Knowledge

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Gear grinding is a precision gear finishing process. It uses a high-speed rotating grinding wheel as the cutting tool to perform fine grinding on the tooth surfaces of gears that have undergone preliminary processing such as hobbing or shaving and have been heat-treated. This process corrects the deformations caused by heat treatment and significantly improves the dimensional accuracy, tooth profile accuracy, and surface quality of the gears. Its core features are high precision and high surface integrity, making it widely used in high-reliability, high-end gear manufacturing fields. Below, we will introduce it from aspects such as working principle, process characteristics, processing equipment, and application scope.

I. Objectives and Principles of Gear Grinding

1) Objectives of Gear Grinding

Enhance Precision: Elevate the gear accuracy grade from IT7-IT8 in the initial processing to IT6-IT4 (or even higher) to meet the demands of high-precision transmission (such as in aviation and precision robot transmissions).

Reduce Surface Roughness: Grinding can lower the tooth surface roughness from Ra1.6-3.2μm to Ra0.4-0.8μm, reducing friction and wear during meshing.

Correct Heat Treatment Deformations: Gears are prone to deformations such as tooth profile warping and tooth alignment deviation after heat treatments like carburizing and quenching or nitriding. Gear grinding effectively compensates for these errors.

Optimize Meshing Performance: By controlling the contact area position on the tooth surface (such as crown modification and tooth alignment modification), it reduces transmission noise and extends the service life of the gear pair.

2) Grinding of gears

The essence of gear grinding is the controlled frictional cutting between the grinding wheel (tool) and the gear (workpiece), and the tooth surface is processed through the combination of two motions:

The main motion: the high-speed rotation of the grinding wheel (the linear speed is usually 30 to 80 m/s, and can reach 100 to 200 m/s in high-speed grinding of gears);

The feed motion: including the axial feed of the grinding wheel along the tooth width direction, the indexing motion of the gear (tooth-by-tooth processing or continuous processing), and the generating motion (simulating the relative rolling of gear meshing, used in generating method grinding of gears).

II. Classification of Gear Grinding

According to the motion relationship between the grinding wheel and the workpiece, gear grinding is mainly divided into two categories: form grinding and generating grinding.

1) Form Grinding

- Principle: The grinding wheel is dressed into a profile that exactly matches the tooth groove of the gear (such as disc-shaped grinding wheels or bowl-shaped grinding wheels). The tooth grooves are ground one by one through the axial feed of the grinding wheel and the indexing movement of the workpiece.

- Characteristics: The equipment is relatively simple (such as form grinding machines), but dressing the grinding wheel is complex; the accuracy is limited by the precision of dressing the grinding wheel (typically IT6-IT5); the production efficiency is relatively low (as grinding is done tooth by tooth or with multiple indexing operations); it is suitable for small-batch, multi-variety gear processing (such as sample gears, gears for special-purpose machines, and instrument gears, etc.).

- Processing range: External cylindrical gears (straight and helical): diameters from 10mm to 2m; internal gears.

- Clamping method: The workpiece is usually mounted on centers, and the design must allow for the movement space at both ends of the grinding wheel.

2. Form grinding of gears

- Principle: The grinding wheel and the gear simulate the meshing motion of a pair of gears (form generating motion). Through the rotation of the grinding wheel and the rolling of the gear, the tooth surface is ground continuously or tooth by tooth. According to the shape of the grinding wheel, it can be further classified as:

(1) Grinding with disc-shaped grinding wheels:

- A "virtual rack" is formed by the end faces of one or two disc-shaped grinding wheels, which meshes with the workpiece gear (form generating motion), and the tooth surface is ground tooth by tooth (the workpiece is indexed once after each tooth is ground).

- Characteristics: Extremely high precision (IT3 to IT5), but low efficiency (tooth-by-tooth processing), suitable for ultra-precision gears (such as machine tool spindle gears, aerospace gears).

(2) Grinding with bowl-shaped grinding wheels:

- The outer cylindrical surface of the bowl-shaped grinding wheel is used as the grinding surface, which meshes with the workpiece gear (form generating motion), and the tooth surface is ground tooth by tooth.

- Characteristics: High precision (IT4 to IT5), good rigidity of the grinding wheel, suitable for high-precision processing of large-module gears (module m > 5mm) (such as gears for heavy machinery).

(3) Grinding with worm-shaped grinding wheels (rolling grinding method):

- The grinding wheel is made in the shape of a worm (forming a worm-gear meshing with the workpiece gear), and through the rotation of the grinding wheel (main motion) and the synchronous rolling of the workpiece (form generating motion), the tooth surface is ground continuously (similar to gear hobbing, but using a grinding wheel instead of a hob).

- Characteristics: High efficiency (continuous processing), high precision (IT4 to IT6), only suitable for external gear processing; suitable for high-precision gear processing in large batches (such as gears for automotive transmissions, aircraft engine gears).

 

 

III. Grinding Process Flow

Grinding of gears requires strict control of each step to ensure accuracy. The typical process is as follows:

1) Pre-processing: The gear blank undergoes forging/casting → rough machining (turning) → semi-finish machining (rolling/cutting), obtaining a gear blank that meets the dimensional and positional tolerance requirements, with allowance for grinding.

2) Heat treatment: Perform carburizing and quenching, nitriding, etc., according to performance requirements (for example, automotive gears need to be carburized and quenched to HRC58-62).

3) Grinding the reference: Finely grind the inner hole and end face to provide an accurate positioning reference for grinding the teeth.

4) Pre-grinding preparation: Clean the workpiece → clamp and position (using a mandrel or fixture to ensure coaxiality) → dressing the grinding wheel (precisely dress the target tooth profile and tooth profile modification, and tooth direction modification is achieved by changing the radial cutting depth of the grinding wheel).

5) Grinding processing:

 - Tool setting: Adjust the relative position of the grinding wheel and the workpiece to ensure the accuracy of the grinding starting point;

 - Grinding: Grind the tooth surface according to the set generating motion or forming trajectory (single tooth multiple feed or continuous indexing);

 - Modification (optional): Through adjusting the grinding wheel trajectory or using additional devices, perform crown modification, chamfering at the tooth end, etc., to optimize the contact area.

6) Inspection and compensation: Use a gear measuring center (CMM) to inspect tooth profile error, tooth direction error, cumulative pitch error, etc. If out of tolerance, adjust the process parameters (such as grinding wheel speed, feed rate) or dress the grinding wheel.

7) Deburring and cleaning: Remove burrs from the tooth surface, clean, and package for storage.

IV. Key Equipment for Gear Grinding

Gear grinding machines are the core equipment for gear grinding processes, and their performance directly determines the processing accuracy of gears. According to structure and application, they are mainly classified as follows:

1. Worm wheel grinding machines (such as KAPP NILES from Germany, Liebherr from Germany, and Qinchuan Machine Tool from China): They can achieve a grinding speed of 10-20 teeth per minute (continuous indexing) and are widely used in automotive gears.

2. Conical grinding machines (such as Gleason from the United States and Reishauer from Switzerland): They use double or single conical grinding wheels and are suitable for large module, high-precision gears (such as gears for aircraft engines).

3. Form grinding machines (such as Mitsubishi from Japan and Harbin Tool & Die Group from China): They have a simple structure and low cost, and are suitable for small-batch, small-module gears (such as gears for instruments and meters).

4. Five-axis grinding machines (high-end models): They integrate multi-axis CNC systems, allowing for flexible grinding wheel dressing and the grinding of complex tooth surfaces (such as shaped gears and non-circular gears), representing the current highest level (such as Tornos from Switzerland and DMG MORI from Germany).

V. Application Scenarios

Due to their high precision and good surface quality, gear grinding processes are widely used in fields with strict requirements for transmission performance:

- Aerospace: Gears for aircraft engine reducers (such as accessory drive gears for turbofan engines), gears for helicopter rotor transmissions (requiring extremely high reliability and lifespan).

- Robotics: High-speed planetary gears for industrial robot RV reducers and harmonic reducers (requiring low backlash and high transmission accuracy).

- Automotive: Gears for new energy vehicle gearboxes, synchronizer gears for automatic transmissions (AT/DCT), and main reducer gears (requiring high load and impact resistance).

- Precision Machinery: High-precision gear pairs for CNC machine feed systems (such as gears for ball screw drives), high-speed spindle gearboxes.

- Energy Equipment: Gears for wind turbine gearboxes (requiring fatigue resistance and impact resistance), gears for marine diesel engines (requiring long lifespan), gears for high-speed gearboxes in gas turbine generators, and gears for high-speed equipment, etc.

 

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