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High-precision gears CNC gear hobbing process measures
Due to the shortened transmission chain, CNC gear hobbing machines significantly reduce transmission errors and enhance machining accuracy. However, for high-precision gears—especially those with a large number of teeth, large diameter, long tooth surface, and high precision requirements—the hobbing process demands strict process control. Therefore, it is essential to manage the gear blank machining, fixtures, tools, machine tool adjustments, and environmental conditions. Otherwise, even with a CNC hobbing machine, achieving high-precision gears remains challenging.
Gear blanks, particularly those with larger diameters, are prone to deformation during mounting. This deformation is difficult to correct on a gear hobbing machine and can lead to a loss of meshing accuracy. Since the gear surface is often machined using the gear hole and end face as reference surfaces, the precision of the gear blank primarily depends on the dimensional and shape accuracy of the hole, as well as the positional accuracy of the hole and end face (as per GB10095-88). For 6-level accuracy gears, the positioning hole's size and shape accuracy must be IT6, with a surface roughness Ra of 1.6μm. The radial runout of the positioning holes should not exceed 0.014mm for gears with a diameter range of 125 to 400mm.
To ensure this, two common methods are used: 1) Direct clamping and turning, completing the hole and reference surface in one operation; and 2) Axis turning, where the face and cylindrical surface are machined based on a previously processed hole. Regardless of the method, the gear blank must meet specified standards and provide a high-precision positioning datum for the hobbing process.
As shown in Figure 1, special tool holders and fixtures are used to secure the gear blank. In Figure 2, a mandrel and holder seat are employed to support the workpiece during machining. These components play a critical role in maintaining accuracy throughout the process.
For the gear hobbing jigs, they must have sufficient strength and meet the required accuracy. After being processed on other machines and mounted on the hobbing machine table, it can be difficult to ensure parallelism. Therefore, a method of turning a positioning plane directly on the hobbing machine is recommended. As illustrated, the hob and arbor are removed, and a special tool rest is fixed on the hob using bolt holes. A radial cutting program is then executed, allowing the machine table to rotate the fixture while the tool feeds radially. The table speed must not exceed its rated limit, and each cut depth should be no more than 0.2mm.
After installation, as shown in Figure 2, the run-out of points A, B, and C must be checked. The distance between A and B varies depending on the length of the gear blank. When multiple gears are machined together, the total length affects the maximum allowable run-out, which is typically 0.006mm for A and 0.003mm for B and C.
The installation and adjustment of the hob system require careful attention. The hob bar should be securely fastened using a tie rod, and after releasing the rod, tension is eliminated without loosening the bar completely. The lever should be tightened just enough to hold the bar in place. This is due to the tension created by pulling the arbor into the spindle taper hole, causing the tension bar to be under tension. During operation, temperature changes cause the spindle taper hole to expand, drawing the arbor deeper into the hole. When cooling stops, the spindle contracts, making disassembly difficult.
As shown in Figure 3, dial gauges are mounted on the machine tool holder to check the arbor’s movement. Rotating the hob bar allows for measuring the bounce, which indicates the machine’s machining accuracy. For a 6-grade machine, the maximum allowable jump is 0.005mm for point A and 0.002mm for point B.
Hob installation and adjustment depend on the gear’s accuracy. Gears with a grade of 6 or higher require an AA-accuracy hob. The position of the hob relative to the machine center must be adjusted using adjusting washers. The hob must be clamped firmly using static friction between the bushing, hob, and washer. Even with an axial keyway, the key connection serves only as a backup drive. For high-precision gears, the hob must be carefully calibrated.
As seen in Figure 4, two dial gauges are fixed on the machine to monitor the rotation of the hob. By rotating the hob and observing the dial gauge readings, any radial runout can be corrected through adjustments to the hob, spacer, or sleeve. The general requirement is a maximum runout of 0.006mm for point A and 0.003mm for point B.
Temperature also plays a significant role in machining accuracy. Both machine and ambient temperatures affect the precision of large gears with long processing cycles. Temperature fluctuations can cause minor shifts in the positions of the machine carriages relative to the workpiece. Studies have shown that temperature has a major impact on gear accuracy, especially tooth-to-tooth accuracy, and can reduce contact spots on meshed gears.
Modern CNC gear hobbing machines often include internal temperature adjustment functions with numerical displays. Therefore, it is crucial to monitor and control indoor temperatures, ideally keeping them stable. The hobbing process described here is specifically for high-precision gears. In practice, appropriate technological measures should be applied according to the specific conditions and requirements of the workpiece to avoid unnecessary costs.