Design process of carbide hob and hard tooth surface hobbing

The carbide hob is used to process the hard tooth surface, which revolutionizes the traditional hard tooth surface finishing process. For high-precision grinding gears, hard tooth surface hobbing can replace the rough grinding process with high efficiency, remove the heat treatment deformation of the gear, leaving a small and uniform margin for fine grinding, and improve the gear grinding efficiency.

For derivative gears, arranging hard tooth surface hobbing before the derivative teeth can remove the heat treatment deformation and achieve the necessary accuracy, and then perform the derivative processing to give full play to the advantages of the finishing process of the derivative teeth process and compensate for the hobbing process. shortcomings.

For hardened gears with ordinary precision, carbide hobs can be used to directly perform fine hob processing to ensure gear processing accuracy. It is an irreplaceable gear processing technology.

 

carbide hob

 

1. Carbide hob design

 

1) Hob material

 

The characteristics of hard tooth hobbing are high workpiece hardness, intermittent cutting process and thin cutting layer. During the cutting process, the tool is subjected to a large impact load, high cutting temperature and strong friction. Therefore, the tool is severely affected. The requirements for the impact toughness of the cutting part materials are very high. Through tests, it is recommended to use the grade YT14. This material has high wear resistance, and high-temperature carbides such as lithium carbide are added to improve the impact and wear resistance of the blade to obtain good cutting performance.

 

2) Heat treatment and coating of hob

 

The heat treatment of the hob adopts salt bath furnace isothermal quenching, salt bath furnace graded isothermal and vacuum quenching. The improvement of the quenching method enables the hardness of the tool to be stably controlled within a reasonable range. The surface of the hob is coated with TiN, TiALN and carbon-coated composite nanomaterials, which greatly improves the durability of the hob.

 

3) Structural form of hob

 

At present, there are three main structures of carbide hobs designed by countries around the world, including integral type, machine clamp type, and welded type.

 

(1) Solid carbide hob

It is characterized by strong rigidity, time-saving mechanical processing, and high precision. However, due to the limitations of the overall pressing process, currently only hobs with an outer diameter of 85mm can be produced, and the cost is high due to the high consumption of expensive carbide. Therefore, it is only suitable to produce hobs with a module m=3mm or less.

 

(2) Machine clamp-type carbide hob

 

The machine-clamp structure is more complex and the clamping reliability is also poor. Especially when processing 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 relatively high. high. This structure can be used for various types of medium module (m1-6) carbide hobs with rake angle g=0°~-30°, and the cutting effect is very good.

(3) Welded carbide hob

 

It is characterized by a simple structure, high connection strength, easy sintering of carbide blades, material saving, and wide application. However, cracks caused by welding stress have always been a factor in unstable product quality, so higher welding technology is required, which has been applied in production in recent years.

 

4) Hob rake angle

 

Due to the poor impact toughness of cemented carbide, edge chipping is easily produced when hobbing hard surface gears. Edge chipping is the main problem that carbide hobs need to solve. For this reason, when designing the hob, a special form with a large negative rake angle is adopted.

There are two points to consider when determining the rake angle:

a. Maintenance of tooth profile accuracy after tool sharpening.
b. Improve the ability of the blade teeth to resist chipping and reduce blade wear.

 

The size of the negative rake angle will directly affect the maintenance of tooth profile accuracy after sharpening and the ability to resist chipping. The larger the negative rake angle, the worse the accuracy retention, but if the negative rake angle is too small, the tool’s ability to resist chipping will be smaller.

Theoretically, as the negative rake angle of the carbide hob increases, the inclination angle of the hob side edge increases, allowing the hob teeth to cut into the metal layer smoothly, thereby reducing the impact and protecting the carbide teeth. The blade will not chip and its durability is significantly improved. However, the larger the negative rake angle value, the more difficult it is to ensure the tooth profile accuracy of the hob.

 

5) Correction of the offset value of the hob front

 

After the straight groove hob is reground, it is customary to stipulate that the front offset value remains unchanged, thinking that this can ensure the accuracy of its tooth shape. But theoretically, it is inconsistent. As the hob regrinding amount increases, the involute error of the hob gear teeth gradually deviates in the negative direction at the tooth top. Through theoretical analysis and experiments, it is found that changing the front offset value during regrinding can ensure the straightness of the hob. The tooth shape accuracy of the grooved hob is improved after grinding, thereby increasing the service life of the hob. In short, only by increasing the rake angle after grinding can the accuracy of the tooth shape after grinding be ensured.

 

6) Determination of the basic dimensions of the hob

 

The basic size of the hob refers to the outer diameter, hole diameter and length of the hob, which should be based on the specification and purpose of the hob.

Determined by factors such as structural form and machine tool conditions.

 

(1) The outer diameter and hole diameter of the hob

 

The advantages of reducing the diameter: It can overcome the vibration caused by variable loads, thereby increasing the speed of the hob and gear, shortening the cutting-in and out-cutting time, and increasing the interval between two sharpenings of the hob.

Disadvantages of reducing the diameter: it will cause the helix angle of the hob to increase, which will lead to larger shape errors of the hob and reduced accuracy.

Large-diameter hobs, because they have larger inner holes and better rigidity, should be given priority in actual design.

 

(2) Hob length

 

The minimum length of the hob should meet the following requirements
a. The hob should have sufficient “blade shifting” length
b. The hob can completely envelop all the tooth shapes of the gear being processed.
c. The tooth load on the hob end should not be too heavy

 

2. Hard tooth surface gear hobbing process

 

With the continuous development of the machinery industry, gears are used more and more widely, and the requirements for gear accuracy and mechanical performance in transmission systems and transmission systems are getting higher and higher. More and more gear transmissions adopt large load-bearing capacities and are resistant to pitting corrosion. Good performance hardened gears. For industrial gears, hard tooth surfaces with a hardness above 350HBS have almost completely replaced soft tooth surfaces with a hardness lower than 350HBS after surface quenching and overall quenching.

The process route for hobbing hard tooth surfaces is as follows:

(1) Ordinary precision gears (levels 7-9): gear hobbing, quenching, hardened tooth surface, semi-precision rolling,

(2) Derived gear (level 6-7): gear hobbing, quenching, hardened tooth surface, semi-precision rolling gear;

(3) Grinding gears (levels 3-6): hobbing and quenching hardened tooth surfaces and semi-precision hobbing gears.

 

There are two main reasons that affect tooth profile accuracy:

 

First, the gear hobbing machine has poor stability and transmission stiffness;

Second, there are certain difficulties in manufacturing high-precision carbide hobs.

The tooth shape of a hob with a large negative rake angle changes greatly after regrinding. To this end, it is necessary to carefully design and calculate the structure and parameters of the hob, analyze the tooth profile accuracy after regrinding, and propose improvement measures.

Gear hobbing is the most commonly used and highly productive processing method in tooth profile processing. It is not only used for tooth processing but also for finishing. However, during the gear hobbing process, errors inevitably occur during the manufacturing, installation and adjustment of the process system. These errors affect the accuracy of the transmission motion, the smoothness of the transmission and the uniformity of the load distribution. Therefore, when designing a carbide hob, in order to meet its use requirements, the material, shape and specification requirements during design are also very strict. If the hobbing tooth surface defects and tooth surface roughness do not meet the design requirements, it will also cause noise during gear transmission, aggravate tooth surface wear, and reduce service life.

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