# Tooth positioning of automobile gear hob

All automobile gear hob teeth must be in the same center, especially for insert hob teeth with undercutting and chamfering functions. The symmetrical cutting edge of automobile gear hob can produce the required tooth shape, and the center line of the cutting surface (the symmetry of the cutting edge) coincides with the radial line. In this position, the automobile gear hob teeth are in the center. Similarly, when the tooth groove between two adjacent automobile gear hob teeth is the center.

For the single head hob, each tooth slot is processed by the teeth on the same thread of the automobile gear hob. Therefore, taking the automobile gear hob tooth as the center, a symmetrical tooth shape can be generated. However, for multi head hobs, only when the hob tooth center on each head is different from the machined part teeth of other head automobile gear hobs can the symmetry of tooth profile be ensured. In order to achieve this, only when the number of teeth of automobile gear hob can be divided by the number of heads.

Centrosymmetry can be achieved in several ways, which largely depends on the accuracy required by the tool. Before the automobile gear hob is installed on the gear hobbing machine, the center line of the automobile gear hob teeth and tooth grooves is based on the axial center. This can be compared or measured by using a micrometer height gauge or an optical comparator. Knowing the outer diameter and indexing circle can be applied to the selected tooth center of automobile gear hob. After each grinding, a new diameter should be generated, which should change the position relative to the central cutting edge of the automobile gear hob tooth.

In order to keep the center of the gear hob unchanged. Similarly, the number of heads and spiral grooves of automobile gear hobs are also affected. See the following expression for the amount of tool channeling under the condition of maintaining the center of the tooth profile:

For straight groove hobs

For spiral groove hob

Where, D = tool shifting amount of automobile gear hob

F = number of chip holding grooves of automobile gear hob

K = coefficient

Lh = tooth direction of automobile gear hob

Nh = number of hob heads of automobile gear

Ph = straight indexing circle of automobile gear hob

λ = Tooth direction angle of automobile gear hob

In all cases, f must be divided by the number of heads of the automobile gear hob. The coefficient K is the number of teeth between two cycles of teeth along the same spiral groove.

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