(1) Outer diameter of helical gear hob
The outer diameter of helical gear hob not only affects the stability of cutting, but also affects the rationality of main parameters such as hob aperture and circumference, hob manufacturing technology and so on.
The selection principle of the outer diameter of the helical gear hob is that when the accuracy of the processed gear is high or the number of teeth is large, the outer diameter of the hob shall be as large as possible. Generally, the outer diameter of the hob can be reduced as much as possible under the condition of sufficient stiffness.
(2) Length of helical gear hob
The minimum length of helical gear hob must meet the following two conditions:
A helical gear hob can envelope the complete tooth profile of the processed gear;
B the load on the edge of helical gear hob shall not be too heavy.
(3) Back angle and back clearance of helical gear hob
The back angle of helical gear hob is obtained from the radial back relief. Relationship between back angle and shovel back:
Where α E shall be greater than or equal to 3 °. α E is actually the projection of the back angle of the top edge of the helical gear hob in the end face. The true back angle of helical gear hob shall be measured in the helix direction. However, because the spiral rise angle of gear hob is generally small, and the rear angle value allows relatively large tolerance, it is completely possible that the rear angle in the helix direction is equal to the rear angle of the end section.
The helical gear hob requiring tooth grinding must have a double shovel back. The hob blade with double shovel back has two different forms:
① Type I, as shown in figure (type I). Shovel back 1K can be taken:
② Type II, as shown in the figure.
(4) Graduation diameter of helical gear hob
The indexing diameter of helical gear hob is determined by the following equation:
Spiral angle of indexing circle of helical gear hob:
Where n — number of heads of helical gear hob.
(5) Pitch of helical gear hob
① Normal pitch of helical gear hob:
Where nm – normal modulus of the gear to be machined, unit: mm.
② Axial pitch of helical gear hob:
(6) Tooth thickness of helical gear hob
Where sn0 – normal spiral tooth thickness of the gear to be machined on the indexing circle, unit: mm.
(7) Tooth top height and tooth root height of helical gear hob
① Addendum height of helical gear hob
② Tooth root height of helical gear hob
The tooth root height h F0 of the helical gear hob is equal to the sum of the tooth top height h a and the top clearance C of the processed gear, that is:
Where c * — top clearance coefficient of the gear to be cut.
(8) Chip holding groove of helical gear hob
The chip holding groove of helical gear hob is divided into straight groove and spiral groove. In order to improve the manufacturing and grinding accuracy of helical gear hobs, helical gear hobs are generally made into straight grooves, which are easy to check.
When the helical angle of helical gear hob γ When Z0 ≤ 50 °, the chip holding groove of the helical gear hob is made into a straight groove, and when the helical rise angle of the helical gear hob γ When Z ≥ 50 °, the chip holding groove is made into a spiral groove.
Depth of chip holding groove H
Type I tooth back:
Type II tooth back:
Calculation of radius r of chip holding groove bottom:
Calculation formula of lead KP of spiral groove:
Groove angle θ ：
When ≤ 9nm, take θ °= 25 ；
When ≥ 9nm, take θ °= 22 。