Altan T. Metal Forming Handbook

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For cold forming processes, the gripper units are mounted on clamp-

ing rails which are fixed in turn on a base rail(Fig. 6.6.1).When reset-

ting for producing a different part, all that has to be exchanged are the

clamping rails including the pre-set grippers. The front grippers are

equipped with proximity switches to monitor part transfer.

Parts with a higher L/D ratio (approx. 3 to 8) must be transferred

between stations by a 3D transfer. This involves the use of an addition-

al horizontal pusher which moves the part together with its retainer

into a position from where it can be raised vertically. Here, too, the lev-

el of the base plate on which the parts are positioned is located below

the transport plane. Shaft-shaped parts with a billet L/D ratio of > 6 can

also be moved into the uplift position using special devices such as

rotating sleeve retainers.

In warm forming, the loading station has the additional function of

segregating parts which either have not been fed in correct synchro-

nization with the press, or which have an incorrect temperature. To

allow these parts to be ejected, an opening is released in the base plate

of the loading station through which the parts drop down into a chute

for removal.

6.6.2Transfer study

Before conducting a transfer study, data on the kinematics of the slide

movement must be available in the form of a time-displacement dia-

gram. The stroke is defined based on the range of parts, the required

forming process and the die layout. The slide curve is given by the stroke

height and press kinematics (eccentric drive, knuckle-joint drive, modi-

fied knuckle-joint drive) (cf.Fig.3.2.3). The slide curve, whose bottom

dead center is at 180°, can only be displaced vertically for transfer study.

The ejector stroke is determined by the range of parts being processed

and the respective position of the forming stations in the die. The

largest necessary ejector stroke is equal to the sum of the inlay depth of

the part in the die and the part length. The ejector stroke can be

achieved by means of a mechanical and an additional pneumatic dis-

placement. The stroke covered mechanically must eject the part, which

is stuck in the tool as a result of elastic deflection of the container. In

addition, the pneumatically generated displacement is able to raise the

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Part transfer

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

part. Our present example has a mechanical ejector curve with a stroke

of 200mm which is travelled through at 80°crank angle: In this case,

no pneumatic ejector is required. The starting point for the ejector

curve can be optionally selected, allowing the curve in the diagram

(Fig. 6.6.2)to be displaced horizontally. The slope of the ejector curve

can be configured in such a way that it corresponds approximately to

the linear portion of the slide curve.

Starting with the transverse movement of the connecting rod, the

slide ejector curve is generated by the corresponding kinematic

(Fig. 6.6.2, dashed line). For the transfer study, this curve can be dis-

placed only vertically.

The curve of the pneumatic ejector, whose actual distance from the

slide curve must be determined with the transfer study, runs parallel to

the slide curve (Fig. 6.6.2, broken line).The slide ejector acts mechani-

cally briefly after the bottom dead center and can be operated pneu-

matically after that.

In addition to these main press motions, in universal transfer devices

the curves for opening and shutting the grippers are also significant. The

opening and closing times can normally be adjusted within certain lim-

its, e.g. within a 30°range, making the curves horizontally displaceable

in the transfer study. The opening stroke is determined by the required

diameter of the upper die (Fig. 6.6.2, opening stroke 100 mm). Where

mostly slender punches are used, this stroke is smaller than for female

dies used on the side of the slide, for example when producing long rod-

shaped parts or parts in closing devices. Depending on the opening

stroke, these movements require a smaller (approx. 30°for around

40 mm) or a larger (approx. 60°for around 100 mm) crank angle range.

Through the forward and reverse movement, lateral transportation

takes place. Forward motion can be initiated as soon as the grippers

engage the part, and must have been completed before the grippers

open, i. e. the upper die elements make contact with the transported

part. The forward and return motions require a crank angle of approx.

80 to 100°. The return motion takes place during the infeed and press-

ing cycles.

Transport studies of 3D transfer devices involve a lift-up motion in

addition to the movements previously described. This motion starts

briefly after the ejector movement and after the closing of the grippers.

It ends with the lowering motion which should have been completed

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Solid forming (Forging)

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

483

Part transfer

Fig. 6.6.2 Two-dimensional transfer study:

1 define opening starting point; 2 determine pneumatic spring travel for fed part;

3 define shortening of ejector bolt in the machine and starting point of

ejector in bed; 4 define closing end point;

5 determine pneumatic spring travel for ejected part

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

just before the opening of the grippers. Here, too, depending on the lift-

up motion, a greater or smaller crank angle range may be required. A

lift-up motion of around 100mm can require a crank angle of around

60°. All the transport motions of the press and transfer device are limit-

ed by maximum allowable acceleration levels, and must be separately

designed for each individual case under consideration.

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Solid forming (Forging)

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

6Solid forming (Forging)

6.7Die design

In industrial practice, extrusion or cold forging dies are frequently sub-

ject to parametric designs and adaptations, making the configuration of

the punches and dies of central importance. Less frequently, for exam-

ple where a new press is used, questions related to die holder design and

spare and active die sets must be addressed. Modern cold and warm forg-

ing dies have a multiple-station configuration. Most of the main forging

operations are completed within one to three stations. However, using

four or even five-station die sets it is possible to execute other sizing,

piercing and trimming operations in one and the same processing step.

Thus, it is possible to approach the objective of net shape or near net

shape production, and to reduce the transport and processing costs

involved in an additional pass through the press.

Figure 6.7.1illustrates the basic schematic layout of a modern multi-

ple-station die for a universal press. An intermediate plate is mounted

on the press bed and distributes the press force in the form of a pressure

cone which increases in size as it acts on the press bed. The intermediate

plate is equipped with recesses for connecting the shafts of the transfer

device clamping boxes, and is able to accommodate sensors for press

force measurement or roller brackets for die set changeover. Hydraulic

die clamps for complete die change operations can also be located on

the intermediate plate.

The die base plate houses the die holding sleeves. As demonstrated in

the example, this plate can also hold hydraulic closing devices and

scrap chutes. However, where these are used, the overall height of the

plate must be configured to be higher. The dies themselves can be

mounted in a retainer block which reaches over the entire width of the

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

die. However, for warm forming and in some cases also for cold form-

ing, the use of single die holding sleeves is preferred. Thus, the thermal

expansion is absorbed in each individual die holder, independent of

other die stations. Using a suitable cooling device, it is possible to pre-

vent the motion of die block, as a result of thermal expansion, relative

to the die halves mounted on the slide.

In principle, the layout of the upper die is similar. However, addition-

al wedge adjustment devices (Fig.6.7.2)are located in the head plate.

These are used for individual height adjustment of the dies. The wedge

adjustment devices have an adjustment height of about 3 to 5mm.

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Solid forming (Forging)

Fig. 6.7.1 Four-station die holder with hydraulic die clamping and closing system

in the second station

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

If no slide height adjustment is available in the press, the wedges must

then have a longer adjustment height of approx. 8 to 12mm in the indi-

vidual stations. Before actuating a wedge adjustment device, the clamp-

ing ring is released, thus allowing the downward motion of the dies. The

upper dies are relatively high, as they are designed to accommodate

pneumatically supported pads for workpiece transfer (Fig. 6.7.3).As a

result, reliable part transfer is ensured. Thus, higher effective stroking

rates and increased output can be obtained. The stroke of the pneumat-

ic ejectors is determined by means of a transfer study (cf. Sect. 6.6.2).

Active and exchange die components are located in the die holders.

The active die components contact the workpiece contour and are sub-

ject to wear. These must be reworked or replaced periodically. The

active die components include the containers, the die inserts, punches

and counterpunches, as well as for example stripper sleeves. The part-

dependent tooling must be exchanged together with the active die

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Die design

Fig. 6.7.2 Section through a die holder with wedge adjustment

(auxiliary columns and transport straps are built in for die holder change)

die holder

die holding sleeves

pressed part

active components

exchange components

part-dependent

exchange components

acitve component clamping

wedge adjustment

transport strap

pneumatic pad

punch

clamping

ring

dies

counter-

punch

auxiliary

column

wedge

adjustment

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

components when setting up the press for a new product. Other uni-

versally used exchange tooling components remain in the holding

sleeves. The exchange tooling include mainly pressure plates, pressure

pins, spring elements and guide sleeves. For the structural design of

active and exchange components of die sets, please refer to the VDI

Guidelines 3176, 3186 Sheets 1 to 3.

6.7.1Die holders

The die holders hold both the active and exchange tooling compo-

nents. Their function is to ensure that the lower and upper die operate

as on-center as possible in relation to each other. The number of sta-

tions, the distance between stations and the holder sizes are generally

grouped together in standardized series. Table 6.7.1 illustrates a modu-

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Solid forming (Forging)

Fig. 6.7.3 Section through a die holder with wedge adjustment and

hydraulic sleeve clamping

die holder

pressed part

die holding sleeves

active components

exchange components

part-dependent exchange

components

wedge adjustment

clamping individual station

active and

exchange

dies

auxiliary

column

intermediate

die sleeve

die holding

sleeve

pneumatic

pad

hydraulic

sleeve

clamping by

means of

wedge clamp

conical retainers

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

lar system of this type for a universal press design frequently used for

forming parts of approx. 0.1 to 15kg. Of course, other component and

die sizes can also be implemented.

The die change system is closely linked with the die holder design.

The range of parts to be produced and the individual batch sizes are the

main criteria that influence the die holder design and the layout of the

die changing system.

The complete die holder change, required when starting work on a

completely new part, involves moving the entire die holder out of the

machine to be replaced by a ready-prepared holder located in a pre-

scribed waiting position. The die head and base plate of the die holders

are hydraulically clamped (Fig.6.7.1).For moving in and out of the

press, the upper and lower part are moved together by means of

auxiliary columns and locked together with safety straps for transport

(Fig. 6.7.2). A third die holder can be assembled and set-up in the die

shop during the pressing or exchanging the die holders. For this

method of die holder exchange, parts of the gripper rails have to be

uncoupled, fixed on the die holder and removed together from the

press. Depending on the size of the holder, the exchange can take any-

where between 20 and 40min. These times include resetting of the

feeding station and, in the case of warm pressing, of the induction fur-

nace as well.

In exchanging the sleeves, the base and the head plate are left in the

press and the sleeves in the table and slide are replaced individually. For

this purpose, the base and head plate are fastened at the bed and slide.

They are equipped with conical centering rings (Fig.6.7.3and6.7.4).

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Die design

Table 6.7.1: Modular system for possible die holder sizes (widths) (bold = frequently used sizes) for

number of and distance between stations

Number of stations Die holder widths with distances between stations in mm

200 250 300 350 400

2 400 500 600/750 700 800

3 600 750 900/1,000 1,050 1,200

4 800 1,000 1,200/1,250 1,400/1,500 1,600

5 1,000 1,250 1,500 1,850 2,000

6 1,200/1,250 1,500 –––

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

Each die holding sleeve is fastened by means of two to four hydraulic

wedge-type clamps (Fig.6.7.3).This design is used for large batch sizes

when the tools wear at very different rates in the individual stations

and have to be individually replaced. In this case, ready prepared die

holding sleeves are located in a prescribed loading position and can be

mounted into the die station by a die changing arm (cf. Fig. 3.4.5).It

takes between 4 and 8 min to replace one sleeve. Figures 6.7.3and 6.7.4

illustrate the use of intermediate sleeves which can be assembled with

axial pre-stressing. Multiple-station forging permits subsequent opera-

tions to be executed in one processing step. As illustrated in Fig. 6.7.4

and 6.7.5,in the case of this specific die holder, the end of the extrud-

ed shaft is sheared to the required length by a wedge-action pusher die

in the fourth die station.

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Solid forming (Forging)

Fig. 6.7.4 Four-station die holder with hydraulic sleeve clamping and shearing station

in the fourth station

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