While quiet fuel cells are replacing the internal combustion engine as hybrid and electric vehicles become more popular, planetary gears and other parts remain essential. These parts also need to be as quiet as the cars they are used in. Keys for improvement are weight reduction, high precision and rigidity.
Considering such changes for the future, skiving technology has attracted attention as a new gear machining method. Gear shaper machining and broaching are traditionally applied for internal gears and hobbing is used as a standard for external gears. Skiving has great potential as an alternative to existing machining methods for both internal and external gears.
The principle of skiving machining was developed in Germany about a century ago. In and after the 1970s, it was also considered in Japan. However, due to the lack of machine tool rigidity, it could not be brought into practical use. Along with technological progress in recent years, however, research and development for skiving has been actively pursued.
The original meaning of “skive” is to thinly peel. The principle of skiving is as follows:
・The tool is set diagonally to the work piece, and a crossing angle for axes is provided between the rotating axis of the work piece and the tool.
・High-speed rotation after synchronizing the work piece and the tool generates sliding at the contact point. This sliding flakes off material to form the teeth of the gears.
Skiving realizes advantages in a series of processes that existing machining methods lack. For example, skiving makes it possible to create blind-hole internal gears, which broaching cannot accomplish. Furthermore, skiving can perform accurate adjustment of tooth trace for crowning and hollowing, as well as adjustment of thickness and cut in the inner diameter of the hole.
While gear shaper machining is performed by a reciprocating motion, meaning that half of the motion is not used for machining, skiving machining is serial machining performed by rotational motion, which increases efficiency. In addition, skiving generates less vibration than gear shaper machining, which realizes improved accuracy.
The chart below compares the performance of broaching, pinion (gear shaper), hobbing and skiving machining.
Skiving with these products alleviates the need for specialize equipment. It can be performed with combined lathe and machining centre, which makes complete improvement of the production environment possible. Gear machining through hobbing and broaching requires different specialized machines; however, skiving can be performed with general-purpose machines that can perform a significant number of other machining processes.
In order to make skiving even more practical, it is important to improve machining accuracy, productivity and tool life. If the crossing angle of the axes increases, machining speed increases. After carefully checking interference with work pieces, we determine and adjust angles to increase productivity. During skiving, rake angles change, which produces parts with high resistance to cutting and tends to shorten tool life.
As an outstanding solution to product life issues, Mitsubishi Materials is now offering cutting tools made of KHAZ materials with a GV40 coating. Such materials are already used for hobs and pinion cutters. KHAZ is made of high-alloy powder containing fine, high-hardness carbide for excellent wear resistance. The amount of carbide is optimized and gives significantly improved resistance to chipping. Research is also conducted on the application of cemented carbide to tools for future manufacturing. If cemented carbide is used instead of high-speed steel, it may significantly extend tool life. In addition, coating materials for even longer tool life are being researched. Simultaneously, work on applications for smalldiameter gears and improved accuracy is ongoing.
If carbide skiving cutters can be manufactured, heat treated work pieces can also be machined. Finish machining after heat treatment, which is now done by a grinding process will also be possible by a skiving process. This will expand the potential for integrating tools and processes.
Mitsubishi Materials has accumulated design and analytical technology for shaving cutters and manufacturing technology for pinion cutters. In addition to such knowhow for the design and manufacture of tools, there is a commitment to advancing the development of cemented carbide skiving.