Hobbing technology of the hardest tooth surface an

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Hobbing technology of hard tooth surface and design and use of hob

Abstract: This paper mainly expounds the hobbing technology of hard tooth surface and the design of hob. This paper analyzes the hobbing technology of hard face gear, the design of carbide hob with large negative rake angle, and how to correct the front offset value of the hob in order to ensure the tooth profile accuracy

with the continuous development of productivity, gear transmission is developing in the direction of high speed, heavy load and high precision. More and more gears adopt hard tooth surface gears with large bearing capacity and good pitting resistance. For industrial gears in developed countries, the hard tooth surface with hardness above 350hbs has almost completely replaced the soft tooth surface with hardness below 350hbs after surface quenching and overall quenching. Since the 1980s, China has begun to popularize the application of hardened gears

1. Hobbing process of hard tooth surface

hard tooth surface is processed with carbide hob, which innovates the traditional finishing process of hard tooth surface. First of all, for high-precision gear grinding, Gear Hobbing with hard tooth surface can replace the rough grinding process with high efficiency, remove the heat treatment deformation of the gear teeth, but some special samples in the rubber tensile property test can not be finely ground with a small and uniform margin measured by the contact sensor, thus greatly improving the gear grinding efficiency. Secondly, for honing gears, hard tooth surface hobbing is arranged before honing, which can remove heat treatment deformation to achieve the necessary accuracy, and then honing is carried out, so as to give full play to the advantages of honing process finishing and make up for the shortcomings of hobbing. Thirdly, for hardened gears with ordinary precision, carbide hobs can be directly used for fine hobbing to ensure the gear machining precision at the lowest cost, which is of more technical and economic significance for large and medium-sized gears

the process route of Gear Hobbing with hard tooth surface is roughly as follows:

· gear grinding (Level 3 ~ 6): gear hobbing → quenching → semi fine Hobbing with hard tooth surface → gear grinding

· honing gear (Grade 6 ~ 7): gear hobbing → quenching → semi fine hobbing of hard tooth surface → gear honing

· ordinary precision gears (grades 7 ~ 9): gear hobbing → quenching → fine hobbing of hard tooth surface

except for the tooth profile accuracy, other items can reach grade 3 ~ 5 accuracy

there are two main reasons why it is difficult to further improve the tooth profile accuracy. One is the poor stability and transmission stiffness of the gear hobbing machine. The other is that there are certain difficulties in manufacturing high-precision carbide hobs, especially the hobs with large negative rake angle have great changes in tooth profile after regrinding. Therefore, it is necessary to carefully design and calculate the structure and parameters of the hob, analyze the tooth profile accuracy after regrinding, and put forward improvement measures

2. Design of cemented carbide hob

1) hob material

the characteristics of hard tooth surface hobbing are: high hardness of workpiece, intermittent cutting process and thin cutting layer. In the cutting process, the tool bears large impact load, high cutting temperature and strong friction. Therefore, the requirements for impact toughness, wear resistance and heat resistance of some materials cut by the tool are very high. Professor Xiangpu, an expert of cemented carbide hob in Japan, recommended P20 cemented carbide, which is equivalent to China's domestic brand YT14. This material has high wear resistance. High temperature carbides such as lithium carbide are added to improve the impact toughness and wear resistance of the blade, so as to obtain good cutting performance

2) structural form of hobs

at present, there are three main types of carbide hobs designed by countries all over the world: integral type, machine clamp type and welding type

a. integral carbide hob

has the advantages of strong rigidity, time-saving machining and high precision. However, due to the limitation of the overall pressing process, at present, only the hobs with an outer diameter of  85mm can be made, and there are many expensive cemented carbide and high cost, so only the hobs with a modulus of m=3mm or less should be made

b. machine clamped carbide hob

the structure of machine clamped carbide hob is relatively complex, and the clamping reliability is also poor. Especially when machining large module hardened gears, the extrusion force on the tooth surface is large, and the alternating effect is significant, so the clamping requirements for the blade are high. The carbide hob produced by Shaoguan tool factory is of this kind of structure. This structure can be used for all kinds of medium modulus (M1 ~ 6) cemented carbide hobs with rake angles of g=0 ° ~ -30 °, and the cutting effect is very good

c. welded carbide hob

has the advantages of simple structure, high connection strength, easy sintering of carbide blade, material saving and wide application. However, the crack caused by welding stress has always been a factor of unstable product quality, so higher welding technology is required. In recent years, Japan has solved the problem of plywood for container bottom plate (gb/t 19536 ⑵ 004), which has been applied in production. This structure is adopted by tool factories in Chongqing and Hanjiang in China

3) hob rake angle

due to the poor impact toughness of cemented carbide, it is very easy to produce edge collapse when hobbing on hard tooth surface. Edge collapse is the main problem to be solved by cemented carbide hobs. Therefore, the special form of large negative rake angle is adopted in the design of hobs

two points should be considered when determining the rake angle:

· the retention of tooth profile accuracy after tool grinding

· improve the ability of the cutter teeth to resist edge collapse and reduce the wear of the blade

Table 1 recommended rake angle value

hrc> 60 ~ 62

- for example, the size of negative rake angle in metal tension and compression test of 20 ° ~ -30 °

in material test will directly affect the retention of tooth profile accuracy after tool grinding. The hydraulic system of tension test machine adopts a single oil tank made of welded steel plates and the ability of anti collapse blade. The greater the negative rake angle is, the worse the accuracy retention is. However, if the negative rake angle is too small, the less the tool's ability to resist edge collapse

theoretically, with the increase of the negative rake angle of the carbide hob, the inclination angle of the side blade of the hob increases, which makes the hob teeth cut smoothly into the metal layer, thus reducing the impact, protecting the carbide teeth from edge collapse, and significantly improving the durability. The same conclusion has been reached in the tests and applications at home and abroad. However, the greater the negative rake angle, the more difficult it is to ensure the tooth profile accuracy of the hob. According to the tooth surface hardness of the gear to be machined, the recommended value of rake angle is shown in Table 1

3. Deviation value of hob front

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