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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 improve machining accuracy. However, for high-precision gears—especially those with a large number of teeth, large diameter, long tooth surfaces, high precision requirements, and extended processing cycles—strict process control is essential during the hobbing process. Therefore, it is crucial to manage gear blank machining, fixtures, tools, machine tool adjustments, and environmental conditions. Otherwise, even with a CNC hobbing machine, producing qualified high-precision gears remains challenging.
Gear blanks, particularly those with larger diameters, are prone to deformation when mounted, which is difficult to correct on a gear hobbing machine. This 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 mainly requires high dimensional and shape accuracy of the gear hole, as well as positional accuracy between the hole and end face (as per GB10095-88). For gears with 6-level accuracy, the size and shape accuracy of the positioning holes must be IT6, with a surface roughness of Ra 1.6μm. The radial runout of the gear positioning holes and the round runout tolerance for gears within a range of 125 to 400mm should not exceed 0.014mm.
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 (using the previously processed hole to complete the face and cylindrical turning). Regardless of the method, the gear blank must meet the required specifications and provide a high-precision positioning datum for the hobbing process.
As shown in Figure 1, several components such as the screw holder, special turning tool holder, lathe tool, fixture, and hobbing machine table are involved in the setup. Similarly, Figure 2 illustrates the mandrel and holder seat used for precise alignment.
Tooth hobbing jigs must have sufficient strength and meet the accuracy requirements of the gear hobbing process. After the jig's positioning base is processed on other machines and mounted on the hobbing machine worktable, ensuring parallelism can be challenging. One solution is to turn the positioning plane directly on the gear hobbing machine. As illustrated, the hob and arbor are removed, and a special tool rest is fixed on the hob using its bolt holes. A radial cutting program is then executed, allowing the machine table to rotate the fixture while the tool feeds radially. It’s important to keep the table speed below its rated limit, and each cutting depth should not exceed 0.2mm. Once installed, as shown in Figure 2, the run-out at points A, B, and C must be checked. The distance between A and B depends on the length of the gear blank. When multiple gears are machined simultaneously, the total length of the blanks should meet the general requirement of a maximum run-out of 0.006mm at A, and 0.003mm at B and C.
The installation and adjustment of the hob system require careful attention. The hob bar should be securely fastened into the hob, tightened with a tie rod, and then the rod should be released to eliminate tension without causing the cutter bar to fall. Retighten just enough to hold the cutter bar in place. This is due to the tension created by pulling the arbor into the spindle taper hole, which places the tension bar under tension. During machine operation, temperature increases cause the spindle taper hole to expand, drawing the arbor deeper. When cooled, the taper contracts, making disassembly difficult.
As shown in Figure 3, dial gauges are fixed on the machine tool holder, touching the cylinder surface of the arbor. Rotating the hob bar allows checking for bounce, which reflects the machine's machining accuracy. For a 6-degree accuracy machine, the maximum allowable jump is 0.005mm for point a and 0.002mm for point b.
Hob installation should be based on the gear's accuracy, with high-precision gears requiring AA-grade hobs. The position of the hob must be adjusted relative to the machine center, using adjusting washers to secure it. The clamping should rely on static friction between the hob, bushing, and washer. Even with an axial keyway, the key connection typically serves as a backup drive. For high-precision gears, careful calibration of the hob is essential. As shown in Figure 4, two dial gauges are fixed on the machine, touching the circular surfaces on both ends of the hob. Rotating the hob and observing the dial gauge indicates the amount of runout, which can be corrected by adjusting the hob, spacer, or sleeve orientation. The general requirement is a maximum runout of 0.006mm for a and 0.003mm for b.
Temperature also plays a significant role in machining accuracy. Both machine and ambient temperatures affect the precision of gear machining, especially for large gears with long processing times. Temperature changes can subtly shift the position of various carriages relative to the workpiece, impacting gear accuracy. Studies show that temperature has a major effect on tooth-to-tooth accuracy and reduces contact spot areas on meshed gears. Most CNC gear hobbing machines have internal temperature control and display functions, so monitoring and controlling indoor temperature is essential for maintaining precision.
In summary, the hobbing process described here is specifically tailored for high-precision gears. In real-world applications, appropriate technological measures should be taken based on specific workpiece conditions and requirements to avoid unnecessary costs and ensure quality.