A Study Into Cold Rotary Forming of Precisison Metal Components

7/24/2019 A Study Into Cold Rotary Forming of Precisison Metal Components

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SIMTech technical reports Volume 8 Number 2 Apr-Jun 2007

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A study into cold rotary forming of precision metal components

C. C. Wong, A. Danno, K. K. Tong, M. Tsuyoshi, K. B. Lim, and M. S. Yong

Abstract In the work reported in this paper, asimple flow forming and form rolling facility wasestablished to investigate the feasibility of formingthin (thin wall cups) and gear profiles. The resultsshowed that it is feasible to adopt a two step formingprocess, bending and flow forming to enable mate-rial flow along the mandrel in order to form a thin wallcup component using two different profiles and

adopting an axial roller movement. Quality of thecups formed depends on the diameter reduction,

starting disc thickness of the blank and the number of

passes in the flow forming stage. For form rolling ofspur gear profile, the accuracy of the tooth profiledepends on the roller indentation, blank diameter andmatching of the phase angle of the rollers.

Keywords: Rotary forming, Flow forming, Form roll-ing, Gear, Roller path

1 BACKGROUND

In recent years, there is a growing demand forlight weight and higher value-add components by the

OEMs of transportation industries. These can beachieved with lower density materials and newstructural designs. As component shapes are becom-ing more complicated, machining is not a cost effec-tive process for producing these components andshould be minimized as a production operation. As aresult, precision forging, or net-shape forging, hasbecome increasingly popular due to savings in mate-rial, energy and finishing steps. Precision forging issometimes described as close-tolerance forging to

emphasize the goal of achieving, solely through theforging operation, the dimensional and surface finish

tolerances required in finished parts.

However, as manufacturers strive to reduceweight and cost, many of the new components, be-cause of their shape complexity and complicated tooldesign and high load requirements, are challengingthe current precision forging technologies beyond itscurrent level of technology. In order to meet this re-quirement, there is a renewed interest in incrementalforming, especially rotary type incremental formingprocesses, such as swaging, cross-wedge rolling, ringrolling, rotary forging, conventional spinning andflow forming. As these processes involved plasticdeformation of small volume of the workpiece at atime, the power and working forces required are re-

duced significantly, allowing more complicatedcomponents to be produced on relatively small ma-

chines using simple tool shapes. In addition, tool lifeis much improved as compared with forging proc-esses.

Although developments in incremental forminghave expanded manufacturers options in the designof a particular component, a major problem withalmost all incremental rotary deformation processeshas been the high cost of very specialized equipment.Therefore, the aim of this research is to develop amore generic incremental cold rotary forming tech-nology utilizing the concept of flow forming and form

rolling for wider applications. Thus, we have adoptedthe term cold rotary forming which encompasses theflow forming and forming rolling processes, in thisreport.

2 OBJECTIVE

The work described in this report represents aninitial stage of this research, which seeks to obtainfundamental and basic understanding into the coldrotary forming (flow forming and form rolling) ofaxisymmetrical hollow aluminum components pro-filed, small gears and non ferrous materials. The ob-jectives of this study are to investigate the flow

forming of thin wall cups and the form rolling of spurgear profile.

3 METHODOLOGY

3.1 Flow Forming of Thin Cup Profile

In this work, a Mazak NC lathe was utilized as aflow forming machine. Only one roller was used ineach experiment. A roller tool was designed and builtto accommodate the lathe tool post, as shown in fig.1.The mandrel was clamped using the lathes chuck andthe workpiece was fixed onto the mandrel and tight-

ened by a bolt. In addition, in order to minimize radialdeflection of the mandrel during flow forming opera-

tion, a mandrel holder was designed and fixed ontothe lathe bed. Figure 1 shows the experimental set up.

Two different rollers were used as shown in Fig.2. RollerA(shown in Fig. 2(a)) has an approach angleof 60 and the second roller, rollerB, has an approachangle of 20, shown in Fig. 2(b). In order to reduce theloading on the machine and prevent severe radialdeflection of the roller tool, an annealed aluminumalloy, A6061 was used as the workpiece. The hard-ness of the aluminum disc is approximately 49 HV.Flat disc blanks of diameter 80 mm and thickness of 5

mm and 10 mm were used as the starting workpiece.


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Formeddisc

blank

Roller ath

Mandrel

Work iece

Radius reduction

Fig. 1. The experimental set-up of flow forming test.

(a) RollerA (b) RollerB

Fig. 2. The designs and geometries of the rollers.

Two flow forming steps were proposed in thisexperimental study to investigate the feasibility offorming thin wall cups from flat disc blanks. In the

first step, rollerA(Fig. 3(a)) was proposed to bend

the disc blank to the preset diameter into a cup shapeproduct. In the second step, roller B(Fig. 3(b)) wasused to flow form the wall of the cup onto the mandrelto obtain uniform wall thickness, desired internaldiameter and height.

For both forming sequences, the mandrel and theworkpiece were rotating and the roller was fed alongthe workpiece parallel to its axis at a preset interfer-ence (diameter reduction) for a pre-defined length.The roller path for both sequences is shown in Fig. 3.The rotation of the workpiece was fixed at 250 rpmand the axial feed rate of the roller was set at 1 mm/s(0.24 mm/rev). Cutting oil was used at the interface

between the roller and the workpiece as well as theinterface between the workpiece and mandrel. The

initial thicknesses of the workpieces investigatedwere 5 mm and 10 mm.

(a) Bending of flat disc blank

(b) Forming to achieve desired internal diameter and wall

thickness.

Fig. 3. The roller path of the flow forming sequence.

3.2 Form Rolling of Spur Gear

For the form rolling of spur gear in this study,Tsugami rolling machine was used. The diameter ofthe roller die and blank was calculated based on themeshing condition between the bank and the roller.

The workpiece was first placed on a workpiece holderand subsequently clamped at its center-line portion bytwo centering stocks with pneumatic pressure. Theworkpiece was then fed into the rotating roller diesaxially. The roller indentation was prescribed by set-ting the distance between two roller axes. The axialfeed rate of the workpiece is 140 mm/min. Figure 4shows the external view of working area after rollingoperation.

Approach

angle

Noise

radius

Roller

Mandrel Original workpieceMandrel holder

Approach

angle

Formed cup

Roller

Mandrel

Roller


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Fig. 4. The external view of working area after rolling

operation.

Before form rolling, the roller die was first ro-tated to its initial rotational position, i.e. at zero de-grees. Then in order to correct the mismatch betweenphase angles of both rollers, a very small indentationwas made on the blank by rolling and the mismatch inthe pitching marks induced on the blank was meas-ured approximately by a vernier caliper. The initialdifference in roller phase angle was 0.925. The ma-terial of blank was a low carbon chromium alloy (JISSCR415, 0.13~0.18%C, 0.15~0.35%Si, 0.60~0.85%Mn, 0.90~1.20%Cr) with hardness of around 200 HVafter annealing (850C 4 hr, FC).

4 RESULTS & DISCUSSION

4.1 Flow Forming of Thin Cup Profile

Figures 5(a) and (b) shows the final deformedshape of the disc blank after the bending process forstarting thicknesses of 5 mm and 10 mm (at differentdiameter reductions). For both starting thickness ofthe disc blanks, it can be seen that at diameter reduc-

tion above 3%, a cup shape component was producedby simply translating the roller in the axial direction

after a certain diameter reduction was set. For bothstarting disc thicknesses, too small a reduction will

result in insignificant cup height and wall thickness.This is due to the high rigidity of larger wall thicknesswhich hindered the bending mode.

(a) Starting wall thickness = 5 mm

(b) Starting wall thickness = 10 mm

Fig. 5. Deformed shape for with diameter reduction forstarting thickness of 5 mm and 10 mm.

Figure 6 shows the variation of cup height andwall thickness with diameter reduction. It can be seenfrom the figure that for both starting disc thicknesses,cup height increased linearly with increased diameter

reduction. However, for diameter reduction above19%, the height of the cup increased drastically. This

is because for diameter reduction above 19%, thematerial that was being deformed comes in contact

with the mandrel at the beginning of roller axialtranslation. This forced the material to flow axiallyalong the mandrel, thereby elongating the formed cup.On the other hand, for diameter reduction less than19%, the cup was practically formed in the air, i.e.without any support on the inner walls of the cup (seeFig. 3(a)), and there is no contact between the innerwall (internal diameter of the formed cup) and themandrel, and the cup formed was parallel to thehorizontal axis of the mandrel. The reason for thisphenomenon is that the rigidity of the cup formed bythe roller is able to withstand the localized deforma-tion that is induced by the roller during the formingprocess.

It can also be seen from Fig. 6 that for variousdiameter reductions, the variation in wall thickness

for disc thickness of 5 mm and 10 mm is not verysignificant compared to cup height. Moreover, tallercups were produced for starting disc thickness of 10mm. The taller cups produced using larger startingdisc thickness may be explained by the fact that highervolume of material was displaced axially compared tosmaller disc thickness. In other words, the height ofthe cups is directly affected by the diameter reduction.

On the other hand, the diameter reduction does notaffect the wall thickness. Wall thickness is largelyaffected by the nose radius of the roller which deter-mines the amount of plastic deformation inducedalong the wall. As a result, the variation in wallthickness for both starting disc thickness of 5 mm and

10 mm is not significant as the same roller nose radiuswas used.

Roller gear Roller die

red = 3% red = 14% red = 17% red = 20% red = 22%

red = 3% red = 12% red = 15% red = 19% red = 22%


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0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Diameter Reduction (%)

CupHeight&W

allThickness(mm) Cup height, To=5mm

Cup height, To=10mm

Wall thickness, To=5mm

Wall thick ness, To=10mm

Inner diameter of

tube touches the

mandrel

Forming

limit

Fig. 6. Variation of cup height & wall thickness with di-

ameter reduction.

For both starting thicknesses of 5 and 10 mm, itcan be seen from Fig. 7 that a step (difference in in-

ternal diameter) is formed along the inner wall of thecup. This step is more prominent in cups producedfrom larger diameter reduction. This is due to thebending mode of the flange, which caused the exte-rior of the flange that is in direct contact with theroller to flow faster that the interior surface that isfacing the mandrel. This also happen when reductionis larger than 19% but the size of the step formed wassmall due to more axial material flow.

Fig. 7. Formation of inner step along the internal diameter ofthe cup.

A critical forming limit occurred at diameter re-duction of 25%. For both starting disc thicknesses of 5

mm and 10 mm, severe breakage occur during theinitial forming stage for diameter reduction above

25%, as shown in Fig. 8. This may be due to the heavymaterial accumulation in front of the roller for highdiameter reduction, resulting in material flowingpredominantly in the radial direction as the rollermoved axially. In addition, the heavy accumulation atthe front of the roller, from high diameter reduction,gave rise to very high axial stress. This in turn causessevere bulging which led to instability and ultimatelycracking of the flange in front of the roller.

In order to elongate the cup along the mandreland to control the dimension of the formed cup in step

1, flow forming process was proposed as a secondstep to obtain the net shape product. Figure 9 shows

the percentage increase in internal diameter with cupdepth having 5 mm initial disc thickness and diameterreduction of 22% during the first step. As this step issimilar to the flow forming of cylindrical tubes, thethickness reduction for the flow forming operationwas recommended to be controlled at 20% to 30% so

as to prevent circumferential flow due to too low areduction and bell mouthing defects due to too higha reduction.

From the figure, it can be seen that after the firstpass, the internal diameter of the cup was uneven andincreases along the height of the cup. However, the

accuracy of the internal diameter was improved witheach subsequent pass and the dimension of the inter-

nal diameter is tending towards uniformity along thecup height at about 3rd or 4th pass. It is believed thatthe internal diameter will be uniform if the materialcan flow along a longer mandrel as compared to theone used in this study.

Fig. 8. Cracking due to large diameter reduction.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 5 10 15 20 25 30

Cup Height (m )

%i

ncreaseinInternalDiameter 1st pass , Tred=24%

2nd pass, Tred=29%

3rd pass, Tred=29%

Tred=Thickness reduction

Fig. 9. Percent increase in internal diameter for flow form-

ing of cups for initial starting disc thickness of 5 mm anddiameter reduction of 22%.

4.2 Form Rolling of Spur Gear

Figure 9 shows the form rolled spur gear adoptingthe through feed process. In order to examine theprofile of the teeth and to evaluate the filling of thematerial in the roller die, the gear was sectioned (cut)in the middle portion. Of all the samples formedadopting the experimental conditions mentioned inthe section 3.2, there are cases of under-filling and

over-filling of the teeth.

Step along inner wall of the cup

when material is flowing in air

Cracking


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Fig. 10. Formed spur gears.

Figure 11 shows a cross-section in the case of

under-filling which is mainly due to insufficient rollerindentation which will give rise to insufficient fill-upof gear teeth during the forming process. However forthe case of under-filling, no defects were observed.

Fig. 11. The cross-section of form rolled gear teeth with

under-fill.

However, in cases of over-filling, the lappingdefects occurred at the root and along the flank of theteeth, as shown in Figs. 12a) and 12b). These defects

are largely due to the miss-match of roller phase angle.In addition, lapping at tooth root is also due to exces-sive indentation of the roller die.

Fig. 12. The cross-section of form rolled gear teeth withover-fill and defect.

To eliminate lapping defects, a new instrument of2-gear system was designed to measure more accu-rately the mismatch between the phase angles of tworollers at the initial stage (see Fig. 13). The 2-gearsystem is clamped between the front and tail stock

(similar to clamping of the blank). To check themismatch of the roller die, the gear system set and

meshed between the two diametrically opposite dies.Adjustments in meshing had to be made to ensure nobacklash between them. Once there is no backlash, thenut was tightened to fix the gears in place.

The aligned gears were then placed on a centrework bench, as shown in Fig. 13. To measure the

degree of mismatch, the following steps were carriedout:1) The width over 5 teeth (XoandX) was measured

by micrometer;Xois the thickness for one gearwhileXis the total thickness of both gears. Thedifference in Xand Xois the mismatch in pitch

phase between the two rollers.2) The mismatch (in degrees) was then input into the

machine code to adjust the phase angles of theroller dies.Figure 14 shows the profile of the spur gear after

adjusting the roller phase angle. It can be seen thatthere is no visible lapping effect on the teeth. This

confirmed that lapping defects are largely due tomismatch of the phase angle of roller die pitch.

Fig. 13. The setup of a new instrument of 2-gear sys-

tem to measure the mismatch between the phase an-gles of two rollers at the initial stage.

Fig. 14. Measuring of a mismatch between 2 roller phases.

Figure 15 shows the flank line error on the leftand right side of the formed gear. From the flank lineprofile, a crowning of flank line on both sides of theteeth flank can be seen as well as a slight taper in teeththickness. This is due to the low forming load duringthe initial and final stage of rolling as the axial contactlength between blank and roller dies decreases in theinitial and final stage of rolling.

Underfill

Lapping defects

Xo

X

a) At tooth flank b) At tooth root


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Fig. 15. The formed spur gear after adjusting the roller

pitch.

In addition to flank line error (Fig. 16), the tooththickness and pitch error were also measured. Theresults indicated that the errors plotted are generallySine-like or Cosine-like curves which indicate theerror is mainly due to eccentricity of teeth frank inreference to the centre of gear. Nevertheless, most ofthe errors measured are within the limits of JIS 10class standards.

Fig. 16. The flank line error.

5 CONCLUSIONS

In this work, the feasibility of forming a thin wallcomponent by a two step flow forming process, usingmulti-pass flow forming in the second step has beendemonstrated. In addition, form rolling of spur gearprofile was investigated. Based on the outcomes, thefollowing conclusions may be drawn:

Flow forming of thin cup profile:

A roller with an approach angle of 60 can beused to produce an initial cup shape from a flatdisc blank.

In the first step, reduction above 25% will resultin severe cracking of the disc at roller contactarea for starting disc thickness of 5 mm and 10mm.

In the first step, diameter reduction above 19%will allow the material to flow axially along themandrel rather than radially, thereby achieving

greater height.

In first step, wall thickness of the wall depends onnose radius of the roller and cup height dependson the initial diameter reduction.

The diameter of the mandrel has to be changedaccordingly to the required internal diameter for

second step so as to prevent unnecessary flowforming passes, which will lead to galling effectsdue to excessive work hardening.

Multi-pass flow forming in second step can im-prove the dimensional accuracy and the uni-formity of the internal diameter.

Form rolling of gear profile:

The amount of roller indentation is important toprevent under-filling or over-filling defects in the

formed gear. Matching of the phase angle between two dia-

metrically opposite rollers is important to preventlapping defects.

With the correct parameters, formed spur gearaccuracy can match those produced by machin-ing.

6 INDUSTRIAL SIGNIFICANCE

Although developments in various types of rotaryforming processes have expanded manufacturersoptions in the design of a particular component, a

major problem with almost all incremental rotarydeformation processes has been the high cost of veryspecialized equipment. This study has showed that thecombination of flow forming and form rolling proc-

esses can be employed to produce higher added valuecomponents towards net shape. Advantages of this

development are:

Use as an additional operation (replacing secon-dary machining) to existing forming route.

Replacement of complicated fabrications (e.g.forming and welding) by single piece parts withconsequent improvement in mechanical proper-ties and reduced cost.

Generation of complicated shapes using simpleand cheap tool forms programmed to move incomplex paths. Thus the final shape of the

product depends on tool path rather than expen-sive tools in the case of parts with complex ge-

ometries.

Net or near net shape could be obtain because theformed component is not depend on the tool-maker but on the tool path program.

REFERENCES

[1] M. Jahazi, G. Ebrahimi, The influence of flow forming

parameters and microstructure on the quality of a D6acsteel,J. Mater. Process. Technol., vol. 103 (1-3), pp.362-366, 1997.

[2] C.C. Wong, J. Lin, T.A. Dean, Effects of roller path

and geometry on the flow forming of solid cylindricalcomponents,J. Mater. Process. Technol., vol. 167, pp.

344-353, 2005.[3] J. Yao, M. Makoto, An experimental study in paraxial

spinning of one tube end,J. Mater. Process. Technol.,

vol. 128(1-3), pp. 324-329, 2002.

Documents

A Study Into Cold Rotary Forming of Precisison Metal Components