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Alternative die designs in net-shape forging of gears
Journal of Materials Processing Technology 150 (2004) 48–55
Alternative die designs in net-shape forging of gears J. Cai a , T.A. Dean a , Z.M. Hu b,∗ a
Manufacturing & Mechanical Engineering, School of Engineering, The University of Birmingham, Birmingham B15 2TT, UK QinetiQ Limited, Future Systems Technology, Cody Technology Park, Ively Road, Farnborough, Hampshire GU14 0LX, UK
b
Abstract This paper deals with the aspects of die design in the gear forging process. It discusses alternative tool designs which may be used on a press with only one moving slide and ejection system. It examines the influences of different designs on the metal flow and load requirements through experiments and finite element simulation. The effects of friction on the loads and mode of metal flow are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Net-shape; Gear forging; Finite element
1. Introduction Precision forging of gears has been the subject of considerable effort in the last few decades in the forging industry. Various technologies have been employed to forge bevel, spur and helical gears, in an attempt to reach the finished form without finish machining. The aim of precision forging is to produce gear forms which require no subsequent operations to make them fit for purpose. If forging is possible at room temperature (cold forging) accurate tooth forms can be achieved. Currently bevel gears, particularly those for automobile differentials, are cold forged to finished form on the teeth. Spur and helical gears of higher quality, such as those for automobile gear-boxes, cannot be cold forged, but have to be forged at elevated temperature, either warm or hot, resulting in dimensional inaccuracy. Additional machining procedures to correct this inaccuracy will add tremendous difficulty and cost to the process, which then becomes not economically competitive. A possible commercially viable routine being developed is a combination of warm and cold forming processes. This paper deals with the aspects of the warm forging process. It contains discussion of alternative tool designs that may be used on a press with only one moving slide and ejection system. In various arrangements of the tool sets, the frictional forces on the interface between the tool and the workpiece act differently and affect the manner in which ∗ Corresponding author. Tel.: +44-1252-397016; fax: +44-1252-397298. E-mail address: [email protected] (Z.M. Hu).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.01.019
cavity filling takes place. In this paper, the influences of different designs on metal flow and load requirements are examined through experiments and finite element simulation. The effects of friction on the forging loads and mode of metal flow are also discussed.
2. Completely closed cavity tool-set design The function of a tool-set is to enable a part of specified shape to be formed. Basic knowledge of the design of forging tools for net or nearer net-shape forging has accumulated over the past 30 years. Aspects of the design and the use of this type of tooling were analysed and discussed in the early days of research and development by Dean [1]. Hollow cylindrical billets are often used to forge net-shape axisymmetric and hollow parts such as gears used in power transmission systems. The precision shape can be formed with parallel bores, by using completely closed dies with mandrels. Various tool-set designs are possible for finishing the forging of gears in completely closed cavity dies [2]. Four working elements are required to forge hollow shapes; a punch, a counter punch (ejector), a container and a mandrel. A generic assembly of the elements is shown in Fig. 1 where only a cylindrical cavity is illustrated for simplicity [3]. The punch forms the top surface of the cavity and is attached to the moving ram of the forging machine. The counter punch is attached to the stationary machine bed. The container and mandrel which define the periphery and bore of a forging, respectively, can be attached to either part of the forging machine, either rigidly or with allowance for vertical movement. The basic function of the
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
49
Fig. 2. Forging billet and forged gear.
low or solid; the container may be attached to the ram or the machine bed as a fixed or a movable item; the mandrel may be attached to the anvil or punch and be fixed or movable; the anvil can be penetrating or non-penetrating. In this paper, the effects of the arrangement of the tooling on die filling were analysed in detail using the case of gear forging. The die cavity filling and load variations were modelled using finite element software and tested through forging trials.
3. Alternative die designs for gear forging Fig. 1. Elements of a tool for the forging of hollow parts.
tool is that the workpiece is deformed by relative closure of punch and counter punch. The mode of the flow of the workpiece depends on the relative movements of the tool elements and therefore, cavity filling and the positioning of a compensation space (if it is used) will be affected by tool kinematics. The sequence with which a workpiece will fill a cavity can be determined by noting the direction in which friction forces act. The possible combinations of tool elements were thoroughly discussed and analysed in [3]. The punch can be hol-
The commonly used preform for the forging of a gear is a hollow cylindrical billet with its external diameter close to the root diameter of the gear. A billet and the forged gear are shown in Fig. 2. The 27-teeth gear is of 36 mm height with a module of 4.233 mm. The gear is to be warm forged. The gear material is AISI 8620H. There are several possible die designs for this type of part and in this paper studies of die movement are dealt with. In the two tool-set designs considered, the die is either fixed rigidly to the machine bed or attached to the machine bed by a light spring such that the die can move together with the ram/punch. In both cases, the punch is moving down with
Fig. 3. Schematics of gear forging tool designs with a flat punch and counter punch. (a) I: die is elastically attached to machine bed; (b) II: die is rigidly fixed to machine bed.
50
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
Fig. 4. Schematics of gear forging tool designs with a chamfered punch and counter punch. (a) III: die is elastically attached to machine bed; (b) IV: die is rigidly fixed to machine bed.
the ram while the counter punch is fixed to the machine bed during the forging process. Schematics of the two designs are shown in Fig. 3, where the die is elastically connected in (a) and rigidly fixed in (b). On the right is the arrangement of the tool elements before the deformation and on the left is their arrangement after deformation. The main difference in these two designs is the effect of the frictional forces in the interface between the workpiece and the die once the material fills the tooth die cavity. In the fixed die design, the punch will move into the die cavity during forging and tooth forms have to be made on the punch, while in the moving die design, the punch will not enter the die cavity, but pushes the die downwards, so that no tooth forms are needed: a simple cylindrical shape can be used. The dimensions of the forged tooth form depend not only on the expansion due to the elevated temperatures of the billet and the tools but also on the elastic expansion of the forging tools due to radial pressure which is related to the forging load required to complete the forged shape. In gear forging, the mid-section of the gear teeth fill in advance of the top and bottom faces. It is in the last stages of forging that top and bottom corners of the teeth are filled and it is this filling that requires high forging force [4]. A significant increase in load arises when the workpiece reaches the die. As analysed in [4], the final punch movement of 0.3 mm, 1.2% of the billet deformation, to fill the corner was accompanied by a load increase of nearly 50%. Easing flow into the corners by die design techniques or designing gears which do not have sharp corners can dramatically reduce forging load requirements and tool stresses. The benefit will be less distortion, better dimensional accuracy and longer tool life. One possible design is to leave a chamfer in the corner of the tooth tips to accommodate extra metal and reduce the amount of die/workpiece contact in the final forming stage. The excess metal formed on the teeth can be easily faced off in a simple post-forging operation. Possible similar de-
signs are shown in Fig. 4, both the punch and counter punch being modified with chamfers on the tooth tips. The gear forging process using the four designs has been analysed using the finite element method, considering the filling of the die cavity, and load requirements. The adopted design is used to conduct forging trials and the experimental results are compared with those from finite element simulations.
4. Formation of tooth form The four designs in Figs. 3 and 4 have been analysed using the finite element method. Because of symmetry, a portion corresponding to half a tooth was used for analysis. The case with moving die and chamfered punches is shown as an
punch toothed die
mandrel
billet
counter punch
Fig. 5. FE model for gear forging simulation.
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
example in Fig. 5. The geometrical model and FE meshes were generated using PATRAN. Abaqus/Explicit was used for the analysis. The gear was warm forged with the billet heated to 950 ◦ C and the tools heated to 200 ◦ C. Water based graphite was used as billet and die lubricants and a frictional factor of 0.1 was used in finite element simulation [5].
51
The stages of tooth forming simulated using finite element analysis are shown in Figs. 6 and 7 for the four alternative die designs. In the early stages of gear forging, the mode of the metal flow is close to open die upsetting. Thus, the material in the middle section flows faster than the material in the top and bottom region because of the friction force on the punch and counter punch. As forging continues, the
Figs. 6 and 7. Tooth formation using two die designs with flat punches: (a) fixed die; (b) moving die. Fig. 7. Tooth formation using two die designs with chamfered punches: (a) fixed die; (b) moving die.
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
partly formed tooth is in contact with the toothed die surface, which is either fixed rigidly with machine bed or moving together with the punch when the die is elastically attached to the machine bed. The frictional resistance from the toothed die surface acts differently according to the die movement. When the die is fixed, the frictional force opposes the metal flow downwards and the top region fills more rapidly. The upper part of the tooth is formed prior to the middle and bottom regions. When the die is elastically attached to the machine bed, it moves down with the punch during forging. The frictional force then assists the material to flow downwards and the material in the bottom parts flows faster than that in the middle and top region. Hence the forming of the tooth is from bottom to the top until the die cavity is filled. The differences between the flat and chamfered punches are evident only in the final stage of the forging when the corners are filled. The top corner is formed prior to bottom corner when the die is fixed and vice versa. With the use of flat punches, all the top and bottom corners can be fully filled with metal. In the case of chamfered punches, the formed chamfer will be removed afterwards and full filling of the die corners is not necessary, given that the full tooth volume is formed. Therefore, only the top corner is fully filled when the die is fixed while only the bottom corner is filled when the die is elastically attached. 5. Forging loads Forging loads are carefully examined because excessive load will cause the die to expand, impairing the accuracy of the forged parts and reducing die life. Forging loads using the four alternatives of die designs are shown in Fig. 8, where the punch load of moving die design include the load acting on the punch and the load acting on the die because the die is pushed downwards by the punch. The metal of the billet flows sideway during forging. The flow mode is the same for all four designs before the metal touches the die surface at a punch stroke of 15 mm.
25000 fixed die &flat punches max.load = 24933 kN
punch load (kN)
52
fixed die &chamfered punches max.load = 15192 kN
10000
moving die &chamfered punches max.load = 12520 kN
0
5
10
15
20
25
punch stroke(mm)
Fig. 8. Forging loads using different die designs.
Once the metal is in contact with the die, the frictional force on the die interface will either assist it to flow downwards or oppose the metal flow, resulting in a difference in the punch loads. In the case of chamfered punches, the moving die arrangement requires lower loads than the arrangement of fixed dies. The maximum load for the fixed die system is 15 192 kN while that for the moving die design is 12 520 kN; the former is thus 21% greater. If flat punches are used, extremely high loads are required to fill the top and bottom corners. When the die is fixed, the load needed to fully fill the corners increases sharply to 24 933 kN, an increase of 64% when compared with chamfered punches. An even larger increase of load can be seen when the die is moving. The load for flat punches becomes 90% greater than that with chamfered punches, amounting to 23 881 kN. The effect of friction on the load can also be seen from the loads on the punch and the counter punch. The difference between the punch and counter punch load is the frictional force between the die and the billet. As shown in Fig. 9, when the die is rigidly fixed to the machine bed, the frictional resistance on the die–workpiece interface opposes the punch 16000
14000
punch & counter punch load (kN)
punch & counter punch load (kN)
15000
0
punch max load = 15192 kN counter punch max load = 12904 kN
10000 punch load
8000
counter punch load
6000 4000 2000
14000 counter punch max load = 14859 kN
12000 10000
punch max load = 12520 kN
8000 punch load
6000
counter punch load
4000 2000 0
0 0
(a)
moving die &flat punches max.load = 23881 kN
5000
16000
12000
20000
5
10 15 20 punch stroke (mm)
0
25
(b)
5
10 15 20 punch stroke (mm)
25
Fig. 9. Punch and counter punch loads of gear forging: (a) fixed die with chamfered punches; (b) moving die with chamfered punches.
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
forging load
14000
FE prediction 12000
punch load (kN)
movement and thus, increases the punch load. The counter punch load is lower because the die reacts on the machine bed. When the die is elastically attached to the machine bed, the frictional force on the workpiece–die interface is in the same direction as the punch movement, hence the punch load is lower, but the counter punch load is higher because it balances the load from both the punch and the die. Considering the load capacity and metal flow, a moving die design with chamfered punches, design IV, will be used for the forging trials in the development of the net-shape manufacturing of gears.
53
Experimentation
10000 8000 6000 4000 2000 0 0
5
10
15
20
25
30
punch stroke(mm)
6. Forging trials
Fig. 11. Verification of FE predicted forging loads (moving die and chamfered punches).
A forging die set was made to design IV, where the mandrel is connected with the punch. The die was attached to machine bed through a spring. The counter punch is also used as ejector after forging. This tool-set was assembled on a 12 000 kN mechanical press in the Advanced Forging Laboratory at the University of Birmingham, as shown in Fig. 10. Forging trials were carried out under the same conditions as used for the finite element simulation. The forging loads acting on the punch were recorded through a load cell. Fig. 11 shows a comparison of the forging loads from experimental measurements and finite element prediction. It is evident that a very good agreement was obtained between the numerical analysis and actual forging. Fig. 12 shows the forming stages of the gear in the operation. The similarity of the metal flow is apparent from the
Fig. 12. Tooth formation in the gear forging process (moving die and chamfered punches): (a) forging trial; (b) FE prediction.
tooth formation. Both clearly show that the tooth form was formed from the bottom to the top when the die is elastically attached to the machine bed.
7. Discussion and summary
Fig. 10. Gear forging die set (moving die and chamfered punches).
There exist several alternatives of the tool design for a certain part. A particular design should be chosen with respect to the geometry to be forged as the relative movements of the element affect the flow of the workpiece and
54
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
the manner in which filling occurs. This is more evident when a large frictional force exists in the tool–workpiece interface because the friction condition plays an important role in the tool design. When no friction exists in the interface, the relative movement of tools has no effect on the metal flow and the forces required to fill the cavity. For the examples of chamfered punches, both the punch and counter punch load are 12 500 kN whether the die is either rigidly fixed or elastically attached to the machine bed. The metal flow pattern is also independent of the relative movements of the tools. The friction effects on the formation of the tooth were studied using the design III in which the die is elastically attached to the machine bed and both the punch and counter punches are chamfered. The tooth formations at the final
stages are shown in Fig. 13 where frictional factors of m = 0, 0.1 and 0.2 were employed for finite element simulations. When there is no friction, the mode of metal flow is close to a free upsetting process, Fig. 13(a). The middle region of the tooth was formed prior to the top and bottom corners. As there is no frictional resistance in the vertical direction, the top and bottom corners were filled uniformly at the same time. When friction exists in the interface, the movement of the die will assist the metal flow downwards and the bottom corner was formed much earlier than the top corner. At the end of the process, the bottom is fully filled while a chamfer was formed at the top, Fig. 13(b). As discussed earlier, the ends of gear can be easily faced off to obtain a perfectly formed gear. The non-uniformity increases with friction. As shown in Fig. 13(c), the top corner was formed at an even
Fig. 13. Effects of friction on the forming of tooth (moving die and chamfered punches): (a) frictional factor m = 0; (b) frictional factor m = 0.1; (c) frictional factor m = 0.2.
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
later stage when the frictional factor m = 0.2. A slightly larger chamfer was also formed at the top corner.
Acknowledgements The authors would like to thank the Engineering and Physical Science Research Council of the UK for financial support through Grant GR/L96714/01 of the project of net-shape forging of gear forms. The authors are also grateful to Corus, Dana, David Brown, Eaton, New Holland (UK), Ford, UEF, BGA and Mills Forgings for their support of the research work.
55
References [1] T.A. Dean, The feasibility of flasless forging, Metall. Met. Forming 44 (1977) 488–498, 542–544. [2] C. Tuncer, T.A. Dean, Design alternatives for precision forging hallow parts, Int. J. Mach. Tools Manuf. 27 (1) (1987) 65–76. [3] T.A. Dean, The relation of die design to process characteristics in near net-shape forging, in: Proceedings of the Third International Conference on Industrial Tools, Slovenia, Rogaska Slatina, Celje, 2001, pp. 25–30. [4] Z.M. Hu, T.A. Dean, Some aspects of net-shape forging of gears, in: Proceedings of the Second International Seminar on Precision Forging, Osaka, Japan, 2000. [5] J. Cai, Precision forging and post processing of spur gears, PhD Thesis, The University of Birmingham, UK, 2001.
Alternative die designs in net-shape forging of gears J. Cai a , T.A. Dean a , Z.M. Hu b,∗ a
Manufacturing & Mechanical Engineering, School of Engineering, The University of Birmingham, Birmingham B15 2TT, UK QinetiQ Limited, Future Systems Technology, Cody Technology Park, Ively Road, Farnborough, Hampshire GU14 0LX, UK
b
Abstract This paper deals with the aspects of die design in the gear forging process. It discusses alternative tool designs which may be used on a press with only one moving slide and ejection system. It examines the influences of different designs on the metal flow and load requirements through experiments and finite element simulation. The effects of friction on the loads and mode of metal flow are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Net-shape; Gear forging; Finite element
1. Introduction Precision forging of gears has been the subject of considerable effort in the last few decades in the forging industry. Various technologies have been employed to forge bevel, spur and helical gears, in an attempt to reach the finished form without finish machining. The aim of precision forging is to produce gear forms which require no subsequent operations to make them fit for purpose. If forging is possible at room temperature (cold forging) accurate tooth forms can be achieved. Currently bevel gears, particularly those for automobile differentials, are cold forged to finished form on the teeth. Spur and helical gears of higher quality, such as those for automobile gear-boxes, cannot be cold forged, but have to be forged at elevated temperature, either warm or hot, resulting in dimensional inaccuracy. Additional machining procedures to correct this inaccuracy will add tremendous difficulty and cost to the process, which then becomes not economically competitive. A possible commercially viable routine being developed is a combination of warm and cold forming processes. This paper deals with the aspects of the warm forging process. It contains discussion of alternative tool designs that may be used on a press with only one moving slide and ejection system. In various arrangements of the tool sets, the frictional forces on the interface between the tool and the workpiece act differently and affect the manner in which ∗ Corresponding author. Tel.: +44-1252-397016; fax: +44-1252-397298. E-mail address: [email protected] (Z.M. Hu).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.01.019
cavity filling takes place. In this paper, the influences of different designs on metal flow and load requirements are examined through experiments and finite element simulation. The effects of friction on the forging loads and mode of metal flow are also discussed.
2. Completely closed cavity tool-set design The function of a tool-set is to enable a part of specified shape to be formed. Basic knowledge of the design of forging tools for net or nearer net-shape forging has accumulated over the past 30 years. Aspects of the design and the use of this type of tooling were analysed and discussed in the early days of research and development by Dean [1]. Hollow cylindrical billets are often used to forge net-shape axisymmetric and hollow parts such as gears used in power transmission systems. The precision shape can be formed with parallel bores, by using completely closed dies with mandrels. Various tool-set designs are possible for finishing the forging of gears in completely closed cavity dies [2]. Four working elements are required to forge hollow shapes; a punch, a counter punch (ejector), a container and a mandrel. A generic assembly of the elements is shown in Fig. 1 where only a cylindrical cavity is illustrated for simplicity [3]. The punch forms the top surface of the cavity and is attached to the moving ram of the forging machine. The counter punch is attached to the stationary machine bed. The container and mandrel which define the periphery and bore of a forging, respectively, can be attached to either part of the forging machine, either rigidly or with allowance for vertical movement. The basic function of the
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
49
Fig. 2. Forging billet and forged gear.
low or solid; the container may be attached to the ram or the machine bed as a fixed or a movable item; the mandrel may be attached to the anvil or punch and be fixed or movable; the anvil can be penetrating or non-penetrating. In this paper, the effects of the arrangement of the tooling on die filling were analysed in detail using the case of gear forging. The die cavity filling and load variations were modelled using finite element software and tested through forging trials.
3. Alternative die designs for gear forging Fig. 1. Elements of a tool for the forging of hollow parts.
tool is that the workpiece is deformed by relative closure of punch and counter punch. The mode of the flow of the workpiece depends on the relative movements of the tool elements and therefore, cavity filling and the positioning of a compensation space (if it is used) will be affected by tool kinematics. The sequence with which a workpiece will fill a cavity can be determined by noting the direction in which friction forces act. The possible combinations of tool elements were thoroughly discussed and analysed in [3]. The punch can be hol-
The commonly used preform for the forging of a gear is a hollow cylindrical billet with its external diameter close to the root diameter of the gear. A billet and the forged gear are shown in Fig. 2. The 27-teeth gear is of 36 mm height with a module of 4.233 mm. The gear is to be warm forged. The gear material is AISI 8620H. There are several possible die designs for this type of part and in this paper studies of die movement are dealt with. In the two tool-set designs considered, the die is either fixed rigidly to the machine bed or attached to the machine bed by a light spring such that the die can move together with the ram/punch. In both cases, the punch is moving down with
Fig. 3. Schematics of gear forging tool designs with a flat punch and counter punch. (a) I: die is elastically attached to machine bed; (b) II: die is rigidly fixed to machine bed.
50
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
Fig. 4. Schematics of gear forging tool designs with a chamfered punch and counter punch. (a) III: die is elastically attached to machine bed; (b) IV: die is rigidly fixed to machine bed.
the ram while the counter punch is fixed to the machine bed during the forging process. Schematics of the two designs are shown in Fig. 3, where the die is elastically connected in (a) and rigidly fixed in (b). On the right is the arrangement of the tool elements before the deformation and on the left is their arrangement after deformation. The main difference in these two designs is the effect of the frictional forces in the interface between the workpiece and the die once the material fills the tooth die cavity. In the fixed die design, the punch will move into the die cavity during forging and tooth forms have to be made on the punch, while in the moving die design, the punch will not enter the die cavity, but pushes the die downwards, so that no tooth forms are needed: a simple cylindrical shape can be used. The dimensions of the forged tooth form depend not only on the expansion due to the elevated temperatures of the billet and the tools but also on the elastic expansion of the forging tools due to radial pressure which is related to the forging load required to complete the forged shape. In gear forging, the mid-section of the gear teeth fill in advance of the top and bottom faces. It is in the last stages of forging that top and bottom corners of the teeth are filled and it is this filling that requires high forging force [4]. A significant increase in load arises when the workpiece reaches the die. As analysed in [4], the final punch movement of 0.3 mm, 1.2% of the billet deformation, to fill the corner was accompanied by a load increase of nearly 50%. Easing flow into the corners by die design techniques or designing gears which do not have sharp corners can dramatically reduce forging load requirements and tool stresses. The benefit will be less distortion, better dimensional accuracy and longer tool life. One possible design is to leave a chamfer in the corner of the tooth tips to accommodate extra metal and reduce the amount of die/workpiece contact in the final forming stage. The excess metal formed on the teeth can be easily faced off in a simple post-forging operation. Possible similar de-
signs are shown in Fig. 4, both the punch and counter punch being modified with chamfers on the tooth tips. The gear forging process using the four designs has been analysed using the finite element method, considering the filling of the die cavity, and load requirements. The adopted design is used to conduct forging trials and the experimental results are compared with those from finite element simulations.
4. Formation of tooth form The four designs in Figs. 3 and 4 have been analysed using the finite element method. Because of symmetry, a portion corresponding to half a tooth was used for analysis. The case with moving die and chamfered punches is shown as an
punch toothed die
mandrel
billet
counter punch
Fig. 5. FE model for gear forging simulation.
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
example in Fig. 5. The geometrical model and FE meshes were generated using PATRAN. Abaqus/Explicit was used for the analysis. The gear was warm forged with the billet heated to 950 ◦ C and the tools heated to 200 ◦ C. Water based graphite was used as billet and die lubricants and a frictional factor of 0.1 was used in finite element simulation [5].
51
The stages of tooth forming simulated using finite element analysis are shown in Figs. 6 and 7 for the four alternative die designs. In the early stages of gear forging, the mode of the metal flow is close to open die upsetting. Thus, the material in the middle section flows faster than the material in the top and bottom region because of the friction force on the punch and counter punch. As forging continues, the
Figs. 6 and 7. Tooth formation using two die designs with flat punches: (a) fixed die; (b) moving die. Fig. 7. Tooth formation using two die designs with chamfered punches: (a) fixed die; (b) moving die.
J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
partly formed tooth is in contact with the toothed die surface, which is either fixed rigidly with machine bed or moving together with the punch when the die is elastically attached to the machine bed. The frictional resistance from the toothed die surface acts differently according to the die movement. When the die is fixed, the frictional force opposes the metal flow downwards and the top region fills more rapidly. The upper part of the tooth is formed prior to the middle and bottom regions. When the die is elastically attached to the machine bed, it moves down with the punch during forging. The frictional force then assists the material to flow downwards and the material in the bottom parts flows faster than that in the middle and top region. Hence the forming of the tooth is from bottom to the top until the die cavity is filled. The differences between the flat and chamfered punches are evident only in the final stage of the forging when the corners are filled. The top corner is formed prior to bottom corner when the die is fixed and vice versa. With the use of flat punches, all the top and bottom corners can be fully filled with metal. In the case of chamfered punches, the formed chamfer will be removed afterwards and full filling of the die corners is not necessary, given that the full tooth volume is formed. Therefore, only the top corner is fully filled when the die is fixed while only the bottom corner is filled when the die is elastically attached. 5. Forging loads Forging loads are carefully examined because excessive load will cause the die to expand, impairing the accuracy of the forged parts and reducing die life. Forging loads using the four alternatives of die designs are shown in Fig. 8, where the punch load of moving die design include the load acting on the punch and the load acting on the die because the die is pushed downwards by the punch. The metal of the billet flows sideway during forging. The flow mode is the same for all four designs before the metal touches the die surface at a punch stroke of 15 mm.
25000 fixed die &flat punches max.load = 24933 kN
punch load (kN)
52
fixed die &chamfered punches max.load = 15192 kN
10000
moving die &chamfered punches max.load = 12520 kN
0
5
10
15
20
25
punch stroke(mm)
Fig. 8. Forging loads using different die designs.
Once the metal is in contact with the die, the frictional force on the die interface will either assist it to flow downwards or oppose the metal flow, resulting in a difference in the punch loads. In the case of chamfered punches, the moving die arrangement requires lower loads than the arrangement of fixed dies. The maximum load for the fixed die system is 15 192 kN while that for the moving die design is 12 520 kN; the former is thus 21% greater. If flat punches are used, extremely high loads are required to fill the top and bottom corners. When the die is fixed, the load needed to fully fill the corners increases sharply to 24 933 kN, an increase of 64% when compared with chamfered punches. An even larger increase of load can be seen when the die is moving. The load for flat punches becomes 90% greater than that with chamfered punches, amounting to 23 881 kN. The effect of friction on the load can also be seen from the loads on the punch and the counter punch. The difference between the punch and counter punch load is the frictional force between the die and the billet. As shown in Fig. 9, when the die is rigidly fixed to the machine bed, the frictional resistance on the die–workpiece interface opposes the punch 16000
14000
punch & counter punch load (kN)
punch & counter punch load (kN)
15000
0
punch max load = 15192 kN counter punch max load = 12904 kN
10000 punch load
8000
counter punch load
6000 4000 2000
14000 counter punch max load = 14859 kN
12000 10000
punch max load = 12520 kN
8000 punch load
6000
counter punch load
4000 2000 0
0 0
(a)
moving die &flat punches max.load = 23881 kN
5000
16000
12000
20000
5
10 15 20 punch stroke (mm)
0
25
(b)
5
10 15 20 punch stroke (mm)
25
Fig. 9. Punch and counter punch loads of gear forging: (a) fixed die with chamfered punches; (b) moving die with chamfered punches.
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forging load
14000
FE prediction 12000
punch load (kN)
movement and thus, increases the punch load. The counter punch load is lower because the die reacts on the machine bed. When the die is elastically attached to the machine bed, the frictional force on the workpiece–die interface is in the same direction as the punch movement, hence the punch load is lower, but the counter punch load is higher because it balances the load from both the punch and the die. Considering the load capacity and metal flow, a moving die design with chamfered punches, design IV, will be used for the forging trials in the development of the net-shape manufacturing of gears.
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Experimentation
10000 8000 6000 4000 2000 0 0
5
10
15
20
25
30
punch stroke(mm)
6. Forging trials
Fig. 11. Verification of FE predicted forging loads (moving die and chamfered punches).
A forging die set was made to design IV, where the mandrel is connected with the punch. The die was attached to machine bed through a spring. The counter punch is also used as ejector after forging. This tool-set was assembled on a 12 000 kN mechanical press in the Advanced Forging Laboratory at the University of Birmingham, as shown in Fig. 10. Forging trials were carried out under the same conditions as used for the finite element simulation. The forging loads acting on the punch were recorded through a load cell. Fig. 11 shows a comparison of the forging loads from experimental measurements and finite element prediction. It is evident that a very good agreement was obtained between the numerical analysis and actual forging. Fig. 12 shows the forming stages of the gear in the operation. The similarity of the metal flow is apparent from the
Fig. 12. Tooth formation in the gear forging process (moving die and chamfered punches): (a) forging trial; (b) FE prediction.
tooth formation. Both clearly show that the tooth form was formed from the bottom to the top when the die is elastically attached to the machine bed.
7. Discussion and summary
Fig. 10. Gear forging die set (moving die and chamfered punches).
There exist several alternatives of the tool design for a certain part. A particular design should be chosen with respect to the geometry to be forged as the relative movements of the element affect the flow of the workpiece and
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J. Cai et al. / Journal of Materials Processing Technology 150 (2004) 48–55
the manner in which filling occurs. This is more evident when a large frictional force exists in the tool–workpiece interface because the friction condition plays an important role in the tool design. When no friction exists in the interface, the relative movement of tools has no effect on the metal flow and the forces required to fill the cavity. For the examples of chamfered punches, both the punch and counter punch load are 12 500 kN whether the die is either rigidly fixed or elastically attached to the machine bed. The metal flow pattern is also independent of the relative movements of the tools. The friction effects on the formation of the tooth were studied using the design III in which the die is elastically attached to the machine bed and both the punch and counter punches are chamfered. The tooth formations at the final
stages are shown in Fig. 13 where frictional factors of m = 0, 0.1 and 0.2 were employed for finite element simulations. When there is no friction, the mode of metal flow is close to a free upsetting process, Fig. 13(a). The middle region of the tooth was formed prior to the top and bottom corners. As there is no frictional resistance in the vertical direction, the top and bottom corners were filled uniformly at the same time. When friction exists in the interface, the movement of the die will assist the metal flow downwards and the bottom corner was formed much earlier than the top corner. At the end of the process, the bottom is fully filled while a chamfer was formed at the top, Fig. 13(b). As discussed earlier, the ends of gear can be easily faced off to obtain a perfectly formed gear. The non-uniformity increases with friction. As shown in Fig. 13(c), the top corner was formed at an even
Fig. 13. Effects of friction on the forming of tooth (moving die and chamfered punches): (a) frictional factor m = 0; (b) frictional factor m = 0.1; (c) frictional factor m = 0.2.
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later stage when the frictional factor m = 0.2. A slightly larger chamfer was also formed at the top corner.
Acknowledgements The authors would like to thank the Engineering and Physical Science Research Council of the UK for financial support through Grant GR/L96714/01 of the project of net-shape forging of gear forms. The authors are also grateful to Corus, Dana, David Brown, Eaton, New Holland (UK), Ford, UEF, BGA and Mills Forgings for their support of the research work.
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References [1] T.A. Dean, The feasibility of flasless forging, Metall. Met. Forming 44 (1977) 488–498, 542–544. [2] C. Tuncer, T.A. Dean, Design alternatives for precision forging hallow parts, Int. J. Mach. Tools Manuf. 27 (1) (1987) 65–76. [3] T.A. Dean, The relation of die design to process characteristics in near net-shape forging, in: Proceedings of the Third International Conference on Industrial Tools, Slovenia, Rogaska Slatina, Celje, 2001, pp. 25–30. [4] Z.M. Hu, T.A. Dean, Some aspects of net-shape forging of gears, in: Proceedings of the Second International Seminar on Precision Forging, Osaka, Japan, 2000. [5] J. Cai, Precision forging and post processing of spur gears, PhD Thesis, The University of Birmingham, UK, 2001.