Journal of Materials Processing Technology 142 (2003) 773–785

Design and finite element analysis of innovative tooling elements(stress pins) to prolong die life and improve dimensional

tolerances in precision forming processes

Muammer Koça,∗, Mehmet A. Arslanba Department of Mechanical Engineering, University of Michigan, PO Box 131121, Ann Arbor, MI 48113, USA

b Quality Engineering and Software Technologies, Schenectady, NY, USA

Received 7 June 2002; received in revised form 22 April 2003; accepted 30 April 2003

Abstract

This paper proposes the use of stress pins as an innovative pre-stressing element, and presents the results of a design methodologyinvestigation through combined finite element analysis (FEA) and design of experiment (DOE) studies for large and non-axisymmetricprecision forming tooling (i.e. as in hydroforming and precision forging of connecting rod) where conventional shrink-fit solutions cannotbe applied effectively and economically. In summary, smart pre-stressing elements (i.e. stress pins) are implemented to overcome veryfundamental and practical problem of prolonging the die life, preventing die failures and improving dimensional part tolerances by applyinglocal and controlled pre-stressing effects.© 2003 Elsevier B.V. All rights reserved.

Keywords: FEA; Stress pins; Dimensional tolerance

1. Introduction

Improving die life is a challenging issue in the economicsof the forming processes such as forging, stamping, hydro-forming and powder forming. Die life briefly can be definedas the number of forming operations that can be achievedsuccessfully, i.e. with the required part quality, using a singletooling (die, insert, etc.). A die may fail by different modesdepending on the type of loading exerted on it. Die failuresin forming processes can be categorized as follows[1,2]:(1) wear, (2) mechanical fatigue (catastrophic), (3) thermalfatigue, and (4) plastic deformation. The crucial factors thatcontribute to the failure modes mentioned above can be listedas follows: (1) die and billet temperature, (2) mechanical,physical and thermal properties of die material, (3) surfacefinish of the die, (4) equipment type used (press or ham-mer), and (5) magnitude of the contact stresses developedduring forming. All these factors are interrelated to eachother. In order to obtain optimum process parameters at acost effective way, designers are frequently confronted withchallenges in making compromises among these factors.

∗ Corresponding author. Fax:+1-646-514-7590.E-mail address: mkoc@umich.edu (M. Koç).

Wear is one of the die failure mechanisms beginningwith the first forming operations and accumulating oversubsequent operations until total accumulated wear reachesa degree to yield parts with out-of tolerances. Die failureby fracture is generally a result of the cyclic loading andunloading of the die under high amplitude tensile stresses.Catastrophic low cycle fatigue failure of forming dies isunpredictable. Especially, during the final stage of fillingof the die cavity, a die insert is subjected to high contactstresses. Plastic deformation occurs wherever the high ten-sile stresses induced by the metal forming loads exceed theyield strength of the die material at that operating tempera-ture. As with wear, the fatigue failure process starts with thefirst forming operation causing microscopic crack particu-larly at the stress concentration region over the time. Thiscrack then propagates due the continuous cyclic high tensilestresses. As crack initiations and crack growth do not haveany apparent effect on the part quality, detection of lowcycle fatigue failure is not possible to detect[1,3–5]. Themaximum principal stress (σ1) is generally tensile along thedie surface. These tensile stresses may cause catastrophicfailure at stress concentration regions of the die, such as fil-lets and corners, at the final stage (i.e. coining or calibration)of the forming. Prevention of catastrophic failure can be ac-complished either by (a) changing the die design and process

0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0924-0136(03)00647-2

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parameters to reduce local maximum principal stresses, or(b) counteracting the high forming tensile stresses usingpre-stressing elements (shrink fits or stress rings).

Similarly, the accuracy and tolerances of the formed partdepend mainly on the following factors: (a) dimensional ac-curacy and surface finish of preform and coining tooling, (b)accuracy of the production equipment (press, machine tool,and other component), (c) volume and temperature controlof the initial blank (billet, tube, flat sheet metal, etc.), (d)elastic deflection of the tooling due to forming stresses, and(e) thermal deflections due to high forming temperatures (i.e.blank material and die). Elastic die deflections are more pro-found in cold forming operations compared to warm and hotforming[6,7]. This is because die deflection due to high tem-perature are dominant in warm forming processes. There-fore, for cold forming processes, elastic deflections shouldbe minimized in order to control the final dimensions andincrease the tool life. Elastic deflection of a die for an ax-isymmetric cold forged part can be expressed as follows:

Ra = Rd + de (1)

de = e − c + s (2)

whereRd is the designed part radius,Ra the actual part ra-dius,de the elastic deflection in mm,e the elastic deflection(expansion) of die due to forming load,c the elastic de-flection (contraction) of die when forming load is removed(springback),s the elastic deflection (expansion) of partafter it is removed from die (springback).

During warm forming operations, thermal deflections(expansion and shrinkage) should be also taken into ac-count[7]. In terms of magnitude, elastic die deflection dueto forming loads (e) is far greater than elastic die deflec-tion (contraction) upon removal of loading (c) and elasticdeflection of part after removal from die (s). Hence, mini-mization of elastic deflection of die due to forming loads (e)is impartial for achieving parts with desired tolerances, andalso prolonging die life as cyclic expansion and contractionof forming dies result in catastrophic failure as explainedbefore[1,2,8]. Therefore, in order to design a tooling withlong life cycle that will produce net shape parts withinthe precision-like tolerances, it is necessary to take intoaccount the parameters described above in terms of both

Fig. 1. Application of strip-wound stress rings for an axisymmetric cold forged bevel gear (a), cross-sectional view of insert, die and strip-wound stressring (b), and actual tooling (c)[28].

die life and tolerance considerations[9]. As the demandsin more complicated part designs and lower unit cost in-crease, in order to stay competitive and profitable, variousmeasures to increase the die life have been sought over theyears. Some researchers have investigated different approx-imate methods to improve the precision of axisymmetricand axisymmetric-like parts[6,10–12]. Precision forming iswidespread in the forging of axisymmetric shapes, like outerand inner races for bearings, and axisymmetric-like shapeslike bevel and spur gears[13,14]. Techniques of predic-tion, reduction and overall design guidelines are long-timeavailable and efficient for axisymmetric parts[6,15,16].Methods to increase die life and precision of parts include:(a) process optimization (i.e. loading profile, lubrication,perform design, etc.), (b) improvement of die surface finishand use of coatings, (c) optimization of die surface shape tominimize stress concentrations, (d) proper selection of toolmaterials, (e) frequent non-destructive testing of tooling todetect cracks, and (f) use of pre-stressing elements as de-picted inFig. 1. Particularly, design of axisymmetric toolingcan be performed efficiently using widely available designguidelines[15–17]. Effect of thermal and die deflections onthe part tolerances, and improvement of part tolerance bychanging die shape was presented in[18,19]. Optimum de-sign of preform tooling and its effect on achieving final partswith tight tolerances as well as reduced contact stresses wereinvestigated and presented in[20]. In order to minimize thepossibility of low cycle fatigue failure, concentrated contactstress values due to die geometry are eliminated or reducedby changing the die profile and perform design[21–25].

Use of stress rings has been in-place for years to re-duce die deflections and tensile contact stresses to in-crease die life [15–17,26,27]. Donfoss company fromDenmark even designed and commercialized strip-woundpre-stressing method to maximize the influence of stressrings. Strip-wound stress rings were used in cold forgingof bevel gears, gear wheels, shafts, etc. to yield in 30–50%reductions in die deflections and 3–10 times increase in dielife, Fig. 1 [28].

The allowable tolerances of the formed part, the forma-bility of the material, and the capacity of the productionequipment/system put constraints for changes in the designof die and process. Thus, optimization of the die profile for

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Fig. 2. Elastic deflections on a hydroforming tooling and part.

less stress concentrations or alterations in the preform de-sign becomes infeasible. As a result, use of pre-stressingelements (i.e. stress rings) usually left as a only prominentchoice of improving die life and accuracy. Pre-stressing ofthe dies is found to be an effective way of counteracting thehigh tensile stresses induced by the contact stresses devel-oped during forming. The use of pre-stressed dies would (a)prevent catastrophic fatigue failure by reducing the ampli-tude of tensile stress components on the dies, thus, longerdie life is achieved and (b) improve dimensional toleranceson the parts by reducing elastic deflections of dies. An in-terference between the mating outer surface of the die insertand inner surface of the stress ring gives rise to compres-sive stresses on the die surface, which is in contact with theformed part and subjected to the highest level of formingstresses. These compressive stresses reduce the effect of ten-sile stresses developed during forming. Furthermore, tightdimensional tolerances on a formed part can be obtainedusing stress rings since they decrease the elastic deflectionof the die [27,28,35]. The following factors are found tobe effective in the design of stress ring (shrink fit) toolingfor precision forging, and thus can be applied to other coldforming processes[15,16]: (1) material properties of die andstress ring (elastic modulus and yield strength), (2) interfer-ence between die and ring (magnitude and form of the in-terference, i.e. uniform or non-uniform), (3) magnitude anddistribution of the contact stresses on the die, (4) die andstress ring configuration (depends on the part shape), (5)temperature distribution on the die and stress ring.

On the other hand, very little effort has been focusedon the development of methods to improve the accuracyof tooling and increasing tooling life for the forming ofnon-axisymmetric, complex and large parts. With an increas-ing demand and recent attempts of applications for preci-sion, light weight and low cost automotive parts, advancedmanufacturing processes such as powder forging and hydro-forming have been improved to a level of mass production

of various automotive parts replacing conventional forgingsand stampings. As these processes widen their applicationareas, the requirement of long life cycle and accurate tool-ing also increases[29–34]. Fig. 2 depicts illustrative elasticdeflections on a large hydroforming tooling and part withcorresponding loading and stressing. In these cases, first ofall, use of pre-stressed tooling has not been considered andtested. Secondly, conventional analytical methods for designof pre-stressed large and complex tooling are not as effectiveas they are for axisymmetric and small parts. Overall designof tooling largely depends on experience and trial-error. Nu-merical simulations have been used during recent years topredict the accuracy of the formed part and to compensatefor the deformation of the tooling[1,25,29,36].

Use of stress rings for large and complex productsis questionable in terms of economics, practicality andmaintainability. Consequently, in this study, an innovativetooling element,stress pins, is recommended for large,non-axisymmetric and complex forming tooling systems(i.e. precision forging and hydroforming) to increase dielife and reduce elastic die deflections as it will be explainedin the next section.

2. Use of stress pins on a large hydroforming die forforming of automotive structural parts

The hydroforming process has found a wide variety ofapplications in the metal fabrication industry due the factthat, with hydroforming, manufacturers are able to producecomplex shaped parts with lightweight and fewer weldsthan with conventional metal forming techniques. Advan-tages of hydroforming include reduced tooling cost, partconsolidation on assemblies, excellent material utilization,fewer operations, and improved part quality[29,36]. Inthe tube hydroforming process, a blank tube, straight orpreformed via bending and/or crushing, is shaped in a die

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Fig. 3. Process sequence of a typical hydroforming operation where a blank tube is placed into hydroform dies, die is closed and ends of the tube aresealed with axial punches. Tube is filled with pressurizing medium (i.e. usually water), via simultaneous and coordinated application of internal pressureand axial feeding, the final desired complex shape is obtained[37].

cavity through the simultaneous and synchronized applica-tion of hydraulic internal pressure and axial compressiveforces from both ends (Fig. 3).

Because of the high internal pressure applications, hy-droforming dies are exposed to very high forming stresses.Contact stresses at the end regions are particularly higher

Fig. 4. Hydroforming tooling for an automotive structural frame part of around 2500 mm long and around 1000 mm wide (a). Use of stress rings (shrinkfits) around such large dies is impossible. Local, selective and smart stress pins can be used in order to counteract tensile forming stresses on the innerfaces of the die (b). Interference between stress pins and holes were 0.2 mm.

and harsher due to the sliding of axial punches into the diefor sealing and axial feeding purposes than middle regions.Because of these high and cyclic stressing, hydroformingdies are subject to low cycle fatigue failure and also undergodeflections that leads to out-of-tolerance products. Stresspins are proposed to be used on a hydroform tooling for

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Fig. 5. An internal pressure value of 150 MPa is applied onto inside of the hydroforming tooling to represent the maximum forming pressure. Only asection of the tooling was analyzed using the symmetry conditions.

Fig. 6. Comparison of distribution of displacement inX direction and maximum principal stress values with and without stress pins.

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precision forming of structural frame rails used in sport util-ity vehicles (SUVs) and trucks as illustrated inFig. 4 tocounteract the diminishing effect of forming loads. Typicaland approximate dimensions of such a hydroform tooling are2500–3000 mm×500–1500 mm×300–600 mm (L×W×D).The die material is usually of made 1045 low carbon steel(E = 210 MPa andv = 0.3), and contact surfaces of diesare mostly flame hardened, polished and sometimes coatedfor better surface finish and increased wear resistance. Asseen fromFig. 5, only a section of the tooling was ana-lyzed using the symmetry conditions to save time and com-putational cost. During hydroforming, internal pressure isincreased gradually to its highest values for calibration andtight dimensional control of the part. Hence, in this study,only the maximum internal pressure was used to representthe most severe conditions of the hydroform die. A pressurevalue of 150 MPa was adopted based on discussions and in-put from a real hydroform experimentation on a part similarto the one inFig. 4 [38].

2.1. Finite element analysis (FEA) results

Fig. 6 presents the distribution of maximum principalstress and deflection inx-direction for a hydroforming die

Fig. 7. (a) Comparison of deflection along the contour line in the larger arm of the hydroforming tooling various conditions. Because of the stress pins,maximum die deflection is reduced from 0.11 to 0.04 mm. (b) Comparison of maximum principal stress distribution along the contour line. Maximumstress value at the fillet of the die is reduced from 350 to 170 MPa.

with and without stress pin. As it is seen from these dis-tributions, a substantial reduction in the deflection and in-duced stresses on hydroform die is achieved with use of localpre-stressing elements. As also presented inFig. 7 graphi-cally along a representative line (i.e. beginning at the middleof the die cavity and ending at the edge), maximum prin-cipal stresses observed at the fillet region of the dies arereduced from 350 to 170 MPa. Similarly, deflections at theedge of the hydroforming die decreased to 0.04 mm from0.11 to 0.12 mm levels (threefold of reduction). Such reduc-tions in stress amplitudes would definitely be reflected inan increase in the fatigue life of the hydroforming dies as,simply, three main factors of the low fatigue life are stresseshave the following characteristics: (1) tensile, (2) cyclic, and(3) high amplitude in addition to various other factors suchas material, surface finish, stress concentrations, etc.[39].An overall improvement in terms of reducing the severity ofstressing and deflection is possible with appropriate designof stress pins. Design and selection of: (a) size (i.e. radiusand length), (b) positioning and distance to critical zones(i.e. high stress and deflection regions), (c) interference, and(d) material of the stress pins for increased benefits will bediscussed in the next section using design of experiments(DOEs) techniques.

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Fig. 8. (a) DOE factors identified for further investigation to reveal theireffects on reducing die deflection and contact stresses. (b) Responses andtheir measurement locations.

3. Design of smart pre-stressing elements (stress pins)for a hydroforming tooling using DOEs methods

In order to increase the effect of stress pins in reducingthe amplitude of the induced stresses and deflections dueto forming loads, the following factors were identified tobe influential and candidates for further investigation: (1)radius of stress pin, (2) interference between pin and hole, (3)distance of stress pin to the highest deflection and/or stressedregion, and (4) depth of the stress pin as shown inFig. 8. Inthis section, we will present the results of a DOE study todetermine the effects of above factors quantitatively.Table 1represents the selected factors, their ranges of variation andlevels used in this study based on a central composites design

Table 1Factors, ranges and levels used in the CCD fractional factorial design

Factor Levels

Low (−1) Middle (0) High (1)

Radius (mm) 30 40 50Interference (% of radius) 1 2 3Distance (mm) 130 140 150Depth (mm) 80 100 120

(CCD) with three levels and four factors resulting in a totalof 27 runs. Radius of pin was varied between 30 and 50 mmwhile interference was selected to be at 1, 2 and 3% levelsof the radius.

Responses to the design are determined as: (1) maximumdeflection (Ux) at the edge of the hydroform die and (2)maximum principal stress (S1) at the fillet region of the dieas illustrated inFig. 8. All the analyses were performed inANSYS[40]. An APD macro was written to run the simula-tions continuously without any human interruptions.Table 2tabulates the overall design and corresponding results.

As a result of DOE study, it is found that, in contrast todistance (X) and depth (D), both radius (R) and interference(I) are the most significant factors in reducing the inducedstresses and deflections at the critical regions of the hydro-forming dies as illustrated inFig. 9a. This conclusion islimited to the ranges of the factors as indicated inTable 1.Fig. 9(b)–(e) further illustrates the response surface modelsof maximum deflection and maximum stress as functions ofall factors.Eqs. (3) and (4)describe the regression modelsof maximum die deflection at the edge (i.e. atL = 1) andmaximum principal stress at the fillet (i.e. atL = 1). There-fore, when designing stress pins for precision forming dies,radius and interference should be given a careful consider-ation as long as distance and depth are in compliance withother design and manufacturability issues.

For instance, in order to achieve a zero deflection at theedge of the die, as illustrated inFig. 9a, a radius of around37 mm, and an interference of around 1.6% should be usedwhereas distance of the pin location from the edge shouldbe more than 150 mm and depth of the pin should be lessthan 80 mm. This set of conditions would then result in amaximum principal stress of around 80–100 MPa at the diefillet.

Deflection= −0.0197− 0.0623R − 0.0426I

+0.00987X − 0.0107D (3)

Maximum stress= 42.133− 102.893R − 71.53I

+11.893X − 2.043D (4)

whereR is the stress pin radius,X the distance,I the inter-ference, andD the depth.

4. Use of pre-stressing elements on a precision forgingtooling of a connecting rod

In order to investigate the applicability and feasibility ofstress pins on a different tooling and process, 3D stress anal-yses of a connecting rod tooling were performed using: (a)monobloc con-rod die, (b) con-rod die with stress rings, and(c) con-rod die with stress pins in this study. Effect of stressring in reducing deflection and tensile stresses on the surfaceof the die were verified with FEA analysis as illustrated in

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Fig. 9. Results of DOE: (a) main effects on die deflection and maximum stress. Response surfaces: (b) die deflection and (c) maximum principal stressare minimized with increasing interference and radius. (d) and (e) Effect of distance and depth on die deflection and maximum stress are less significantcompared to radius and interference.

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Table 2Overall DOE and results

Factor Response

Run Radius Interference Distance Depth Deflection at edgeL = 1 Maximum principal stress at die fillet

1 0 1 0 1 −0.0702 76.852 0 −1 1 0 0.0312 229.433 −1 0 0 1 0.0325 251.614 1 0 0 1 −0.01 25.145 1 0 1 0 −0.0731 43.9446 0 0 0 0 −0.0164 158.57 −1 0 −1 0 0.032 248.498 0 0 1 −1 0.0048 176.029 0 −1 −1 0 0.0184 226.86

10 0 1 −1 0 −0.0723 87.18811 0 −1 0 −1 0.0334 233.0812 0 1 1 0 −0.0462 92.5213 1 0 −1 0 −0.1032 37.54814 −1 1 0 0 0.0145 210.615 0 −1 0 1 0.0193 228.6516 −1 0 1 0 0.0429 250.6417 −1 0 0 −1 0.0451 252.4318 0 0 1 1 −0.017 153.6719 0 0 −1 1 −0.0355 152.8420 0 0 0 0 −0.0191 155.0221 0 0 −1 −1 −0.0152 169.222 0 0 0 0 −0.0157 110.0423 1 0 0 −1 −0.0689 67.18324 1 1 0 0 −0.1525 −40.3725 −1 −1 0 0 0.0613 288.1726 1 −1 0 0 −0.0213 149.2527 0 1 0 −1 −0.0419 111.62

Figs. 10–13. For all cases a pressure value of 400 MPa in thecrank-end region and 200 MPa in the I-Beam and pin-endregions was applied to reflect the maximum induced stressesbecause of the forming as presented inFig. 10. Only one-halfof the con-rod die was modeled due to symmetry while somenodes at the bottom were constraint in all directions to rep-resent the bolting of the die onto the bolster at the press bed.Material properties of a typical tool steel (E = 210 MPa andv = 0.3) were used throughout this study. Effect of stressrings (shrink fits) was demonstrated by comparing the diedeflections and stress values at the die-part interface as pre-sented inFig. 11. In this representative case, die deflectionat the crank-end region of the connecting rod die was re-duced from 0.22 to 0.15 mm with only 0.5 mm of absoluteinterference value that corresponds to 0.1% interference.

Fig. 10. (a) Connecting rod, (b) monobloc con-rod die and (c) con-rod die with stress ring tooling. A uniform pressure value of 400 MPa was applied atthe crank-end region while 200 MPa was applied at the I-Beam and pin-end regions to represent the induced stresses due to forming.

Similarly, maximum principal stress on the die wasreduced from 586 to 490 MPa. With increased levels of in-terference, improvements expected to be higher. Design ofpre-stressing elements in terms of their shape, material, lo-cation, interference are important parameters that need to beoptimized for a given die and stressing conditions. As a re-sult, different tooling configurations (i.e. circular die-circularring, elliptical die-elliptical ring, etc.) and interference op-tions are suggested since the loading on the die is verycomplex and non-uniform as illustrated and compared inFigs. 12 and 13. Die and stress ring shapes other than rounddesign can be considered. As it can be seen fromFigs. 12and 13, round die-round ring configuration with an inter-ference of 0.3% offers the least die deflection as the sameconfiguration with 0.5% interference cause opposite die

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Fig. 11. Effect of stress rings and stress pins on the die deflections and induced stress values. Comparison of (a) die deflection and (b) maximum principalstress induced on the die surface along the indicated line due to forming loads. Distribution of overall deflections and maximum principal stresses arealso depicted. As it is seen, due to the geometry and loading complexities, stress pins are not as effective as stress rings on the given connecting roddie in terms of reducing the die deflections and induced contact stresses on the die.

deflections. In addition, use of non-uniform interferencewould be another suggestion to be effective in counteract-ing high tensile stresses developed on the particular regionsof the die by forging loads. On the other hand, it is ratherdifficult and expensive to have non-uniform interference.

5. Use of stress pins for non-axisymmetric precisionforging tooling

In order to overcome the practical and economical diffi-culties with using stress rings for precision forming of largeand complex parts, stress pins are introduced as an alterna-tive to the stress rings to support the tooling. Stress pins arelocal and selective pre-stressing tooling elements acting ina similar manner with stress rings (shrink fits). Since theirlocation, size and number can be changed and designedin any combination, they bring flexibility and smartnessin using pre-stressing on tooling for precision forming.

Fig. 14 depicts the stress pins on the same connecting rodtooling used in the previous section. As it is presented inFig. 14, three stress pins are selectively inserted aroundthe crank-end region of the connecting rod (i.e. where thelargest die deflection and tensile stresses occur according topreviously presented results) for comparison purposes un-der the same loading and material conditions. Various stressanalyses were performed with varying stress pin diameter,interference values and location.

As a result, it is found that stress pins are not as effectiveas stress ring in circumventing the die deflections and tensilestresses occurring due to forming loading in the connectingrod tooling studied. As shown inFig. 11, even higher de-flections and stress values resulted with use of stress pins in-stead of stress rings. Obviously, because of the geometry ofthe connecting rod die and the condition of the non-uniformforming loads, effect of stress pins are in a negative mannerin terms of reducing deflections and induced stresses onthe die. Their negative effect may be eliminated by varying

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Fig. 12. Comparison of different stress ring configurations in terms of reducing elastic die deflections on a connecting rod tooling.

Fig. 13. Deflection of con-rod die insert at the forming region inZ direction under both assembly and forming loading for (a) analysis #2(b), (b) analysis#3(b) and (c) analysis #4(b).

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Fig. 14. Use of stress pins around the crank-end region of a precision forging tooling for connecting rods. Stress pin (1) has a radius of 50 mm andthere is an interference of 0.25 mm. Radius of stress pin (2) is 40 mm with an interference of 0.1 mm. Stress pin (3) is with a radius of 30 mm and aninterference of 0.1 mm.

some or all of the design elements such as interference,radius, location and material. However, it is unlikely thatstress pins in this kind of geometry will be a useful toolingelement as they are found to be one for large hydroformingtooling.

6. Discussion and conclusions

In the present study, an effective use of stress pins areproposed and validated through a combined FEA and DOEsstudy as an innovative approach to support the precisionforming dies for reducing the amplitude of tensile stressesand deflections induced by the forming loads. Stress pinscan be substantially effective in prolonging the die life andimproving the tolerances on the parts. Their ease of instal-lation, cost-effectiveness in terms of both material and laborare superior advantages over stress rings (shrink fits). For agiven die, part and process design, more stress pins can bealways added as needed to increase the rigidity of the dieand reduce adverse effects of forming loads. However, itwas shown that stress pins may not be always an appropriatefor all kind of die and part geometries as well as loading,as presented in the connecting rod tooling case. Die, partdesign and loading conditions will influence the selection ofstress pins.

As a result of this study, radius of the stress pin and theinterference between pin and hole were found to be the mostsignificant factors that reduce the deflection and stress am-plitudes at the critical regions of the die. With these findingsin mind, design of stress pins for other hydroforming tool-ing can be simplified into a methodology for efficient andrapid design.

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