As an internal gear manufacturer, I often encounter challenges in processing hardened tooth surfaces of small and medium-modulus internal gears. Traditional grinding processes are unsuitable due to structural limitations, and alternative methods like nitriding after gear shaping offer poor efficiency and precision. This led me to explore electrical discharge machining (EDM) techniques, which can handle conductive materials regardless of hardness. Specifically, I developed an EDM forming milling method that improves surface quality and processing efficiency for internal gears. In this article, I will detail the methodology, apparatus design, experimental investigations, and conclusions drawn from our research.
The EDM forming milling process involves rotating a disk-shaped electrode to replicate its profile onto the workpiece. Unlike conventional EDM, this approach ensures uniform electrode wear, better discharge debris removal, and enhanced cooling, which collectively stabilize the discharge process. For internal gears, this method addresses issues like secondary discharges and low efficiency in deep, narrow slots. Prior to EDM forming milling, I use wire EDM for rough machining to reduce processing time. The core principle involves positioning the electrode relative to the gear slot, followed by rotational and feed motions to machine each tooth sequentially. Key advantages include reduced sensitivity to electrode loss and improved surface integrity, making it ideal for internal gear manufacturers seeking high-quality outputs.
To implement this, I designed a compact milling head attachment for standard EDM machines. The apparatus includes a high-precision bearing system to maintain electrode rotation accuracy, with axial precision ensured by FK10 precision bearings and radial support from angular contact ball bearings and deep groove ball bearings. A mercury slip ring (model A1M2) facilitates current transmission up to 20 A at speeds to 2,000 rpm, while a DC brushed gear motor (42GP-755) provides rotation via synchronous belts and pulleys. Electrical isolation is achieved using a nylon transition plate. This setup allows machining of internal gears with a minimum pitch diameter of 150 mm, using a T2 copper electrode and 45 steel workpiece under oil dielectric. The electrode rotation speed is adjustable from 0 to 330 rpm, and gear parameters include a module of 3 and 50 teeth. For internal gear manufacturers, this design offers a practical solution for complex geometries.
In our experiments, I conducted single-factor tests to understand basic process规律. Surface roughness (Ra) was measured with a laser confocal microscope, and processing efficiency was evaluated via machining time (t). The single-pulse discharge energy (W) is critical and can be expressed as:
$$ W = \int_{0}^{t_{\text{on}}} u(t) i(t) dt $$
where \( u(t) \) is the inter-electrode spark maintenance voltage, \( i(t) \) is the discharge current, and \( t_{\text{on}} \) is the pulse duration. This equation highlights how electrical parameters influence outcomes. For pulse width variation at 6 A peak current and 200 rpm electrode speed, results showed that increasing pulse width raised Ra but reduced machining time, as larger energy pulses create bigger craters. Similarly, varying peak current at 30 μs pulse width and 200 rpm demonstrated that higher currents degrade surface finish but boost efficiency. Pulse interval tests at 30 μs pulse width, 6 A, and 250 rpm indicated minimal impact on Ra but significant effects on stability and time, as shorter intervals increase discharge frequency but risk incomplete deionization.
| Pulse Width (μs) | Ra (μm) | t (s) |
|---|---|---|
| 18 | 3.9 | 218 |
| 30 | 4.6 | 198 |
| 60 | 5.2 | 139 |
| 80 | 5.6 | 120 |
| Peak Current (A) | Ra (μm) | t (s) |
|---|---|---|
| 3 | 3.4 | 332 |
| 6 | 4.6 | 240 |
| 7.5 | 4.8 | 200 |
| 9 | 5.0 | 120 |
| Pulse Interval (μs) | Ra (μm) | t (s) |
|---|---|---|
| 20 | 3.8 | 180 |
| 40 | 3.5 | 240 |
Based on these findings, I designed an L9(3^4) orthogonal experiment with three factors: pulse width (A), peak current (B), and electrode speed (C). Each factor had three levels, and open-circuit voltage was fixed at 280 V. The goal was to optimize for surface roughness and efficiency, considering the trade-offs observed. For internal gears, achieving a balance is crucial for manufacturers aiming to minimize post-processing. The factors and levels are summarized below:
| Parameter | Factor | Level 1 | Level 2 | Level 3 |
|---|---|---|---|---|
| Pulse Width (μs) | A | 18 | 30 | 60 |
| Peak Current (A) | B | 2 | 3 | 6 |
| Electrode Speed (rpm) | C | 100 | 150 | 250 |
The orthogonal test results and range analysis revealed that peak current had the most significant impact on Ra, followed by pulse width, while electrode speed showed minimal influence. This aligns with the energy equation, where current and duration dominate discharge characteristics. Electrode rotation, within the tested range, did not alter discharge mechanisms but aided debris expulsion. The optimal parameter combination was A2B2C1 (pulse width 30 μs, peak current 3 A, speed 100 rpm), yielding a Ra of 3.6 μm and t of 356 s. This combination offered a good compromise for internal gear manufacturers prioritizing surface quality and time.
| Experiment No. | Parameter Combination | Pulse Width (μs) | Peak Current (A) | Speed (rpm) | Ra (μm) | t (s) |
|---|---|---|---|---|---|---|
| 1 | A1B1C1 | 18 | 2 | 100 | 2.0 | 900 |
| 2 | A1B2C2 | 18 | 3 | 150 | 3.2 | 680 |
| 3 | A1B3C3 | 18 | 6 | 250 | 4.1 | 240 |
| 4 | A2B1C3 | 30 | 2 | 250 | 2.4 | 840 |
| 5 | A2B2C1 | 30 | 3 | 100 | 3.6 | 356 |
| 6 | A2B3C2 | 30 | 6 | 150 | 4.7 | 207 |
| 7 | A3B1C2 | 60 | 2 | 150 | 3.0 | 720 |
| 8 | A3B2C3 | 60 | 3 | 250 | 4.2 | 306 |
| 9 | A3B3C1 | 60 | 6 | 100 | 5.5 | 157 |
Range analysis for Ra involved calculating arithmetic means for each level. For factor A (pulse width), means were I = 3.1, II = 3.57, III = 4.24, with a range of 1.14. For B (peak current), I = 2.46, II = 3.67, III = 4.77, range = 2.31. For C (speed), I = 3.63, II = 3.7, III = 3.57, range = 0.13. This confirms that B is most influential, and C least. The discharge efficiency with electrode rotation was estimated at 50% due to carbon film formation, suggesting ideal machining time could be as low as 180 s under optimal conditions. For internal gears, this efficiency gain is significant for manufacturers.
To validate the method, I conducted comparative tests with the optimal parameters but without electrode rotation. With high-speed jump enabled, Ra was 3.8 μm and t was 238 s; without jump, Ra worsened to 4.2 μm and t increased to 260 s. This demonstrates that electrode rotation in EDM forming milling consistently improves surface quality and efficiency by stabilizing the discharge environment. The rotation mitigates issues like debris accumulation and abnormal arcing, which are common in traditional EDM for deep slots in internal gears. Thus, for internal gear manufacturers, this method offers a reliable alternative to conventional processes.
In conclusion, the EDM forming milling technique effectively addresses the challenges of machining hardened small and medium-modulus internal gears. Through single-factor and orthogonal experiments, I identified optimal parameters that balance surface roughness and processing time. The designed apparatus ensures precision and practicality for industrial applications. Compared to traditional EDM, this method enhances surface quality and efficiency, making it valuable for internal gear manufacturers. Future work could explore higher rotation speeds or alternative electrode materials to further optimize performance for a wider range of internal gears.