Selection of basic worm of gear hob

Gear hob is the most widely used high-efficiency tool. Gears with any number of teeth with the same modulus can be processed with the same gear hob. From the working principle of gear hob, the gear hob itself is an involute cylindrical helical gear with small number of teeth (Z0 = 1-3) and large helix angle (beta0 close to 90 °). In order to form the cutting edge of the gear hob, many chip holding grooves are milled on the gear hob to form many shorter cutter teeth. The basic structure of hob is shown in Figure 1: 1 – Basic worm surface; 2 – rake face; 3 – gear hob blade; 4 – side shovel spiral surface; 5 – back of tooth tip shovel.

The basic worm of gear hob is the spiral surface where the side edge of gear hob is located. The basic worm of gear hob has the following three basic forms:

(1) Involute worm

From the theoretical analysis, the section shape of the involute worm in the end section is involute, so the involute worm should be the basic worm of the gear hob, but it has great difficulties in accurate manufacturing and inspection, so it is rarely used in actual production, as shown in Figure 2.

(2) Normal straight profile worm

Normal straight profile worm is the basic worm of gear hob, and its tooth profile is a straight line in the normal section, as shown in Figure 3. When the helix rising angle is large, its tooth profile error is larger than that of Archimedes worm.

(3) Archimedes worm

Using Archimedes worm as the basic worm, its axial profile tooth shape is a straight line, which is very convenient for finishing and inspection. The tooth profile of the gear processed by this gear hob is not involute (in theory), and there is this principle error, but at present, most of the hobs used in gear hobbing are still Archimedes worm hobs. Because the spiral rising angle on the indexing cylinder of the gear hob is very small, the gear processed by gear hobbing has a small tooth profile error, and the distribution of the error can cause a certain trimming to the gear profile, which is conducive to the transmission of the gear. The selected gear hob is Archimedes worm hob, as shown in Figure 4.

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