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Journal of Materials Processing Technology 185 (2007) 139–146

Precision forging processes for high-duty automotive components

B.-A. Behrens a, E. Doege a, S. Reinsch b, K. Telkamp b, H. Daehndel a, A. Specker b,∗a IFUM-Institute for Metal Forming and Metal Forming Machine Tools, University of Hannover, Germany

b IPH-Institute for Integrated Production Hannover Ltd., Germany

bstract

Precision forging is defined as a flashless near net-shape forging operation which generates high quality parts concerning surface quality andimensional accuracy. In the past, precision forging processes have been industrially established for axis-symmetric parts, e.g. gearwheels andteering pinions. Further development of the technology to more complex parts, e.g. helical gears, connecting rods and crankshafts is expectedo lead to a wider implementation into industry. In particular high-duty automotive components offer a wide application field for the precisionorging technology. Advantages like shortened production cycles which are achieved by eliminating machining operations and the saving of raw
aterial contribute to the ongoing cost-saving trend in the automobile industry. The design process for complex precision forged parts has to be
urpose-built to consider the distinctions of this technique. This article describes the adopted methods and development process of a precisionorging process exemplarily considering a helical gearwheel and a crankshaft.

2006 Elsevier B.V. All rights reserved.

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eywords: Precision forging; Crankshaft; Gearwheel; Production process

. Introduction

Because of the excellent mechanical properties, namely thendisrupted and stress corresponding fibre orientation and theigh productivity, bulk forming is a very important productionechnique. Despite of these advantages, this technology is inermanent competition to other procedures, e.g. casting or sin-ering. Further on, there is an increasing thread of substitution ofulk formed products with non-ferrous metals, like aluminium oragnesium. Beside the technological competition, producers of

rop forged parts have to deal with severe conditions concerningkeener international rivalry and increasing product variations

1,2,11]. Among organisational optimisations it is necessary totrengthen the competitiveness of the bulk forming technology.ne way to enhance the engineering potentials of this technology

s to establish the precision forging technique as a near-net shaperocess. By the approximation of the drop forged part geometryo the final component geometry, precision forging enables theossibility to reduce time- and cost-intensive cutting processes3,4]. In the context of a Collaborative Research Centre (SFB89), settled at the University of Hannover, a process chain for
he precision forging of a helical gearwheel and a three cylinderrankshaft is developed.
∗ Corresponding author. Tel.: +49 511 27976375; fax: +49 511 27976888.E-mail address: [email protected] (A. Specker).

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924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2006.03.132

. Principle of precision forging processes

Precision forging is the flashless hot forging of near net-shaperoducts using closed dies. The tools for this process generallyonsist of a lower and an upper punch, one or more dies and aevice to hold the dies closed during the forming process.

The dies enable a translatoric motion for opening and closinghe dies. The punch enters into the closed die through integrateduiding assemblies. At the start of the process, the work piece isnserted in the open die sinking. In the next step the tool closesithout deforming the part. Subsequently the punches move into

he die to form the hot material to its final geometry. After theomplete filling of the form the punches return, the dies opennd the finished product can be taken out.

To achieve the complete filling of the form the work pieces supposed to have the exact volume needed for the finishedart. Too much material will cause an overflowing of the die andherefore an overload which may destroy the die.

.1. Precision forging of helical gears

Precision forging of helical gear has been widely researched
t the IFUM—Institute for Metal Forming and Metal Formingachine Tools.Figs. 1 and 2 show a variety of tool concepts for the precision
orging of helical gears [5,6].

mailto:[email protected]

dx.doi.org/10.1016/j.jmatprotec.2006.03.132

140 B.-A. Behrens et al. / Journal of Materials Processing Technology 185 (2007) 139–146

Fig. 1. Tool concept for the precision forging of helical gears from punchedsemi-finished parts.

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Fig. 2. Tool concept for the precision forging of gears from raw parts.

Helical gears, which are precision forged with tools workingccording to these concepts are shown in Fig. 3.

The demands on helical gears used in automobiles concern-ng noise development and limited installation space are veryigh [7]. Especially the noise reduction causes a necessary fin-shing machining of the helical gears. Therefore an allowance onhe helical gears has to be provided through the precision forg-ng process. In order to minimize this finishing machining thellowance has to be small-sized and very accurate. At allowancest about 0.1–0.2 mm even the shrinking behaviour of the partffected by the forging temperature and the utilized raw partaterial has to be embraced throughout the process design and

evelopment.In Fig. 4 a helical gearwheel is shown which is used as stan-

ard model part in the Collaborative Research Centre (SFB 489).

Fig. 3. Gearwheels precision forged at the IFUM.

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Fig. 4. Helical gear used for the examinations.

To reach the allowances of 0.1–0.2 mm mentioned above,ven slightest changes in the precision forging process have toe included into the design and concept of the process. One ben-fit when using precision forging is, that the process heat can besed for an integrated heat treatment. For an integrated heat treat-ent the material of the part has to have enough carbon content.oday’s standard materials for gear wheels are case hardeningteels which need a carburisation prior to an annealing. To usehe forging heat the part has to be made of heat treatable steel.he altered material choice greatly influences the formation of

he deviations of the part and therefore of the predeterminedllowances due to a different shrinking behaviour [8,9]. Fig. 5hows the results of examinations about the influence of temper-ture and material choice on the deviations of a part are shownxemplified through the tip diameter of the standard model partelical gearwheel.

The examinations carried out at the IFUM showed that thease hardening steel 16MnCr5 (1.7131) and the heat treat-ble steel 42CrMo4 (1.7225) nearly have the same shrinkingehaviour. Helical gears forged from the heat treatable steel00Cr6 (1.3505) had smaller tip diameters than those of thewo other steels examined. The diameters for the root circlehowed an identical behaviour. These examinations were carriedut with other helical gear wheels with both greater and smallerodule as well as gear wheels forged from tube line sections

nd gear wheels forged from solid line sections. All investiga-
ions showed comparable results with gear wheels forged from6MnCr5 having the greatest tip and root circle diameters andear wheels forged from 100Cr6 having the smallest tip and rootircle diameters.
ig. 5. Tip diameter of the standard helical gear addicted to temperature andaterial.

B.-A. Behrens et al. / Journal of Materials Processing Technology 185 (2007) 139–146 141

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3.2. Traditionally forged crankshafts

ig. 6. Geometries used for the investigations of the influence of the surfacerea.

The shrinking behaviour of a part is not only influenced byhe chosen material of the part but also of the surface to massatio. To investigate the influence of the surface of a part, theodel part helical gear has been forged using different variants

f the upper and lower punch. By combining these variants ofhe punches four different geometries of the part with increasingurface are forged using the same tooth system (Fig. 6). Twof the four geometries (geometry 2 in Fig. 6) were identicalside that upper and lower punch geometry were changed. Theorgings were carried out under identical conditions using theame mass for all parts.

The investigations showed that a greater part surface leads tohigher heat dissipation to the tool. Due to this, the shrinkingf the part after the forging is reduced (Fig. 7).

The results of these investigations were used for the designnd development of the tools for long parts. Especially for longarts the compensation of the shrinking is more complicatedecause of the uneven mass distribution compared to axis sym-etrical parts like gearwheels.

.2. Precision forging of long parts

Former research projects by IPH—Institute for Integratedroduction Hannover Ltd., Germany have proved the feasibilityf the precision forging technology even for flat long pieces. Therototype geometries have been symmetrical and asymmetricalonnecting rods (Fig. 8).

On the basis of the research projects, a further step to trans-er the technology to even more complex components has beenade. The crankshaft is representative for non-axially symmet-

ical long parts. Due to the highly uneven mass distribution, such

ig. 7. Tip diameter of the model helical gear addicted to the surface of the part.

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ig. 8. Principle of the moving sequence in a typical precision forging processconnecting rod).

omponents represent significant challenges for pre-formingrocesses. In contrast to the single stage forming of rotationalymmetric parts, the layout of a precision forging process foromplex geometries is due to a multistage forming process muchore complicated.

. Precision forging of crankshafts

.1. Function and design of crankshafts

The crankshaft is transferring the oscillating movement ofhe piston into a rotary movement. Each crank arm, which isivided into the crank lever and the counterbalance, is joined bymain bearing respectively a crank bearing with the neighbourrank arm (Fig. 9).

The counterbalance compensates the uneven working forcesf the driving mechanism. The torques, which are loaded ontohe crankshaft via the crank pins, twist the structure of therankshaft body and necessitate therefore a torsion proof layout.

The process chain of traditionally forged crankshafts startsith the heating of the cut slug to a forging temperature of

Fig. 9. CAD-model of a crankshaft with an elementary cell.

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The next development step consists of the design of a forgingsequence for a one cylinder crankshaft. This is composed of twohalf main bearings, a crank pin and two counterbalances.

42 B.-A. Behrens et al. / Journal of Material

250 ◦C. Further on, the heated slug is drop-forged in one toour process steps to its final geometry. Afterwards the flash,hat may embody up to 30% of the input mass, is removed. Thenishing covers processes like metal cutting, heat treatment andard-cutting operations [4,10]. Due to the very uneven mass dis-ribution along the main axis, the design of the forging dies in theorming sequence has to ensure a form filling with a simultane-usly minimum rate of cost-intensive flash. Due to the clipping,he materials fibre orientation is interrupted at the cuttingdges.

Typically, the machining allowance of the forging in theones of the main bearing and the crank pin amounts 1–3 mm iniameter. Partially this allowance is needed to ensure that struc-ural changes are removed through machining. Furthermore themount of unbalance of the crankshaft claims a component-pecific machining allowance.

.3. Characteristics of precision forged crankshafts

The application of the precision forging technology torankshaft forging operations creates the technological poten-ial to reduce costs and to shorten the manufacturing process. Inddition to the elimination of the clipping process, omitting theeating and recycling of the flash material leads to a reduction ofanufacturing costs of approximately 1.5%. However, transfer-

ing the precision forging technique from rotational symmetricorgings (e.g. bevel gears) to more complex components such asrankshafts is a special challenge.

In comparison to gearwheels or connecting rods crankshaftsave a substantially higher weight, more than any other prod-ct ever forged with this technology. The highly asymmetricass distribution along the main axis, compared to the mass

istribution of connecting rods and gears, demands a thoroughpproximation of the preform geometry. The layout of the pre-orming process is therefore an important contribution to theuccess of the whole design process.

The application of precision forging processes for crankshaftsliminates one of the main disadvantages of the conventionalorging process. Due to the fact that no clipping process takeslace after the actual drop forging, the segregation zone of theutting remains in the neutral axis of the component. Thereforencreased physical properties in comparison to conventionallyorged crankshafts can be expected. The minimization of theass of forging results in another further advantage. Since

xcess material does not have to be pressed into the flash the nec-ssary press power is reduced. Therefore the size of the requiredorging press can be scaled down in some cases. In case of annvestment for a new forging line the costs can be reduced con-iderably.

.4. The development process of precision forging ofrankshafts

Design of a process chain for the precision forging ofrankshafts has to consider the engineering features of the com-onent. An integrated approach considering the design of theorging sequence and the tool design by means of CAE-chain

cessing Technology 185 (2007) 139–146

nd theoretical verification through the simulation with finitelement programs becomes vital.

Complete construction and design of the precision forgingrocess are done in a closed CAE-chain. The geometry of theorging sequence as well as the tool layout are designed withhe CAD-system Pro/ENGINEER. The forming processes areimulated with the FE-programs MSC.SuperForge and Forge3.n analysis of the thermal and elastic strain of the dies andunches during the forming process is carried out. The toolsre meshed and the initial thermal and mechanical boundaryonditions are defined for each process. Temperature field of theorging is transferred from step to step. Furthermore, an analysisf the material flow of the steel during the complete forgingequence is carried out. Analysis also comprises the search fororging mistakes such as folds and under-fillings as well as thenalysis of the strain rate.

.5. Elementary cell

Since the complexity of precision forging of crankshaftss very high, the development process takes place in sev-ral development steps. Each step is concluded with theerification of the developed process by means of practicalests.

Because crankshafts consist of a regular structure, the firsteries of tests contain the precision forging of an elementaryell. An elementary cell is composed of a main bearing, onerank lever and one crank pin.

The modular build-up of the tool allows the production of aomplete crankshaft using eight of these modules. The experi-ental verification was done at the IPH. During the experiments

he verification of the precision forging process for the elemen-ary cell was achieved (Fig. 10).

The work pieces were made of C45, an industrially usedrankshaft steel. The material was cut from a rod with a massolerance of ±3 g. The forging temperature was TS = 1250 ◦C.o prevent scale formation, the heating process took place in anlectrical oven fitted with airtight metal hoods.

.6. One cylinder crankshaft

Fig. 10. Tested forging sequence of the elementary cell.


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Fig. 11. Precision forging sequence of a one cylinder crankshaft.

Different from the elementary cell the raw part is a squareection part. It is formed to the final geometry in two intermediateteps (Fig. 11).

The asymmetric mass distribution of the one cylinderrankshaft is concentrated in axial and radial direction. Theadial asymmetry is caused by the internal structure of the crankrms and the counterbalances.

For the first intermediate step a single punch tool concept wassed. The principle layout of the tool is, except the unappliedower punch similar to the tool concept of the final precisionorging process. For the final precision forging process a tooloncept with four punches – two in the upper die and two in the
ower die – was applied (compare Fig. 12).
To achieve a short material movement in the precision forgingrocess it is necessary to accomplish that the main part of the

ig. 12. Tool concept for final forming stage in the precision forging of a oneylinder crankshaft.

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ig. 13. An optimised preforming process leads to a better mass distributionnd therefore to better forging results for the precision forging step.

equired mass disposition takes place during preforming steps. Aell-adapted preform has been created in two preforming steps.or this purpose conventional preforming methods have beenodified.Hereby the consideration of preferably equal and short form-

ng paths takes a centre stage during the optimisation process. Anterative advancement of the preform geometry is accomplishedy FEM-based simulations. Fig. 13 shows the beginning and theesult of the optimisation process for the second preform of thene cylinder crankshaft.

In consequence of the optimisation the forming distance wasut down by 15 mm. Besides the saving of press space, the opti-isation leads to better forging results due to equally forming

aths.The tool concept for the production of the second intermedi-

te good is based on a forging operation with a partial open die.uring the forming stage, bearings and pins are moved in their

orrect position. The diameter of the main bearing and the crankin was reduced by 1 mm in diameter. Also the thickness of thereformed counterbalance has been reduced. This is necessaryo be able to insert the second preform into the final forgingie.

In preparation for the precision forging of the completerankshaft the forging operation was designed with consider-tion of the angle of forging. Since the second preforming stepf the complete crankshaft is a tri-directional forging operation,ne along the longitudinal axis and two along the respectiveateral axis of the counterbearings, its counterpart of the oneylinder crankshaft is a bi-directional forging operation. Insteadf aligning the lateral axis of the one cylinder crankshaft hori-ontally with the dies, its axis is inclined with an angle of 30◦compare Fig. 14).

During the forming process the tool components responsibleor the compression movement along the longitudinal axis griphe main bearings and the crank pin.

The compression movement proceeds simultaneously withradial tool movement of the middle grip plate along the

ateral axis of the counterbalance to induce the formation of the

144 B.-A. Behrens et al. / Journal of Materials Processing Technology 185 (2007) 139–146

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fof the clamps, (3) the movement of the clamps, (4) the opening ofthe clamps, (5) the unloading of the dies and (6) the return of theclamps to their starting position. The movement of these clampsis driven by a wedge gear mechanism. The wedge gears are next

ig. 14. Tool concept for the second preforming step of a one cylinder crankshaftith middle grip plate for the radial motion of the crank pin along the lateral

xis of the counterpart.

ounterbalances and the displacement of the crank pins out ofhe centreline of the crankshaft.

The bi-directional movement of the tool components isealised with the redirection of the vertical ram movement. Thushe complex forging operation can be realised with a conven-ional vertical-acting press technology.

The chronology of the different tool component kinematicss of immense importance to assure the desired forming. Theequired kinematics are realised using a very precisely designededge gear mechanism. Different speeds and distances can be

ealised by adjusting the angle of the wedge and its counterpart.double threaded gearing mechanism is also possible to expand

he usable forming paths.

.7. Three cylinder crankshaft

The development of the forging sequence and the tool layoutor a three cylinder crankshaft are carried out similar to the pre-ision forging process of the one cylinder crankshaft (Fig. 15).

For the redistribution of the mass in the radial direction in therst step a lateral extrusion process concentrates the material to

he counterbalances. The dies for the compression along the lon-itudinal axis are partially profiled for the asymmetric contoursf the counterbalances preform. During this first precision forg-ng process the diameter of the main bearings and the crank pinsre formed out, provided with a shrinkage of the final geometryimensions.

The second process produces the compression of the crankevers and the translation of the crank pins to an eccentric posi-ion (Fig. 16, bright for the main bearings and dark for the pins).

For the simultaneous compression of the crank levers alonghe longitudinal axis and the translation of the crank pins, grip
lates are clamping the main bearings and the pins. The move-ent of the clamps in three directions – one along the lon-
itudinal axis and two along the respective lateral axis of theounterbearings – produce the correct deformation.

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Fig. 15. Precision forging sequence of a three cylinder crankshaft.

The grip plates are divided into two parts, allowing the open-ng and closing of the tools to load and unload this forming step.or a three cylinder crankshaft it is necessary to clamp the threeins and the four main bearings at the same time. Thereforehe second tool has seven clamps, each one with an individual

oving character.The chronological order of the single procedures during this

orming step is: (1) the loading of the open dies, (2) the closing

ig. 16. Principle tool for the second preforming process of a three cylinderrankshaft.


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mclosing force profile for each process. To retract the tool compo-nents after the precision forging process all frames are connectedwith chains to pull the different components to the initial positionagain.

ig. 17. Principle tool for the precision forging process of a three cylinderrankshafts.

o the clamps and powered by the ram. All tool componentsre kinematically controlled for the special requirements of theorged crankshaft type.

The final tool for the precision forging process is a horizon-ally divided tool with punches integrated in the upper and theower die. The intermediate part geometry of the previous pro-ess features the shrunk diameter and the correct position of theearings and pins. Only the crank lever is not yet differentiatednto crank arm and counterbalance. The dies represent the geom-try of the main bearings and the pins. The punches are designedo form the geometry of the crank levers (Fig. 17).

A heating and cooling system controls the temperature of theurfaces between the dies and the punches. The chronologicalrder of the single procedures during this forming step is: (1)he loading of the open dies, (2) the closing of the dies, (3) the

otion of the punches, (4) the opening of the dies, and (5) thenloading of the dies.

.8. Generating the closing force

During the precision forging processes the dies have to keeplosed to avoid the formation of horizontal thin flash. Therefore,t is necessary to generate a vertical force which presses the uppernd the lower die onto each other. There are different possibilitieso put on the needed closing force. One option is to include diskprings between the ram and the upper die, respectively, theable and the lower die. When the upper and lower die comen contact with each other during the forging operation theytop moving and subsequently the punches penetrate into theorging die. During this operation the springs between the diesnd the punches are compressed and induce the closing forcento the dies. Depending on the geometry of the forged parts,he generated force has to be adjusted to the closing forces torevent the disengagement of the dies during the ongoing forgingperation.

An important consideration for the design of a system tonduce the closing force is the energy consumption of the forg-
ng machine. If the closing force is produced with disk springshe energy is stored in them. The amount of energy stored in theprings can be much larger then the amount of energy neces-ary for the forming process. During the design of the closing
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ig. 18. Deforming stages of a sample aluminium body to load up a closingorce on the dies for a precision forging process.

orce inducing system it is therefore essential to match the pro-uced closing force with the needed closing forces. Using diskprings, there are only little possibilities to apply a special pro-le of the induced force, if the mandatory force at the end of

he process is fixed. To be able to adapt the produced closingorces the disk springs can be substituted with deformable alu-inium bodies. These bodies are produced from the plastic alloylMgSi1 (compare Fig. 18).Their geometry was computed using FEM-programs to find

he necessary force function for the process. These forces aresed to keep the dies closed. The energy for the production ofhe closing forces is linked to the plastic deformation of theluminium bodies. The amount of the energy and the heightf the momentary force depend on the geometry of the bodies.mall forces are produced at the beginning of axial deformation,

ncreasing to high forces at the end. This matches to the closingorce requirements of the precision forging process.

The use of plastic deformation bodies is suited to the experi-ental investigation because it is possible to produce a specific

ig. 19. Schematic tool construction of a forming machine for precision forgingperations with integrated gas springs responsible for applying the closing force.

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[10] K. Lange, Umformtechnik, in: Handbuch fur Industrie und Wissenschaft,vol. 2: Massivumformung, 2nd Ed., Springer, Berlin, 1988.

46 B.-A. Behrens et al. / Journal of Material

For industrial use it is necessary to integrate a special toolor this task into the forming machine. For this purpose a tooloncept for a forming machine was developed by Muller Wein-arten in which the components responsible for applying thelosing forces are an integral ingredient of the tool construction.as springs are used to apply the closing force (Fig. 19).Employing a different gas pressure, the gas springs are able

o be adjusted to different application cases.

. Conclusion and future areas of research

In order to fulfil the demand of economic and ecologic forgingrocesses it can be necessary to shorten the process chain byliminating machining operations. This requires high precisionorging operations regarding the surface quality and dimensionalccuracy. One suitable solution is the flashless precision forgingechnology.

The presented precision forging technologies for a gearheel and a crankshaft consist of multistage processes includingreforming operations and a final forging. Preforming steps cane carried out in closed dies or with a partly open tool system.he principle of the tool concept for the final forming is theeparation of the closing of the dies and the forming throughovable punches.The developed technology for the precision forging of an

lementary cell is transferable to the precision forging of ahole crankshaft. Design of a process chain for a one cylinder

rankshaft represents an intermediate step which suits theurpose of formation of knowledge.

[

cessing Technology 185 (2007) 139–146

eferences

[1] Informationsstelle Schmiedestuck-Verwendung imIndustrieverbandDeutscher Schmieden e.V. (IDS), Schmiedeteile, Gestaltung, Anwendung,Beispiele, Hagen, 1995.

[2] VDI-Z Integrierte Produktion, Schmiedeteile aus Aluminium, SteigendeNachfrage sorgt fur kontinuierliches Wachstum 143 (6) (2001)72–73.

[3] R. Balendra, Net-shape forming: state of the art, J. Mater. Process. Technol.115 (2) (2001) 172–179.

[4] R. Douglas, D. Kuhlmann, Guidelines for precision hot forgingwith applications, J. Mater. Process. Technol. 98 (2) (2000) 182–188.

[5] E. Doege, R. Bohnsack, C. Romanowski, Tool technologies for nearnet shape forgings, in: International Conference on Forging andRelated Technology ICFT’98, ImechE, Birmingham, 1998, pp. 311–321.

[6] E. Doege, R. Bohnsack, Closed die technologies for hot forging, J. Mater.Process. Technol. 98 (2) (2000) 165–170.

[7] F. Klocke, H.K. Toenshoff, E. Brinksmeier, C. Evans, T. Howes, I. Inasaki,E. Minke, J.A. Webster, High speed grinding—fundamentals and state-of-the-art in Europe, Japan and the USA, Annu. CIRP 46 (2) (1997) 715–724.

[8] R. Bohnsack, Untersuchungen zum Praezisionsschmieden von Laufverzah-nungen, Dissertation, University of Hannover, 1999.

[9] M.H. Sadeghi, T.A. Dean, Analysis of dimensional accuracy of precisionforged axis-symmetric components, Proc. Inst. Mech. Eng. 205 (1991)171–178.

11] S. Reinsch, B. Muessing, B. Schmidt, K. Tracht, Advanced manufacturingsystem for forging products, J. Mater. Process. Technol. 138 (1–3) (2003)16–21.