Maching Principle and Error Analysis of Cylindrical Gear

In mechanical manufacturing, there are many gear processing methods. According to the principle of tooth profile travel, it can be divided into forming methods, such as milling teeth on the milling machine with a milling cutter; generating methods, such as hobbing gears on gear hobbing with a hob.

1. Machining principle of generating methods

As for gear processing, the generating method is one of the most common types. It includes gear shaping, hobbing, shaping. The generating method takes advantage of the principle that tooth profile meshes to cut tooth profile. Suppose that using one of a pair of intermeshing gears(or gears and racks) as the tool, and the other as the wheel blank, and the two are still driven by the transmission ratio, while the tool is cutting. The gear tooth profile conjugated with the tool profile is machined on the wheel blank.

2. Cylindrical gear machining error

The error of hobbing gear mainly comes from the periodic error of machine tool- tool - wheel blank system, it can be divided into:

(1) Eccentricity

The eccentricity is divided into geometric eccentricity and kinematic eccentricity. The geometric eccentricity results from the installation eccentricity caused by the misalignment between the axis of the reference hole of the tooth blank and the rotating axis of the gear hobbing table. Geometric eccentricity is the change of distance of gear blank relative to the hob. When testing the central position of reference hole of gear, the radial runout error of the gear ring occurs periodically during one rotation of the gear, and the tooth pitch and profile also change periodically.

The kinematic eccentricity results from the installation eccentricity between machining error of indexing turbine of gear hobbing and the axis of indexing turbine and the axis of the working table. The movement eccentricity causes the uneven speed of gear blank relative to the hob and tangential dislocation of each profile of a processed gear. When processing gear, the central distance of worm changes periodically, which is equivalent to the change of pitch radius of the turbine, while the linear velocity of the worm is constant. Therefore, the speed of the turbine changes periodically. When then angular velocity increases, the pitch distance, and the common normal line will become longer. When the angular velocity decreases, the pitch distance and common normal line will become short, which will make gear produce tangential errors.

(2) The high-frequency error of machine tool transmission chain

When processing gears, it is mainly affected by the error of each transmission element in the indexing chain, especially the installation eccentricity of the indexing worm and the influence of the axial jump. The speed of the turbine changes many times within a circle, the machined gear produces pitch deviation and tooth profile error. When processing helical gear, in addition to the indexing chain error, there is also the differential chain error.

(3) Machining error of hob

The machining error of hob mainly refers to the manufacturing error of the hob itself, such as the base pitch and tooth profile. They will be reflected on each tooth of the machined gear during the process of processing gear, which will make processed gear produce a base pitch deviation and tooth profile error. In the hobbing process, the amount of cutting is also very important. When processing gears, choosing a relatively large cutting amount and excessive cutting speed will lead to the damage of cutting edge of the hob. The slight is the deformation, which will affect the accuracy requirement of the workpiece. The serious is the damage of the tool, even the scrap of the workpiece. So, in the production process, it is very important to choose the appropriate cutting amount and corresponding cutting speed, which will not only protect hob but also improve the qualification rate of the workpiece.

(4) Installation error of hob

The eccentricity of the hob will make the machined gear produce a radial error. The inclination and axial jump of the hob tool holder guide or tooth blank axis relative to the rotation axis of the worktable is the inconsistency between the feed direction of the hob and theoretical direction of the gear teeth, which directly produces the skew of the tooth surface. Because the stroke of the tooth profile is the result of hob cutting the tooth blank periodically. So the machining error with periodicity is the character of gear error.

Conclusion

We should set solutions based on the corresponding gear processing problem. Only comprehensively analyze the working principle and error of cylindrical gear can we solve the problem faster.