Gear hob is a kind of cutting tool used to process gears by generating method.
It is widely used in gear manufacturing.
It can be used to process spur gears, helical gears, standard gears and modified gears.
The machining range of gears is very large.
Gears with modulus of 0.1 ~ 40mm can be machined with hobs.
（1） Characteristics of gear hobs
Fig. 6-1 is the working principle diagram of machining gears with gear hobs.
The hob axis is inclined at an angle to the workpiece section ψ.
The rotary motion of the hob is the main motion.
When machining spur gears, the workpiece rotates one tooth (when the hob is single head) or several teeth (when the hob is multi head) every time the hob rotates, so as to form a generative motion, that is, a circular feed motion.
In order to cut out the tooth profile on the full tooth width of the gear, the hob also needs to have a feed movement along the gear axis.
When cutting the helical gear, in addition to the above movement, it is necessary to give the workpiece an additional rotation.
Fig. 6-1 working principle of gear hob
Hobs are expensive metal cutting tools, which require high thermal hardness and wear resistance. Therefore, the quenching temperature is relatively high, but slightly lower than that of turning tools. They are usually classified at 1225 ~ 1230 ℃, 580 ~ 620 ℃, and the metallographic grade is 9.5 ~ 10.
The first tempering should be treated with 350 ~ 380 ℃ or secondary bainite, and then 550 ℃ × High temperature tempering for 1H for 3 times, overheating for 1 ~ 2 grades, or even 3 grades, and the ultimate microstructure hardness can reach more than 65HRC
（2） Classification of gear hobs
As shown in Fig. 6-2, gear hobs can be divided into integral hobs and inserted hobs according to different structures.
It can be divided into rough machining hob and finish machining hob according to the purpose, and it can be divided into high-speed steel hob and cemented carbide hob according to the cutting part of the hob;
According to the number of thread heads of hobs, they can be divided into single head hobs and multi head hobs.
In addition, according to the characteristics of the workpiece to be processed, various improvements can be made to the structure and tooth shape geometry of the gear hob, such as the small pressure angle hob, the wave edge hob, the unequal tooth high-speed hob, the full cutting hob, as well as the pre shaving gear hob, the pre grinding gear hob and the pre shaving gear hob.
The latter two types of cutter teeth are narrow, so that after hobbing to the depth, the tooth thickness of the workpiece has the margin for grinding or shaving.
By structure:
- Integral hob
- Inserted gear hob
By application:
- Rough machining hob
- Finishing hob
By hob cuts some materials:
- High speed steel hob
- Carbide hob
By the number of hob heads:
- Single head hob
- Multi head hob
1. Integral high speed steel gear hob
Medium and small module hobs are made into an integral structure, as shown in Fig. 6-3.
There are two basic types of integral high-speed steel gears.
Type I is applicable to Grade 3A hobs specified in JB/T3227-2013 general technical conditions for high precision gear hobs and grade 2A hobs specified in GB/T6084-2016 general technical conditions for gear hobs;
Type II is applicable to hobs with 2A, A, B and C precision specified in GB/T6084-2016.
The integral high-speed steel gear hob is made of single head, right-hand rotation, zero rake angle and straight groove.
Fig. 6-3 integral high speed gear hob
2. Insert gear hob
Large module and medium module hobs can be made into insert structure, which can not only save high-speed steel, but also ensure the heat treatment performance of the blade and improve the service life of the hob.
Due to the continuous improvement of forging and heat treatment process, the insert gear hob with modulus above 20mm is gradually replaced by the integral high-speed steel gear hob.
(1) Insert gear hob: fig. 6-4 shows an insert gear hob with a modulus of 9 ~ 40mm, which is made into a single head, right-hand rotation, zero rake angle and straight groove form.
The Accuracy grades are 2A, A and B.
The involute basic worm shall be used for the tooth shape inspection.
Fig. 6-4 insert gear hob
a) With axial keyway
b) Keyway with section
(2) Positive rake angle inserted gear hob: fig. 6-5 shows a positive rake angle inserted gear hob with a modulus of 9 ~ 40mm, which is made into a single head, right-hand rotation, and straight groove with a rake angle of 7 ° at the top circle.
The involute basic worm shall be used for the tooth shape inspection.
Fig. 6-5 positive rake angle inserted gear hob
a) With axial keyway
b) Keyway with section
3. With cutting taper hob
When cutting helical gears with large helix angle, the inclination angle of the hob axis is large, the projection length of the hob on different planes is shortened, and the first few teeth of the hob cutting into the gear blank bear excessive overload load.
In order to make the load distribution between the hob working teeth more uniform, when the helix angle of the gear to be machined is greater than 20 °, a cutting cone should be machined at one end of the hob.
The position of the cutting cone shall be determined according to the helical direction of the gear to be cut, the helical direction of the hob and the feed direction during gear hobbing.
4. High efficiency rough cutting gear hob with wave edge
During rough gear hobbing, the chip shape changes greatly, which causes the cutting force to change greatly, and causes the vibration of the machine tool, which limits the production efficiency of rough gear hobbing.
After the hob tooth shape shovel is grinded into a wave edge, the chip can be cut off.
The change of chip shape is small, the change of cutting force is small, and the vibration of the machine tool is small, which can improve the cutting parameters, and the effect is more remarkable when cutting hard tooth surface gears.
The wave depth is 0.8 ~ 1.2mm, and the wave distance is 7 ~ 12mm.
The ripple and the straight surface shall be smoothly connected to avoid sharp corners.
The following types of waveform structures are often used:
(1) Full ripple type, with staggered ripples, as shown in Fig. 6-6.
Fig. 6-6 full ripple type
(2) The rows are corrugated, and the ripples are not staggered, as shown in Fig. 6-7.
Fig. 6-7 corrugated partition
(3) With polished teeth, the ripple is not staggered before and after, as shown in Fig. 6-8.
Fig. 6-8 tooth profile with polishing
5. Machine clamp and bonding large module gear hob
The large module inserted gear hob has heavy blade weight and needs high-speed tool steel with large cross-section for forging.
However, the carbide segregation of high-speed tool steel raw materials increases with the increase of cross-section, and the carbide segregation is high.
Not only the service life of the blade made of it is low, but also cracks are easy to appear in the process of forging, heat treatment, grinding and use.
Therefore, the large module inserted gear hob is expensive and of low quality.
Clamping and bonding large module gear hobs overcome the above shortcomings by changing a row of blades into a tooth of a blade.
Large module gear hobs can be made of small section high-speed tool steel.
6. Multi head gear hob
The use of multi head gear hobs can significantly improve the hobbing productivity, especially when hobbing helical gears with a large number of teeth.
When using multi head hobs with the same spiral direction of gears, the angle of the tool holder is smaller than that of single head hobs with similar diameters, so the load on the edge teeth of the hobs will also be reduced.
Therefore, multi head hobs with medium and small modulus are widely used in rough hobbing. The number of hob heads is usually 2 ~ 3.
With the increase of the number of hobs, the cutting load increases correspondingly, and the cutting times of enveloping a tooth surface decreases proportionally.
In order to improve the rigidity of the hob and reduce the edge of the gear surface to be cut, the outer diameter and aperture of the hob should be increased correspondingly, and the number of circumferential teeth should also be increased.
It should also be noted that:
(1) When the number of teeth to be cut Z2 and the number of hob heads Z1 are prime numbers to each other, the number of hob circumferential teeth ZK and the number of hob heads Z1 should also be prime numbers to each other.
In this way, the manufacturing error of hob during gear hobbing will not be fully reflected on the gear, which is beneficial to improve the gear accuracy.
(2) When there is a common factor between the number of gear teeth Z2 and the number of hob heads Z1, the number of hob circumferential teeth ZK should also be divided by the number of hob heads Z1, or there is a common factor.
This is because when the number of hob circumferential teeth cannot be divided by the number of hob heads, if one tooth aligns with the gear center, any tooth of other heads cannot align with the gear center, resulting in asymmetric rolling of other teeth.
Due to the small number of teeth in the hobbing envelope of multi head hobs, the error caused by this asymmetry is often considerable.
In this case, when the multi head hob is used for rough gear hobbing, the gear hobbing allowance should be properly considered.
（3） Selection of gear hobs
The accuracy grade of gear hob is divided into: 3A, 2A, A, B and C.
When the gear tooth accuracy is grade 6 or above, the hob with Grade 3A accuracy shall be used for machining.
The accuracy of pre shaving, pre grinding and pre shaving hobs is generally class A and class B.
The accuracy grade of gear hob and the accuracy of machined gear are shown in table 6-1, and the accuracy grade of scraping hob is shown in table 6-2.
Table 6-1 accuracy grade of gear hob
| Gear hob accuracy class | Accuracy of machined gear（GB/T 10095.1-2008） |
| 3A | 6 |
| 2A | 7 |
| A | 8 |
| B | 9 |
| C | 10 |
Table 6-2 precision grade of scraping hob
| Accuracy grade of scraping hob | Accuracy of machined gears (45~60HRC hardened gears) |
| A | Finishing of general precision gears or semi finishing of high precision gears |
| ZA | For hardened gears of Grade 7 and 8 (GB/T10095.1-2008), the tooth surface roughness value is Ra=0.63~1.25um, and the service life is 50~80m after each grinding. |
The key points for gear hob selection are as follows:
(1) Select according to the accuracy of the gear to be machined.
Integral hob is recommended for gears with grade 7 accuracy and above, and blade hob can be used for other gears.
(2) Select according to the hardness of the gear to be machined.
If the hardness of gear blank is >300HBW, it is recommended to use blade hob.
(3) Select according to gear batch and efficiency.
If the batch is large and there is a high efficiency requirement, the blade hob shall be used.