AGMA 09FTM04-2009 pdf free.AGMA Technical Paper New Developments in Gear Hobbing By Dr. O. Winkel, Liebherr- Verzahntechnik GmbH.
Modern tool design
Although also the machine tools have been improved a lot in the last decades, the major impact on the hobbing technology was related to the tool development. If we look on the improvements in the past there, the focus was on the one hand on the substrate materials and on the other hand on the coating systems (figure 1). Both developments to- gether lead to much higher cutting speeds and/or longer tool life. Even processes like dry hobbing became reality.
Coming from the conventional HSS substrates (e.g., EMo5Co5 or M35) with TIN-coating, the use of carbide hobs seemed to be necessary to realize dry hobbing applications. But after a phase of enthusiasm, problems with the process reliability regarding the tool life of the reconditioned carbide hobs (e.g., due to cobalt leaching during stripping) stopped the trend. Then the introduction of the more heat-resistant T1AIN-coatirigs in combination with higher alloyed and more homogeneous PMHSS substrates brought the dry cutting back on track. Nowadays, dry-cutting with PM- HSS as well as carbide is state-of-the-art. And since the AICrN-based coatings have been introduced successfully in the gear hobbing market a couple of years ago, speeds and feed could be increased even more in many applications.
Besides their more homogeneous structure, the main advantage of the powder metallurgical HSS substrates is the ability to contain greater amounts of alloys. As shown in figure 2, the remaining content of iron was reduced from about 70% for a standard substrate (e.g., ASP2O3O, S590 or Rex45) to a minimum of 55-60% for the so-called bridge materials” (e.g., ASP2O8O, S290 or Rex 121). Those extremely alloyed substrates are more similar to carbide material then PM-HSS, which gives on the one hand the advantage of higher wear resistance but on the other hand the disadvantage of much worse resharpening. Therefore, medium alloyed substrates like ASP2052, S390 or Rex 76 are a good compromise for high performance hobbing applications.
Regarding the different coating systems. figure 3 shows a comparison of the most important characteristics .(1J While the higher hardness of the TiCN-.coating compared to the TiN-coating showed potential to improve the tool performance in wet cutting. the low red hardness (maximum service temperature) of both coatings was not really sufficient for dry hobbing applications. Here the TiAIN- and especially the new AICrN-coatings have proven their performance ability. With maximum service temperatures of 900 — 11O0C in combination with their thermal isolating effect to the substrate material, a new level of dry cutting could be reached.AGMA 09FTM04 pdf download.