The current status of gear shaving technology and its comparison with gear grinding technology

As is well known, gear shaving and grinding are two different gear finishing methods. Gear shaving is performed before heat treatment (which is also the reason for its limitations), while gear grinding is performed after heat treatment. In recent years, with the continuous improvement of machining accuracy, the continuous improvement of CNC machine tools, and the rapid development of machining software, gear finishing technology has been continuously developed, and the precision, efficiency, and various functions of gear grinding have reached extremely high levels. Due to the use of new ceramics and CBN grinding wheels, both form grinding and generating grinding have made great progress. Therefore, some people once thought that gear shaving would be almost completely replaced by gear grinding in a few years, because indeed many gears that were previously shaving are now being ground. However, contrary to expectations, gear shaving is still the most important gear finishing method in global gear production.

2. Comparison of shaving and grinding processes

There are many reasons why gear shaving and gear grinding processes coexist. Due to space limitations, only a simple analysis and comparison will be provided below:

(1) Reduction of transmission noise

The main causes of noise in transmission devices include: ① gear precision (including tooth profile, tooth direction, tooth pitch, etc.); ② device precision (precision of housing bore system, center distance and its variation); ③ gearbox structure (transmission rigidity and deformation), etc.

For a long time, gear manufacturers have relied entirely on gear grinding to improve the overall quality of gears. However, while grinding is virtually flawless in ensuring gear machining accuracy, its high processing and investment costs make it difficult to obtain high-quality gears at a lower cost. Furthermore, high-precision gears do not necessarily guarantee reduced transmission noise.

With the development of CNC gear shaving technology, and thanks to advanced CNC gear shaving grinders, we can now complete the grinding of various profiled gear shaving cutters in just a few tens of minutes. At the same time, modern gear shaving machines can control all their movements through programming. In other words, we are now capable of gear shaving processes that were impossible just a few years ago. Although the quality of some gears after heat treatment cannot be fully controlled (quenching can cause deformation in some gears, especially those with uneven structures, commonly seen in vehicle and tractor transmissions), by controlling the shaving precision to level 5 and implementing anti-deformation measures, we can minimize heat treatment deformation of automotive gears, especially planetary gears used in automatic transmissions, thereby stabilizing the finished gear dimensions.

Advanced gear shaving processes can improve tooth direction accuracy by 2-3 grades (DIN standard), tooth profile accuracy by 2-3 grades (DIN standard), and tooth pitch accuracy by 1-2 grades (DIN standard), making the surface roughness of shaving close to that of grinding (Ra 0.4-0.6μm). Through meticulous pre-shaving and shaving processes, gears with a precision of grade 5 (DIN 3962) can be obtained (the inherent characteristics of the gear should be suitable for the use of gear shaving).

To reduce gear meshing noise, we usually need to consider the theoretical standards and actual conditions of tooth profile and tooth direction. During gear shaving production, the meshing condition of the meshing gears must be checked, and sometimes special tooth profile modification is required to reduce transmission noise. Now with fully CNC gear shaving cutter grinding machines, such modification is easy to achieve.
(2) Elimination or reduction of heat treatment deformation

In medium and large-scale production, by analyzing and studying the deformation of gear tooth profile and tooth direction caused by heat treatment, compensation (anti-deformation) can be added during gear shaving to eliminate or reduce heat treatment deformation.

Since the theoretical tooth direction may differ from the actual measured tooth direction, the ideal gear tooth direction accuracy can be achieved by controlling the shaving parameters and special shaving cutters in the shaving process. At the same time, transmission noise can be reduced, and gears of the same quality as those produced by high-cost grinding processes can be produced using low-cost shaving processes, thus meeting the quality requirements of various transmission gears.

(3) Advantages of CNC machine tool gear shaving process

① It can complete various tooth profile and tooth direction modification to obtain gears with high-precision machined surfaces and good tooth profiles;

② Gear shaving, as a widely adopted gear machining process, features flexible and rapid processing, high production efficiency, and a high degree of automation. Gear shaving machines can be integrated into automated production lines; cutting tools can be used to their fullest potential.

③ Low processing costs (including machine depreciation and tooling costs) and low maintenance costs can reduce costs.

(4) Scope of application of gear shaving process

According to gear standard DIN3962, the main applicable scope of gear shaving is:

①DIN 7～8 grade: Application scope: gears for industrial vehicles, agricultural machinery, tractors and machine tools with low quality requirements; automotive gearboxes, low-speed reducers, etc.;

②DIN 6 grade: If grade 5 precision is achieved through initial gear shaving, grade 6 can be easily achieved after heat treatment. Applications: Gearboxes for various automobiles (including those with high quality requirements), industrial vehicles, machine tools, and reducers. This is the most widely used precision grade.

③DIN5 grade: After gear shaving, the precision of the finished gear is guaranteed to remain unchanged by controlling deformation during heat treatment. Applications: Aircraft gear systems, high-quality automobiles, buses, trucks, measuring instruments, etc.

Besides, some gears cannot be ground and therefore require shaving, such as small-tooth gears connected to large gears, gears with shoulders, synchronizers, and other components. For example, shaving the gears on the second-speed gearbox shaft is necessary because the gears being shaved are primarily used in vehicle transmission systems. Since the second-speed gear cannot be ground, shaving is essential (generally, gears machined with gear shapers cannot be ground).

Of course, for some gear systems that transmit extremely high torques (such as those in heavy trucks), although the required precision can be achieved using gear shaving, they are currently mostly processed by profile grinding. This is because profile grinding makes the root of the gear smoother, making the gear more robust and greatly reducing the risk of gear breakage during transmission. Therefore, gears with larger modules (4-8mm) are usually ground. In addition, gears of DIN4 grade and above must be ground.

In recent years, a rapidly developing new process—internal honing—has driven the widespread adoption of gear shaving. Because CBN wheel honing removes relatively little metal, the pre-honing error of the honed gears cannot be too large. Furthermore, CBN wheels are very expensive. If heat treatment causes significant gear deformation, a deformation-compensating shaving process should be used before heat treatment to remove a large amount of material and effectively reduce heat treatment deformation. This will facilitate the subsequent honing process.

(5) Comparative Examples

When machining planetary gears for automatic transmissions, grinding can produce a grade 5 (DIN standard) gear in about one minute; while shaving requires 13-15 seconds less machining time to obtain the same grade gear than grinding. Furthermore, shaving requires only one machine tool, while grinding requires at least three, making grinding twice as expensive as shaving.

Given the above, all costs and profits should be estimated before selecting a gear machining process.