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Hardened tooth surface finishing has made some progress in machining accuracy and efficiency, and some new process methods have emerged. Below, we briefly introduce the new process methods and equipment for hobbing, gear shaping, gear shaving, gear honing, and gear grinding of hardened tooth surfaces both domestically and internationally.

In recent years, various hard surface finishing techniques have made some progress in terms of machining accuracy and efficiency, and some new hard surface finishing methods have also emerged.

1. Hardened gear hobbing process

Due to the low efficiency and high cost of gear grinding, foreign countries have begun to research hardened gear hobbing technology in recent years. The use of carbide hob cutters can improve processing efficiency, and it has been applied in production in Japan, Germany, and other countries. In recent years, China has also actively researched hardened gear hobbing technology and made some progress, with some factories already applying it in production.

Some domestic manufacturers have adopted the hardened tooth surface hobbing process to replace rough grinding, achieving a processing efficiency approximately 5 to 6 times higher than that of conical grinding wheel gear grinding machines. The hardened tooth surface hobbing process requires a hobbing machine with good rigidity and high precision. Only with the correct coordination of gear cutting technology, cutting tools, and machine tools can the gear cutting process achieve an optimal state, and the precision of hardened tooth surface hobbing can reach Grade 7. The major issue with hardened tooth surface hobbing is the low tooth profile accuracy and unstable machining precision. The main reasons for this are:

1) When using a carbide hob to machine a hard tooth surface with a hardness of around 60HRC, the hardness difference between the tool and the workpiece is very small, and the hob is prone to wear, which directly affects the tooth profile accuracy;

2) As gear hobbing is an intermittent cutting process, it is often accompanied by forced and self-excited vibrations, as well as impacts of force and heat, which can easily cause the carbide hob to break;

3) Due to the use of negative rake angles in carbide hob cutters, coupled with the small cutting thickness and high hardness of the workpiece, the radial cutting force is significant. From a process system analysis, although the gear hobbing machine has strong rigidity, the tool holder and the spindle are two weak links in terms of rigidity. Radial deformation has a significant impact on accuracy.

2. Hardened tooth surface gear shaping process

Some foreign companies have achieved preliminary results by grinding the top edge of carbide gear shapers to form a larger negative rake angle, allowing the side edges to have a larger inclination angle, thus creating a scraping process. However, it is very difficult to use carbide gear shapers to produce gears with a precision of Grade 6. In addition to similar issues as those encountered with carbide hob cutters and gear hobbing, there are also many problems such as the inability to increase the gear shaping speed to the optimal cutting speed suitable for carbide tools, vibration caused by reciprocating gear shaper movements, and substandard accuracy of the gear shaper's transmission chain, which have prevented their practical application in production.

Based on the ordinary gear shaping process, some domestic factories have also conducted some research on the hard gear surface gear shaping process. Currently, gears with a medium-hard tooth surface and a grade 7 precision, around 48HRC, can be produced. The main measures taken include: 1) improving the rigidity and precision of the machine tool, and enhancing the precision of the transmission chain; 2) using high-precision gear shapers (grade AA gear shapers), and strictly controlling the installation eccentricity of the tool; 3) improving the precision of the gear blank and fixtures; 4) selecting reasonable feed times and machining feed rates.

3. Hardened tooth surface shaving process

In recent years, Japan has successfully experimented with machining gears with a precision level of 8 and a hardness of 60 HRC using hard alloy gear shaving cutters. To enable the shaving cutter to penetrate the hard tooth surface of the workpiece, not only was the involute helical surface edge on the shaving cutter narrowed, but also the number of cutting edges per tooth was reduced to 1 or 2. The original major advantage of soft tooth surface shaving is the presence of multiple cutting edges and the involute helical surface, which maintain stable meshing, resulting in high shaving efficiency and ensuring precision. However, structural modifications to the hard alloy gear shaving cutter have diminished these two advantages. Additionally, the characteristic of shaving is that the cutting thickness is very small, and the cutting edges of hard alloy are generally dull, making it difficult to perform shaving processing; thus, it is difficult to be practically applied in production.

In recent years, China has also been researching the hard gear shaving process. Currently, it is capable of shaving medium-hard gears with a hardness of around 48 HRC and a precision of Grade 7, achieving high processing efficiency. The main measures taken are as follows:

1) Select tool materials with good shaving performance for making shaving cutters;

2) Improving the manufacturing precision of gear shaving cutters and adopting modified or negative-modified gear shaving cutters can not only reduce or eliminate the concavity phenomenon in the tooth profile of the shaved gear, but also enable the shaving of drum-shaped gears;

3) Improve the machining accuracy of the gear blank before and after heat treatment, and strictly control the quenching deformation during heat treatment;

4) Fine-tune the machine tool, enhance its rigidity, and select appropriate shaving parameters.

4. Honing process

Honing is currently the primary method for machining high-precision hardened gears; however, it is relatively difficult to improve the honing accuracy to level 6. Currently, some domestic factories have already honed gears with level 6 accuracy. Factories in Germany also produce gears with level 6 accuracy using honing, employing a series of measures such as rough and fine gear hobbing, shaving with modified shaving cutters, and strictly controlling heat treatment deformation to ensure the accuracy before honing, and using modified honing wheels for honing.

The conventional honing process, utilizing disc-shaped honing wheels, boasts high machining efficiency, capable of processing one gear in 1 to 2 minutes. In recent years, Japan has introduced a novel honing method, the worm gear honing process, accompanied by the launch of new worm gear honing machines. The working principle of worm gear honing involves using a worm-shaped honing wheel to hone the tooth surface of gears. Compared to the pre-honing accuracy, it can enhance the accuracy by 1 to 2 levels. Currently, over a dozen patents for worm gear honing technology have been published in countries like Japan, the United States, the United Kingdom, and Switzerland, predominantly utilized in the manufacturing of automotive transmission gears. In recent years, China has organized research on worm gear honing technology, conducted extensive studies, and has seen the production of worm gear honing machines by Nanjing No.2 Machine Tool Plant and Changjiang Machine Tool Plant. These machines have begun to be applied in production, achieving an accuracy of 6 to 7 levels and an average production speed of 3 to 6 minutes per piece. Generally, worm gear honing wheels utilize conventional abrasives and are categorized into soft honing and hard honing types. In recent experimental research, honing processes using electroplated diamond worm gear honing wheels and electroplated CBN (cubic boron nitride) worm gear honing wheels have been explored.

In recent years, foreign countries have also developed the internal gear free honing process, and Switzerland has produced internal gear honing machines. This process uses an internal gear honing wheel to process external gear workpieces, with an average single-piece work time of 1 to 2 minutes. The accuracy after honing can be improved by two levels, generally reaching levels 6 to 7. If the accuracy before honing is improved, gears with higher accuracy can be honed.

5. Gear grinding process

Gear grinding is divided into two major categories: generating gear grinding and form gear grinding. Generally speaking, generating gear grinding has lower efficiency (except for worm wheel gear grinding), higher gear grinding costs, complex machine tools, and expensive prices, which limit its wide application in production and are only used in a few precision machinery and tool industries. Form gear grinding, on the other hand, has the advantages of simple machine tools, higher efficiency, and lower costs. However, due to the unresolved issue of grinding wheel dressing in form gear grinding in the past, the application of this process in production has also been hindered. Nowadays, countries around the world are actively researching new gear grinding methods and gear grinding machines that are high-precision, high-efficiency, multifunctional, and stable in performance.

5.1 Generating gear grinding

The generating gear grinding method can be further divided into single-tooth indexing generating gear grinding and continuous generating gear grinding, as shown in Figure 1.

Figure 1 Classification of gear grinding methods using the generating method

The Swiss MAAG gear grinding machine utilizes a disc-shaped grinding wheel for gear grinding. Apart from maintaining its high-precision advantage, it has undergone improvements in structure and grinding methods, resulting in increased efficiency. To enhance processing efficiency, MAAG has evolved its grinding method from the conventional 15°/20° grinding method of the HSS series gear grinding machines to the 0° grinding method, utilizing the BC mechanism for grinding and shaping gears, thus forming the new SD series. Subsequently, based on the 0° grinding method, MAAG further developed the K grinding method, further improving efficiency. Despite these advancements, the efficiency of MAAG gear grinding machines remains low, often taking several hours to grind a single gear, leading to high costs, especially for large-sized gear grinding machines, which exhibit even lower efficiency.

The transmission chain of a conical grinding wheel gear grinding machine is long, and the involute tooth surface is enveloped point by point, affecting machining accuracy and stability. The working hours per piece are generally 0.6 to 3 hours; however, conical grinding wheel gear grinding machines have good versatility and are widely used in production. The world's most reputable companies producing conical grinding wheel gear grinding machines are the Hofler Company and the Niles Company in Germany. These two companies have developed a series of gear grinding machines with a diameter of 3500 mm.

The large-flat grinding wheel gear grinding machine has a short transmission chain, a relatively simple structure, and high machining accuracy; however, its machining efficiency is low, and it is mostly used for machining gear cutting tools such as shaving cutters and gear shapers, as well as gears with high precision. Relevant manufacturers in various countries are producing this type of gear grinding machine, but there has been no significant development in its structure and performance over the years.

Among various gear grinding methods, the worm wheel gear grinding machine boasts high efficiency, typically capable of grinding one gear in just over 10 minutes. It is suitable for processing gears with 5-6 stages, tooth count greater than 8mm, or diameter greater than 600mm. However, there are still issues to be addressed.

In recent years, Japan has also seen the emergence of CNC worm wheel gear grinding machines that utilize electroplated CBN (cubic boron nitride) worm wheel grinding wheels. These grinding wheels can continuously process tens of thousands of gears without requiring dressing, and can be replaced with new ones after being worn out, resulting in high processing efficiency. It takes about 1 minute to grind one gear, and it is also capable of grinding profiled gears. Currently, various forms of generating gear grinding machines have been produced in China, but the product size series is not yet comprehensive.

5.2 Form grinding

In order to address the issue of efficient and economical gear grinding for large quantities of gears, many countries around the world (such as the aforementioned Swiss company MAAG, as well as Germany, Japan, the United Kingdom, the United States, and others) have been actively researching form grinding processes in recent years. Form grinding achieves a precision comparable to that of generating gear grinding, and its productivity is many times higher (although still lower than that of worm wheel gear grinding). The application of powerful and efficient grinding to form grinding will further enhance its efficiency. Form grinding is generally suitable for the mass production of gears with a precision level of 5 to 6, and is particularly suitable for the processing of gears with large modulus, few teeth, wide width, and various modified tooth profiles.

The first problem to be solved in form grinding is the creation of a universal form grinding wheel dresser. In recent years, many patents and achievements in this area have been published, as shown in Figure 2. Most foreign form grinding machines adopt a four-bar mechanism dresser based on a master model; however, this type of dresser has poor accuracy and stability, requires a large number of master models, and is cumbersome to adjust. Due to the generally complex structure of accurate involute dressers, which affects accuracy, many approximate involute dressing methods have been developed. When dressing grinding wheels with a single diamond pen, it is difficult to solve the impact of diamond pen wear on the accuracy of the dressed tooth profile; therefore, the diamond roller dressing method has been developed. Due to the complexity and high cost of manufacturing shaped diamond rollers, the diamond abrasive belt pressing roller method has been developed abroad. In recent years, CNC form grinding wheel dressers have been developed both domestically and internationally, using a three-coordinate closed-loop system to ensure accuracy and automatically adjust and compensate for diamond pen wear. However, these devices are complex and expensive, and have not yet been formally applied in production.

Figure 2 Classification of dressing methods for form grinding wheels

Future development of hardened gear finishing technology

With the advent of CNC technology, the original internal linkage transmission chain for forming the involute profile of gears in the design of forming movements of hardened gear finishing equipment has been realized through mechanical mechanisms. CNC technology enables each movement of the machine tool to be equipped with a servo motor. Through the CNC system of the CNC machine tool, pulse commands are sent to each servo motor. Upon receiving the command, each servo motor drives the ball screw to achieve machine tool movement through motor rotation. This eliminates the mechanical mechanism errors of the original internal linkage transmission chain for forming the involute profile of gears, thereby improving the movement accuracy of the machine tool. New CNC gear hobbing machines, gear shapers, gear shaving machines, and gear grinding machines will be widely used.