The working principle of worm gear hob is to simulate the meshing process of worm and worm gear.
According to the working principle, non involute gear hobs can be divided into generating hobs and forming hobs.
1. Worm gear hob
（1） Characteristics of worm gear hob
The worm gear hob is equivalent to the original worm.
It only makes cutting edges on it.
These cutting edges are distributed on the spiral surface of the original worm.
This worm is called the basic worm of the worm gear hob.
Therefore, the worm gear hob is correct as long as its basic worm type and basic parameters are consistent with the working worm.
（2） Classification of worm gear hobs
The main parameters of worm gear hob include module, tooth shape angle, dividing circle diameter, spiral rising angle, spiral direction and the number of thread heads.
Worm gear hob is a special tool. It is necessary to design and manufacture a special hob for processing each specification of worm gear.
According to the structure, ordinary worm gear hobs can be divided into two types: sleeve type with hole and integral handle type.
Worm gear hobs are classified according to tooth shape, including Archimedes worm gear hob (ZA worm gear hob), normal straight profile worm gear hob (ZN worm gear hob), involute worm gear hob (ZI worm gear hob) and cone wrapped worm gear hob (ZK worm gear hob), etc.
1. Worm gear hob with hole sleeve
Fig. 6-9 shows the worm gear hob with hole sleeve.
The diameter and other parameters of the worm gear hob are the same as those of the working worm.
When the outer diameter of the worm gear hob is relatively large, the sleeve structure shall be adopted, and the axial key or end face key can be used for connection.
Fig. 6-9 worm gear hob with hole sleeve
2. Integral handle type worm gear hob
Fig. 6-10 shows the integral handle type worm gear hob.
This type of structure is applicable to worm gear hobs with small diameter to ensure that the strength of the tool body meets the working requirements.
Fig. 6-10 integral shank type worm gear hob
（3） Selection of worm gear hob
The selection of worm gear hob is considered from the aspects of structural form, outer diameter of hob, number of teeth on the circumference of hob, front angle and back angle of hob, etc.
(1) On the condition that the strength requirements of the cutter body are met, the worm gear hob with hole sleeve is preferred;
Otherwise, the worm gear hob with integral handle is selected.
(2) The outer diameter of the worm gear hob is larger than that of the original worm, ensuring that there is a radial clearance, and it is used as the compensation value for the reduction of the outer diameter of the hob after regrinding.
(3) Under the condition that the hob teeth have sufficient strength, the number of circumferential teeth shall be increased as much as possible, so that the worm gear tooth surface has high surface quality.
(4) The rake angle of worm gear hob is generally 0 °, and the rough machining hob can take a positive value.
(5) The back angle of the tooth crest in the spiral direction of the hob can be taken as 10 ° ~ 12 °.
When the strength of the hob is low, the smaller value is taken, otherwise the larger value is taken.
3. Non involute gear hob
（1） Characteristics of non involute gear hobs
The working principle of generating hob for machining non involute tooth shaped workpiece is the same as that for machining involute gear.
The gear cutting process of hob is equivalent to the meshing process between the basic worm of hob and workpiece.
The forming hob is processed according to the forming principle.
When processed with this hob, there is no transition curve on the workpiece, so the workpiece with no transition curve at the tooth bottom is usually processed with forming hob.
（2） Classification of non involute gear hobs
Non involute gears include spline shaft, sprocket, circular arc gear, cycloid gear, ratchet, synchronous belt wheel and cam.
The processing method of this kind of parts is similar to that of involute gears.
It can be processed by generating method or forming method.
The tools used are called non involute gear tools.
This kind of cutter includes non involute gear hob, non involute gear shaper cutter and generating cutter.
Among them, non involute gear hobs are widely used in production because of the following advantages.
(1) In addition to the rectangular spline, the workpiece with different tooth numbers can also be machined with the same hob within a certain tooth number range.
The tool has good versatility.
(2) Machining is continuous cutting, so it has high productivity.
(3) The tool pitch error does not affect the workpiece pitch error.
(4) It can process long shaft parts.