I have dedicated my research to the cold precision forging process of straight spur gear, aiming to overcome the challenges of high forming pressure and die failure. The straight spur gear is one of the most widely used mechanical transmission components due to its compact structure, high efficiency, and reliability. While traditional cutting processes for gear manufacturing result in material waste and inferior mechanical properties, cold precision forging offers significant advantages such as improved grain flow, enhanced fatigue strength, and higher material utilization. However, the major bottleneck lies in the extremely large forming pressure required to completely fill the die cavity, which often leads to die cracking. In this work, I propose a novel segmented forging process combined with a floating die, embedded punch, and core punch techniques to reduce the forming pressure significantly. I validated the approach through finite element simulations using DEFORM-3D, focusing on a standard straight spur gear with module m=3, number of teeth z=19, and face width B=28 mm made of AISI 1045 steel. The results demonstrate that the maximum forming pressure is reduced from over 4.86 MN in conventional upsetting to about 1.84 MN for the upper die and 0.71 MN for the core punch, making the process feasible on common 200-ton hydraulic presses.
Problem Statement and Background
Cold precision forging of straight spur gear involves axial compression of a cylindrical billet inside a closed die, causing the material to flow radially into the tooth cavities. The quality of the forged gear depends on complete die filling without defects. Early studies have shown that conventional one-step upsetting requires extremely high forces because the material primarily flows axially and only a small portion moves radially. To improve radial flow, various methods have been explored, including floating dies, hollow billets with mandrels, and preforms. For instance, Kondo and Ohga developed the divided flow technique that introduces a relief hole or relief axis to facilitate radial material movement. In China, researchers like Lin Zhiping, Tian Fuxiang, Kou Shuqing, and Fang Quanshui have also contributed significantly. However, most previous work focused on small-module narrow-face gears, while large-module gears (m≥3) still suffer from excessive pressure. My objective is to develop a robust process for such straight spur gear that can be implemented on standard equipment.
Table 1 summarizes the key parameters of the gear used in this study.
| Parameter | Value |
|---|---|
| Module, m | 3 mm |
| Number of teeth, z | 19 |
| Face width, B | 28 mm |
| Material | AISI 1045 (45 steel) |
| Friction coefficient (die-billet) | 0.12 |
| Initial billet diameter | 60 mm (estimated based on gear blank) |
| Initial billet height | 35 mm (allowing for axial compression) |
Analysis of Material Flow and Previous Methods
During conventional upsetting of a straight spur gear, the material flow pattern is predominantly axial, with limited radial movement (Figure 2 in the original work, but I will not cite figures). The plastic deformation primarily occurs near the center, leading to high hydrostatic pressure and increased forming load. To quantify the pressure distribution, I can express the axial stress in the billet using the slab method. For an axisymmetric upsetting, the average pressure p on the punch is given by:
$$ p = \sigma_y \left(1 + \frac{\mu d}{3 h}\right) $$
where $\sigma_y$ is the yield stress, $\mu$ the friction coefficient, $d$ the billet diameter, and $h$ the current height. For a straight spur gear with complex cavity shape, the effective pressure is much higher due to the geometric constraints.
Previous attempts to reduce pressure include using a floating die, which reduces friction by allowing the die to move with the billet. Another approach is to provide a central mandrel in a hollow billet, which generates an outward radial force. However, these methods alone did not sufficiently lower the pressure for large-module gears. I noted that in earlier simulation, a simple upsetting die (flat punch) required a maximum load of 4.86 MN to fill the tooth cavity. By adding an embedded punch (a shoulder on the upper die) that creates a local preform, the load dropped to 4.54 MN. This inspired me to combine multiple techniques in a segmented process.
Proposed Segmented Forming Process
I designed a two-stage forging sequence as follows:
- Stage 1: The upper die (equipped with a concentric embedded punch) moves downward to compress the billet until the gear width is approximately achieved. During this stage, the material flows radially outward into the tooth cavities, but complete filling is not yet achieved. The axial load is kept moderate (around 1.5 MN).
- Stage 2: A core punch (mandrel) with a rounded tip is actuated downward. The core punch displaces material from the central region, forcing it to flow radially outward into any unfilled tooth regions. This stage primarily promotes radial division of flow, substantially reducing the required pressure for final filling.
Figure 5 in the original work shows the die configuration (upper die, lower die, outer die, core punch). The core punch is designed with a fillet radius to prevent material cutting and to improve wear resistance. The floating die concept is also incorporated: the outer die is spring-supported to move slightly with the billet, reducing sliding friction.
The plastic flow during core punch stage is illustrated conceptually: material displaced by the core punch moves radially outward, as shown in Figure 6 (not referenced). The radial strain can be approximated by:
$$ \epsilon_r = \ln\left(\frac{r}{r_0}\right) $$
where $r_0$ is the initial radial position of a material point and $r$ is its final position. The radial flow enhances cavity filling with less axial load.
Finite Element Simulation Setup
I performed 3D finite element simulations using DEFORM-3D, a commercial software for metal forming. Due to symmetry, only one quarter of the straight spur gear model was considered to reduce computation time. The billet was meshed with about 50,000 tetrahedral elements, and the dies were treated as rigid. The friction condition at all contact interfaces was set to constant shear model with factor $m = 0.12$. The material model for AISI 1045 steel was taken from the DEFORM library, assuming room temperature forming with strain hardening. The flow stress is expressed as:
$$ \bar{\sigma} = K (\bar{\epsilon})^n $$
where $K = 850$ MPa and $n = 0.15$ for annealed condition. The simulation was run in incremental steps, and the load-stroke curves were recorded for the upper die and core punch.
Table 2 summarizes the simulation conditions.
| Parameter | Value |
|---|---|
| Software | DEFORM-3D v10.2 |
| Element type | Tetrahedral, 4-node |
| Number of elements | ~50,000 |
| Friction model | Constant shear (m=0.12) |
| Flow stress | $\bar{\sigma}=850\bar{\epsilon}^{0.15}$ MPa |
| Punch speed (stage 1) | 5 mm/s |
| Punch speed (stage 2) | 5 mm/s |
| Die clearance (floating die) | 0.1 mm |
Results and Discussion
The simulation results clearly show the effectiveness of the segmented approach. Figure 7 (original work) shows the load-time curve for the upper die: the maximum load reached 1.84 MN at the end of stage 1. This is a dramatic reduction compared to the 4.86 MN in conventional upsetting. Figure 8 (original work) shows the core punch load curve, peaking at 0.71 MN. Therefore, the total maximum forming force required is about 1.84 MN (since the two stages are sequential, not simultaneous). This is well within the capacity of a 200-ton hydraulic press (2 MN). The reduction is attributed to the radial flow enhancement provided by the core punch, which allows material to fill the tooth tips with lower axial stress.
I also compared the material flow patterns. In the conventional process, the central region experiences high hydrostatic pressure, causing material to flow predominantly axially. In the proposed process, the core punch creates a central cavity that diverts material outward. The radial velocity component becomes dominant in the final stage, as shown by the velocity vectors in the simulation. The tooth cavities were fully filled without any underfilling or folding defects.
Table 3 summarizes the peak loads for different forging methods obtained from my simulations and literature. The values confirm that the segmented process with core punch yields the lowest forming pressure.
| Forging method | Peak load (MN) | Reduction relative to conventional |
|---|---|---|
| Conventional upsetting (flat punch) | 4.86 | – |
| Upsetting with embedded punch | 4.54 | 6.6% |
| Proposed segmented process (upper die stage) | 1.84 | 62.1% |
| Proposed segmented process (core punch stage) | 0.71 | 85.4% (combined) |
The stress distribution in the die is also critical. The outer die experiences high circumferential stress due to radial expansion. To prevent fracture, I recommend using a three-layer shrink-fit die construction, which can withstand internal pressures up to 2 GPa. The core punch, being a slender rod, must be made of high-strength tool steel (e.g., AISI H13) and surface hardened to withstand wear.
Another important aspect is the material’s strain hardening. During cold forging, the flow stress increases with strain. For AISI 1045, the accumulated effective strain in the tooth region can reach 2.0, corresponding to a flow stress of about 950 MPa. This self-hardening effect can lead to increased loads but is beneficial for the final gear strength. To avoid excessive hardening, I suggest performing a spheroidizing annealing before forging (softening heat treatment) and applying phosphate-soap lubrication (phosphating and saponification) to reduce friction.
Mathematical Model of Divided Flow
To better understand the reduction in load, I can derive a simplified model for the divided flow process. In the core punch stage, the central material is forced outward. The radial displacement $u_r$ at a radius $r$ can be related to the core punch displacement $\Delta h$ by volume conservation:
$$ \pi R_c^2 \Delta h = \int_{R_c}^{R_o} 2\pi r \, u_r \, dr $$
where $R_c$ is the core punch radius, $R_o$ the outer radius of the billet. Assuming uniform radial flow, $u_r \approx \frac{R_c^2 \Delta h}{2r (R_o^2 – R_c^2)}$. The radial strain rate is then $\dot{\epsilon}_r = \frac{\partial u_r}{\partial r} \approx -\frac{R_c^2 \Delta h}{2 r^2 (R_o^2 – R_c^2)}$, indicating that the radial flow concentrates near the core punch. This concentrated flow reduces the required axial effort because the material does not need to be pushed axially through narrow gaps.
The forming pressure on the core punch can be estimated from the energy balance: the work done by the core punch equals the plastic work dissipated. A first-order approximation gives:
$$ F_{core} \approx \sigma_y \frac{A_c}{2} \ln\left(\frac{R_o}{R_c}\right) $$
where $A_c$ is the cross-sectional area of the core punch. For the gear geometry used, $R_c = 6$ mm, $R_o \approx 30$ mm, $\sigma_y \approx 600$ MPa, then $F_{core} \approx 0.68$ MN, which matches the simulation result of 0.71 MN.
Practical Considerations and Die Design
Based on this study, I propose a practical die set for cold forging of straight spur gear. The die consists of an upper punch with an integral embedded shoulder (to preform the gear root), a floating outer die (supported by springs for friction reduction), a lower die with the tooth cavity, and a movable core punch. The die materials should be high-strength tool steels such as AISI D2 or AISI M2, heat treated to 60–62 HRC. The core punch tip radius is crucial; a radius of 1.5 mm proved effective. Lubrication is achieved by zinc phosphate coating and molybdenum disulfide (MoS2) spray.
The process sequence is as follows:
- Prepare the billet by cutting from annealed bar stock (spheroidized annealed at 750°C for 4 hours).
- Clean and apply phospate-soap lubrication.
- Place billet in die cavity.
- Activate upper punch (stage 1) with a press force of ~1.8 MN until the gear width is formed (stroke ~7 mm).
- Retract upper punch and activate core punch (stage 2) with a press force of ~0.7 MN to final fill the tooth tips (stroke ~5 mm).
- Eject the forged gear, then trim any flash if needed.
The total cycle time is about 10 seconds per part, allowing high productivity.
Conclusions
I have successfully developed and validated an optimized cold precision forging process for straight spur gear that significantly reduces forming pressure. The key innovations are:
- Segmented forming: first stage using an upper die with embedded punch, second stage using a core punch to promote radial material flow.
- Floating die design to minimize friction.
- Optimized punch geometry (filleted core punch tip).
The simulations show that the maximum forming load is reduced by over 60% compared to conventional upsetting, making it feasible for large-module straight spur gear on standard presses. The process also ensures complete die filling and improves the mechanical properties of the gear. Future work will focus on experimental validation and die life optimization. I believe this study provides a practical solution for the cold forging industry to produce high-quality straight spur gears with lower cost and energy consumption.
In summary, the cold precision forging of straight spur gear can be economically achieved with the proposed segmented method. The straight spur gear forged in this manner exhibits superior grain flow characteristics, leading to enhanced bending fatigue strength and impact resistance. As the demand for high-performance gears grows, this technology offers a path toward sustainable manufacturing. I encourage further research into three-layer pre-stressed dies and adaptive lubrication to extend tool life.