blank out of the die

onto the reverse outfeed conveyor. Transport systems making use of

magnets are used to stack the contoured blanks emerging from the out-

feed conveyor. Integrated in the line is also a blank turning station. The

lines are designed in such a way that both, contoured blanks punched

from uncut blanks and side members formed from contoured blanks,

can be manufactured.

Important accessories for use with fully automatic lines include a

blank lubricating or spraying device at the press infeed, a hole pattern

camera to detect punch breakage and a device for edge milling. Fully

automatic lines achieve cycle times of 3 strokes per minute. Taking into

account the need for two passes and without considering the set-up

time of around 15 min required for the change from blanking to bend-

ing, this results in an average line output of some 1.5 side members per

minute. Die changeover also takes place in the longitudinal axis of the

press.

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

In the case of presses with material transport transversely to the longitu-

dinal press axis, the sheet plates are guided between the press uprights

by means of a chain conveyor and a gripper feed device in synchro-

nization with the press cycle. After each stroke, the remaining plate

pushes the newly punched contoured blank onto the reverse outfeed

conveyor belt. When an operation has been completed, the waste strip

of the old plate must be disposed of together with the blanking waste

from the new plate.

The equipment used for destacking the bales of plates or contoured

blanks and the other elements of the line are the same as those used in

lines transporting the material in the direction of the longitudinal press

axis.

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Sheet metal forming lines

Fig. 4.4.13 Layout of a line for the manufacture of truck side members

= 50,000 kN, blanking shock damping and stroke limitation, 11 bed cushions,

25 slide ejectors, maximum off-center load: 25,000 kNm with electronic control

of parallelity)

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

Die concept

The blanking and bending die each comprise a basic die equipped with

trimming and piercing modules in accordance with the various side

member lengths. Side members are structured in such a way that cer-

tain groups of holes, for example those used to fasten the front and rear

suspension mounting, are identical and only change relative to the

wheel base. In the area of the driver’s cab at the front end of the side

member, the contour and hole patterns are identical. The modules are

accordingly adjusted and positioned according to the requirements of

each individual side member type:

– front module for the area of the driver’s cab,

– main module, for example for the rear axle area,

– intermediate module to bridge different side member lengths and

– the end module.

If at all possible, right and left-hand side members should have the same

hole pattern. Only then it is possible to minimize the effort required for

drawing and mounting piercing hole punches when changing set-up

from right-hand to left-hand side members. The shearing edges required

for blanking the side member contour do not have to be reset, as for

example the left-hand contoured blank is produced from the right-hand

contoured blank by simply rotating it in the turning station. However,

for the bending process, bending dies, forming edges and ejector beams

in the asymmetrical areas must be changed for a new set-up.

As the position of the front module does not change, if the length of

the side members decreases, the position of the workpieces in the

blanking and bending dies becomes increasingly off-center. If a single-

sided coping cut is to be carried out at the same time, e.g. for example

when trimming the front, an additional off-center load is applied. The

off-center forces must be accounted for, either by a balancing module or

by providing a suitable press concept. Otherwise an unacceptable

amount of slide tilting action will occur.

To reduce the blanking force, the edges of the blanking die and

punch are configured in a saw-tooth arrangement and the piercing hole

punches are staggered on several blanking levels. The aim is to achieve

uniform loading over the entire length of the workpiece at every blank-

ing plane.

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

During the bending process, the web surface of the side member is

supported above the bed cushion by means of a support beam. By vary-

ing the bed cushion pressure levels, it is possible to influence the flat-

ness of the web surface, and in particular the rectangularity of the ribs.

A stripping device in the die ensures controlled stripping of the angled

side members from the bending punch. Parallel ejection is achieved

using the support beams.

Segmented bed cushion

As a result of the variable side member length, the cushions at the end

zones of the side members are only partially in use. A tilting moment

occurs here which must be balanced by means of stable gibs.

If all bed cushions are activated at the same time during the upstroke

of the slide for ejection, the cushions which have the smallest load reach

the top dead center first. However, the side members must be ejected in

parallel formation in order to avoid damage. There are two alternatives

here: to eject the side member while maintaining continuous contact

with the top die during the upstroke of the slide, or to control the syn-

chronous ejection speed of all the cushions. If side members are formed

with right-angled bends, different cushion strokes are required.

Control of parallelity and cylinder deactivation

The pistons of the cylinders that control parallelity are located at the

outside of the slide. During the working stroke, they operate against the

mean pressure of the servo valves (cf. Fig. 3.3.5). The off-center loading

of the dies generates a tilting moment on the slide. In the event of a

deviation from parallelity, the pressure is increased in the control cylin-

der that is on the loading side while pressure on the opposite side is

reduced. A supporting moment occurs which acts opposite to the tilt-

ing moment. In the case of side member tools, the off-center loading of

the slide can be large due to the arrangement of the individual mod-

ules. In some cases, tilting of the slide can no longer be prevented by

control of parallelity at a reasonable cost. In such cases, the control of

parallelism is supported by the selective deactivation of individual slide

cylinders (Fig. 4.4.14). As a result, the center of application of the press

force shifts in the direction of the forces generated in the dies. The larg-

er lever arms of the parallelism control force help to increase the

moment acting opposite to the off-center loading. However, deactiva-

tion of individual cylinders reduces the maximum available die force.

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216

Sheet metal forming and blanking

Fig. 4.4.14 Tilt and parallelity control moments in a press for forming side members

b a

slide cylinder

parallelity

control

cylinder

blanking

shock

damping

center of loading

controllable

moment

controllable

moment

= 5 x F – F

PR1

– F

PR2

PR1

PR2

long side members

short side members

deactivated cylinders

PR2

3/5 F

PR2

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

4.4.4Destackers and blank turnover stations

Destackers

Destackers separate the blanks and feed them into the press in syn-

chronization with the press cycle. Where low stroking rates are

involved, the destacking and feeding operation is performed by feeders.

In the case of higher stroking rates, after destacking the blank is trans-

ferred onto magnetic belts which transport them into the centering sta-

tion, for example the pick up position of the press transfer.

Destackers with feeder and suction cup tooling

In the case of press lines (cf. Sect. 4.4.5) with a stroking rate of about 10

to 12 parts per minute and where suitable blank shapes are being

processed, feeder systems are used. In case of a die change, the entire

tooling is replaced together with the attached suction cups. During

destacking, the tooling suction cups pick up the top blank from a stack.

The operation is supported by fanning magnets whose magnetic field

fans the top-most blanks in order to prevent the blanks from sticking to

each other. The magnets can be positioned manually or automatically

near the stack. Both permanent fanning magnets and electrically pul-

sating magnets are used.

When processing non-magnetic materials, e.g. blanks from alumini-

um or stainless steel, only suction cups can be used. In this case, com-

pressed air is blown in from the side of the stack to enhance the sepa-

ration of the individual blanks.

Destackers with magnetic belts

This design is equipped with one or two cylinder-actuated destacking

units provided with vacuum cups or magnets (Fig.4.4.15). These are

lowered between the magnet belts on the top-most blank of the respec-

tive stack, and then raise and transfer the suspended blank to the mag-

netic belt conveyor. Here, too, fanning magnets are used to support the

removal of individual blanks. Using these systems, up to 25 relatively

small and up to 15 larger parts per minute can be fed into the press.

To ensure that the press is able to operate continuously when chang-

ing from one stack to the next, either two destacking stations or systems

with a reserve magazine are used. In the case of dual stacking feeders,

switchover from the finished to the new stack takes place automatically.

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

Another possibility is to work with a reserve magazine. Here, the

remainder of the stack is supported and destacked off the reserve forks

while a new stack is added to it from underneath. Double blanks are

detected electronically and automatically separated out of the system

without bringing the entire line to a standstill.

In the case of non-magnetic materials, an integrated feeder with suc-

tion cups is used. Here, after destacking, the top-most blank is positioned

from above onto a conveyor belt. A lower cycle rate is expected when

using this type of handling system.

Another configuration used to transport medium-sized parts

involves the use of magnetic belts mounted on a lifting bridge, which

are lowered onto the stack of blanks. Thus, the top-most blank of the

stack is picked up. This type of system is driven by means of a crank

mechanism (Fig. 4.4.16).The maximum achievable cycle rate with this

method is about 16 parts per minute.

Prior to the forming operation, frequently the blanks must be

washed, lubricated, centered and oriented. These operations are inde-

pendent of the selected destacking method.

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Sheet metal forming and blanking

Fig. 4.4.15 Destacker of a large-panel transfer press for maximum blank dimensions of

3,800 31,800mm with a destacking bridge

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

Washing

To ensure the maximum possible part quality, the blanks – particularly

those used to produce visible parts such as doors and roofs etc. – are

washed and cleaned prior to forming. The washing/cleaning process is

performed using cleaning emulsions or oils in conjunction with clean-

ing brushes. The blanks are fed towards the washing machine through a

conveyor belt (Fig.4.4.22). The blank is mechanically washed in the

washing/cleaning machine by a pair of roller brushes. During this

process, the washing medium is continuously applied to the blank by

means of nozzles. Downstream from the pair of roller brushes is a pair of

squeezing rollers made of non-woven fabric. These reduce the cleaning

medium uniformly and assure that there is only a minimal residue. The

high throughfeed velocity based on the cycle rate is up to 150 m/min.

For less complex parts, subsequent lubrication can be eliminated if the

residue of the washing emulsion is sufficient for drawing, and the medi-

um used is suitable for this purpose.

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Sheet metal forming lines

Fig. 4.4.16 Destacker with magnetic belt for maximum blank dimensions of 2,100 3750mm,

suitable for coated and non-coated blanks

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

Oiling/greasing

For selective lubrication of blanks, oil spraying and greasing devices are

integrated into the destacker. These can be used to apply the lubricant

partially on the upper and/or lower side of the blank. Modern oiling

devices are computer controlled and capable of partial greasing or oiling

of approx.200 mm length at throughfeed velocity of up to 150 m/min.

Centering/orientation

Before the blank can be automatically transported into the die, it must

be aligned or centered in line with the required position. Inaccuracies

may occur when positioning a blank. These may be due to the location

on the stack or transport and they are corrected by means of end stops

or lateral pushers. In order to obtain a high degree of flexibility con-

cerning the dimensions and geometry of the blanks, all the positioning

devices can be programmed and power adjusted. As a result, the set-up

times are reduced.

Turnover devices for blank stacks

Blanks intended for further processing in forming lines are parted from

the coil stock on blanking lines (cf. Sect.4.6.2) and layered to blank

stacks. In order to save tool costs, where symmetrical parts are involved,

only a single blanking die is produced. Thus, it is possible, for instance,

to use the blanks for the left-hand fender also for the right-hand side by

simply turning over the stack. A distinction is made between the fol-

lowing blank stack turnover concepts:

– Friction-locked stack turning device: The stack is pressed with suffi-

cient force so that friction holds the blanks together and then it is

turned over.

– Positive-locked stack turning device: When turning in a drum or fork

turning device, the stacks are mechanically supported.

Drum turning device: for blanks with complex geometries

Many blanks with irregular geometry, for example trapezoidal parts,

may slip out when the stack is being turned over. Therefore, the stack

must be supported on two sides by means of end stops. In addition,

turning devices are often required to occupy only a minimal space and

a single location to accommodate stacks both before and after turning.

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

Altan T. Metal Forming Handbook

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