Elsevier Editorial System(tm) for CIRP Annals - Manufacturing Technology Manuscript Draft Manuscript Number: 2011-F-Keynote PaperR1 Title: Mechanical servo press technology for metal forming Article Type: Annals 2011 - Vol. 60/2 Corresponding Author: Prof. Kozo Osakada, D., Corresponding Author's Institution: Osaka University First Author: Kozo Osakada, D., Order of Authors: Kozo Osakada, D.,; Ken-ichiro Mori, Dr.; Taylan Altan, Dr.; Peter Groche, Dr.

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Mechanical servo press technology for metal forming

K. Osakada (1)

a*, K. Mori (2)

b, T. Altan (1)

c, P. Groche (1)

d

a School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan

b Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi, Aichi, Japan

c Center for Precision Forming, Ohio State University, Columbus, OH, USA

d Technische Universität Darmstadt, Darmstadt, Germany

Abstract.

Recently several press builders developed gap and straight-sided metal forming presses that utilise the mechanical servo-drive

technology. The mechanical servo-drive press offers the flexibility of a hydraulic press (infinite slide (ram) speed and position control,

availability of press force at any slide position) with the speed, accuracy and reliability of a mechanical press. Servo drive presses have

capabilities to improve process conditions and productivity in metal forming. This paper reviews the servo press designs, servo-motor

and the related technologies, and introduces major applications in sheet metal forming and bulk metal forming.

Keywords: forging, stamping, servo press

1. Introduction

1.1. Characteristic features of mechanical servo press

Electro-mechanical servo-drives have been used in machine

tools for several decades. Recently, several press builders, mainly

in Japan [7,46] and Germany [1], developed metal forming

presses that utilise the mechanical servo-drive technology. The

mechanical servo-drive press offers the flexibility of a hydraulic

press (infinite slide (ram) speed and position control, availability

of press force at any slide position) with the speed, accuracy and

reliability of a mechanical press [102].

Figure 1 shows two types of servo presses developed at early

stages [80]. In the design of Figure 1a, the rotational motion of

the servo-motor is transmitted to the slide using a timing belt and

ball screw. The press slide is moved up and down by the

reciprocating motion of the motor and ball screw. The tilting of

the press slide is detected by linear sensors and corrected by

adjusting the motion of each individual motor as needed. Thus, it

is possible to maintain slide/bolster parallelism under off-centre

loading of the press. In the linkage design, seen in Figure 1b

[62,63], the knuckle-joint mechanism is activated by the ball

screw.

Due to the feed-back control of slide position of the servo

press, scatter of the bottom dead centre is kept very small as

shown in Figure 2, in which the result for the press of Figure 1a

is given, and the product accuracy is kept high irrespective of

working time or temperature change [12].

Because all the press motions such as starting, velocity

change and stopping, are done only by the servo-motor, the

mechanical servo press has a simple driving chain without

flywheel, clutch and brake that are essential for a conventional

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(a) Ball screw type servo press

(b) Ball screw and knucle-joint type servo press

Figure 1: Flexibility of slide (ram) motion in servo drive presses [80].

Figure 2: Change of position of bottom dead centre with working time [12].

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mechanical press, and thus the maintenance of the servo press is

simplified.

Compared with the hydraulic servo press which has been

used for many years, the mechanical servo press has higher

productivity, better product accuracy and better machine

reliability without noise of hydraulic pump and complicated

piping.

The most important feature of the servo press is flexible

slide movement. Figure 3 presents the typical motions used with

the servo presses [3]. Since a servo press can realize almost all

the motions, the following merits are considered to be brought

about by choosing a motion suitable for each purpose.

Figure 3: Typical slide motions of servo-press [3].

1) The accumulated know-hows with the existing presses can

be inherited because the motions such as crank press and

linkage press can be duplicated by a servo press.

2) Impact loading is avoided and the tool life is extended.by

reducing the touching speed of the tool to the work piece.

3) Lubrication is often improved and the working limit can be

extended by using a pulsating or oscillating slide motion.

4) Touching and break-through noise is reduced by stopping

the slide for a short time or reducing the slide speed.

5) Vibration of sheet metal can be reduced by an optimised

slide motion and the shape of sheet metal product is

stabilised.

6) The accuracy of the product can be improved by controlling

the slide parallelism and choosing an optimum slide motion.

7) Higher productivity is possible by shortening of a forming

cycle with a partial short stroke around bottom dead centre

as well as a high speed return motion as shown in Figure 4.

Figure 4: Decrease in cycle time as well as in impact speed using a servo

press (150% increase in output) [15].

The digital control of the servo press enables harmonic

movements of various devices such as hydraulic, air and servo-

motor controlled cushions with the main slide. By utilizing this

function, servo presses are often employed to extend the forming

limit and to improve product accuracy. On a multi-axis servo

press, two or more processes may be completed in a stroke by

moving the driving axes cooperatively.

Since a servo-motor can recover kinetic energy during

deceleration, some servo presses store the energy during dynamic

braking in the capacitor or in the independent flywheel. The

stored energy is taken out and used during short energy peak

demands of the next stroke, which results in a decrease of the

voltage in the intermediate circuit [17].

1.2. Historical background

Although the servo-motors were used in metal cutting

machine tools as early as in the 1950s, they were not mounted on

the metal forming presses for a long time because their power

was not strong enough for the large force required in metal

forming. High powered AC servo-motors were realised in the

1980s with the development of the strong magnets. Together with

the development of transistor controllers, they were applied to

injection moulding machines in the late 1980s, and the

production of servo-motor driven injection moulding machines

increased rapidly in the 1990s [89].

High powered servo-motors were also applied to CNC

powder compacting machines. In powder compaction, many axes

should be driven independently to achieve uniform density in a

product with non-uniform heights. Tsuru et al. [134,135] and

Kishi et al. [53] developed a 6 axis CNC powder compacting

machine on the basis of servo-motor driven injection moulding

machine.

Since some metal forming processes, such as enclosed die

forging, necessitated complicated motions of multiple slides,

hydraulic servo-presses were built in the 1970s, but only a small

number of presses was constructed because the areas of their

application were limited.

In 1987 a 100kN press brake (bending machine) driven by

an AC servo motor, shown in Figure 5, was developed by

Toyokoki to carry out accurate bending [20] by measuring the

stroke with the taco-generator of the motor and the load with the

motor torque. Similarly to later servo presses, the force was

transmitted through a ball screw. This press is considered to be

the first commercial servo press, and since then press brakes

driven by electro-hydraulic [2] and purely mechanical [38,44]

servo systems have been manufactured by increasing the

maximum load as the power of servo motor has been raised.

Figure 5: 100kN press brake driven by servo motor built in 1987 [20].

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Because the power of servo motor was not large enough, a

general purpose servo press was not built until 1997 when

Komatsu HCP3000, shown in Figure 6, appeared. The driving

mechanism of this press is given in Figure 1a. In this press, the

maximum load of 800kN was transmitted through two ball

screws. Since then various types of servo presses have been

developed in Japan, Germany, Taiwan, Spain [27] and China.

Figure 6: 800kN servo-motor driven press developed in 1997 [80].

In presses, servo-motors can be used in two different ways.

Either they are used as high speed motors or as low speed and

high torque motors. To provide high forces needed for forming,

high speed motors require timing belt, linkage and gears to

transform the high speed rotational motion into the low speed

motion. The low speed and high torque servo-motors allow direct

drive of the eccentric or crank mechanism and thus there is no

need for belt, linkage or ball screw drives [14]. From around year 2000, linear motors began to be used for

small servo presses. The motion of the linear motor press is fast

and is controlled accurately, but the power of the press is limited.

The linear motor driven presses are mainly used for high speed

punching of polymer tapes and metallic foils [97,121], and are

also applied in micro-forming and precision coining [32,138].

2. Designs of servo-motor driven press

2.1. Types of mechanical press

Conventional presses are classified into stroke controlled

presses (crank press, knuckle-joint press, linkage press), energy

controlled presses (screw press, hammer) and force controlled

presses (hydraulic press) [64]. The common feature of stroke

controlled presses and the screw press is that the rotational

movement of motor is mechanically converted into the linear

slide movement. To convert the movement, various mechanisms

with crank, knuckle-joint, linkage and screw, shown in Figure 7,

are used. The crank press is the most commonly used model, and

the knuckle-joint and the linkage presses are used to modify the

shortcomings of the crank press, i.e. to reduce the slide velocity

around the bottom dead centre and to extend the slide travel of

large available force.

Since the power of a conventional motor is not large enough

to carry out a metal forming operation directly, the energy

supplied by the motor is stored into the fly wheel as the rotating

kinetic energy as shown in Figure 7 and is released in a short

time when forming is carried out. In the cases of crank, knuckle-

joint and linkage presses, the positions of top and bottom dead

centres and the velocity characteristics are determined by the

driving mechanism. In the energy controlled screw press, the

rotational movement of the flywheel is changed to the linear

motion with a screw, and the slide stops when the energy stored

in the flywheel is consumed completely, and thus the bottom

dead centre cannot be pre-determined.

Figure 8 shows the time-stroke curves of the mechanical

presses. The slide velocity of the screw press is kept almost

constant, but once forming starts the slide is decelerated very

rapidly. The crank press moves with a sinusoidal motion due to

the simple crank system. In the knuckle-joint and linkage presses,

the velocities near the bottom dead centre are lowered compared

to the crank press [22]. Since the torque applied to the driving

axis is limited by the twisting strength of the axis, high maximum

working load is possible when the slide position is low.

In the case of the hydraulic press, the pressure of the

working oil is raised by the motor, and the slide is driven by the

oil pressure. Due to the limit of the motor power, the slide

movement is usually slow. By using the flow control servo

valves and air accumulators, it is possible to control the velocity

and position of the slide, as used in the hydraulic servo presses.

Figure 8: Stroke-time diagrams of mechanical presses.

The high powered servo-motor enabled direct driving of the

mechanical press without using a flywheel and clutch, but the

fundamental driving mechanisms remain the same as

conventional presses. In the following, the presses driven by

servo-motors are reviewed.

2.2. Servo-motor and ball screw driven presses

The first servo presses illustrated in Figures 5 and 6

employed ball screws to reduce the friction of the screws.

Differently from the traditional screw press, this press does not

require a flywheel and clutch, and the slide velocity can be

controlled during forming. Later, the screw press that simply

(a) Crank (b) Knuckle-joint (c) Linkage (4) Screw

Figure 7: Conventional mechanical presses (S: Slide, B: Bolster).

.

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replaced the motor with a servo-motor keeping the flywheel was

developed [22].

An important feature of the ball screw type servo press is

that the maximum force and the maximum slide speed are

available at any slide position, and thus it can be applied to

almost all the forming methods. This feature is especially useful

for forming with a long working stroke such as extrusion, and for

forming that needs a high speed motion at the end of forming. As

will be introduced later, the majority of the present servo presses,

such as the crank, knuckle-joint, and linkage presses, use crank

mechanisms to achieve large press forces, and the slide velocity

is slowed down around the bottom dead centre although it is

controllable to some extent.

The maximum load is limited by the torque capacity of the

servo motor, the reduction of the belt drive and the load carrying

capacity of the ball screw. One of the solutions to increase the

power of the ball screw type servo press is to increase the number

of motors and driving axes (spindles). Figure 9 shows the

5000kN ball screw type servo press with four spindles build by

Hoden Seimitsu Kako (HSK) [144]. In this press, the four

spindles are moved independently, and thus the inclination of the

slide to any direction can be corrected [43, 91]. Further, as shown

in Figure 10, it is possible to drive multiple axes by several

servo-motors in order to carry out multiple processes in a single

stroke [90, 92] as will be explained in section 4.5 [39].

(a) Driving Axes [39] (b) Perspective view [144]

Figure 9: Four axis 5000kN ball screw type servo press.

Figure 10: Multiple moving axes in ball screw type servo press [92].

Servo presses with four spindles were developed also in

Germany. Figure 11 shows the presses from Heitkamp and

Tumann [41] as well as Synchropress [126]. They use servo

motors positioned above (Figure 11a) or below (Figure 11b) the

slide for each planetary roller thread spindle. The slide is guided

by linear bearings. Sensors recognize a tilting of the slide due to

eccentric loading. Since the spindles are not linked mechanically,

the position of the slide can be kept parallel to the table of the

press by an appropriate control of the servo motors. Press forces

from 630kN up to 2500kN are used for both transfer and

progressive tools.

2.3. Servo-motor driven crank presses

Since the ball screw type presses were expensive, crank

presses driven by servo-motors began to be manufactured. Figure

12 shows the crank type driving mechanism [40]. In this case, a

high torque servo-motor is attached directly to the press drive

shaft.

Figure 12: Direct gear driven gap press with servo drive, where the high

torque servo-motor is directly coupled to press drive shaft [40].

By combining the servo driving mechanism and the existing

press structure, gap (C-frame) presses shown in Figure 13 were

developed by Aida Engineering [127,128] and Amada [118,119].

The structural stiffness and the gear-eccentric drive were the

same as the conventional crank presses while only the flywheel,

clutch and main motor were replaced with the servo-motor.

When the main shaft rotates at a constant speed, the stroke-time

curve is the same as the conventional press (see Figure 8).

(a) H+T [41] (b) Synchropress [126]

Figure 11: Direct drive ball screw type servo presses produced in Germany.

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To increase the power and to keep parallelism of the slide

under off-centre loading, crank presses with two connecting rods

were produced as shown in Figure 14 [21, 57].

Figure 14: Crank press with two crank shafts (Komatsu) [57].

For crank presses with larger forming forces, many servo-

motors can be attached to one driving gear shown in Figure 15.

By utilizing this motor arrangement, a 50000kN hot forging press

is under construction.

(a) Four motors with 2 gears [104]

(b) Four motors with 1 gear [116]

Figure 15: Crank shaft systems with multiple servo-motors attached to a gear.

2.4. Servo-motor driven linkage presses

Linkage mechanisms are often used for presses to reduce the

slide velocity and to increase the load capacity of a given motor

torque around the bottom dead centre as shown in Figure 8, and

they are often applied to cold forging presses. For servo-motor

driven presses, linkage mechanisms are considered to be

advantageous in increasing the approaching and returning speeds

by slowing down the slide speed in the working region in a stroke,

and having large available load over a relatively long working

stroke. Yossifon et al. investigated various linkage [146] and

double knuckle [147,148] mechanisms suitable for metal forming

processes on servo presses.

Figure 16 shows a 6300kN servo press for cold forging

driven by two servo-motors and the linkage mechanism [8].

Figure 16: 6300kN linkage servo press (Komatsu) [8].

Groche et al. [30,31] proposed a three degree of freedom

servo press which enables three dimensional movement of the

slide, which can be rotated during a stroke as shown in Figure 17.

By tilting the slide, a product which necessitates plural tool

actions from different directions can be produced.

(a) Linkage mechanism (b) CAD image

Figure 17: Three degree of freedom servo press [31].

Meng et al. [78] proposed a link mechanism using a

conventional constant speed motor and a servo-motor. In this

mechanism, the main function of forming is carried out by the

conventional motor, and a servo-motor is used to vary the

velocity of the slide during a stroke depending on the purpose of

forming. Using this concept of combining a conventional motor

and a servo motor, Du et al. [18] and Li et al. [65] examined

some other linkage mechanisms.

(a) Aida Engineering [127] (b) Amada [120]

Figure 13: Crank shaft type servo presses in Japan.

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2.5. Hybrid servo press with ball screw and knuckle- joint

As shown in Figure 8, the knuckle joint and the linkage

mechanisms are used to increase the press power with the crank

shaft mechanism, and they can be combined with the ball screw

mechanism for servo presses. The servo press shown in Figure 1b

is a 5000kN hybrid press with ball screw drives and knuckle-joint

mechanisms [61]. Figure 18 shows a 25000kN servo press driven

by a different type of combination of a ball screw and knuckle-

joint mechanisms [4,131].

Figure 18: 25000kN Hybrid type servo press (Amino) [6].

2.6. Servo-motor driven hydraulic press

By the direct drive volume control (DDVC) of hydraulic

system (Figure 19a) with the AC servo-motor, the flow rate and

direction of oil flow are controlled without using flow control

valves. Some hydraulic presses driven by servo-motors have

been developed [143] because the high power servo-motors

realised higher slide speed compared with the conventional servo

press with flow control valves. Figure 19b shows the hydraulic

servo press made by Kawasaki hydromechanics.

(a) DDVC hydraulic system

(b) Perspective view

Figure19: CCVD hydraulic servo press (Kawasaki Hydromechanics) [51].

3. Control and systems of servo presses

3.1. Servo-motor

Servo-motors are available as AC or DC motors. In the

1980‟s servo-motors were mainly DC motors because their large

currents could only be controlled by silicon-controlled rectifiers.

As transistors became capable of controlling and switching larger

currents at higher frequencies, the AC servo-motor became

common. Early servo-motors were specifically designed for

servo amplifiers [24]. Today most motor manufacturers offer a

wide range of motors, which are designed for applications that

use a servo amplifier or a variable-frequency controller. That

means that a motor may be used in a servo system in one

application, and used in a variable-frequency drive in another

application. Various manufacturers define any closed-loop

system that does not use a stepper motor without feedback

system as a servo system. Hence any simple AC induction motor

connected to a velocity controller may be denoted as a servo-

motor [54].

Torque motors represent a type of servo-motors, which

provide high torques. A torque motor is in principle a large

scaled servo-motor with a hollow shaft and optimised torques.

The layout of a torque motor is shown in Figure 20. It works like

a regular synchronous motor. The moving part is a drum with

fixed magnets on the inner surface. The stator consists of a large

number of magnetic coils integrated in an iron stack. These coils

are star-connected and provided with three-phase current. The

required velocity is controlled by the frequency. Due to the large

number of poles a high torque at a very low speed can be

achieved. To reduce cogging to a minimum the permanent

magnets inside the rotor are fixed. There is no need for

gearboxes, worm-gear drives, and other mechanical transmission

elements since the rotor is directly connected to the shaft.

This combination is, in connection with pre-loaded ball

bearings, absolutely free of play. Depending on the used control

system the rigidity of the drive can be increased. Further higher

power, precision, angular speed and acceleration can be realised

[133].

Figure 20: Layout of a torque motor [133].

The advantage of torque motors (broken line) compared to

conventional asynchronous motors without control (solid line) is

explained in Figure 21. Characteristic torque curves and the

operating area for both motor types are shown. The asynchronous

motor is reaching its maximum torque with an increasing number

of revolutions while the torque motor directly provides the

maximum torque. This is possible because the torque motor has

no slip. This is the major of the reason why servo presses are

primarily driven by torque motors. Furthermore the characteristic

curve of a linear motor is identical to the shown torque curve.

Linear motors are employed as well in several types of servo

presses [33,42].

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Figure 21: Motor Characteristics.

3.2. Servo control

The control of a servo press can be differed between the

control of the motor position and the control of the motor

moment. To control the slide movement usually an internal

control loop is employed. Therefore the motor position is

monitored and fed into the controller. A force limiter or a default

force function is used to perform defined forming operations and

to avoid overshooting of the slide [23]. Figure 22 displays an

internal control loop for a single input single output system.

Figure 22: Schematic of an internal control loop [23].

This scheme can also be employed for the control of more

than one motor as multiple input multiple output system. In that

case the variables represent vectors instead of scalars. Further

linear sensors can be used to measure the current position of slide

(Figure 23). This can be used to detect and compensate slide

tilting in multi drive presses. Therefore the measured tilting is fed

back into the controller [1].

To further improve the press accuracy for unpredicted

changes an iterative learning control (ILC) can be used for

repetitive movements. Therefore an additional external control

loop is employed [97]. Figure 24 displays the scheme of an

iterative learning control [65]. The slide position is measured and

fed back to the controller. The determined deflection is used to

adjust the following stroke. Hence environmental fluctuations,

machine deformations or changes in load, such as the break

through during blanking, can be countervailed.

Figure 23: Multi motor servo presses using linear sensors for compensation of slide tilting.

Figure 24: Schematic of an iterative learning control [65].

Conventional presses usually employ a flywheel that is

connected to the main gear via a clutch brake. Therefore most

attention has been paid to the on/off operation of the clutch brake

[37]. For most servo presses the energy is directly transmitted.

Hence an overrun of the servo-motor directly leads to an overrun

of the slide‟s motion. Thus additional digital control becomes

necessary also for safety reasons. To avoid errors in position

sensing, multiple position measuring is employed, combining

absolute slide position sensors, pulse coders and, if applicable,

angle sensors in the gear. Further redundant control loops are

employed to avoid incorrect outputs [37].

3.3. Energy storage

In generating a full press load by using servo motors, a large

amount of energy is taken from the electric circuit. An analysis of

different operating press modes shows, that most energy is

required during the forming and acceleration processes.

Furthermore it is detected, that a large amount of energy is

produced during the deceleration of the slide. Conventional

mechanical presses store this energy in a flywheel. In hydraulic

presses the energy is lost. Currently there are two options to store

energy in servo presses. Either the energy is stored in a flywheel

or fed back to the electricity network by using capacitors. By

storing the energy inside the press, the size of the main line

connection can be reduced by 30% to 70% [59]. Figure 25 shows

the modification possibilities of a press with flywheel to a press

with capacitor.

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The example in Figure 26 illustrates power storage and

output in a servo-mechanical press system over the course of a

cycle. Operating with two main motors, each with a maximum

output of 175 kW, the excess energy is stored in an energy-

storage device during deceleration and is drawn upon when the

press requires more than 175 kW. Because the stored energy is

used during peak power requirements, the load on the facility

network remains at approximately 50 kW. Two available

methods to save the energy generated resulting from the

deceleration of the slide are capacitor storage or motor/generator

connected to a rotating mass. The more reliable and longer-

lasting method is a motor/generator, but on-going developments

in capacitor capabilities will likely make capacitors the preferred

storage option of the future [111].

Currently capacitor banks with a capacity of 132F are used

for presses of the mid-range engine-power class. Up to three

capacitor banks can be connected to increase the storage capacity.

The application of capacitors is also possible for already existing

presses by retrofitting them.

Figure26: Power storage and output in a servo mechanical press system [111].

3.4. Manufacturing information system

A manufacturing information system supports manu-

facturing functions such as production scheduling, plant design

and inventory management [125]. Therefore, information from

different sources is required and consolidated in this system.

Material and personnel data, inventory and vendor information as

well as engineering specifications are processed in a

manufacturing information system [125]. This information is

defined as an intangible object, which flows value-carrying

through the product development process and the manufacturing

sequence using software tools like CAD/CAM or data sheets

[26]. The possibility to exchange information between different

production systems, departments or even production sites is one

of the most important advantages of a manufacturing information

system. The control system of a servo press represents a part of a

manufacturing information system. Data can be transferred

between different presses and production sites. The knowledge of

experienced operators is combined in the manufacturing

information system and is accessible to all operators in the

company. Hereby a prototyping process can be easily reproduced

and checked at different production sites even overseas. Using

this control possibility the conventional evaluation method of

different machine parameters is simplified. This leads to a

reduction in time and effort for the setup from prototyping to

high volume production (Figure 27).

3.5. Internal connections

In the following the connections of servo presses with other

elements of the production system are differentiated in internal

and external connections. Internal connections are located inside

the system boundary defined around the press itself. Here

applications of die cushions, ejector and multi slide systems will

be presented. External connections are referred to elements

outside of the system boundary such as handling system, press

feeder and the arrangement of multiple presses.

A servo-motor supplier [1] shows the possibility to apply

the servo-motor drive principle to the design and control of a die

cushion. In Figure 28 the schematic configuration of a servo die

cushion is shown [9]. The servo-motor underneath the working

Figure 27: Effect of visualisation and exchange of machine data [36].

Figure 28: Schematic of servo drive die cushion [9].

Figure 25: Servo press with energy storage [59].

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space is linked to a screw by a timing belt. By powering the

motor and turning the screw the movement of the die cushion is

realised. The pressure sensor between the elements of the die

cushion controls the position of the assembly. The application

area of this cushion is similar to hydraulic cushions, which are

used in conventional mechanical and hydraulic presses. To

optimize the metal flow in the flange, between the die and the

blank holder multiple point die cushions are required. With the

shown servo-motor an enlarged number of cushion pins can be

implemented. The most important advantage of the servo die

cushion is the possibility to regenerate energy when it is pushed

down by the upper die. Hereby the servo-motor act as a generator

which recuperates the energy back into the system and provides it

for the next stroke.

For die cushions for deep drawing air and hydraulic ones

have been used. Since the die holding force of the air die cushion

cannot be controlled, and the response of hydraulic die cushion

with a servo valve is slow, a servo motor controlled hydraulic die

cushion was developed. Figure 29 shows the NC control

hydraulic die cushion attached to a servo press [77]. It is reported

that the die cushion can reduce the impact loading at the time of

contact and 70% of the energy used for die cushion is recovered.

Figure 29: Electro-mechanical die cushion for servo press [77].

Another example of an internal connection is a multiple

slide press. As described in [16] a number of slides is driven

independent from each other by single servo-motors. With this

design, shown in Figure 30, every slide can provide a unique

path-time diagram for the different forming operations in the

work space. The servo-motors in the press are staggered to each

other which minimises the distance between the different slides.

To move the work pieces from one forming operation to the

following no external transfer system is required, which is

necessary for a conventional press line. Based on this fact the

servo driven multi press is characterised by its high productivity.

In comparison with known multi slide presses [28] the accuracy

of the servo-driven press is increased because every slide is

equipped with its own guidance and not linked to a common

drive. The control system for a servo multiple slide press is less

complex than for a multiple slide hydraulic press [117]. Besides,

efficiency of hydraulic drives is lower because of their high

energy consumption.

3.6. External connections

3.6.1 Servo feeders

To allow transfer systems to be synchronized to the press

motion servo feeders are used. The feeding function can be

integrated into the press control, avoiding collision with the slide

during the feeding motion [19]. Due to the absence of a

mechanical drive coupling wear and maintenance effort are

reduced [49].

Especially, for forging applications servo-motor actuated

transfer fingers are applied. These allow carrying out changes in

their opening and closing movement as well as traverse

movement without mechanical adjustment. Further the correct

gripping can be monitored by the position and force information

of the servo system. Hence no additional sensors become

necessary [49].

3.6.2 Connection of presses

For the production of more complex parts, multiple servo

presses are combined with press lines. Hence productivity is

increased and problems regarding eccentric loads can be limited.

In a transfer press line one transfer is employed for the part‟s

handling between all presses. In a tandem line each press is fed

from an external device. Hence the feeder can be easily matched

to the press movement (Figure 31) [96,128].

Figure 31: Connection of tandem press lines with presses [129].

To optimize cycle times of press lines a phase shift

between the motions of successive presses is employed

(Figure 32). Therefore the influence of the transport time on

the cycle time is limited. Furthermore employing a phase shift

allows smoothening peaks in energy consumption.

Figure 30: Schematic design of a multiple slide servo press [28].

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Figure 32: Phase shift in motions of subsequent presses [129].

While a mechanical transfer press usually provides a high

productivity, a transfer press line provides a high flexibility.

Since each press in a tandem line is fed by an individual device,

the height of the dies does not have to be matched [13].

Furthermore each press can be used independently from the

others, allowing shortening the line or production on single

presses as shown in Figure 33.

Figure 33: Individual forming motions of presses in a transfer line [13].

3.6.3 Large press lines

By extending the idea of connecting presses, large presses

can be connected to transfer press lines for plate forging and

sheet drawing of auto body panels. The gaps between the presses

are set to be small only to accommodate the transfer devices and

robots.

For plate forging, seven 6300 N forging presses, explained

in Figure 16, are connected to each other without gaps as shown

in Figure 34. The total press force is 42,000kN [8].

Tandem servo press lines for draw-forming of automobile

body panels have been built by Toyota (with Komatsu), Honda

(with Aida Engineering [132]) and BMW (with Schuler [115,

123]). Figure 35 shows the servo press line in Suzuka plant of

Honda [77]. This tandem line is composed of 4 servo presses

(23000kN drawing press force) and 4 servo feeders for conveying

panels from one process to the next. It is reported that more

complicated shapes of body panels can be formed by this line

with faster speeds than conventional press lines because the

motions of die cushion and transferring equipment can be

optimised for each product.

Figure 36 shows a layout of a servo press line of the BMW

factory in Leipzig [115]. The total press force is 103,000kN, the

drawing press force is 25,000kN, the length of the press line is 34

m (total length is 98m), and the stroke per minute is 17.

Figure 35: Servo press line for automobile panels (Honda-Aida) [77, 132].

Tandem servo press lines are believed to increase the

efficiency of small quantity production and to reduce the time for

preparation of production and to stabilize the product quality and

to increase the yield of material usage. Because of large freedom

of slide motions and related equipment such as servo die cushion,

process simulation is essential.

Figure 34: 42000kN servo press line with seven 6300kN presses for plate

forging (Komatsu) [8].

Figure 36: Layout of servo press line in BMW Leipzig [115].

11

4. Application of servo press to sheet metal forming

Press forming of sheet metal, often called „stamping‟,

includes bending, deep drawing, stretching and shearing. Sheet

metal forming is usually carried out at room temperature, but to

form high strength steel sheets hot stamping began to be used in

the last decade.

4.1. Bending

To reduce the weight of car, high strength steel sheets are

more and more employed as the material of body panels. In

bending of high strength steels, spring-back is extremely large,

and the dimensional accuracy of the formed products

deteriorates. To decrease the amount of spring-back in V-bending

of various steel parts, Mori et al. [82,83] used a servo press and

examined the slide motions: holding and bottoming shown in

Figure 37a. The angle of spring-back in the 980 MPa level ultra-

high strength steel sheet was considerably reduced by the

bottoming motion, which gives a small amount of excessive

deformation at the bent corner, whereas spring-back was not

improved by the holding motion without giving excessive

deformation as shown in Figure 37b. Kaewtatip et al. [50] used

the bottoming motion to reduce spring-back in U-bending of high

strength steel sheets.

At the final stage of the bottoming motion, the bent corner is

indented by the punch tip and the stress state may be changed

from pure bending to compression, and the spring-back

associated with the bending stress may be reduced.

(a) Slide motions

(b) Effect of slide motion and forming speed

Figure 37: Reduction in spring-back by bottoming in V-bending of 980 MPa

level ultra-high strength steel sheet [83].

In U-bending, Suganuma [128] used the re-striking motion

(Figure 37c) with a servo press by compressing the side wall to

the height direction in the second strike as shown in Figure 38a.

The re-striking process was realised by installing a device to

change the tool distance between the bottom surface and the

flange surface after the first strike. From the Figure 38b it is clear

that spring-back is suppressed by re-striking. In the re-striking,

the stress state of the sheet becomes compressive in the height

direction of the sheet and the stress gradient in the thickness

direction, which is the main reason of spring-back in bending,

may be reduced.

In bending of sheet metal, the bent parts often undergo

stretch flanging, which causes large tensile stress, and fracture

tends to take place. Fracturing strain of the flange made in

bending of an ultra-high strength steel sheet was increased by

improving the sheared surface of the blank and using forming a

servo press [81]. To improve the quality of the sheared surface,

the clearance between the punch and the die in shearing before

the stretch flanging process was optimised, and the sheared edge

was smoothed with a conical punch.

4.2. Deep drawing

4.2.1 Deep drawing with various slide motions

In deep drawing, the occurrence of wrinkling is the most

important defect that should be avoided and a blank holding force

is applied to suppress the occurrence of wrinkling. But a large

blank holding force increases the frictional force which tends to

cause fracture of the sheet. Thus it is important to reduce the

friction to achieve a successful deep drawing operation.

Tamai et al. [130] tried to improve the formability of high

strength steel parts in deep drawing by detaching the tools from

the die cushion periodically from the sheet as shown in Figure

39a. It was found that the sheet was automatically re-lubricated

when the tool was detached. A 590 MPa level high strength steel

sheet was successfully dawn by using the detaching motion as

Figure 39b.

(a) Movement of tools

(b) Products

Figure 39: Improvement of deep drawability of high strength steel sheet by

detachment of tools from the sheet during forming [130].

(a) Re-striking of bent channel

without re-striking with re-striking

(b) Products

Figure38: Reduction in spring-back by re-striking in bending of channel

shaped product [128].

12

Komatsu et al. [58] reported a case where wrinkling in deep

drawing was prevented by applying the stepwise drawing motion

shown in Figure 40a on a servo press with a constant clamping

force. As shown in Figure 40b, the occurrence of wrinkling was

eliminated by applying a smaller (about 1/3) flange clamping

force than the conventional motion with the stepwise motion. No

clear reason is given why wrinkling disappears by the stepwise

motion. Although wrinkling would start with the low clamping

force in the downward movement of the stepwise motion, the

initiated small wrinkle may be suppressed or released by the

clamping force during the upward movement of the main slide. A

similar effect for preventing the occurrence of wrinkling by the

pulsating internal pressure was reported for hydroforming of

tubes [85].

(a) Stepwise motion

(b) Products

Figure 40: Prevention of wrinkling in deep drawing by stepwise slide motion

[58].

A sheet product, which ruptures with the conventional crank

motion, is successfully formed by optimizing the slide motion of

a servo press as shown in Figure 41 [95]. In this motion, the

punch touches the sheet with a slow speed, and the slide

movement is once reversed between the pre-forming stage of the

top portion and the drawing stage of the rectangular portion.

Since a complicated motion is used, the most influential motion

for preventing fracture is not clear.

(a) Controlled motion

(b) Crank motion (c) Controlled motion

Figure 41: Prevention of rupture in stamping by controlled slide motion [95].

Hagino et al. showed that a stainless steel sheet was

successfully stamped into a fuel cell separator with a servo press,

and the dimensional accuracy was improved by striking two

times at the bottom dead centre [34]. Since rupture of stainless

steel sheet in deep drawing tends to be caused by local

temperature rise at a high stamping speed, low forming speed is

desirable. By reducing the slide speed just before the contact of

the punch and the work piece on a servo press, deep drawing of a

stainless steel sheet was successfully carried out with a high

productivity without causing rupture [122].

4.2.2 Deep drawing with variable blank holder force

By combining a servo press and a hydraulic or servo motor

driven die cushions (see Figure 28), it is possible to vary the

blank holder force (BHF) during a process, as is done in the large

servo press lines of automobile body panels. Figure 42 shows an

example of the motion of a servo die cushion when applied to

deep drawing [9]. The position controlling and force controlling

can be switched during a process. The blank holder force is

changed to prevent wrinkling of flange while moving the cushion

with the same speed as the slide.

(1)Preliminary Acceleration

(2)Pressure Change(4) Position Locking

(5) Shock less Stop

Position Pressure PositionControl

(5) Returning to

Waiting Position

Press

Slide

Motion

Die Cushion

Stroke

Maximum

StrokeForming

Stroke(3) Preparatory Lift

(1)Preliminary Acceleration

(2)Pressure Change(4) Position Locking

(5) Shock less Stop

Position Pressure PositionControl

(5) Returning to

Waiting Position

Press

Slide

Motion

Die Cushion

Stroke

Maximum

StrokeForming

Stroke(3) Preparatory Lift

Figure 42: Motion of servo die cushion used for blank holding in deep drawing [9].

To achieve large deformation in deep drawing by avoiding

wrinkling and fracture, optimum BHF was searched for with a

servo press by keeping the BHF at different constant levels [145].

Since the optimum BHF varies during an operation, variable

BHF began to be studied by Manabe et al. [69] and many

researches [100] have been carried out to determine the BHF

history for drawing of deep cups without fracture and wrinkling.

Since the BHF history cannot be decided easily with experiment,

some methods using the FEM simulation were carried out for a

round cup [142] and a square cup [55]. Koyama et al. optimized

the process by combining FEM simulation and a data base [60].

Figure 43 shows a concept of determining BHF history by

including logic of control in the finite element simulation [106].

Figure 43: Concept of determining optimum history of blank holding force [106].

13

4.2.3 Deep drawing with heated tool

Magnesium alloy sheets exhibit poor ductility at room

temperature but when they are heated to a temperature between

200 and 300 ºC, they become ductile and deep drawing is

possible. To avoid cooling of the heated Mg blank by contacting

with the cold tools before deep drawing starts, heated tools are

used. Oyamada et al. [114] adjusted the contacting time of the

blank with the heated tools. Kaya et al. [52] included a heating

process in the slide motion of a servo press, i.e. the sheet was

sandwiched between the die and the blank-holder having inside

heaters just before drawing operation as shown in Figure 44.

(a) Apparatus with heater in die

(b) Slide movement

Figure 44: Warm deep drawing of magnesium alloy sheet with heated dies. The numbers given in the lower part of the figure denote the stages of the process [52].

4.3. Ironing

Koga et al. [56] applied a stepwise motion to the ironing of

cups, by calling this “vibratory ironing”. As shown in Figure 45,

a larger reduction in thickness was attained with the vibratory

ironing method.

(a) No stepwise (b) Stepwise

Figure 45: Large reduction in thickness in ironing of aluminium cup by stepwise motion [56].

4.4. Shearing

Murata et al. [93, 94] proposed a “progressive stair die” in

which shearing of a sheet is done at multiple slide strokes by

using a punches with different lengths as shown in Figure 46.

Since the available press force is not changed by the slide

position in the case of the ball screw type servo press, it is not

necessary to carry out shearing near the bottom dead centre,

differently from the conventional mechanical press. By shearing

at different slide positions, the maximum shearing force is kept

low. Together with the good parallelism of slide of the servo

press, Figure 46a, the low sharing force leads to accurate

shearing of products with complicated shape such as Figure 46b.

Conventional press Servo press

(a) Progressive stair die working

(b) Product

Figure 46: Shearing by progressive stair die [94].

Otsu et al. [113] proposed a slide motion to reduce the noise

in shearing by calling it “variable punch speed”. As shown in

Figure 47a, the punch is stopped midway during shearing and

then accelerated to finish shearing. Figure 47b shows the change

of noise level by the stopping position in the variable punch

speed motion. The noise level is lowered especially when the

punch is stopped at the late stage of shearing because the load

before breakthrough is reduced, and the horizontal vibration is

restricted during intermediate stopping by holding the punch in

the half-made hole. Junlapen et al. [47] also examined the

reduction in blanking noise with various motions.

(a) Punch motion

(b) Noise level

Figure 47: Reduce in noise of shearing by two-step punch motion on servo

press [113].

14

In the shearing of plates and sheets, it is known that burr

formation is prevented if the shearing direction is reversed twice

as shown in Figure 48 [66]. Julapen et al. [48] used a servo press

to realize this process and showed that burr-free shearing was

possible if the working condition is carefully chosen.

(a) Slide motion

(b) Sheared surfaces

Figure 48: Burr-free shearing of aluminium alloy sheet [48].

4.5. Sheet forming with multiple driving axes

In the progressive and transfer dies mounted on a press, as in

the general cases employed in sheet metal forming, a product is

completed by changing the dies as Figure 49a. To complete a

product without transferring the forming position, a one shot

stamping process with a servo press having some driving slides

was proposed by HSK as shown in Figure 49b [92]. The process

is composed of several stages which are realised by using

multiple slides on a servo press explained in Figure 10. For the

one shot stamping, the process design and die structure are

critically important but they are not disclosed for this case.

(a) Progressive and transfer dies

(b) One shot

Figure 49: One shot stamping process having some slides for controlling motion in one press [92].

4.6. Hot stamping

Hot stamping of quenchable steel sheets is effective in

solving the problems encountered in forming of the high strength

steel sheets. By heating the sheets, the forming load is lowered,

spring-back is prevented and formability is greatly improved. In

addition, the hot stamped parts can be hardened by contacting

with the cold dies just after forming (die quenching), and ultra-

high strength steel parts having a tensile strength of

approximately 1.5GPa are obtained with a low forming load.

To attain the slide motion for die quenching process, which

holds the dies at the bottom dead centre for a while by

sandwiching the sheet, servo presses are suitable [84]. In

addition, a high slide speed of the mechanical servo press is

advantageous to prevent temperature drop during stamping

although the hydraulic presses having a slow slide speed are

mainly employed in the present hot stamping operation.

In conventional hot stamping, the steel sheets are heated in a

furnace and oxidation of the formed sheets is unavoidable. To

prevent oxidation in hot stamping, Mori et al. [86] developed a

heating process using rapid resistance heating as shown in Figure

50. The resistance heating equipment is synchronised with a

servo press, and the sheet is stamped after only 0.2s of the end of

heating. It is reported that oxidation is considerably reduced by

stamping the sheet on a servo press just after heating by the

resistance heating process.

Figure 50: Hot stamping process using rapid resistance heating [86].

A warm and hot punching method [88] was developed by

combining the resistance heating method with a servo press to

reduce the punching load of ultra-high strength steel sheets as

shown in Figure 51. The resistance heating equipment is

synchronised with the servo press. As the heating temperature

increases, not only the punching load decreases but also the shiny

burnished surface increases.

Figure 51: Hot punching of 980 MPa level ultra-high strength steel sheet

using resistance heating [88].

15

When a sheet heated at 900 ºC is drawn at a low slide speed,

the sheet is ruptured due to local temperature drop. The

formability in the hot stamping is improved by increasing the

slide speed as demonstrated in Figure 52.

(a) 26mm/s (b) 149mm/s

Figure 52: Improvement of formability in hot stamping by increase in slide

speed in deep drawing at 900 ºC [84].

The combination of servo press and resistance heating was

also applied to hot spline forming of high strength gear drums

used in automobile transmissions [136]. At first a sheet is drawn

into a deep cup, and then a spline is formed on the side wall of

the cup by a gear drum die. As shown in Figure 53, the side wall

of the cup was heated by resistance heating to decrease the

forming load and to increase the formability. Since the cross-

sectional area of the side wall of the ironed cup is uniform in the

electrification direction, the temperature of the side wall is

uniformly distributed and this helps to form the can with gear

drum easily.

(a) Resistance heating of cup

(b) Cold stamping (c) Hot stamping

Figure 53: Hot spline forming having resistance heating of side wall of

980MPa level high strength steel cup [136].

A die quenching method for producing ultra-high strength

steel parts having strength distributions was developed [87].

Since the quenchable sheets are not hardened by air cooling, the

strength distribution is controlled by limiting the contacting area

with the tools to the portions requiring a high strength as shown

in Figure 54. The contacting parts were locally quenched by the

grooved tools when the slide is kept at the bottom dead centre for

a given time, while the non-contact portions were left without

quenching. A servo press was employed to control the holding

time at the bottom dead centre.

(a) Resistance heating (b) Holding at bottom dead centre

Figure 54: Tailor die quenching in hot stamping for producing ultra high strength steel formed parts having strength distribution [87].

5. Application of servo press to bulk metal forming

Since the early servo presses had low loading capacities,

they were employed mainly for cold forging that requires low

forging force due to the small product size [10,12]. Recent

development of strong servo motors enabled construction of large

servo press for hot forging of large product size [104].

5.1. Free forging

Free forging is frequently carried out as the first process of

die forging under the name of “upsetting”, and has been used in

incremental bulk forming because various shapes could be

flexibly formed with a small number of tools [29]. In free forging,

deformation is not strongly restricted by the tool shape and the

forming pressure is low, but the forming pressure in upsetting of

thin plate is extremely high due to the restriction by friction.

Maeno et al. applied the oscillating or pulsating motion of a

servo press to plate upsetting [68]. In the unloading stage of the

oscillating motion, the periphery of the work-piece leaves from

the tool surfaces as shown in Figure 55a and liquid lubricant is

sucked into the gaps reducing the friction in the next loading

stage. Figure 55b illustrates the load-stroke curve in upsetting of

an aluminium plate of 2 mm in thickness and 20 mm in diameter.

By the oscillating motion, the load is lowered especially at large

reductions.

(a) Elastic deformation of tool during loading and unloading

(b) Load stroke curve in forging with and without oscillation

Figure 55: Mechanism of lubrication in upsetting with oscillating load and

change of load in upsetting of thin plate [68].

16

Traditionally incremental forging has been performed as

hammer forging, and is used now to forge very large products

such as rotors of electric generators by using large hydraulic

presses with capacities of 50MN - 200MN. Manipulators are

combined with the presses to handle large work-pieces.

By combining a servo-press with a robot, this process may

be used as an efficient method for small batch production of

smaller parts by using robots to handle the work-piece in

positioning and posturing. Wang et al. [141] demonstrated the

case of the robot in combination with a servo press for

incremental forging as shown in Figure 56.

(a) Equipment

(b) Product

Figure 56: Free forging with robot and servo-press [141].

5.2. Closed die forging

Die forging is used to change the shape of a bulk metal to

the shape of die cavity by compressing between the upper and

lower dies. In closed die forging, the product shape is the same as

the shape of die cavity, and the pressure tends to be increased

excessively. Since the servo press can detect the working load

and stop the slide motion before the bottom dead centre in case of

excessive loading, it is advantageous for close die forging.

For close die forging of a magnesium plate, it is necessary to

heat the plate up to 200 - 300 ºC to deform it without causing

brittle fracture [98], but the heated plate is easily cooled down

during transferring from the furnace to the die. To carry out

heating and forming of a Mg plate with heated tools, the process

shown in Figure 57 [73] was developed. The Mg plate is

sandwiched between the high temperature dies for a while, and

then forming is carried out with the same dies [74]. For this

movement of the upper die in this process, the servo press was

effectively used. Since the Mg alloy exhibits significant work

softening at 200 - 300 ºC, it is desirable to deform the work-piece

without restriction at the beginning of forming until the flow

stress drops sufficiently [70,75].

Figure 57: Forging of Mg alloy with heated tools [73].

By using heated dies, Shiraishi et al. [124] proposed a plate

forging method for magnesium covers with ribs and flanges as

shown in Figure 58a. By applying a back pressure to the central

part of the work-piece on a servo press, the ribs are formed

successfully without contraction defect as shown in Figure 58b.

(a) Work piece and product

(b) Cross-section of rib

Figure 58: Forging of magnesium cover with rib and flange [124].

Coining is a die forging method in which a thin plate is

compressed between closed dies with shallow cavities at room

temperature. Because the work piece is thin, very high pressures

are necessary.

By using a servo press, coining of a thrust bearing shown in

Figure 59a which has shallow grooves formed by the hammering

motion in which the tool hit the work-piece repeatedly without

changing the slide position [20]. When the hammering pressure is

high (392kN) as shown in Figure 59b, the grooves are completely

shaped, but the groove depth is incomplete when the pressure is

not high enough.

(a) Hydrodynamic thrust bearing

(b) Effect of number of hammering on groove depth

Figure 59: Coining of thrust bearing by hammering motion [20].

5.3. Forward extrusion

Free extrusion is a method used to reduce the cross-sectional

area of rod shaped work-piece by pushing through the die hole

without restricting by container. This method can be successfully

applied when the compressive pushing stress is lower than the

yield stress and the elastic buckling limit of the rod.

17

In “FM forming” or “Axial Forming” patented by Felss [25],

the die or the billet is oscillated during extrusion, i.e. the die or

the billet moves forward about 2 mm and then back 1 mm in each

step (Figure 60a). The work-piece is re-lubricated with liquid

lubricant when the tool retreats. By using the oscillating motion,

the forming load in free extrusion of a spline shaft was reduced

by about 40% as shown in Figure 60b [101], and the load drop

extended the forming limit set by buckling of the shaft. Although

this result was obtained with the specially designed machine, the

oscillation mode can be realised by a servo press.

(a) Slide motion (b) Load history

Figure 60: FM Forming with oscillating motion [101]. .

Pinion gear shown in Figure 61a is often manufactured by

forward extrusion. An important technical problem of this

process is that the temperature of the deforming area gradually

changes as extrusion progresses and the dimensional error of the

teeth is varied along the axis as shown in Figure 61. Ando

showed that it was possible to keep the error to a minimum extent

in the whole length by varying the extrusion speed during an

extrusion process on a servo-press [11].

(a) Pinion gear (b) Distribution of dimensional error of teeth

Figure 61: Dimensional error of helical gear extruded with mechanical and

servo-presses under different forming speeds [11].

5.4. Backward extrusion

Backward extrusion is used to produce cup shaped products,

and is often combined with forward extrusion for making

backward cup and forward cup, or backward cup and forward rod.

One of the problems of this process is that the extrusion pressure

becomes too high when the extrusion ratio is large.

The accuracy of an extruded product is influenced by the

history of deformation speed during forming because different

final temperature distributions, which is affected by the speed

history, causes different amounts of heat contraction in cooling

after forming. Ishiguro et al. examined the punch motions shown

in Figure 62a in backward extrusion and investigated the

variation of outer diameter of the extruded cup [45]. As shown in

Figure 62b, a high-speed stroke condition without multiple

strikes caused the minimum diametric error because the heat

contraction balanced with the elastic spring-back.

(a) Slide motions

(b) Variation of outer diameter to height direction

Figure 62: Variation of outer diameter of extruded product by slide motion

[45].

5.5. Cold forging with multiple driving axes

5.5.1 Enclosed die forging

In enclosed die forging, the billet is enclosed in the die

cavity between the upper and lower dies, and then it is squeezed

out by the upper and lower punches sideways. As shown in

Figure 63, an enclosed die forging operation consists of: (a)

supplying of a billet into the cavity, (b) closing of the upper and

lower dies, and (c) pushing of punches to fill the billet material in

the cavity. To realize this process, multi-axial hydraulic servo

press was developed by Mitsubishi Heavy Industries in 1970s

[79]. Because the contacting areas of the punches do not change

during an operation, the forming load does not increase sharply,

differently from die forging with flash formation. When the

pressure in the die cavity exceeds a certain limit, the upper die

floats up to relief the pressure.

(a) Billet supply (b) Die closing (c) Forging

Figure 63: Method of enclosed die forging process and product.

18

Recently, an AC servo-motor driven mechanical press was

used for cold enclosed forging of bevel gears as shown in Figure

64. It was reported that the die life was extended by 3 times and

the energy consumption decreased to a half [5] by substituting a

hydraulic press to a mechanical servo press.

Figure 64: Cold forging process of bevel gear on mechanical servo press [5].

5.5.2 Cold forging with axially driven container

In many closed die forging processes, the product corners

cannot be filled well. To fill the corners in closed die forging

with a straight cylindrical container, a method to drive the

container in parallel to the punch as shown in Figure 65a was

proposed [108,109]. In this process, the second slide drives the

container while the main slide compresses the billet. The

frictional force between the work-piece and the container helps to

push the material into the corners with the movement of the

container, and the punch pressure to fill the cavity is reduced

almost to a half when the oscillation mode is used as shown in

Figure 65b.

(a) Motion of container

(b) Punch pressure

Figure 65: Closed die forging with container driving [107].

This method was applied to forming of an outer spline and

gear. By moving the container to the axial direction, the frictional

force over the teeth of gear or spline is changed from the radial

direction to the axial direction, and this brings about lower filling

pressure and more uniform deformation of the teeth. [110,140].

In extrusion of a double cup product with the usual fixed

container, one of the cup walls reaches the objective length first

and is stopped by the stopper tool, and then only the other wall is

extruded. The punch pressure is raised extremely when the flow

is changed to one side extrusion. When the container is moved to

the axial direction as Figure 66, the extruding velocities to both

sides can be controlled in such a way that both walls reach the

objective lengths at the same time, and the extrusion pressure is

kept low during the whole process [108]. This idea was extended

to using a moving container with tapered inside surface, where

the extruded cup is elongated by ironing [139].

(a) Before deformation (b) After deformation

Figure 66: Forward-backward combined extrusion with container driving

[109].

5.5.3 Piercing against counter pressure

Piercing is often employed to make a hole in the ring shaped

billet for cold forging. When a long billet is pierced, the volume

of the discarded slab cannot be neglected and the surface quality

of the pierced surface is poor.

Nishiyama et al. [99] applied a counter pressure from the die

exit to the slug (part to be thrown away) as shown in Figure 67a.

By the slide motions of a servo press shown in Figure 67b, the

counter pressure is applied to the slug and the deformation mode

is changed from simple shear to extrusion of a forward bar-

backward cup, and the length of the slug is shortened.

(a) Piercing method

(b) Motions of main and sub slides

Figure 67: Apparatus and slide motions for piercing against independently

driven counter punch to cause back pressure [99].

19

Matsumoto et al. applied this method to piercing of a

magnesium alloy at room temperature [72] with a servo press. By

applying a counter pressure, brittle fracture is prevented and a

long smooth shear surface is obtained, and the length of the slug

is shortened as shown in Figure 68.

Figure 68: Effect of pressure on smooth shear surface and slug length in

piercing of Mg alloy against counter pressure [72].

5.5.4 Extrusion against counter pressure

Because a servo press can be operated with synchronization

of air and hydraulic cylinders, and moving the secondary slide

axis with a controlled force, it is convenient to use it for the

processes that apply a force to the tools during forming.

In forward extrusion, the shape defects shown in Figure 69a

tend to occur. When the billet height is short, the cavity defect is

caused on the rear surface of the product. If a counter force is

applied to the extruded end by a retreating counter punch driven

by a servo cushion as Figure 69b, the rear cavity may be

suppressed and the front surface may be flattened.

As shown in Figure 69c, the rear cavity disappears when the

pressure is about 0.3 times as high as the flow stress, and the

front end becomes completely flat when the counter pressure

exceeds the flow stress [35,105].

(a)Shape defect (b) Pressure supported counter punch

(c) Effect of counter pressure on defect formation

Figure 69: Prevention of extrusion defects by using pressure supported punch [105].

When a work-piece is extruded through multiple exit holes,

the extruded lengths are different from each other. By using a

counter tool supported by a proper pressure, it is possible to

obtain the same extruded lengths as shown in Figure 70 [103]. It

was found that the extruded lengths could be equalised with an

average counter pressure less than 5 % of the flow stress.

(a) Extrusion method against pressure supported tool

(b) Extruded products

Figure 70: Forward extrusion against pressure supported tool [103].

Otsu et al. proposed an extrusion method of a forward rod

and a backward cup by driving the two punches using a double

axis servo press [112]. The speed of the counter punch was

controlled to form the length of the forward rod accurately. As an

extension of this method, reversed slide extrusion shown in

Figure 71 was proposed, in which the extruded forward rod is

pushed back while the main punch was kept at the final position.

By employing this motion after combined extrusion, the

dimensional accuracy and the flatness of the front end were

improved.

(a) Initial (b) Extrusion (c) Reverse motion

Figure 71: Extrusion with reversed slide motion [112].

5.6. Hot die forging

In hot die forging, it is generally believed that high slide

speed is desirable to avoid cooling of the billet and heating of the

dies due to the direct contact of the hot billet and the cold dies.

On the other hand, it is known that the flow stress is strain rate

sensitive at high temperatures, and a low speed deformation

brings about a low forming force as is utilised in isothermal

forging with the dies kept at a high temperature. Thus the

appropriate forging speed may be determined by the temperature

and heat conductivity of the dies. Maeno et al. examined the

effect of the slide speed in spike forging by using a servo press

for an aluminium alloy. As shown in Figure 72, the spike height

is the largest for v = 15 mm/s becomes the largest because the

20

equivalent stresses near the corner of the spike portion for v = 6

and 75 mm/s are larger than those in the flash portion, and thus

the flow into the spikeportion becomes small [67].

In this way, servo presses may be used at an optimum speed

depending on the balance of cooling by the dies and heat

generation by plastic deformation.

Figure 72: Effect of slide speed on spike height for spike forging obtained

from experiment with Al-Si alloy [67].

5.7. Combination of forging and joining

The new plastic joining method for fixing a cold bar with a

hot forged plate shown in Figure 73a was studied by using a

servo-press. During indentation of the cold bar into the hot plate,

a high speed movement of the bar prevented cooling of the hot

plate and resulted in a low indentation pressure and high bonding

strength as shown in Figure 73b [71].

(a) Joining method by i\\\\\\\\\ndenting cold bar into hot disc

(b) Effect of indentation velocity

Figure 73: Effect of indentation velocity on forming pressure and bonding

stress in joining of cold bar with hot forged plate [71].

5.8. Improvement of workability in forging

In upsetting tests of a magnesium alloy AZ31B at 473K, the

effect of the strain rate change during an operation shown in

Figure 74a was studied. Slide motion (A) was the natural motion

of the link type servo press (Komatsu) without speed

modification. Motion (B) accelerated the speed at a reduction of

20%, and motion (C) decelerated the strain rate. As shown in

Figure 74b, it was found that slide motion (C) caused a higher

ductility than (A) and (B) by about 30% [76].

(a) Strain rate change in upsetting on servo press

(b) Effect of strain rate on ductility

Figure 74: Effect of strain rate change on ductility of AZ31B at 200 ºC [76].

By utilizing the function of the servo-press that the forming

velocity can be freely changed, the best working condition to

give a large working limit was searched for Ti by Venugopal et al.

[137]. The deformation mode was plotted on the map of

temperature and strain rate and the optimum temperature and

strain rate avoiding instable deformation was found for

commercially pure titanium.

6. Concluding remarks

6.1 Advantages of mechanical servo press

In this review, various servo press designs and their

applications are introduced. The apparent advantages of the servo

press can be summarised as follows:

(1) Because all the press motions such as starting, velocity

change and stopping, are done only by the servo-motor, the

mechanical servo press has a simple drive chain without a

flywheel, clutch and brake that are essential for a

conventional mechanical press.

(2) The flexible slide movement enables the most suitable

motion for each forming method or product. The optimised

motion can extend the forming limits, enhance the

productivity, and improve the product accuracy.

(3) Due to the reduction of friction loss by clutch and brake,

reuse of the stored kinetic energy during deceleration and

the optimised slide motion, the energy consumption in

forming is reduced.

(4) A servo press with multiple driving axes enables to

incorporate in-die operations that can reduce the number of

forming steps and to form more complex and accurate parts

in one press.

21

(5) Due to the computer control of servo-motors, many servo

presses can be connected by driving with different motions

in order to maximize the production efficiency.

6.2 Industrial trend

At present, the servo presses are used mainly in the fields of

sheet metal forming to extend the productivity and the forming

limit, and also to improve the dimensional accuracy of the

products. Some auto makers have built tandem press lines for

stamping of automobile bodies to attain greater formability and

better productivity. The industrial trends of mechanical servo-

drive presses indicate as follows:

(1) In the last decade, more than three thousands of servo

presses with relatively small capacities, up to 400 ton, have

been built in both C frame and straight side frame designs.

(2) Several press lines with large straight side sheet metal

forming presses up to 3000 ton capacity have been built and

used in automotive production. They offer considerably

larger stroking rate than the conventional transfer presses for

stamping of large body components.

(3) It can be expected that the application of servo presses will

increase rapidly since these presses realize higher

productivity as well as quality improvements.

6.3 Future prospect

Due to the rapid changes in information technology and

progress of global production, servo presses will be integrated

into the large scale manufacturing systems. Although the future

prediction may be easily differed by various factors, the

followings are the present prospects of the authors:

(1) New and innovative forming methods will be developed by

using the flexible slide motion and multiple driving axes.

(2) To optimize the complicated slide motion of servo presses,

computer simulation of forming processes will become

essentially important.

(3) Similarly to the cutting machine tools, majority of the present

presses will be replaced by servo presses, and the number of

driving axes of each servo press will be increased.

(4) Servo presses will be used on the same floor as the cutting

machine tools to make smooth production lines because they

do not cause strong vibration and impact noise.

(5) Servo presses will form integration platforms for the

combination of several different manufacturing processes.

(6) Servo presses will be preferably used for global production

because the same slide motions can be realised irrespective of

the location of production without depending on the skill of

the operators.

Acknowledgements

The authors would like to express their sincere thanks to:

Prof. J. Endou (Kanagawa Inst. Tech.), Mr. M. Nagakura

(Amada-Toyo Co.), Prof. M. Shiraishi (Kinki University), Dr. R.

Matsumoto (Osaka University), Prof. T. Ishikawa (Nagoya

Univ.), Mr. T. Nakano (Aida Engineering), Prof. A. Verl (ISW,

Stuttgart) for providing with information related to the servo

presses technology, and Dr. J.M. Allwood, University of

Cambridge), Prof. A.E. Tekkaya (Technical University of

Dortmund), Prof. F. Vollertsen (BIAS) and Prof. J. Jeswiet

(Queen's University) for giving advice during preparation of the

manuscript.

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