In his article, “Uncover Hidden Potentials in Hobbing,” which appeared in the June 2014 issue of Gear Solutions, Dr. Friedrich Momper explained how G90 — a new tooling material from Gleason — closes the performance gap between HSS materials and tungsten carbide. He also describes how the utilization of the G90 material can save up to 25 percent in production costs through either increased production rates or extended tool life. The intent of this article is to provide additional test results to further support Momper’s findings and to consider additional benefits of the G90 material.
The Transition to G90 Hobs
Automotive manufacturers and suppliers, farm, and heavy equipment manufacturers, as well as aerospace component makers, around the world have been involved in the testing and validation of G90 hobs through a variety of applications and gear-cutting scenarios. Finding the boundaries and peak performance zones for all of the variables introduced to these test hobs has been challenging, and in some cases the results were different than what was expected.
The most consistent and predictable improvements have been observed in tests where the objective was to increase tool life. This is best accomplished by replacing an existing HSS hob with a hob made from G90 material, while leaving all other cutting parameters the same. Increasing production rates, while achievable, does introduce a host of additional variables that must be considered. For example, if the existing HSS hob was already being used at an aggressive feed rate, increasing this further with a G90 hob could result in chip evacuation problems. In some instances it may be necessary to redesign the hob to better accommodate increased feeds and speeds.
Successful testing and hob design enhancements have now led major high-volume gear manufacturers to transition their hobs from traditional HSS to G90. Entire manufacturing plants are making this change, resulting in cost savings of hundreds of thousands of dollars per year.
Best Applications for G90
One high-volume gear manufacturer recently conducted a study aimed at determining the cost benefit of replacing a carbide hob with a G90 hob for a specific application (Table 1). The new G90 hobs were designed to match the existing carbide hobs as closely as possible. Due to material size availability, the outside diameter of the G90 hob had to be made slightly smaller than the outside diameter of the carbide hob. This resulted in a reduction in usable (sharpenable) tooth length by more than a millimeter. To compensate for this difference, the G90 hobs were made slightly longer in order to provide a greater usable shift zone.
This change from carbide to G90 required adjustments to the machining parameters. The Hob Speed (rpm) was reduced by nearly 50 percent; however, due to the fact that the G90 material is less brittle (i.e. higher edge toughness) than carbide, the axial feed rate (millimeters per revolution) was doubled (as indicated in Figure 1).
This generated a slight reduction in cycle time.
At the adjusted feeds and speeds, the G90 hob performed as well as the carbide hob. The cost savings is realized when comparing the lower price of the G90 hob to higher price of the carbide hob. (Table 2 provides the parameters from this study.) The quality, takt time, and overall performance of the G90 hob remain at the levels produced by the carbide hob.
It is because of these positive results seen from the ongoing testing that a steadily increasing number of high-volume gear manufacturers are changing from the traditional carbide hob to hobs made of G90 material. However, G90 is not the best material for all situations. Carbide hobs are still recommended and preferred for some applications, including those that require high cutting speeds, and when cutting prehardened material — where the highest red hardness and wear resistance is needed.
Dry vs. Wet Hobbing
Momper’s article thoroughly explains how one of the advantages of G90 is its ability to perform well when exposed to elevated cutting temperatures. This important characteristic provides the opportunity to run the hobs at higher feeds and speeds than can be accomplished when using traditional HSS hobs. The G90 material has a special microstructure that, along with its improved high-heat performance, lends itself to dry cutting applications. While the material is an excellent choice for this environment, chip packing — one of the biggest disadvantages of the dry hobbing process — still exists and can even be more apparent at the increased speeds and feeds. Due to the increase in the rate of material removal, the chips that are cut by the hob may not be evacuated quickly enough and can become lodged in the gash. This is known as chip packing and can be a common occurrence in dry hobbing applications.
Special considerations and modifications are commonly seen in hobs that are specifically designed for dry hobbing. The likelihood of chip packing increases as feeds, speeds, and resultant material removal rates are increased, which is often the goal when switching to a G90 hob. In these situations it may be necessary to make additional design modifications that are intended to help chip evacuation. This may include a larger gash size or some extra grinding to ensure the surfaces, that chips may come into contact with, are smooth to allow for better chip flow.
Although it is recommended that G90 be used in dry cutting applications, using coolant can sometimes be beneficial in problematic hobbing scenarios. In these cases, coolant would be introduced primarily to evacuate chips from the gashes in the hob. While G90 is intended for dry hobbing, test results have proven that the use of coolant does not have an adverse effect on the performance of the hob. Coolant can be used to help flush chips out of the cutting zone and prevent them from becoming lodged in the gashes. Image 1
Increasing Tool Life and Production Rates
We have seen that G90 can increase tool life or increase production rates but why not do both? Generally speaking, gear manufacturers make the switch to G90 to decrease their production costs. This is achieved by increasing the number of parts cut by the hob, increasing the rate of production, or by decreasing the cost of tooling. Tests have verified that G90 hobs can cut up to 30 percent more gears than a traditional HSS hob or run as much as 50 percent faster while cutting the same number of gears as a traditional HSS hob. When switching from a traditional HSS hob to a G90 hob, manufacturers choose to either maintain the same feeds and speeds but cut more parts per hob use or increase the speeds and feeds and cut the same number of parts in a shorter amount of time. While manufacturers have seen substantially decreased costs by going “all-in” with one of these approaches, it may be most cost effective to take advantage of all of these benefits at once, depending on the annual usage, price of hobs, and production rates. This can be accomplished by targeting feeds and speeds that are higher than those recommended for the HSS hobs, but low enough to still provide an extended tool life. The benefits may not seem as great in any one area, but the overall improvement should yield a lower cost-per-part and save money in the long run.
The right machine, a suitable coating, and proper refurbishing are all necessary to get the full potential from G90. Regardless of the application, the hobbing machine must provide sufficient stiffness and rigidity (see Figure 1) or chipping and premature wear could occur. The coating is also very important to the hob’s performance. The coating must provide high wear resistance, thermal shock stability, and high hot hardness (AlCroNite®Pro applied to G90 provides these benefits). In order to achieve continued high performance, the hobs must be refurbished (sharpened and recoated) by a reliable and experienced company such as Gleason. Improper reconditioning will result in poor performance, profile errors, or potentially premature hob failure.
While it is clear that there are still many applications where carbide hobs are the highest performing tool, G90 has been found to bridge the gap between carbide and traditional HSS.
There are multiple ways G90 can be more cost effective than hobs made of other materials. Whether G90 is being selected to replace a carbide hob or a traditional HSS hob, there are several factors that need to be thoroughly evaluated. Speeds and feeds need to be properly calculated to accomplish the desired outcome; the hob design needs to be reviewed to maximize performance; and the use of coolant needs to be considered as an option to help eliminate chip packing. The application, gear material, and the volume of gears to be cut are also key factors when determining how to make the most out of the advantages of G90 hobs. To sort out the many variables with the hobbing process, Gleason has an optimization program that compares these options on a cost per part basis to assist customers in identifying which option provides the best total solution.