Altan T. Metal Forming Handbook

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ledeburitic 12% chromium steels (1.2379) and carbides fully comply

with these requirements.

Coatings are applied mainly by specialized companies. This means

that particular attention must be paid to the preparation of dies by the

die manufacturer. Prior to coating, the work surfaces must be free of

grooves and polished to a surface roughness of R

< 1 mm. The limit for

optimum coating of internal holes is around l (length) = d (diameter)

when using the PVD technique. With the CVD technique, there are no

known limitations. The limits for the ratio between the diameter D and

thickness H of coated panels in cases where a “deflection” of < 0,02 mm

is required after treatment, are as follows when using the CVD tech-

nique:

D < 7 · H for steels and

D < 15 · H for carbides

and when using the PVD technique

D < 20 · H

It is only possible to coat pre-stressed dies in cases where the insert is

pressed out, coated and pressed in again after coating. In order to pre-

vent the formation of beads, sharp edges must be broken beforehand.

In cases where a mean deviation of the mandrel length L and diameter

D after coating of < 0.01 mm is required, the CVD process should be

used in preference for the following dimensions

L < 10

D in the case of steel

L < 15

D in the case of carbide

and the PVD process for the length

L < 20

Reliable improvement in die life, as a result of coating warm forming

dies with the CVD or PVD techniques, has not been demonstrated yet,

classical nitriding and welding-on techniques are generally used to

enhance performance. The problem faced here is that the base materi-

als have lower degrees of hardness compared to the requirements of

501

Die design

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

cold forging, and the necessary supporting effect of the substrate mate-

rial is not achieved. Once a continuous increase in the hardness of the

substrate material through to the actual coating has been successfully

achieved, more success can be expected. It may be possible to achieve

this result using plasma nitriding and subsequent coating with CrN or

TiAlN, or through the development of special sandwich coatings.

6.7.4Die closing systems (multiple-action dies)

Recent developments in the field of die closing systems have substan-

tially increased the range of parts that can be produced using the cold

and warm forging methods. Flashless forging of spiders, spherical hubs,

tie rod ends, T-fittings for radiator construction, rotors and parts with

different gearing has only become possible with the development of die

closing systems.

With an additional closing force initiated in the die set, female dies

are closed before the bottom dead center of the slide is reached.

Somewhat later, the material is then formed, flashless, in the closed die

by penetration of the punch. Flashless pressing is only possible if the

pre-form can be completely located in the dies when these are closed.

The closing force itself can be initiated by means of elastomers,

hydraulic systems, or mechanically by the kinematics of the press.

Elastomers offer the advantage that their design is simple and their

spring characteristic can be adjusted within certain limits by altering

the material composition. However, they only permit limited travel.

The elastomers also have to be exchanged periodically. Hydraulic clos-

ing systems generate high closing forces within a minimum of space,

and can thus be positioned adjacent to each other in multiple-station

dies. By adjusting the pressure, a greater closing force range can be cov-

ered. However, their design is more complex, and they call for a num-

ber of safety precautions to protect both the operator as well as the dies.

In modern presses, mechanical initiation of closing motions can be

achieved over a large crank angle range. However, the investment here

is substantially higher than that needed for elastomer or hydraulic sys-

tems.

Figure 6.7.9illustrates the basic layout of a hydraulic closing system

integrated fully into the die. In the provided example, there is a closing

502

Solid forming (Forging)

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

system integrated into both the upper and the lower die. This type is

used predominately in cases where there is only one forging step or

when only one station of a multiple-station die is required to work with

a closing system (cf. Sect. 6.7.1). Typical workpieces produced in this

die include gearing elements which are being formed from pre-formed

blanks (rings, lenses). Closing systems can also be accommodated in

separate neutral plates (Fig. 6.7.10)which always remain in the press

irrespective of the actual die in use. Depending on the die set, mounted

in the press, the closing systems which are currently required can be

activated and connected by means of pressure pins to the female die. A

typical application for this system is the production of tie rod heads

and T-fittings which require up to five closing systems. Additional parts

include inner races of constant velocity joints where an additional

piercing process is required and the grooves, formed in the previous sta-

tion, must maintain this shape accurately during the piercing opera-

tion.

The closing force can be individually adjusted according with the

(annular) piston surfaces. The press forces which are introduced down-

503

Die design

bed

clamping plate

hydraulic cushion

lower die

hydraulic cushion

upper die

annular piston

cushion pins

top

advancing die

bottom

daylight

slide

Fig. 6.7.9

Single-station die set with

hydraulic closing system at

the top and bottom

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

wards into the press bed via a counterpunch are absorbed centrally.

This determines the minimum inside diameter of the annular piston.

As a guideline, the closing force can be estimated to be 0.6 to 0.9 times

the punch load. The required closing force depends on whether a com-

pletely closed (or trapped) die set is being used or forging takes place

through extrusion shoulders into a free space. In the case of complete-

ly closed (trapped) die sets, the closing system also serves to apply an

axial pre-stress on the parting line of the dies, that are subjected to high

loads.

504

Solid forming (Forging)

daylight

bed

slide

oil cushion annular piston

neutral plate

for the closing

system

neutral plate

for the closing

system

counterpunch

cushion pins

Fig. 6.7.10 Multiple-station die set with hydraulic closing systems

at the top and bottom of each station

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

6 Solid forming (Forging)

6.8 Presses used for solid forming

6.8.1 Choice of press

The most suitable press for solid forming is selected by taking into

account the following factors:

– part geometry and material,

– batch sizes and

– the user’s existing equipment (e.g. annealing and phosphating lines).

On the basis of this information, process engineers can select the most

economical production method and recommend a suitable press.

As a general rule, mechanical presses are recommended for large pro-

duction lots and batch sizes. The maximum possible part length

depends on the slide stroke and the available ejector stroke. Another

determining factor is the time-motion curve of the press and the auto-

matic feed system. In the case of shaft-type parts, the maximum trans-

port length applies instead of the maximum part length. For the pre-

liminary stages, this dimension is often greater than that of the finished

part. For parts with an internal contour, the penetration depth of the

punch must be taken into consideration. As a rough guideline, the max-

imum possible part or transport length suitable for continuous auto-

matic operation of mechanical presses can be calculated using the fol-

lowing formula:

for parts with internal contour: 0.35 to 0.45 times slide stroke

for shaft-type parts: 0.45 to 0.60 times slide stroke

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

Using special devices such as separately driven feed systems with lift-

ing stroke, it is possible to manufacture lengths up to 0.8 times the slide

stroke. In this case, presses operate in the automatic single-stroke mode

at a production output of approx. 15 to 18 parts per minute. In general,

a transfer study is required in setting up automated mechanical presses

in order to ensure a reliable sequence of motions between the slide and

feeding system (cf. Sect. 6.6.2).

In comparison to sheet metal forming, solid forming requires sub-

stantially longer forming strokes in the partial and nominal load range,

and accordingly a greater press energy capacity (cf. Sect. 3.2.1). In addi-

tion, powerful ejector systems are generally used both in the press bed

and the slide (cf. Fig.3.2.12).When using multiple-station dies, a two-

point slide drive is beneficial to enhance the off-center loading capabil-

ity of the press (cf. Fig. 3.1.3).

Regarding production lot sizes, hydraulic pressesoperate at substan-

tially lower speeds than mechanical presses. However, they have

longer slide and ejector strokes as well as high press forces and they are

considerably simpler to implement with a lower investment volume

(cf. Fig. 4.4.2).For these reasons, operation in the single stroke mode is

also economical. The application of hydraulic presses varies consider-

ably. Concerning the operating sequence and necessary press force, the

differences between mechanical and hydraulic systems are minimal.

The working speed of the slide is a particularly important factor in

determining the economical application of hydraulic presses. High

approach and return speeds of approx. 400 to 500mm/s substantially

increase the press cycles (cf. Fig. 3.3.1). Even more important is the

highest possible velocity under load (approx. 40 to 60 mm/s). These

values call for the use of pumps and motors with adequate capacities

(cf. Fig. 3.3.3).

Equally fundamental to economical forming are reliable control sys-

tems using proportional valves for precise control of the slide motion.

Positive stops that can be automatically adjusted are used for sizing and

coining operations.

Figure 6.8.1gives rough guideline values for the required press forces

depending on the formed part, the material and the production

process. Material-dependent differences for the required forming force

are considerably lower for warm and hot forming than for cold forming

(cf. Fig. 6.1.5).

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Solid forming (Forging)

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

A number of different presses used for solid forming are presented in

the following. For a description of the fundamental principles of press

design, please refer to Sects. 3.1 to 3.3.

6.8.2Mechanical presses

Mechanical presses are used primarily in conjunction with automatic

feed systems for large production series of over 1.5 million parts per

year and batch sizes of more than 10,000 parts. Both vertical and hori-

zontal press designs are used. Hourly production rates, achieved using

vertical systems, range between 1,800 and 3,600, while the correspond-

ing figure for horizontal presses lies between 3,600 and 9,000. The sys-

tems that are most widely used are discussed below.

Knuckle-joint presses with bottom drive

The principle layout of this type of press with nominal press forces from

1,600 kN is described in section 3.2.2(cf. Fig. 3.2.5).Figure 6.8.2illus-

507

Presses used for solid forming

Fig. 6.8.1 Guideline values for press force requirement

pressed part diameter[mm]

20 30 40 50 60 70 80 90

100 110 120

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

20000

22000

24000

18000

16000

14000

15000

42CrMo4cold forming

C10cold forming

C10hot forming

Cf53/100Cr6hot forming

multiple-step forming

press force

single-step forming

[kN]

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

trates a typical press. Here, the press frame moves the upper die up and

down. In the case of single-station dies, part transfer takes place from

the back towards the front (or vice-versa). For multiple-stage dies, the

press frame incorporates lateral openings which provides space for the

rails of the transfer system and for part feed.

One of the benefits offered by this design principle is the low center

of gravity, and the minimal machine weight. These factors result in

lower capital investment compared to presses, offering similar capa-

bilities, but with the top drive. Slide strokes are in the range of 150 to

250mm (in special designs up to 450mm) and they are substantially

larger than those available in coining or blanking presses.

Multiple-station presses with modified top knuckle-joint drive system

The main characteristic of this type of press with rated press forces from

4,000 kN is its modified knuckle-joint drive (cf. sect. 3.2.2). In addition

to the advantages mentioned above, this design principle offers the ben-

efit of a process-adjusted low slide velocity in the work range (Fig. 6.8.3

508

Solid forming (Forging)

Fig. 6.8.2

Knuckle-joint press with bottom drive

(nominal press force 6,300kN, four

stations with transfer feed)

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

and cf.Fig.3.2.3).This is supplemented by acceleration of the slide

downward and upward to achieve an overall higher stroking rate. As a

result, the velocity of the slide when impacting the workpiece material

is reduced and remains approximately constant during forming. As a

result, the dynamic loads on the press and die are reduced. For extru-

sion processes particularly, this helps to extend the service life of dies

and reduce the stress on the drive system. Another positive side-effect is

a reduction of noise development by about 6 to 8 dB(A).

A further benefit in comparison to conventional drive systems is a

three to four-fold increase in the deformation stroke, with a drive

torque value comparable to that of conventional mechanical presses.

This makes production of press parts, requiring a high level of defor-

mation energy and a large nominal working stroke (> 20 mm) very eco-

nomical (cf. Sect. 3.2.1). The eight-track slide gib operates according to

the “fixed-loose bearing” principle, thus compensating for thermal

expansion in the slide. This makes this press ideally suited for warm

and hot forming (cf. Fig. 3.1.7).The slide stroke ranges between 250 and

400mm (special designs up to 520 mm) (Fig. 6.8.4).

509

Presses used for solid forming

Fig. 6.8.3 Slide velocity curves for mechanical cold forging presses

0°

90°

180°

270°

360°

crankangle[°]

-70

-60

-50

-40

-30

-20

-10

eccentric

drive

modified

knuckle-joint

drive

knuckle-joint

drive

slide velocity [m/min]

slidestroke315mm

strokingrate40/min

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

Multiple-station presses with eccentric drive

The main field of application for these presses with nominal press

forces from 6,300 kN lies in warm forming (680 to 820 °C) and auto-

mated flashless forging (Fig. 6.8.5).Due to their large slide stroke of 630

to 800mm, these presses are also ideally suited for cold forming of

long, shaft-type pressed parts. The slide is guided in the same gib sys-

tem as illustrated in Fig. 3.1.7. This guidance concept also permits nar-

row gib clearances in warm and hot forming applications, meaning

that it is no longer necessary to provide for an additional clearance to

compensate for the unavoidable thermal expansion of the slide.

510

Solid forming (Forging)

Fig. 6.8.4 Multiple-station press with modified knuckle-joint top drive

(nominal press force 10,000kN, four stations, loading system and transfer feed)

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