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Hardened gear surface finishing has made some progress in machining accuracy and efficiency, and some new process methods have emerged. Here is a brief introduction to the new process methods and equipment for hobbing, gear shaping, gear shaving, gear honing, and gear grinding of hardened gear surfaces at home and abroad.

 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 tooth surface hobbing process

Complete list of gear machining methods. Let's see how many of them you know?

 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 Level 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, making the hob 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 spindle are two weak links in terms of rigidity, and radial deformation has a significant impact on accuracy.

2. Hardened tooth surface gear shaping process

A comprehensive guide to gear machining methods. Let's see how many of them you know?

 Some foreign companies have achieved preliminary results by grinding the top edge of carbide gear shapers to create a larger negative rake angle, allowing the side edge to have a larger inclination angle, thus forming 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 carbide hob cutters and gear hobbing, there are also many problems such as the inability to increase the gear shaper speed to the optimal cutting speed suitable for carbide tools, vibration caused by reciprocating gear shaper movements, and substandard precision of the gear shaper's transmission chain, which prevent its practical application in production.

 Based on the ordinary gear shaping process, some domestic factories have also conducted research on the hard gear surface shaping process. Currently, gears with a medium-hard tooth surface and a grade 7 precision, capable of withstanding a hardness of around 48HRC, can be produced. The main measures taken include: 1) enhancing the rigidity and precision of the machine tool, and improving the accuracy 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

A comprehensive guide to gear machining methods. Let's see how many of them you know?

 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 is the involute helical surface edge on the shaving cutter reduced to a narrower width, but also the number of cutting edges per tooth on the shaving cutter is reduced to 1 or 2. The major advantage of soft tooth surface shaving is the presence of multiple cutting edges, which, coupled with the involute helical surface maintaining stable meshing, results in high shaving efficiency and ensures precision. However, structural modifications to hard alloy gear shaving cutters have diminished these two advantages of shaving. Furthermore, 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 challenging to implement in practical 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 level of 7, achieving high processing efficiency. The main measures taken are as follows:

 1) Choose tool materials with good shaving performance for shaving cutters;

 2) Improving the manufacturing accuracy of gear shaving cutters and adopting modified or negative-modified gear shaving cutters not only reduce or eliminate the concave 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 Level 6 accuracy gears 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 utilizes disc-shaped honing wheels, which achieve high processing efficiency, capable of honing one gear in 1 to 2 minutes. In recent years, Japan has introduced a new honing method, the worm gear honing process, and launched a new type of worm gear honing machine. The working principle of worm gear honing involves using a worm-shaped honing wheel to hone the gear tooth surface. Compared to the pre-honing accuracy, it can improve the accuracy by 1 to 2 levels. Currently, worm gear honing technology has been patented in more than ten countries including Japan, the United States, the United Kingdom, and Switzerland, and is widely used in the manufacturing of automotive transmission gears. In recent years, China has organized research on worm gear honing technology and conducted extensive studies. Worm gear honing machines have been produced by Nanjing No.2 Machine Tool Plant and Changjiang Machine Tool Plant, and 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 use ordinary abrasives and are divided into soft honing and hard honing types. In recent years, experimental research has been conducted on honing processes using electroplated diamond worm gear honing wheels and electroplated CBN (cubic boron nitride) worm gear honing wheels.

 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 working time of 1 to 2 minutes. The accuracy after honing can be improved by 2 levels, generally reaching 6 to 7 levels. 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 lack of a satisfactory solution to the problem of grinding wheel dressing in the past, the application of form gear grinding technology in production has 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.

A comprehensive guide to gear machining methods. How many of these methods do you know?

Figure 1 Classification of gear grinding methods using the generating method

 The Swiss MAAG gear grinding machines utilize disc-shaped grinding wheels for gear grinding. In addition to maintaining their high-precision advantage, they have 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 enhancing efficiency. Despite these advancements, the efficiency of MAAG gear grinding machines remains low, often taking several hours to grind a single gear, resulting in 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 generated point by point through envelope, which affects the 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 reputable companies producing conical grinding wheel gear grinding machines are Hofler and Niles 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 machining gears with 5 to 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 Maag company in Switzerland, as well as Germany, Japan, the United Kingdom, the United States, etc.) 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 in form grinding will further enhance efficiency. Form grinding is generally suitable for the mass production of gears with a precision of 5 to 6 levels, and it 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 regard have been published, as shown in Figure 2. Most foreign form grinding machines adopt a profiling four-bar mechanism dresser; however, this type of dresser has poor accuracy and stability, requires a large number of profiling templates, and is cumbersome to adjust. Due to the generally complex structure of accurate involute dressers, which affects precision, 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, a diamond roller dressing method has been developed. Due to the complexity and high cost of manufacturing shaped diamond rollers, a diamond abrasive belt pressing wheel method has been developed abroad. In recent years, both domestically and internationally, a numerical control form grinding wheel dresser has been developed, which adopts a three-coordinate closed-loop system to ensure precision and automatically adjust and compensate for diamond pen wear. However, this device is complex and expensive, and has not yet been officially applied in production.

Complete list of gear machining methods. Let's see how many of them you know?

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 the forming motion of hardened gear machining equipment was achieved through mechanical mechanisms. CNC technology enables the integration of a servo motor into each motion of the machine tool. Through the CNC system of the CNC machine tool, pulse commands are sent to each servo motor. Upon receiving the commands, each servo motor drives the ball screw to achieve machine tool motion through motor rotation. This eliminates the mechanical errors inherent in the original internal linkage transmission chain for forming the involute profile of gears, thereby improving the motion accuracy of the machine tool. New CNC gear hobbing machines, gear shapers, gear shaving machines, and gear grinding machines will be widely used.