Mechanical property changes of gear surfaces in high-speed hobbing

Abstract

In recent years, high-precision gears are expected to be used in vehicles and robots. Currently, gear hobbing is widely used as a roughing process while shaving and grinding are used as finishing processes to improve the accuracy and surface quality of the gears. The increasing demand for gears requires a gear machining method that is more efficient than the conventional machining processes. In this study, high-speed gear hobbing that achieves accuracy comparable to shaving and grinding was performed at a cutting speed 12 times that of conventional hobbing. Experiments were conducted using Cr-Mo steel as the workpiece and cemented carbide as the hob cutter in the cutting speed range of 200 - 2450 m/min. The relationship between the hardness and the depth from the gear surface was investigated with a comparison with the change in the material structure of the gear surface layer. The results showed that the hardness of the gear surface layer increased in the high-speed hobbing process compared to the conventional hobbing due to the strain and crystal grain refinement caused by the high cutting speed. In addition, experiments were performed at each cutting speed in the range of 400 - 1200 m/min to investigate the possible changes in machining characteristics when the number of machined pieces increased to 30. The results showed that despite the number of machined gears, the surface hardness increased at high cutting speeds. The effects of the number of machined gears on tool wear and surface roughness were also insignificant, indicating that high-speed hobbing is a promising efficient production method of gears with high surface quality.

Keywords

• gear hobbing
• surface integrity
• high-speed cutting
• nanoindentation
• hardness
• crystal structure
• strain distribution