Original woody kissme
High-speed dry gear hobbing has appeared in the world for many years, but its domestic application is not particularly common. Recently, I sorted out the relevant content on high-speed dry cutting and gear hobbing, wrote a study note and saved it. Interested friends can take the time to take a look.
The emergence of dry gear cutting is mainly due to several factors:
In recent years, environmental safety requirements have continued to increase around the world and many companies are seeking to obtain ISO14000 certification to meet international environmental standards. As part of this effort, the shop designs and implements 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 few dry cutting machine tools that do not use coolant entirely, although some convert from wet cutting to MQL (minimum lubrication). Among them, the field of gear cutting, especially gear hobbing, is developing into complete dry cutting machine tools.
Wet cutting gear hobbing
In gear hobbing applications, dry cutting methods using carbide hobs have been advocated in the past, but have not gained widespread acceptance because the problem of tool chipping has not been resolved. Since then, due to the progress of surface treatment technology and the emergence of high-speed steel coated hobs and dry hobbing machines, gear dry hobbing technology has developed rapidly.
Advantages of dry cutting
In fact, coolant can produce oily contaminants in the factory and negatively impact the environment. Dry cutting not only solves these environmental problems, but also can effectively improve cutting efficiency and reduce life cycle costs with its low operating costs.
From an environmental perspective, the benefits of using dry hobs have prompted companies like Mitsubishi, Gleason, Liebherr and others to investigate the use of high speed steel hobs for hot cutting. dry, which are more stable than carbide hobs. Factory testing begins at the same speed as a normal wet cooktop with a high speed steel cooktop.
The results were surprising. High-speed cutting seemed to be an ideal cutting method because no abnormal wear or temperature increase occurred on the parts by increasing the cutting speed to twice the normal cutting speed. However, lack of coolant also creates significant problems that hinder the practical application of dry cutting. These problems include flying chips and heat generated by chips, which has prompted machine tool manufacturers to develop a new type of dry hobbing machine. The following uses a three-edged high-speed roller as an example to discuss the functions of the machine tool and the tool requirements required for dry cutting.
The capacity of the main motor of the machine tool is an important issue. In wet hobs, high speed steel hobs are generally used for sharpening workpieces (hardened steel) at cutting speeds of 80 to 160 m/min. However, the cutting speed of dry hobbing is 160-400m/min, which is 2-2.5 times that of wet cutting, or even higher. Since the cutting power is directly proportional to the cutting force and spindle speed, the dry trimming machine requires more than 2-2.5 times the capacity of the main motor of the dry trimming machine. traditional water. That is, it is possible to convert a traditional cutting machine into a dry cutting machine, but in many cases the cutting conditions are limited by the capacity of the main motor.
Additionally, measures should be taken to prevent chips from dispersing and accumulating. In wet gear cutting, coolant is used to clean the cutting point, cool parts, tools and accessories, and expel chips to the outside of the machine. On the other hand, in dry rolling, these functions are provided by air and shields.
One strategy employed uses a fully sealed cutting area. Cutting chips carrying kinetic energy bounce around the cutting zone and pass through gaps as small as a few millimeters, are ejected from the cutting zone and accumulate on machine parts. Therefore, to avoid deterioration and breakdown of machine performance, the cutting area must be completely sealed.
On the column side, due to the vertical and rotational movement of the hob headstock, a double cover design (one mounted on the hob saddle and the other on the column) is used to prevent chips from flying away. Additionally, all pipes entering the machine are connected via relay modules that separate the inside and outside of the cutting area. Of course, cable and pipe sections placed in the cutting area must be protected from hot cutting debris.
On the tailstock side, in order to prevent chips from escaping during gear cutting, the opening of the separation cover used for rotating the spindle is closed by a baffle. The top of the bed is covered with a steep-slope stainless steel cover with a low friction coefficient, which can collect the falling chips into a chip conveyor installed at the bottom of the bed to prevent the chips from flying out of the bed. cutting surface and build up.
Then there is the blade cover. Stainless steel cutter head covers prevent flying chips from accumulating on the cutter head or entering gaps between the main engine or machine components.
Dry honing does not have the ability to discharge coolant, which can easily cause chips to adhere to the cutting tool, resulting in defects on the gear tooth surface and chip deposition.
Machine tool manufacturers have provided corresponding countermeasures to solve these problems. Most use flat blow nozzles to generate airflow to remove waste. The guard installation angle should take into account chip bounce and the inclined surface of the fixture to prevent chips from entering the cutting area. Additionally, blown air 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 spinning with it. the hob.
When dry cutting, chip evacuation measures are crucial because most of the heat generated during dry cutting is transferred to the cutting chips. Additionally, because the process does not involve coolant, the heat generated by the machine must be controlled by other means. These measures include:
Inner cover: To prevent heat transfer to the fuselage, a low thermal conductivity stainless steel cover is installed for thermal insulation through an air layer.
Dust collector to release cutting heat: In order to prevent the heat of the chips from being transferred to the machine body by convection, the machine uses a dust collector to capture the tiny chips generated during the dry cutting process to provide a Air flow to remove cutting heat from the machine.
Hose and cable protection: Use steel braided hoses for hydraulic and other connections, as flying debris at high temperatures can cause oil or air leaks. To prevent breakage due to chips, the connection cables are protected by heat-resistant plastic sheaths. Controlling the heat generated by the cell requires the use of non-contact seals. The oil seal used on the spindle, which generates a lot of heat, was replaced with a non-contact seal, which significantly reduced the heat generated.
The bearings are lubricated with an oil mist. When rotating at high speed, apply a large amount of lubricant to the bearings to dissipate heat, and some heat will also be generated due to stirring losses. However, applying too little lubricant is not effective in dissipating heat. That’s why we use an oil mist lubrication method to ensure the bearings generate minimal heat. This method also prevents foreign objects from entering the joint due to the difference in atmospheric pressure.
Features of dry rolling using high-speed hobs
First of all, how is dry rolling done? Dry honing gears using traditional TiN-coated hobs will experience abnormal wear, but TiAIN-coated hobs wear much less under the same conditions. This is due to the following reasons:
Firstly, a coating film with excellent wear resistance is used. When dry cutting, the hob teeth are exposed to extremely high temperatures due to lack of coolant. Changes in coating film composition at high temperatures. In TiN coating, the Ti component of the coating film is oxidized and converted to a brittle TiO2 component, making it unable to maintain its original wear-resistant properties. In contrast, in the case of the TiAlN film, Al is selectively oxidized at a depth of approximately 0.5 μm from the surface, and a rigid film is produced. Studies have shown that due to the role of this film, the TiAIN coating has high wear resistance.
Second, a protective film is created by depositing cooking chips. When cutting gears under water, the extreme pressure additives contained in the coolant prevent deposits of gear cutting chips. However, when dry cutting, the cutting chips are easily deposited to protect the tool surface and reduce wear. Figure 7 shows an image of the composition of the cutting edge after cutting. The white part consists of hob chips (Fe), which can be seen deposited on the cutting surface of the hob.
To compare the different types of hobs, Table 2 shows the hobs used in various gear hobbing methods. Although carbide hobs can be used for gear hobbing at extremely high speeds, they are prone to chipping due to their low toughness. Dry cutting using high-speed hobs offers high productivity and tool stability and is therefore by far the most advantageous method of gear hobbing.
Important notes on using dry baking trays
As previously stated, the capacity of the main engine must be increased. If the cutting speed increases from 100 m/min to 200 m/min by introducing dry cutting, the required cutting power will be doubled, coupled with the increase in the no-load power of the machine, so that the capacity of the main engine must be increased.
Steps can also be taken to minimize chip adhesion. One approach is to coat to reduce the affinity between tool and workpiece and prevent hobbing chips from adhering to the tool edge. It should also be combined with suitable air jets to remove chips adhering to the cutting edge and remove flying chips at the cutting point.
Heat buildup increases with longer cycle times and more heat builds up in the workpiece. In other words, the longer the tool is in contact with the workpiece, the hotter the workpiece becomes. Increasing the cutting speed or feed rate can reduce the workpiece temperature. Therefore, it is necessary to set optimal hobbing parameters based on the relationship with workpiece accuracy, tool life, etc.
recent developments
The maximum speed for dry cutting of high speed steel used to be 200 m/min. However, with the development of new coating films with better high temperature oxidation resistance properties 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. For hobbing the rear cutting edge of a workpiece (m2.25, 52T, 23°LH, B35 mm) using a hob (3 threads, 14 edges) under hobbing conditions specific (cutting speed 250 m/min, axial feed) Wear condition 2.4 mm, no moving knife). The new coated cooktops wear half as much as TiAIN cooktops.
Finally
In the area of dry cutting, higher cutting speeds will be sought in the future. For cutting tools, in order to withstand high temperature gear cutting, it is necessary to develop base materials with excellent heat resistance and coating materials with excellent oxidation resistance.
Although the development of dry cutting machines continues, gear shapers are steadily moving toward dry cutting. In addition, there is also a high demand for gear cutting production lines consisting of only dry cutting machines. The development of dry gear shaving machines is therefore worth looking forward to, although gear shaving is gradually being replaced by gear grinding.
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