1 Determine the installation angle of the hob
When hob cutting the displacement gear, the adjustment of the installation angle of the hob cannot be ignored, especially when using a standard hob to process the module and displacement. For gears with a large coefficient, improper adjustment of the installation angle will affect the tooth profile accuracy. As can be seen from Figure 2, when hobbing a standard gear, the indexing circle of the hob is tangent to the indexing circle of the gear. The indexing circle becomes the cutting pitch circle, and the indexing circle coincides with the pitch circle. For standard spur gears, the installation angle of the hob at this time is equal to the hob helix angle. When hobbing the displacement gear, the situation is different. The hob moves a distance of x·m relative to the tooth blank, and x is the displacement coefficient. The pitch circle of the hob does not coincide with the indexing circle of the gear. The diameter of the hob pitch circle increases or decreases by 2x·m. When processing this type of gear, the installation angle of the hob is not equal to the helix angle of the hob. When it becomes The larger the bit coefficient, the larger the installation angle difference. The installation angle must be determined by calculation and comparison.
2 Calculation of the length of the displacement gear hob
When using a hobbing machine to process the displacement gear, if the module is large, the number of teeth is large, and the displacement coefficient is relatively large , the cutting edge length of the standard hob will be insufficient, and phenomena such as knife beating, knife burning, and edge tooth scraping will occur. The hob wears rapidly, has a short service life, has poor tooth surface roughness after hobbing, and has serious tooth profile errors. Why does the above phenomenon occur? By analyzing the hobbing process, it can be seen that the main reason is the insufficient length of the hob. When using a hob to process gears on a hobbing machine, a helical gear meshes with the workpiece (gear to be machined). The section of the hob shaft is like a rack meshing with a gear, as shown in Figure 3. The involute tooth profile is formed by the enveloping shape of the hob cutting edge along the AB and A′B′ meshing lines. In the B′B range, the tool tooth profile and the gear tooth profile are not meshed, and the cutter teeth only expand and cut the gear tooth root. Transition curve part. When the length of the hob is not enough, point A will move to point E inside the top circle. Then the hob teeth in the EA section cannot be enveloped and formed on the meshing line during processing. Therefore, gears with a large number of teeth and large displacements can be processed. The length of the hob must be calculated
3 The main factors affecting the change of the hob length
From the above analysis and calculation, it is not difficult to see that the factors that affect the change of the hob length include the module and the number of teeth. , pressure angle, tooth addendum height coefficient, tooth addendum height variation coefficient and displacement coefficient. But the biggest impact is the displacement coefficient. Since the gear displacement coefficient is very large, the root circle diameter is usually larger than the index circle diameter (positive super displacement), and the tooth tip circle radius exceeds the tooth tip circle radius of the standard gear with the same module and the same number of teeth by several modules. So much. It can be seen from Figure 3 that the EA section of the meshing line extends outward after positive displacement. As the displacement coefficient increases, the more it extends outward, the length of the hob will also double.
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