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In the hobbing process, increasing the number of hobs can significantly boost the table speed, thereby greatly enhancing processing efficiency. As a result, multi-head hobs have become widely used in the machining of medium and small module gears. However, when machining large module gears with multi-head hobs, as the number of hobs increases, the number of teeth per tooth surface of the workpiece decreases proportionally, leading to an increased cutting load on each tooth of the hob. To improve the rigidity of the tool and reduce flank wear, both the outer diameter and the hole diameter of the hob must be increased, along with the number of teeth (chip flutes). Due to the large cutting depths and high material removal rates involved in large module gears—especially for those with large displacement—it is more prone to uneven cutting, excessive tool wear, and thermal damage, which reduces tool life and efficiency. Therefore, applying multiple hobs to large module and large displacement gear machining presents significant challenges.
To address this issue and improve the efficiency of rough hobbing for large modulus and large displacement gears, we designed a large-diameter double-headed hob. After extensive testing and mass production, the results were highly satisfactory, yielding improved productivity and economic benefits. The design was based on specific gear parameters: modulus m = 12 mm, pressure angle α = 20°, displacement coefficient X = 1.3, and tooth count Z1 = 38. The gear blank was forged and processed on a Y30100 hobbing machine.
Through detailed analysis and calculation of the machine's capabilities and the performance of the double-headed hob, we determined the following design specifications: outer diameter De = 200 mm, bore diameter d = 50 mm, chip flute count Z = 10, rounding thread angle Lf = 8°22', straight grooves, 0° rake angle, circular head, tooth thickness DS = 1.6 mm, and right-hand orientation. These parameters were chosen to ensure compatibility with the machine’s indexing worm speed, which is limited to [nworm] ≤ 500 r/min. Using the formula nworm = 99n / 2Z1, where n is the spindle speed and k is the number of heads, we calculated that nworm = 83.3 r/min, which is within the allowable range.
The maximum allowable outer diameter for the hob on the Y30100 machine is [De]max = 220 mm, and the recommended cutting speed for rough hobbing is [v knife] = 20–25 m/min. Using the formula v knife = πn knife De / 1000, we calculated v knife = 20.1 m/min, slightly above the lower limit, but acceptable for the design. It’s important that the number of hobs and the number of teeth on the gear being cut do not share a common divisor to avoid inconsistencies in tooth thickness. However, using three or more hobs can lead to a larger helix angle, which is less favorable for machining. Since this hob is primarily for roughing, efficiency is prioritized over precision, so a double-headed hob was still chosen.
When designing multiple hobs, increasing the outer diameter and the number of chip flutes improves tool rigidity and cutting performance. Choosing Z = 10 chip flutes instead of Z = 11 helps maintain blade thickness and avoids interference during grinding. A positive rake angle improves cutting conditions, especially for large module hobs with large thread angles. However, after analyzing the axial tooth angle and front blade face angles, it was decided to use a 0° rake angle due to its simplicity and better root arc compatibility. Straight grooves were retained instead of spiral grooves to reduce cost and simplify detection, while still achieving good semi-finishing results.
During application, the double-headed hob showed excellent performance. Compared to a single-head hob with De = 180 mm, the double-headed hob doubled the worktable speed without compromising stability or surface quality. The tool remained sharp, and no groove marks were observed on the gear teeth. Trials with small batches confirmed the effectiveness of the design, with a nearly 100% increase in single-piece processing efficiency and a 20% improvement in overall machining efficiency. The semi-fine hob also showed improved durability.
Throughout the process, several operational considerations were noted: ensuring proper belt tension, adjusting clamping tightness between the tool holder and guide rails, lubricating vertical rails regularly, checking and adjusting the worktable float, monitoring worm gear clearance, and maintaining proper lubrication of the headstock. Regular maintenance of these components ensures smooth operation, reduces vibration, and extends tool life.
Overall, the implementation of the double-headed hob has significantly enhanced production efficiency and economic returns, proving to be a valuable advancement in the machining of large module gears.
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