High-speed dry hobbing has existed worldwide for many years, yet its application in China is not particularly widespread. Recently, I have organized relevant content about high-speed dry hobbing into a study note, which I am recording and sharing with everyone.
The Emergence of Dry Hobbing
In recent years, environmental safety requirements have been continuously enhanced around the world. Many enterprises are seeking ISO 14000 certification to meet international environmental standards. As part of this effort, factories are designing and implementing coolant-free metal cutting methods—not only to improve the working environment but also to protect the environment.
However, in turning and milling applications, there are almost no dry-cutting machines that completely do not use coolant, although some machines are shifting from wet cutting to MQL (Minimum Quantity Lubrication). Among them, the gear cutting field, especially the hobbing field, is transitioning to fully dry-cutting machines.
In hobbing applications, dry cutting methods using carbide hob cutters were advocated in the past, but they were not widely accepted due to unresolved issues of tool chipping.
Since then, with advancements in surface treatment technology, the emergence of coated high-speed steel (HSS) hobs and dry hobbing machines has driven the rapid development of dry gear hobbing technology. (See Table 1)
In fact, coolant in factories generates oil contamination and has adverse effects on the environment. Dry cutting not only solves such environmental problems but also effectively improves cutting efficiency and reduces lifecycle costs due to its low operating costs.
From an environmental perspective, the benefits of using dry hobs have prompted companies such as Mitsubishi, Gleason, and Liebherr to research dry cutting using HSS hobs, which are more stable than carbide hobs. Manufacturers conducted tests by increasing the cutting speed starting from the same speed as that of ordinary wet-cutting HSS hobs.
The results were surprising: high-speed cutting proved to be an ideal cutting method. When the cutting speed was doubled compared to the normal cutting speed, no abnormal wear or temperature rise occurred on the workpieces.
However, the lack of coolant also brought significant problems that hinder the practical application of dry cutting. These problems include heat generated by flying chips and cuttings, which has prompted machine tool manufacturers to develop a new type of dry hobbing machine.
Below, taking Mitsubishi’s high-speed hobbing machine as an example, we will discuss the machine functions and tool requirements for dry hobbing.
The main motor capacity of the machine tool is a key issue. In wet hobbing, HSS hobs are usually used to hob workpieces (carburized steel) at a cutting speed of 80 to 160 m/min. However, the cutting speed of dry hobbing ranges from 160 to 400 m/min, which is 2 to 2.5 times that of wet cutting, or even higher. Since cutting power is proportional to cutting force and spindle speed, dry hobbing machines require a main motor capacity that is more than 2 to 2.5 times that of traditional wet hobbing machines. In other words, it is possible to modify a traditional hobbing machine into a dry hobbing machine, but in many cases, the cutting conditions are limited by the main motor capacity.
In wet hobbing, coolant is used to clean the cutting point, cool the workpiece, tool, and fixture, and discharge chips out of the machine. In contrast, in dry hobbing, these functions are performed by air and protective covers. Figures 1 and 2 show a hobbing machine equipped with these measures.
Figure 1: External View
Figure 2: Measures Against Chip Scattering and Accumulation in Dry Hobbing
One strategy adopted is to use a fully sealed cutting area. Cutting chips with kinetic energy bounce inside the cutting area, fly through gaps as small as a few millimeters, eject from the hobbing area, and accumulate on machine components. Therefore, to prevent deterioration of machine performance and malfunctions, the cutting area must be fully sealed.
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Column Side: Since the hob headstock performs vertical and rotational movements, a double-cover design (one mounted on the hob saddle and the other on the column) is adopted to prevent chips from flying out. In addition, all hoses entering the machine are connected through a relay module that separates the inside and outside of the cutting area. Of course, the parts of cables and hoses placed inside the cutting area need to be protected from high-temperature cutting debris.
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Tailstock Side: To prevent chips from flying out during hobbing, the opening of the partition cover used for spindle rotation is sealed with a baffle. As shown in Figure 3, the top of the machine bed is covered with a steep-sloped stainless steel cover with a low friction coefficient. This cover can collect falling chips into the chip conveyor installed at the bottom of the machine bed, preventing chips from flying out of the cutting area and accumulating.
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Hob Head Cover: A stainless steel hob head cover prevents flying chips from accumulating on the hob head or entering the gaps between the main motor and machine components.
Dry hobbing lacks the flushing ability of coolant, which easily causes chips to adhere to the cutting tool—leading to defects on the gear tooth surface and chip deposition.
Machine tool manufacturers have proposed corresponding solutions to these problems; most use flat air-blowing nozzles to generate air flow for removing waste chips (see Figure 4).
The installation angle of the protective cover must take into account chip rebound and the inclined fixture surface to prevent chips from entering the cutting area. In addition, air blowing is used to remove chips accumulated on the top of the workpiece, and a baffle is installed near the outer circumference of the hob to prevent flying chips from being caught and rotating with the hob.
Figure 4: Measures to Prevent Chips from Getting Stuck on Tooth Surfaces
In dry cutting, chip removal measures are crucial because most of the heat generated during dry cutting is transferred to the cutting chips. In addition, since no coolant is involved in the process, the heat generated by the machine must be controlled in other ways. These measures include:
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Inner Cover: To prevent heat transfer to the machine body, a stainless steel cover with low thermal conductivity is installed with a layer of air in between for heat insulation.
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Dust Collector for Dissipating Cutting Heat: To prevent heat from chips being transferred to the machine body through convection, the machine uses a dust collector to capture fine chips generated during dry hobbing, thereby providing air flow to carry cutting heat out of the machine.
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Protection of Hoses and Cables: Hydraulic and other connections use steel-braided hoses, as high-temperature flying chips may cause oil or air leaks. To prevent chip-induced wire breakage, connecting cables are protected by heat-resistant plastic conduits. Controlling heat generated by the machine body requires the use of non-contact oil seals. Replacing the oil seals used on the spindle (which generates a large amount of heat) with non-contact seals significantly reduces heat generation.
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Bearing Lubrication: Bearings use oil mist lubrication. During high-speed rotation, applying a large amount of lubricant to the bearings to dissipate heat will also generate some heat due to churning losses. However, applying too little lubricant is ineffective for heat dissipation. Therefore, the oil mist lubrication method is adopted to ensure minimal heat generation by the bearings. This method also prevents foreign matter from entering the seals due to air pressure differences.
First, how is dry hobbing achieved? As shown in Figure 5, when dry hobbing gears with traditional TiN-coated hobs, abnormal wear occurs; however, TiAlN-coated hobs exhibit much less wear under the same conditions. This is due to the following reasons:
Use of Coatings with Excellent Wear Resistance
In dry cutting, due to the lack of coolant, the hob teeth are exposed to extremely high temperatures. Figure 6 shows the transformation of coating components at high temperatures. In TiN coatings, the Ti component of the coating is oxidized and transformed into brittle TiO₂, which makes it impossible to maintain its original wear-resistant properties. On the other hand, in the case of TiAlN coatings, Al is selectively oxidized at a depth of approximately 0.5 μm from the surface, forming a rigid film. Studies have found that TiAlN coatings exhibit high wear resistance due to this film.
Figure 5: Tool Wear Comparison Between TiN-Coated and TiAlN-Coated Hobs
Figure 6: Oxidation Test of Coatings at High Temperature (800°C, Exposed to Air for 5 Hours)
Formation of a Protective Film by Deposited Hob Chips
In wet hobbing, extreme pressure additives contained in the coolant prevent the deposition of hob chips. However, in dry hobbing, hob chips are easily deposited to protect the tool surface and reduce wear. Figure 7 shows a compositional image of the cutting edge after hobbing. The white areas are hob chips (Fe), which can be seen deposited on the cutting surface of the hob.
Figure 7: Adhered Workpiece Material Protects the Cutting Surface in Dry Hobbing
To compare different types of hobs, Table 2 shows the hobs used in various hobbing methods. Although
카바이드 hobs can be used for ultra-high-speed hobbing, they are prone to chipping due to low toughness. Dry cutting using high-speed hobs offers high productivity and tool stability, making it currently the most advantageous hobbing method.
As discussed earlier, the following considerations are critical:
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Increasing Main Motor Capacity: If the cutting speed is increased from 100 m/min to 200 m/min by adopting dry cutting, the required cutting power will double. Coupled with the increased no-load power of the machine, the main motor capacity must be increased.
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Measures to Minimize Chip Adhesion: One method is recoating to reduce the affinity between the tool and the workpiece, preventing hob chips from adhering to the tool’s cutting edge. Appropriate air blowing should also be used in conjunction to remove chips adhering to the cutting edge and eliminate flying chips at the cutting point.
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Controlling Workpiece Temperature: The longer the cycle time, the more heat accumulates in the workpiece. In other words, the longer the tool is in contact with the workpiece, the higher the workpiece temperature. Increasing the cutting speed or feed rate can reduce the workpiece temperature. Therefore, it is necessary to set the optimal hobbing parameters based on factors such as workpiece accuracy and tool life.
In the past, the maximum speed of dry hobbing using HSS hobs was 200 m/min. However, with the development of new coatings with better high-temperature oxidation resistance than TiAlN, this speed has been increased to 250 m/min, and in some cases, the maximum linear speed has even reached 450 m/min. Figures 8 and 9 show the wear condition of the cutting edge after hobbing a workpiece (module 2.25, 52 teeth, 23° left-hand helix, width 35 mm) using a hob (3 starts, 14 flutes) under specific hobbing conditions (cutting speed 250 m/min, axial feed 2.4 mm, no hob shifting). The wear of the new coated hob is half that of the TiAlN-coated hob.
Figure 8: Comparison of Oxidation Resistance
Figure 9: Tool Wear of TiAlN-Coated Hob
In dry hobbing, higher cutting speeds will be pursued in the future. For cutting tools, to withstand high-temperature hobbing, it is necessary to develop substrates with excellent heat resistance and coating materials with excellent oxidation resistance.
Although the development of dry hobbing machines continues, gear shapers are making steady progress toward dry cutting. In addition, there is strong demand for gear cutting production lines composed entirely of dry cutting machines. Therefore, the development of dry shaving machines is worthy of expectation—even though shaving is gradually being replaced by grinding.
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