Companies in today’s competitive environment are always looking for opportunities to improve their manufacturing productivity. Turning, milling, and drilling operations utilizing throwaway insert solutions allow easy replacement with a more economical one.
In the gear hobbing process, the majority of the tools are reconditioned to achieve the best utilization and cost-effectiveness of an expensive tool. Reconditioning includes a proper regrinding, recoating, and post-treatment process to generate the same tool performance as compared to a new tool.
The recent introduction of new tool material, G90, closes the performance gap between HSS materials and tungsten carbide. The experience gained in manufacturing hobs with this new substrate can also be transferred to the reconditioning process in order to deliver the same or nearly the same performance level as the original tool.
This article will explain successful applications in hobbing combined with the calculation of total lifetime costs of the hobs. Even if the initial costs range from 30 to 40% higher compared to the traditionally used powder metallurgy high speed steels, the utilization of the G90 material will save up to 25% in production costs.
Potentials to Improve the Hobbing Process
The requirements to design and manufacture gear sets often compete with each other. Shorter cutting times combined with longer cutting tool life often cannot be realized even when using the latest state-of-the-art cutting tool materials and coatings. The boundaries presented by the requirements of finished gears are limiting the available options to increase hob performance in green machining.
In gear manufacturing, the selection of the cutting tool material is still a relatively simple process. In more than 80% of the instances, customers will choose PM-HSS (powder – metallurgical high speed steel) combined with PVD (physical vapor deposition) coatings, while 20% of the customers select solid carbide hobs. The switch to tungsten carbide based substrates and optimized coatings have not transferred to the gear cutting world.
Hob performance can be optimized by the application of geometrical factors like the number of gashes or number of starts. Fine tuning options like edge preparation or pre- and post-treatment of coatings of the hob are applied to keep the performance of hobs on a high level (Figure 1).
All these process steps are realized during the manufacturing of a new hob. But what is happening during the reconditioning (regrinding and recoating) of hobs? The customer has experienced a long tool life with the new tool. Will the customer receive the same performance with a reconditioned hob? Hobs reconditioned by the manufacturer will guarantee that the same process steps will be applied during the regrinding and the recoating process. The number of reconditioning cycles is another important factor, which determines the performance of a hob. The total life cycle costs are divided by the number of gears being manufactured present the effect of a more expensive tool with a higher tool life compared to a traditional hob (Figure 2).
Development of G90 Material
The G90 cutting tool material is closing the gap between traditional PM-HSS materials and high performance solid carbide materials. One advantage of G90 is a higher hardness at elevated temperatures. The strength of the material is comparable to traditional PM-HSS materials used in the industry. The microstructure of G90 is quite different from PM-HSS materials, where the hardness is built-up by metal-carbides like vanadium-carbide, chromium-carbide, and tungsten-carbide (Figure 3). In the case of G90, the hardness of the material is achieved by secondary hardening of the µ-phase (compared to an internal densification) during the heat treatment process.
The higher hot hardness of the material can be translated into the utilization at higher cutting speeds common in gear hobbing applications.
The schematic diagram Cutting Length vs. Cutting Diagram explains the ability to increase the cutting speed by at least 30% (Figure 4). High cutting speeds will create a crater wear on the hob teeth. Because of the higher hot hardness of G90, the crater wear will start at a later stage of the tool life. Many customers are looking for a constant tool life e.g. of 5m. Taking this target for the tool life, the G90 material would allow a 30% to 50% increase in tool life. In all cases, the Gleason Alcronite® Pro coating was applied.
The data collected during the first initial tests encouraged Gleason to move forward in testing the G90 material. In the specific case of an automotive customer, which was machining small gears at 160 m/min (480 sfm), 1,800 parts were machined per reconditioning. G90 allowed with cutting speed of 240 m/min (720 sfm) to manufacture 2,000 parts per reconditioning (Figure 5). Even with higher costs of G90 compared to the PM-HSS material, the tooling costs per gear are at the same level. In total the customer could manufacture 35% more parts per shift. By using G90 customers could gain additional capacity on their existing machines. G90 allows customers to significantly increase their tool life. In this case the customer is using a G10 material (comparable to cast HSS-material) (Figure 6). With G90 the customer could increase the number of pieces by a factor of 2.5. In the case of G10, tool life was determined by chipping of the teeth. In case of G90 the cutting teeth show a regular flank wear and a smooth crater wear on the rake face.
In all cases, where the customer is looking to increase the tool life, the calculation of the manufacturing costs per gear is the key to evaluating the economics for G90. In this specific case the cost (price) difference between G10 and G90 is 80%, at first glance a real disadvantage for the new cutting tool material (Figure 7).
Referring to the diagram of reconditioning (Figure 2) the total life cycle costs for a hob has to be calculated. The G90 material allows at least 12 reconditioning cycles. During this life time the G90 hob manufactures 57,600 gears compared to 21,600 gears with G10. The total savings in this case is more than 50% in tooling costs per gear. One might translate this result into the total number of hobs used in his production. The savings by using G90, where it is applicable will uncover big potential for additional savings.
Summary
During the last decade the development efforts were on a low level, on the other hand the PM-HSS materials are facing a strong competition by solid carbide hobs in many applications. The answer to this challenge is G90, which is closing the performance gap between solid carbide hobs and the traditional PM-HSS materials. The results presented in this paper should provide some ideas to our customers how to uncover hidden potentials in gear hobbing.
