Fundamental Research on Hobbing Hard Gears Using Fly-Tools

Abstract

To increase the bearing strength of gears, it is necessary to increase the hardness of the gear material. However, in cutting hard materials, the durability of the hob becomes a problem. Cutting action in hobbing is very complicated and many factors contribute to this durability. Therefore, a fly-tool identical to a single hob tooth was made and this was used to perform cutting on a milling machine so that the size of chips produced was the same as the largest cutting chips produced during hobbing as determined analytically. Although the experiment was carried out on marine-reduction gears, it is considered that the results obtained are applicable to other types of gearing too. For gears up to 400 H_B in hardness, hobbing is possible by selecting an Mo-Co type high-speed steel for hob material, a multi-thread hob, using chlorinated oil, etc. For hardnesses over 500 H_B, cutting is not possible without a carbide tipped hob. However, it is not very advantageous to use carbide hobs on soft gear materials. Among the carbide materials M 40 is the most suitable and it is necessary to make the rake angle negative. Durability of carbide hobs is very sensitive to the cutting speed and it is advantageous to increase the feed rather than the number of hob threads. It was realized that in all cases the rigidity of the machine and the mountings is of great importance.