Journal of

Materials Processing Technology

ELSEVIER Journal of Materials Processing Technology 59 (1996) 144-157

Tool ing and process design to cold forge a cross groove inner race for a constant velocity joint - physical model ing and FEM process s imulation

Victor Vazquez a'*, Kevin Sweeney b, Darrell Wallace a, Christian Wolf f a, Meinhard Ober a, Taylan Altan a

aERC for Net Shape Manufacturing, Ohio State University, Columbus, Ohio 43210, USA bLaboratorium fi~r Werkzeugmaschinen und Betriebslehre, RWTH-Aachen, Aachen, Germany

Industrial Summary

The cross groove constant velocity joint is widely used in the automotive industry. However, the production costs of the cross groove inner race (CGIR) are relatively high due to extensive machining and grinding operations. Substituting machining by cold forging would reduce costs but also imposes new challenges due to the geometric complexity of this part.

The main objectives of the present study are to investigate the material flow in cold forging a CGIR and to optimize the process using a multi-action tooling concept. In order to define and improve the process conditions, physical modeling experiments and two and three dimensional FEM process simulations were performed. This investigation was performed to determine the best tooling motion that may result in the most cost effective production process. The simulation results are expected to provide relevant information for the design of the production tooling.

1. Introduction

Many automotive parts have a complex geometry, such as asymmetric geometric features and undercuts [1]. Since traditional cold forging methods are not always capable of producing complex parts, these parts are often made by processes wh ich require intense machining operations (e.g. milling, gear cutting, and broaching) at relatively h i g h costs. However, the cost of manufacturing such parts may be reduced by multi-action cold forging processes.

Generally, instead of using a multi-action press, a single-action mechanical press is used, equipped with a separate, multi-action tooling to transform the one press motion into two independent punch movements. It is sometimes difficult to eject complex forged parts. Therefore, the die must often be "split" into three or more parts and more than two punch movements may be required.

* Corresponding author

0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PI10924-0136 (96) 02295-9

Extensive work has been done at the Engineering Research Center for Net Shape Manufacturing (ERC/NSM) to study the multi-action forming of complex parts using numerical analysis and physical modeling methods. The use of split tooling, wh ich requires multi-action capabilities, has been investigated for the cases of a bevel gear [2], pipe fittings [3], and the tripot (tripod) component of a constant velocity joint [1]. Most recently, this technique has been applied to the inner race for a cross groove or plunge-type constant velocity (CV) joint (Fig. 1).

1.1 Cross Groove Constant Velocity Joint

Ever increasing requirements of the auto industry made it necessary to develop CV joints that transmit constant velocity with a high torque over a larger angle and speeds up to 6000 rpm. These requirements were not satisfied by previously existing universal joints. The cross groove CV joint is an improvement of the Rzeppa CV joint, which consists of inner and outer races, six balls, and a cage between the inner

V. Vazqzez et al. / Journal of Materials Processing Technology 59 (1996) 144-157 145

and outer races. The six balls are constantly maintained in the intermediate plane Z-Z by means of the cage (Fig. 2).

of steel that is used [6]. The production cost for th is part is relatively high compared to other CVJ's because the grooves for the inner and outer races have to be broached which is a time consuming and expensive process. Therefore, this joint is only used when constant velocity and end motion are specifically required for axle driveshafts.

The inner races for other types of CV ball joints are almost entirely made by cold forging [7-9], which is a very cost effective process. Therefore by changing the process from machining to cold forging the manufacturing costs could be reduced considerably.

Z X

Y

Z Fig. 2: Technical concept of a constant ve loc i ty universal (Rzeppa) joint [4]

Fig. 1: An example of a cross groove universal joint [4]

The cross groove CV joint allows relative ax ia l displacements of the shafts (plunging: in some cases up to 48 ram). The grooves of the outer and inner race cross at an angle of -32.4 ° to get this effect (Fig. 3). This results in a reduction of vibration and noise, a significant reduction in joint size, and an increase in the maximum speed and torque. The joint's biggest disadvantage is that the allowable axia l displacement is inversely related to the a l lowable angular movement between the two shafts. This design offers the most mass efficient CV joint; i.e., the joint has the least mass for the same functionality compared to other joints, and it has a wide application [5].

2. Manufacture of Cross Groove Inner Race

The common manufacturing steps for the inner race are: 1) peeling of bar to control the volume, 2) billet sawing from a cold drawn bar, 3) cold forging 4) broaching of the grooves, 5) induction hardening or carbonizing of the grooves depending on the type

The advantages of cold forging the outer shape of the CGIR are:

• faster process, because the forming time takes only a few seconds

• cheaper, because the high production rate of a forging press eliminates the expensive tooling and time consuming process required for machining

• hardening of the grooves can be ach ieved due to the plastic deformation

Outer Race 32.4 ° \

Inner Race /

Axial Displacement

/ -- ~7- / " Ball

/ / /

Displacement

Fig. 3: Representation of the ball movement in the cross groove CV joint w i th an axial displacement of the outer race (plunging ability)

146 V. Vazquez et al. / Journal of Materials Processing Technology 59 H 996) 144-157

The disadvantages are: • cold forging multi-action tools are

expensive • forging pressures are large in the die cavi ty

and may damage the tool

A small number of advanced companies in Japan, including Toyota [10] and more recently, Aikoku Alpha [9], have begun to produce the inner race by cold forging with multiple-action tooling.

The grooves must be very accurate for the proper functioning of the CV joint, and they must be produced to at least near-net shape so that only one grinding operation is needed to finish the part.

an additional movement to separate the upper and lower die.

The large forging pressure needed to fill the die segment corresponding to a groove seems to be one of the main reasons for obtaining insufficient die f i l l and the desired tolerance of the cold forged groove. Thus, the objective of this study was to develop a new design concept for tooling to improve the tolerance that is achievable in the cold forged groove. It is expected that tooling pressures will be very high and that there might be some diff icul ty in filling the cavity completely, especially at the edge of the groove and the outer surface of the par t (Fig. 4).

3. Research Objectives

The overall goal of this project was to develop a tooling design concept for cold forging the cross groove inner race. This goal was achieved through:

• Literature and patent reviews • Investigation of the potential problems in

the existing processes • Development of new tooling concepts based

on the above investigation • Evaluation of the feasibility of the new

tooling concepts using solid modeling techniques (I-DEAS) and 2D finite element simulation (DEFORM)

• Design and construction of a tooling for physical modeling of a cross groove inner race

• Analysis of results of the physica l modeling experiments to better understand the material flow and to verify 2D numerical simulations

• Evaluation of the physical modeling results using 3D FEM simulations to determine accurate load and strain distributions

4. Tooling Concept

The cold forging of the cross groove inner race is very difficult, because the grooves of the inner race act as undercuts. This makes ejection of the part in the conventional manner impossible. To eject the part, the die must be split into several die segments. These die segments must either be wi thdrawn radially or horizontally, using retractable pins and

Poss ib le areas of underf i l l

Fig. 4. Possible areas of underfill during forging of the cross groove inner race.

It is clear that the metal flow in this part is not symmetric. The first space to be filled is the zone where the distance between the pins is greatest (points 'A' in Fig. 4); the last space to be filled is that where the distance between the pins is the least (points 'B' in Fig. 4). Generally, it is assumed that the filling of the die would be at first in the middle of the billet, if the inner race had para l l e l grooves. Superimposing these two phenomena, the first filled cross section is somewhere between the middle of the billet and the cross section with the greatest distance between the pins.

V. Vazqzez et al. / Journal o f Materials Processing Technology 59 (1996) 144-157 147

x] IX]*

[X]**

original patent patent can be easily modified to apply to this group

patent can be modified, with some problems, to apply to this group

Summary of U.S. Patents

~ ~ Pins w i t h d r a w n Radia l ly

Die Held together by

a Ring

[11]

[12] [13]*

[23]*

Dies Held together by a Mechanism

I active semi passive no patent found die and die and pin pin pin die

active active

[16]* [161" / [16]* [171" [171"

[171" [18]* [18]*

~ Radially [18]* [191 [19]*

'~ E ~ ~ ~ O [11]* [251" [16]* [171 / [16]*

[12]* [19]** [181 [171"

"~ Vertically [131" [18]** [19]* [18]* r~ ~ ~ [11]* / / [131 [161 I15] [14]

[20] Radially with Pins [24] [ 19]* [ 18]*

rePionvSed n i l n ve r t i ca l ly | i

active passive

[20]*

[21]

[22]*

[23]*

[20]*

[211 [22]

[23]

no t poss ib le

p ins i n c l u d e d in die

Fig. 5. Summary of U.S. Patents for use in the tooling design for cross groove inner race. [26]

Fig. 5 gives an overview of the different patents developed for designing the forging tooling. The patents may be classified in two main groups [26]: 1) how the pins move; 2) how they are held together. There are two directions of motion, horizontal and vertical. The vertical direction includes the direction of the grooves. The two general possibilities of holding the tooling together include: a ring holding all the dies together, and some sort of mechanism (such as a hydraulic unit) used to hold individual die segments together. If the latter case, there is a chance that the die or the pins can be used as punches, in an "active" way. If the die and the pins are merely positioned by a mechanism or are stationary over the whole forging process, they are "passive".

In general the tooling can be split in two ways: • six die segments, each segment containing

the inverse form of the groove • an upper and a lower die which split

horizontally and 6 individual pins to form the grooves

Two plausible designs were chosen from a number of conceptual designs for investigation, which were developed based on the patent search and l i terature review. Both concepts employ the use of six die segments which move radially. Fig. 6 a schematically shows the chosen conceptual design, where the die segments are brought together around the billet with a conical ring. The billet is then extruded radially with two counter-acting punches.

148 V Vazquez et aL / Journal of Materials Processing Technology 59 (1996) 144-157

After deformation, the punches and the die segments retract and the formed workpiece is taken out of the die.

Fig. 6b shows a variation of the conceptual design, where a "two-body" punch design provides various alternative punch motions. This conceptual design can take advantage of the "divided flow method" [27], which can reduce the tool pressure significantly while the billet fills the die cavity.

The tool design can use a one body punch or a two body punch. The two body punch process can be divided into three stages (Fig. 7). In the first stage,

the inner punches are extended and the outer punches are retracted. The billet is placed between the inner punches. In the second stage, both punches move down at the same velocity. Therefore the material is forced to flow into the cavity in a backward extrusion manner. In the third stage, there are two possibilities: first, the inner punch can be retracted (as is shown) while the outer punch continues to move until it stops at its final position (Punch Variation 1); second, the inner punch remains extended until forming with the outer punch is finished (Punch Variation 2).

In punch variation 1 the relief hole could help to reduce contact pressure at the tool-billet interface. This is also known as the divided flow method [27].

The advantages of the "divided flow" punch process are:

• The accuracy of the billet weight is not so critical since the excess material can flow into the relief-hole

• The material in the extra volume after the first stage is near the area where f i l l ing problems most likely will occur (at the comer of die wall and outer punch bottom)

conical ." . . . . . . . . . L

A - A |

Fig. 6a: Conceptual design for the forging of a cross groove CV joint inner race, using retractable die segments.

Fig. 6b: Cross sectional view of the tooling concept with two-body punch.

V. Vazqzez et al. / Journal o f Materials Processing Technology .59 (1996) 144-1.$7 149

Punch Process v

bill Jter punch inner pt

ca, e pid .=

Fig. 7: Three stage punch process.

• The tool pressures can be lower than the pressure generated with a single punch

• Die filling problems can be reduced

The disadvantage of the new process are: • The tooling becomes more complicated and

expensive • It is not clear how the flow in axia l

directions will effect the overall f low characteristic of the metal

• The wear on the outside of the pins might be excessive

5. Physical Modeling Experiments

A series of 3D physical modeling experiments with plasticine were performed for each of the alternative tool designs. The tools for the experiments were based on the conceptual design shown in Fig. 6b and the ERC/NSM multi-action servo press's (Fig. 8a) capabilities and limitations. The ERC/NSM press already gives two movements in the vertical direction created by two individually controlled AC servo-motors. The additional motions of the press were achieved through the use of two pneumatic cylinders wh ich are used to actuate the inner punches. The cylinders

were designed to ride, with the outer punches, on the ball screws powered by servo-motors. This a l lowed the option of moving both inner and outer punches together as one, or moving them one after the other. This press was developed specifically for phys ica l modeling of multi-action forging [1].

5.1 Preparation of the Plasticine Billets

Commonly used for physical modeling experiments, plasticine is a specific type of modeling clay. The plasticine is homogenized and de-gassed by mixing and extruding it with a two- screw vacuum extruder [6].

In order to investigate the flow of the b i l le t material during forging it is necessary to construct layered billets. This is a time consuming operation. It involves the following steps: 1) make billets of different colors, 2) slice the billets with a wire, and 3) stack slices to form a layered billet.

A new method for producing layered billets was developed for this project. The steps for this new method are as follows: 1) plasticine sheets of two colors are rolled with a conventional pasta machine, 2) the sheets sit for a day to relieve residual stresses caused by rolling, 3) the sheets are stacked alternating colors using a thin mist of acetone to bond them, 4) the stack of sheets sits for several hours,

150 V Vazquez et al. / Journal of Materials Processing Technology 59 (1996) 144-157

Fig. 8a: ERC Multi-Action Press with physical modeling tooling for the cross groove inner race

Fig. 8b: Plasticine billet in tooling cavity, centered by die segments

and 5) stamp the billets out with a tool similar to a cookie-cutter. This method produces very clean, evenly layered billets with less effort than the previous method.

5.2 Physical Mode l ing Exper iments

The lubrication applied on the tool surface is a mixture of glycerin and liquid soap. The billet was placed in the die cavity between the two inner punches, surrounded by the six punch segments (Fig. 9b). Figs. 9a and 9b show the final stage of deformation for a normal plasticine billet and for a layered billet respectively. Fig. 10 shows the f inal plasticine part for all three variations in punch

movement. The single punch option shows the best fill but also exhibits the most flashing.

6. 2-D Finite Element Method (FEM) Simulations

Using a simplified geometry, two dimensional FEM simulations of the forming operation were performed with DEFORM (Design Environment for Forming). The specified billet dimensions were as follows:

Diameter 44mm Heigh t 30.25 mm Mater ia l AISI 1055

V. Vazqzez et al. / Journal of Materials Processing Technology 59 (1996) 144-157 151

Fig. 9a: Cross groove inner race in plasticine tooling

Fig. 9b: Half of a layered specimen of the cross groove inner race

Fig. 10: Final cross groove inner race from plasticine;

Left: one body punch variation Center: punch variation 2 Left: punch variation I

For this simulation the simplified cavity was filled completely. The simulation was axisymmetric and the material used for modeling was AISI 1050 ( ~ = 971.5E°16N / m m 2). The metal flow, as predicted

by 2D FEM and as predicted by physical modeling, were compared for the single punch variation.

As seen in Fig. 10, there are areas in the par t which do not fill completely. The adequate f i l l ing

152 E Vazquez el al. / Journal o f Materials Processing Technology 5 9 (1996) 144-157

of the cavity depends cn the initial billet size, the punch travel and the friction conditions.

One set of experiments were performed for each of the following cases:

• Punch variation 1: Inner punch is retracted in second stage.

• Punch variation 2: Inner punch remains in fully extended position during the second stage. In this case, it is not allowed to move further into the cavity.

• One body punch variation: Both punches move together to final position in one step.

The results for each of these variations can be seen in Figs. 11-13.

The deformed layered billets, obtained with the punch variation 1, are shown in Fig. 11. The inner punch is retracted after the first three steps shown. The flow lines from DEFORM agree very well w i t h those of the plasticine part. In the far r ight plasticine part, one can see a ridge that was formed as the outer punch moved into the cavity. There is also some evidence of bulging into the open die cavity. There seems to be filling with minimal f lash.

In punch variation 2, seen in Fig. 12, the inner punch remains in the cavity after the first three steps shown. The flow lines from DEFORM match very well those of the plasticine part. In the far right plasticine part, one can not see the ridge t h a t was present in the previous case.

In the one body punch variation, seen in Fig. 13, the inner and outer punches move together as a single punch. The fill is complete with f lash

occurring between the die segments and around the punches, especially in the case where the plasticine billet was slightly oversized.

7. 3-D FEM Simulations

In order to obtain a more accurate prediction of the metal flow, strains, and the contact pressures distribution between the billet and the die, 3D FEM simulations were performed for different variations of the punch movements. DEFORM 3D was used for these simulations.

Due to symmetry of the cross groove only one sixth of the billet must be analyzed. The billet was modeled initially with 4500 tetrahedral elements and 1300 nodes. The number of elements increased during the remeshing steps, performed because of the large amount of mesh distortion occurring in the simulation. The remeshing was done automatically.

The FEM model used is shown in Fig. 14. The billet had a rigid plastic material formulation. The tools were simulated using rigid surfaces. Therefore, tool deflection was not considered in th is optimization. The symmetry condition was simulated with a plane rigid surface at 60 ° w i t h respect to XY plane.

The Process was simulated under the conditions shown in Table 1.

Fig. 15 shows the material flow in the sequence of forming operations and the effective strain for the case with the following double action punch motion: 1) the inner punches pierce a blind hole at the center of the billet (backward extrusion), 2) the inner

F i g . ~ I ~ c h ~Variation 1, plasticine (above) and FEM (below) results. The part is sectioned between the grooves.

K Vazqzez el al. /Journal o f Materials Processing Technology 59 (1996) 144-157 153

Fig. 12: Punch Variation 2, plasticine and FEM results. Part is sectioned between the grooves.

Fig. 13: Punch Variation 3, plasticine and FEM results. Part is sectioned between the grooves.

punches are maintained at the bottom of their stroke while the ring punches move against the billet and form the grooves, and 3) the punches are released, the die segments split and the part is ejected from the tooling.

The most important goal in simulating different motions of the punches is to:

,, reduce the pressure on the outer die in order to minimize die wear or fracture

• achieve cavity filling; particularly in the comers of the grooves, where underfilling is expected.

Table 1. Process conditions for 3D FEM simulations.

Velocity of inner and ring punches

Stroke

Mater ia l

Workpiece temperature

Simulation mode

Shear friction factor

100mm/s

14.1 mm

AISI-1045

20°C

Isothermal

0.1 (constant)

154 V. Vazquez et al. /Journal of Materials Processing Technology 59 (1996) 144-157

Ring )unch Top

Inner 3unch Top

Outer Die

Billet

nner 'unch ottom Ring )unch

[~ottom Fig. 14: Initial mesh of billet and tools

This can be achieved either by optimizing the force c~ the inner punch in the second stage or by retracting the inner punch, so that the material is enabled to flow towards the inside. In addition, the forces will be transferred to the mesh of the die, so that the die deflection can be calculated.

The pressures cn the outer die were calculated with a FORTRAN program. At the end of the stroke, pressure increases considerably. As shown in Fig. 16, the pressure was calculated along a line in the middle of the groove. If the ring punches move too far, the pressure is exceeded at the end of the stroke. The maximum pressure should not exceed a limit of 2500 MPa (360 ksi). In order to relieve the die pressure at the end of the stroke, the inner punches should be held down with a certain force of 0.2 MN, so that cavity filling can be achieved.

In the multi-action forming of the cross grooved inner race a compromise between filling of the cavity and contact pressure must be achieved. The best tooling design will be such that the best f i l l ing of the cavity is achieved with the lowest possible

contact pressure. 3D FEM simulation of the different concepts for the punch motion can help us decide on the best forging process before the actual tooling is designed and built.

8. Conclusions And Future Work

The aim of this project was to design conceptually a tooling which is able to produce the cross groove inner race to near-net shape by cold forging.

Beginning with a patent search and a l i terature review, a systematic approach for this design problem was conducted. This patent search and literature review provided an overview of the state of the art in industry related to multi-action forming of complex parts. In an analysis of the problems associated with the manufacturing of this part , (a) the metal flow and (b)the accuracy in the grooves are crucial.

Focusing on these two critical points, a number of tooling concepts were developed and evaluated. The result was a new punch concept with a new method for positioning the die segments with the help of a positioning ring. The punch concept requires four individual punches, two inner punches and two outer punches, to take advantage of the Divided Flow Method.

This new punch concept has been investigated with a FEM simulation, which showed that the metal flow is stable with changes in process parameters, such as a variation of billet volume. This FEM simulation was conducted in 2 dimensions, and thus a verification by physical modeling was done.

The tooling for physical modeling was designed for the ERC/NSM multi-action press. The design is made in such a way that the tooling can perform several different punch movements.

The flow lines predicted by the DEFORM FEM simulations are in good agreement with those predicted by the physical modeling for all three tested punch variations. Areas in the part w i t h potential filling problems were also identified.

The tooling for the physical modeling of the cross groove inner race is a fully functional mult iple action tooling. It can be used in the study of metal flow in the part, considering various punch motions, friction conditions, billet sizes and tooling geometry.

The two dimensional simulations were performed in full in a fraction of the time needed to simulate a complex three dimensional forging process. Therefore, 2D simulations can be successfully used in the determination of windows of operation for a particular process, whenever plane strain or

V.. Vazqzez et al. / Journal o f Materials Processing Technology 59 (1996) 144-157 155

a) b) c)

1.47

1.33

1.19

1.05

0.91

0.77

0.63

0.49

0.35 1 d) 0.21

Fig. 15: Metal f low during forging (a-c); effective strain distribution at the end of forging (d)

axisymmetry can be assumed with acceptable accuracy. Although 3D simulations of complex parts are more accurate, at the moment they require a larger amount of simulation time to achieve satisfactory results. This situation may change over

the next couple years so that 3-D FEM becomes a practical reality for simulating 3-D metal flow.

The continuation of this project will include the design of a final "real" multi-action tooling for cold forging a selected inner race.

156 V. Vazquez et al. / Journal o f Materials Processing Technology 59 (1996) 144-157

3500

3000

2500

2000

1500

1000 -20

- - e - - 5 0 % St roke

k . - ~ - - 9 0 % St roke . .

-15 -10 -5 0 5 10 15 20

a)

Plane

e Line

b)

Fig. 16: Pressure distribution in the middle of the groove (a) along the trace line (b).

Acknowlegements

The authors would like to thank DANA Corporation for their interest in the project and their help in building the physical modeling tooling. Also we extend our appreciation to the Machine Engineering Group of Toyota Motor Company for their advice in the design of the tooling concept.

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Patent #5,096,111, Nippon Steel Corporation, Tokyo, Japan, March 17, 1992.

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v

September, 1993, Osaka, Japan (8.1-8.10).