ELSEVIER

Abstract

Journal ofMaterials Processing Technology 63 (1997) 134-139

V-shaped sheet forming by deformable punches

L C Zhang', G Lub and S C LeongC

a Department ofMechanical and Mechatronic Engineering,The University ofSydney, NSW 2006, Australia

h School ofMechanical and Manufacturing Engineering,Swinburne University ofTechnology, VIC 3122, Australia

C School ofEngineering, Nanyang Polytechnic, Yishun Campus, Singapore 2776

Joumalof

MaterialsProcessingTechnology

The technique of elastic-plastic sheet stamping has been employed extensively in the forming of a variety of engineering components.In addition to the conventional processes with unitary rigid punches and dies, a technique of sheet stamping by deformable forming tools(SSDFT) has been established and designed into a wide range of press working. Though the technique has many potential advantagesover the conventional one, the process involved has not been studied carefully and proper guidelines are not available for practicalmanufacturing design.

The present paper describes experimental investigations into the deformation mechanisms of V-shaped sheet forming by defornJablepunches and rigid dies. Emphases are placed on the improvement of springback in terms of the material properties and thickness of thesheet metals, geometry and properties of the deformable punches, and geometry of the rigid dies. The study reveals that the reduction ofspringback ratio is mainly due to the interface actions offered by the punches. The deformation of the punch material during stampingalters the deformation mechanisms of the sheet and makes the springback ratio mainly negative. The development of the contact zonedemonstrates that the plastic flow of the punch material is basically three dimensional. For a given work material, therefore, the selectionof punch material is of great importance.

Keywords: V-shaped sheet forming, deformable punch, plastic bending, springback

1. Introduction

Sheet stamping process has been extensively employed in themanufacturing of a wide range of metallic engineering productcomponents. As a result, there has been much research recentlyinto the detailed mechanics involved in the process in order tounderstand and solve problems in relation to elastic springback,elastic and plastic wrinkling and material tearing [1-3].

The approaches to stamping problems, mainly of springback,have been experimental [4-7], analytical [8-11] and some haveused finite element analyses [12-15]. However, despite theprogress made in the past few decades, problems of large elasticspringback, tendency of tearing and scratching, and inflexibilityof the products using conventional rigid punches and dies stillremain. Recently, a new technique of sheet stamping usingdeformable punches and/or deformable dies has emerged and thisoffers a potentially promising alternative to produce componentswhich are otherwise difficult to make using the conventionalstamping tools. In order to efficiently design the deformable toolsfor a desired final product, one needs to understand themechanical process involved in a typical SSDFT. There is,however, a lack of research in this area except for some isolatedstudies [16-19].

0924-0136/97/$15.00 © 1997 Elsevier Science SA All rights reservedPII S924-0136(96)02613-1

The present paper describes part of our series ofinvestigations into the SSDFT. It involves the mechanism studiesof V-shaped bending of metal sheets using a deformable punchand a rigid die. Emphases are placed on the improvement ofspringback in terms of the material properties and thickness ofthe sheet metals, geometry and properties of the deformablepunches, and geometry of the rigid dies. The contact featuresbetween both the sheet and punch and sheet and die are obtainedsystematically using Prescale Films.

2. Experimental set-up

The punch and die used for the tests are schematically shownin Fig. 1 together with the dimensions and definitions of thevariables. The dies and punches' main body were made of mildsteel, but the leading edge of the punches was machined into agroove and a soft wire (of lead in this case) was mserted, whichmade the punch deformable. In a matching set of punch and die,they both had the same value for angle e. Lead wires of threediameters were used, i.e. 0.75, 1.5 and 3.0mm, and the rigid dieshad radii of 0.5, 0.75 and 1.0 respectively. A summary of thepunch and die dimensions is given in Table l.

L.G. Zhang et al. / Journal of Materials Processing Technology 63 (1997) 134-139 135

50 Table 2: Specimen materials and dimensions. t has three valuesfor each material.Material 8_60° 8-90° 8=120°

Rigid Die

M~ '0_(''''

'.Vorkp1ec= ~

(c)

66x30xt80x30xt

66x30xt80x30xt

66x30xt80x30xt

38x30xt50x30xt

38x30xt50x30xt

38x30xt50x30xt

22x30xt30x30xt22x30xt30x30xt

All the tests were performed with an Instron machine(model 4302) to which the punches and dies were securedby means of adapters. The crosshead speed was set at 25mm/min., which can be regarded as quasi-static. A 10 kNload cell with an accuracy of I% was used. During eachtest, the load-displacement curve was obtainedautomatically. The angle bent after unloading at variousstages was measured and, for typical tests, the contact areawas studied by means of a Fuji Prescale film. The testprogram involved combinations of different punches, diesand materials.

3. Experimental results and discussion

mild steel 22x30xt30x30xt

brass

copper

The mechanism of sheet stamping process by a deformablepunch upon a V-shaped rigid die was explored experimentally. Inthis section, the typical load-displacement curves are presentedtogether with successive photographs of the specimen at variousstages. The final bent angle is ultimately related to the springbackof a test specimen and relevant factors are discussed; they arepunch displacement, deformable punch radius, rigid die radiusand rigid die angle. Phenomenon of springback andspringforward is mentioned and discussed. Finally the contactarea at the interfaces is shown.

(a)

(b)

Softwu",: R ..

90

\"01 ..

2018168 10 12 14

Punch displacement (mm)

Fig. 2. Typical load-displacement curves.

3. I Load-displacement characteristics

A typical set of load-displacement curves is shown in Fig. 2for three materials with a deformable punch (DP90-0.75) and arigid die (RD90-0.75). Here DP90-0.75 denotes a deformablepunch with included angle 8=900 and a radius of 0.75mm;RD90­0.75 stands for a rigid die of angle 900 with a radius ofO.75mm.

(--0-- SheeunelaJ O.Smm - -0-- . Brass O.Smm ---0-- Copper OAmm I0.1 J

0.09

t:.08

0.07

Z 0.06

~'C 0.05

'"0...J 0.04

0.03

0.02

0.01

Tools Deformable punch Rigid die

8 Rdp (mm) Wdp(mm) Rd(mm) Wd (mm)

600 0.75 40.0 0.5 23.7

0.75 50.0 0.5 40.490° 1.5 50.0 0.75 40.6

3.0 50.0 1.0 40.8

1200 0.75 80.0 0.5 69.5

Table I: Parameters of the deformable punches and rigid dies

Three different materials, namely mild steel, brass and copperwere used and each had three thicknesses in the range of 0.3 to0.9mm. The sheets were cut into rectangular specimens of width30mm to be placed either on or in the die, depending on thelength cut. The dimensions of the specimens are given in Table 2and the mechanical properties of the metal sheets were obtainedfrom standard uniaxial tensile tests.

Fig.!. Geometries for the deformable punch (a) and rigid dies (b); andthe tooling set-up (c).

136 L.c. Zhang et aL I Journal of Materials Processing Technology 63 (1997) 134-139

20

II <I,

I

:6 18'"12'0

II

In the primary zone, the force rise rapidly from zero to pointA at an initial small punch travel of 2 mm. This mainlycorresponds to the elastic behaviour of the material where theload increases almost linearly with the punch displacement untilthe onset of the plastic deformation.

In the secondary zone, the bending force somehow rises topoint B and then gradually falls until it reaches point C asindicated in Fig. 4. The increase in load is due to the developingand spreading of the plastic zone while the decrease results fromthe relatively large rigid-body displacement and the lowerdeformation resistance of the specimen in this stage. Point C canbe considered as an onset point of coining.

In the coining zone, the force rises sharply with themovement of the punch. This is the final process of the V-diebending. In the case of bending with a rigid punch and die, thespecimen has full contact with the punch and die surfaces. With adeformable punch, however, the specimen has almost full contactwith die surfaces (depending on the radius of the die) and thepunch surface is separated by the deformed softwire (lead), seeFig 3. The contact zone will be illustrated later in the section.

Generally, the force magnitude required for V-bending varieswith the material parameters and tool geometries. From the load­displacement characteristics shown in Figs 2 and others obtainedfor different tool combinations, the following generalobservations can be made, though some of them may beexpected:

3. Bending force decreases as the die opening Wd increases.This is evident as the lever arm is getting longer for a largerWd, which for a given specimen leads to a smaller force.

1. Bending force increases with the material thickness and itsYoung's modulus. In general, for the same material, the greaterthe material thickness, the higher is the plastic bending momentof the cross section, hence a higher bending force is required.

2. Bending force increases with large radius. This is due to thechanges in the contact condition which leads to a change in thelever arm. The present experimental results are in goodagreement with those from a finite element study by Lange etat [12] where it was found that the force increases with punchradius.

Punch displacement (mm)

Fig. 4. Three stages of a general load-displacement curve.

0"110.09 r '0.C8 r IIam f

!2 0.C6 t AI

C.C5 T ~.a l,r....l 0,04 I

IC.CC r0.02 r0,01 f00------l..------------------"-------1

a

Fig 3 presents the photographs of a specimen of mild steeltaken during a test for DP90-0.75 and RD90-0.75.The angle of Vshaped specimen during loading, a, was measured. The specimenwas unloaded at each stage in order to measure the angle ofspecimen after unloading, ~. Thus before a test the flat specimenhas a value of 1800 for a.

All the load-displacement curves for the different toolcombinations exhibit similar characteristics, as shown forexample in Fig 2. In general, a load-displacement curve can bedivided into three zones, namely, the primary zone, the secondaryzone and the coining zone as illustrated in Fig. 4.

Fig. 3. Photographs of a specimen during test at a displacement of 8, 16and 18mm, respectively.

L.c. Zhang et aL / Journal of Materials Processing Technology 63 (1997) 134-139 137

-0-- SheetmelaJ O.5mm . - Qo •. Brass O.5mm --O-Copper OAmm - .:--:-- . OatlJm

Deformable punch radius (mm)

Fig. 6. Influence of deformable punch radius on 13: 8 =90°.

2.51.3

,;- . - . - . - . ~- - - . - . - . - . - - - . - - ~

0.5

The curves show a similar trend for all the materials studied.The accuracy of the bent component is largely affected by thedeformable punch radius. From Fig 6, the optimum value for thedeformable punch radius is about 1.5 mm for all the threespecimens.

8 00I~ :6

8Ic:::. ~ t

~2180 .!...I "'__-.......-'-_-----.---~-

a

3.2 Influence ofdisplacement on a and ~

4. The bending force prior to the coining process is not affectedby the die radii ranging from 0.5 mm to 1.0 mm. This is due tothe fact that there is no contact between the specimen and thecurved die region.

The int1uence of the punch displacement D on the bentangles can be illustrated in Fig. 5. where a and ~ are plottedagainst D for a deformable punch and a rigid die. The diffetencebetween the two angles represents the angular springback. Fromthe plots, the springback value reduces with the increase of punchdisplacement until point B where the angle after unloading, ~, isless than that before unloading, a. Further increase of punchdisplacement leads to an increase of both a and ~ with a beinglarger than~. For a rigid punch, the maximum punch travel is20.00 mm limited by the die opening depth. For bending with adeformable punch, the punch travel can be more than 20.00 mm,depending on the extent of deformation on the softwire lead.Similar experiments were conducted for brass and copper sheets.

The load-displacement curves are particularly useful as aguide to determine the force capacity requirement of a stampingmachine.

3.4 Influence afrigid die radius on ~1SO

,.0 a..en

130OJ

2-., 120e;,c 110'"01C 100~c

90.,aJ

80

70

a

Fig. 7. shows the plot of ~ versus die radius Rd with punchDP90-0.75. This indicates that ~ is little affected by the rigid dieradii between 0.5 mm to 1.0 mm.

........ :1~

"-enOJ >+- • _. - .>+- . _. - ·x

\ 2-

---~ /d

B 8S

c:::. H....... ~ .... _-=-210 12 ,. 16 18 20 22 80 C --a 0

75

Rigid die radius (mm)

1250.5 0.750.25

70 +----~-__~--~---~---_+_--a

Fig. 5. Angle bent during loading and after unloading: DP90-0.75,RD90-0.75. mild steel, t=O.5mm.

-0-- a. (angle during loading) - -0- . /3 .(angle ahet unloading) IPunch displacement (mm)

Comparing the final bent angle from the two processes, it canbe seen that V-bending with deformable punch is more accuratethan the conventional method. This may be due to the moredistributed force transmitted due to the lead deformation. Thenegative springback, or springforward, after points A and B 10

the two plots is discussed in detail later in the section.

3.3 Influence ofdeformable punch radius on ~

The influence of the deformable punch radius on angle ~ canbe seen from the experimental results plotted in Fig 8 for threedifferent materials; ~ value was obtained at the end of each test.Here the included angle e for both the punch and die is 900 witha die radius of 0.5mm.

---0- Sneetmetal O.Smm --0--Capper O.4mm • - '6- - . Brass O.5mm -)+- 4 Oah.Jm

Fig. 7. Effect of rigid die radius on 13: 8=90°.

3.5 Influence ofpunch and die angle eon ~

In Fig 8 ~ is plotted .. against punch and die angle e withRp=0.75mm and Rd=0.5mm. The datum line represents the idealcase where the springback is zero. The region above the datumline is springback region while below this is the springforwardregion. It is evident that springforward occurs for all the casesstudied. The difference between the datlim line and the finalangle 13 increases as angle e increases and this implies thatspringforward becomes larger as e increases.

1.075

0.975 -0- SMeetmetal 0.5mm - -a- . Brass 0.5mm -O--Copper OAmm

3. 7 Mechanism ofspringback and springforward in V bending

However, one can explain this by carefully following the bendingprocess of the specimens observed.

Refer to the photographs in Fig 3. The sheet is initially under3 point bending with the both support ends and the punchloading being point up to say 18mm. During this period, theplastic zone develops only in the central region and the rest of thesheet remains elastic. Upon unloading, most of springback occursin the central plastic zone where the bending moment ismaximum. This causes a positive springback.

Further loading from this point leads to the both ends of thesheet to lose contact with the die and the contact points betweenthe sheet and die move substantially towards the centre. Thiscorresponds to the increase of the punch force as the lever arm isreduced. As the punch moves down further, the two ends of thespecimen move toward the punch faces, a result of rotation in thesupport points of the specimen. This is still a case of 3 pointbending and the springback is also positive.

Two additional loading points appear when the two ends ofthe specimen contacts the faces of the punches. The specimennow is under deformation similar to a 5 point bending. As thepunch pushes down, the two ends of the specimens are forcedtowards the die surfaces. At this stage, plastic deformation mayoccur in the two contact regions of the specimen with the die.Therefore, upon unloading the elastic recovery there may resultin an apparent negative springback, see the last photo in Fig. 3.This explains, qualitatively, the forming of the springback andspringforward. This experimental observation is in goodagreement with the velocity field of a V bending obtained from afinite element analysis for rigid punch cases (13].

3.8 Contact area

As mentioned earlier, springback is caused by the elasticredistribution of the internal stresses upon unloading. The elasticredistribution of the internal stresses is in turn dependent on themagnitude and direction of forces acting on the specimen duringbending. In order to determine the manner and amount of forceacting on the specimen, Fuji Prescale films were used. Theworking principle of Prescale films is as follows. Once appliedwith pressure, the chemicals within the film reacts and this turnsthe colour of the film into red. The intensity of this colour (red) isa unique function of the value of the pressure applied.

A deformable punch (6=90°) with a radius of 3.0 mm (DP90­R3.0) was used together with a rigid die (RD90-RO.5) to bend a0.5mm thick brass specimen. The shape and size of the contactarea between the deformed softwire (lead) and specimen as wellas between the specimen and die surfaces were attempted duringthe bending process utilizing both the medium and high pressurePrescale films. Fig 10 shows the deformation of both thespecimen and the lead. The Prescale films are illustrated in Fig.11.

The contact area increases as the deformation proceeds. Onthe top surface of the specimen, there is full contact with thedeformed lead. However, the contact zones at the bottom surfaceof the specimen gradually spreads with the central regionuntouched. The deformation may be likely caused by theinteraction between the bending and stretching due to thefrictional effect. An interesting feature is that for both top andbottom surfaces of the specimen, the overall contact zone size issimilar. Thus, to a large extent the pressure applied to thespecimen via the lead may be directly transmitted to the die faces

0.90.80.7

./

./

0.6

L.C. Zhang et al. ! Journal of Materials Processing Technology 63 (1997) 134-139

•• '6- .. 8rass O.Smm . ,/

-~"':""40atumline //./"./ .y. ...

./ "/. ,.../ .-";/,..

,..'~.

~.'

Die angle (deg)

0.50.40.3

/

./ .<l./

/

./

/

/

;.( Springrorward region

<l

50 70 80 90 100 110 120

Springback region

--<>- SMeetmetal 0.5mm

- {J - Copper OAmm

0.95 J-__+--_---'---+----+----t---+---..­02

Fig. 8. Effect of e on p.

3.6 Springback ratio K

The above results of springbackJspringforward have beenpresented in terms of the values of a and ~ individually. It canalso be expressed in terms of the ratio of the two angles, orspringback ratio, detined as K=~/a. Springback ratio of less thanunity is denoted as "springback" and that of more than unity istermed as "springforward". A unity springback ratio means thatthe workpiece is perfectly bent without any springback orspringforward.

It would be straightforward to re-plot the previous figures onthe effect of various parameters in terms of this new parameter K.Fig 9 shows the effect of specimen. It may be seen that K isalways larger than unity and a thicker material possesses a lowervalue ofK. All the three materials exhibit a similar trend

Thickness (mm)

Fig. 9. Effect of thickness onspringback ratio K.

138

120

110

100

Ci'":2- 90

= 80 I

"!50

50 I4Q

50

It is well known that springbackJspringforward is caused bythe elastic recovery of bending deformation in metal sheets uponunloading and the permanent bending is caused by the plasticdeformation. From the experimental results, it was observed thata specimen could experience either springback or springforwardwhen unloaded at different stages in the process of V bending,see Fig. 7. This may be difficult to understand at first glance.

>c:: 1.05

.2e

.:0: 1.025<.J

.QOlC

~(fJ

L.G. Zhang et al.lJournal of Materials Processing Technology 63 (1997) 134-139 139

just underneath the same region of' the specimen. Thedeformation of lead is not in a plain strain condition with aproportion around the end region being "squeezed" out.

Fig. 10. Photograph of deformed lead and specimen.

n=20mm D=2lmm D=22mmFig. II. Impression of contact area obtained from film; top: between leadand specimen; bottom: between specimen and die. 0: punchdisplacement.

It is evident that the shape and size of the contact areachanges as the punch displacement increases. At punchdisplacements greater than 18 mm, a slight increment in punchtravel will cause a significant increase in punch force. This forceis in .turn transmitted onto the specimen via the deformed lead.Hence trom the shape, size and the colour density of the Prescalefilm, pressure distribution on the specimen can in principle bedetermined. However, because the films got saturated easily dueto the high pressure involved, accurate systematic pressuredistribution is difficult to obtain. nevertheless, the results givereliable information on the size and shape of the contact zones.

4. Conclusions

A large number of experiments have been performed anddetailed observations have been carefully made in this study ofV-shaped bending using a deformable punch. Especially, theglobal deformation process of the specimen, load-displacementcurves, and size and shape of the contact zones in the interfaceshave been investigated. Much attention has been paid to themeasurement of springback or springforward values and theeffect of the relevant parameters on it has been thoroughlystudied. The obtained clear physical understanding of theproblem will be good guidelines for engineers and it will form asolid base for our further theoretical studies.

Acknowledgment

This work was partly supported by an ARC Small Grant. Ms JueWang formatted the text.

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