Altan T. Metal Forming Handbook

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The guidance ratio, i. e. the ratio of guide length to slide length, can

be reduced to 1: 3 in large-scale presses with a height restriction. Eccen-

tric press forces can only be partially absorbed by the gibs in this case.

For hot forming applications, the expansion of the slide due to temper-

ature must be taken into consideration in the choice of slide gibs. In

this case, guidance systems are used which permit expansion of the

slide in one direction without generating additional stresses (Fig. 3.1.7).

For ejection of parts that may stick to the upper dies, the slide can

be fitted with pneumatic, hydraulic or mechanical knock-out pins

(cf. Fig. 3.2.12).

3.1.3Drive systems for deep drawing presses

Single and double-action presses are used for sheet metal forming. The

double-action pressfeatures, next to the drawing slide, also a separate

blank holder slide (Fig. 3.1.8, cf. Fig. 4.2.2).Both slides are actuated from

above.

After the forming process, the part must, as a rule, be turned upside

down for the next blanking and forming operation. This requires a

Press types and press construction

Fig. 3.1.7

Eight-track slide gib

in cross formation

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

Fundamentals of press design

blank holder

connecting rod

blank holder

slide

blank holder

press bed

blank

holder

drive

(toggle

drive)

slide connecting

rod

drawing slide

draw punch

drawn part

female die

foundation

press crown

eight-element

slide drive

system

Fig. 3.1.8 Double-action mechanical press

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

Press types and press construction

Fig. 3.1.9 Single-action mechanical press with draw cushion

drawing slide

blank holder

slide connecting

rod

female die

draw punch

drawn part

hydraulic

draw cushion

pressure pin

press crown

eight-element

slide drive system

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

bulky, cost-intensive turning device to be installed downstream from the

double-action press. There are advantages in forming a part in the reverse

position, i. e. the bottom of the part facing upwards. In this case part ejec-

tion is simpler through a slide cushion in subsequent forming stations.

The alternative is to work with a draw cushion or nitrogen cylinders.

Furthermore, the centering in subsequent stations is improved, since the

parts can be positioned over their entire inner shape by the bottom die.

In addition, the sensitive surfaces of the material – for example when

working with automotive body panels – are less likely to sustain damage

during transportation in the press and when centering in the die. It is

also possible to install the blanking punches at a lower level for subse-

quent operations if the part has previously been turned over.

In the case of single-acting deep drawing presses,in contrast, the

blank holder force is transmitted from underneath by a draw cushion

(Fig. 3.1.9and cf. Fig. 4.2.3). Both the forming and the blank holder

force are applied by the slide, as the slide forces the draw cushion down-

wards. After the drawing, without “turning over”, the part is ideally

positioned for the subsequent operations.

When used together with a hydraulically driven slide movement, the

draw cushion can also be used foractive counter-drawing (Fig. 3.1.10and

cf. Fig. 4.2.4).From the point of view of energy savings, it is beneficial

that the slide holds the blank while the draw cushion applies the form-

ing force.

In order to achieve as great a degree of flexibility as possible, double-

action presses can also be configured with a draw cushion.

3.1.4Draw cushions

The function of the draw cushion is to hold the blank during deep

drawing operations in mechanical and hydraulic presses (cf.Sect.

2.1.4). The cushion thus prevents the formation of wrinkles and, as a

result of the ejection function, raises the parts to the transport level

during the return motion of the slide. Modern draw cushions can be

either hydraulically or pneumatically actuated. A major difference

between the two is the larger force of the hydraulic draw cushion. The

cushion force is initiated during downward travel of the slide by, for

example, four hydraulic cylinders via the lifting bridges and com-

Fundamentals of press design

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

Fig. 3.1.10 Single-action hydraulic press with active draw cushion for counter drawing

slide

cylinder

slide

ejector

counter

drawing die

active draw

cushion

draw cushion

cylinder

slide

draw punch

pression columns into the corner areas of the blank holder on the die

(Fig. 3.1.11and cf. Fig. 4.4.39).When the slide reaches the bottom dead

center, the draw cushion is moved back to the upper position either in an

uncontrolled, i.e. together with the return motion of the slide, or a con-

trolled movement, i.e. after a prescribed standstill period, by the lift

cylinder acting in the center of the lifting bridge. Impact damping is

achieved by means of optimized proportional valve technology, ensuring

that the draw cushion reaches its upper end position without vibrations.

The benefit here is that the draw cushion force can be controlled on a

path-dependent and a time-dependent basis during the drawing process.

It is possible, for instance, not only to reduce and increase the overall

blank holder force, but the pressure levels of the four cylinders can also

be modified independently of each other due to the short pressure

response time within a range of 25 to 100% of the rated pressure. As a

result, the blank holder pressure can be individually controlled over the

entire deep drawing process at each of the four corner points of the die.

The precision control of the blank holder follows a prescribed force ver-

sus time profile (Fig. 3.1.12).

Press types and press construction

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

This offers the diemaker and the machine operator the capabilities

for optimization of the drawing process and for the reduction of die

start-up times. The stretch forming process, for example, can be initiat-

ed by briefly raising the blank holder force, or the material flow corre-

spondingly facilitated by reducing the pressure in certain areas of the

blank holder (cf. Sect. 2.1.4). To improve part quality, output and tool

life as well as reducing noise, the draw cushion can be pre-accelerated.

This means that the draw cushion is already set in motion before the

slide impacts the blank. This reduces the relative speed between the

slide and blank holder considerably as compared to an uncontrolled

draw cushion. Due to the reduced impact of the blank holder, the num-

Fundamentals of press design

Fig. 3.1.11 Design of a hydraulic draw cushion

female die

drawn part

blank holder

pressure column

lifting bridge

punch

moving bolster

foundation

displacement cylinder

lifting cylinder

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

ber of press strokes can be increased without compromising part qua-

lity, while at the same time the noise level is reduced by as much as

8dB(A).

An important characteristic of the hydraulic draw cushion with dis-

placement cylinders is that the blankholding force, in contrast to the

pneumatic draw cushion, is not fully applied instantanously on appli-

cation of the slide, and is only built up completely with the start of the

piston movement through displacement of the oil column in the cylin-

ders. This allows the vibrations commonly encountered on impact of

the slide, as indicated in Fig.3.1.13,to be avoided. The negative influ-

ence of vibrations and noise, and also on the drawing process itself, are

eliminated. This is of particular significance when producing large

sheet metal components with high stroking rates. It is possible to large-

ly avoid superimposed vibrations affecting the drawing process at slide

impact speeds of, for example, 40% with a relatively large lead of the

Press types and press construction

Fig. 3.1.12 Progression of blank holder force in a hydraulic draw cushion with a constant

displacement pressure of 100% back right, 80% back left and front right and

pressure changeover from 80% to 25% front left

1 s

displacementpressure

drawcushiontravel

slidetravel

time

pressure/

travel

front

left

back

right

back

left

front

right

dampingphase

100%

25%

80%

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

pneumatic draw cushion (Fig. 3.1.13, top). However, it is only marginal-

ly possible to achieve a constant blank holder force at high impact

speeds (Fig. 3.1.13, bottom).The conditions for the hydraulic draw cush-

ion with displacement cylinders (Fig.3.1.11)are significantly more

favorable. Since the pressure build-up does not take place until the

beginning of the piston movement, even at maximum impact speed

the blank holder force peaks only minimally and is quickly dissipated

(Fig. 3.1.12). Accordingly, in general terms a lead of the blank holder in

the die of some 30mm is sufficient to ensure that the entire drawing

process takes place at a constant blank holder force.

Fundamentals of press design

impactspeed40%

time

blank

holder

force

rated

blank

holder

force

leadtravel

pointofimpact

withdrawcushion

startofdrawing

delayed

impactspeed100%

time

blank

holder

force

rated

blank

holder

force

startofdrawing

delayed

pointofimpact

withdrawcushion

leadtravel

Fig. 3.1.13 Progression of blank holder force of a pneumatic draw cushion depending

on impact speed

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

3 Fundamentals of press design

3.2 Mechanical presses

3.2.1 Determination of characteristic data

The following paragraphs will deal with basic principles concerning

force, work and power of mechanical presses.

If a weight of 75kg is suspended from a crane rope, a force of around

750 N (force F = Mass m · gravitational acceleration g) acts on the rope.

As long as the weight is not raised, no work is performed, as the work

W [Nm] is the product of force F [N] and distance h [m]:

If this weight is now raised by 1m, the work performed amounts to:

This work can be obtained from the weight again when it drops by 1m.

However, the magnitude of the resulting force depends on the distance

over which the work takes place:

If, therefore, a force of 750N is uniformly exerted to raise the weight

over a distance of 1 m, the same force will be expended if the entire

work is performed when lowering the weight over a distance of 1 m.

A different situation results if the weight initially drops freely for half

WFh=⋅

WNmNm=⋅=750 1 750

FWh= /

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

the distance and then the entire work of 750 Nm is expended over the

last 50cm = 0.5 m. The force is then given by:

Since the work has been performed over only half the distance, the

force has doubled. If the entire work is performed over only one tenth

of the distance (i. e. 0.1 m), the force rises tenfold:

In Fig. 3.2.1, the force exerted over the relevant distance is indicated for

all three cases. The results are three rectangles whose areas correspond

to the product of force F ·distance hand thus to the work W.If a curve is

traced through the corners of the rectangles, provided the existing work

of 750 Nm is performed evenly over a certain distance, it is possible to

directly read the magnitude of the force applied. With a forming dis-

tance of 0.2 m, for instance, the force applied is 3,750 N (Fig. 3.2.1).

In summary, it is generally valid that

–force and work are two terms which can be related to each other only

by means of a third variable, i. e. distance.

–If a force is applied over a certain distance, work is performed. This

corresponds in the force-distance graph to the area of the rectangle

below the force curve.

–If a certain amount of work is performed, the distance over which it

is performed determines the magnitude of the generated force.

The relations described here using the example of a weight, apply gen-

erally to the field of press construction. They are applied also in the

forging hammer. The hammer has a certain weight and is raised over a

certain distance. This gives it a defined quantity of potential energy.

When it gives up the work stored in it, i. e. transfers its energy to the

die, the magnitude of its force at the moment of impact depends on the

displacement over which deformation takes place. This force can be

very large if the displacement of the workpiece is very small, and can

even result in damage to the tool and the press. On the other hand, in

Fundamentals of press design

FNmmN==750 05 1,500/.

FNmmN==750 01 7500/. ,

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