Design and Application of Double Hob for Large Modulus and Large Displacement Gears

In the hobbing process, increasing the number of hobs can significantly enhance the speed of the worktable, thereby greatly improving 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 on the workpiece decreases proportionally, leading to an increase in cutting load per tooth. To improve the rigidity of the hob and reduce flank wear, both the outer diameter and the hole diameter of the hob must be increased, along with the number of chip flutes. Due to the larger cutting depths and higher material removal rates required for large-module and high-displacement gears, especially in rough hobbing, uneven cutting, excessive tool wear, and burning are more likely to occur. This reduces tool life and affects the overall machining quality. Therefore, the application of multiple hobs in such cases is challenging. To address this, we designed a large-diameter double-headed hob to improve the efficiency of rough hobbing for large-modulus and high-displacement gears. After testing and mass production, it delivered excellent results and significant economic benefits. The design parameters for the double-headed hob were based on the following gear specifications: module m = 12 mm, pressure angle α = 20°, displacement coefficient X = 1.3, and number of teeth Z₁ = 38. The workpiece was forged and processed on a Y30100 hobbing machine. Through analysis of the machine's capabilities and the hob's performance, the non-standard double-headed hob was designed with the following specifications: outer diameter De = 200 mm, hole diameter d = 50 mm, number of chip flutes 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. The allowable speed of the indexing worm on the Y30100 hobbing machine is [nworm] ≤ 500 r/min. The relationship between the worm speed nworm, spindle speed n, number of heads k, and the number of teeth Z₁ is given by: nworm = (99 × n × k) / (2 × Z₁) Substituting the design parameters into the formula gives nworm = 83.3 r/min < [nworm]. When machining steel workpieces, the maximum allowable outer diameter for the hob is [De]max = 220 mm, and the allowable cutting speed for rough hobbing is [vknife] = 20–25 m/min. The cutting speed calculation formula is: vknife = π × n × De / 1000 Using the selected outer diameter De = 200 mm, the calculated cutting speed is vknife = 20.1 m/min, slightly above the allowed value of 20 m/min. To avoid excessive tool wear, the lower limit of the cutting speed was used in the design. To ensure no common denominator between the number of hobs and the number of teeth on the gear being cut, which could cause inconsistent tooth thickness, a double-headed hob was chosen. Although using three or more hobs may lead to larger helix angles that are less favorable for machining, the primary purpose here is rough rolling, where efficiency is prioritized over precision. Thus, a double-headed hob was deemed appropriate. Increasing the outer diameter and the number of chip flutes improves tool rigidity and cutting performance. However, selecting too many chip flutes (e.g., Z = 11) reduces blade thickness and increases the risk of interference during grinding. Based on the considerations of roughing operations, Z = 10 was found to be the most suitable choice. A positive rake angle enhances cutting conditions, particularly for large modulus hobs with large thread angles. Using a straight groove positive rake hob design method, the axial tooth angle is calculated as: ctgαqz = ctgαcosLf(1 - Zk Ke / 2πrf²cosgf) For the left and right front blade face tooth angle: tanαq = (tanαcosgf ± sinLfsinf) / cosLf Where α is the normal tooth angle, Lf is the rounding thread angle, Zk is the number of chip flutes, K is the radial amount, e is the eccentric position of the front surface, rf is the rounding radius, gf is the rake angle, ae is the top edge radius, and ge is the top edge rake angle. Compared to a 0° rake angle hob, the axial tooth angle of a straight groove positive rake hob increases by 48'. However, due to the limitations of the root arc radius and the smaller radius of the straight-edge hob, the 0° rake angle was retained for practical reasons, despite some challenges in detection. When the thread angle Lf > 5°, spiral grooves perpendicular to the thread direction (i.e., with a helix angle bf = Lf) should be used. For a straight groove 0° rake angle hob, the axial tooth angle is calculated as: ctgaz = ctgαcosLf For spiral groove hobs, the left and right axial tooth angles are calculated as: ctgalz = ctgaz ± KZk/T Where T is the chip kerf lead, calculated as T = πdf/tanLf, and df is the hob diameter. The average axial tooth angle of a straight-toothed hob and a helical groove hob is 18'. During machining with a straight groove 0° rake angle, the effect of one-side tooth thickness is 27 × tan18' = 0.14 mm. This does not affect semi-finishing accuracy, and since spiral groove hobs are more expensive and harder to sharpen and detect, a straight groove chip pocket was retained in the design. To achieve semi-finishing, only the effective tooth profile area is machined, while the root portion is left untouched, thus improving the durability and efficiency of the semi-fine hob. The design incorporated the tooth side allowance and directly machined to the tooth depth, with a circular arc root. Our company previously used a single-headed hob with an outer diameter of De = 180 mm for rough hobbing. The machining parameters included a spindle speed of nknife = 32 r/min, feed rate f = 0.79 mm/r, cutting depth ap = 27 mm, and down milling. The table and workpiece speed was nwork = nknife / Zwork = 0.84 r/min. After switching to a double-headed hob, the same machining parameters were used. The table and workpiece speed increased to n = nknife / Z = 1.68 r/min, resulting in higher machining efficiency. Throughout the process, the spindle tool holder remained vibration-free, and the table and workpiece operated smoothly without noticeable noise, smoke, or abnormal temperature rise. Post-processing inspection showed no groove marks on the workpiece tooth surfaces, and the hob exhibited proper wear. Trial runs (4 pieces), small batch processing (20 pieces), and medium batch processing (60 pieces) all demonstrated good rough-rolling results. Single-piece processing efficiency nearly doubled, and semi-finish rolling also performed well, with improved shape accuracy and about a 20% increase in machining efficiency. The durability of the semi-fine hob was significantly enhanced. Overall, the production efficiency more than doubled, yielding substantial economic benefits. During the hobbing process, several key maintenance and operational considerations should be noted: - Regularly adjust or replace the main motor belt to prevent looseness, which can cause uneven rotation of the table and affect the machining process. - Adjust the clamping tightness between the knife holder bar and guide rails to avoid vibration caused by gaps. - Ensure proper lubrication of the vertical rails before and during processing to prevent crawling. - Regularly test and adjust the floating of the worktable, as long-term wear can create gaps that lead to vibration. - Check and adjust the clearance between the worm gear and worm pair to avoid uneven cutting. - Always ensure proper lubrication of the worm and worm wheel to prevent damage to the machine tool. - Replace the special lubricant for the spindle every 3 to 6 months, as the headstock oil is closed and does not circulate. These steps help maintain consistent and efficient hobbing performance, ensuring high-quality gear production.

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