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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].
9
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].
10
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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