In recent years, environmental safety requirements have been raised around the world, and many companies are seeking to acquire ISO14000 certification to meet international environmental standards. As part of this effort, coolant-free metal cutting methods are being devised and implemented in factories, and not only to improve the working environment, but also to protect the environment. In turning and milling applications, however, there are almost no complete dry-cutting machines that do not use coolant at all, although some are shifting from wet cutting to MQL (minimum quantity lubrication). Amid this, the shift to complete dry-cutting machines is progressing in the field of gear cutting, and especially hobbing.
In hobbing applications, a dry cutting method using a carbide hob was advocated in the past, but it was not widely accepted because the problem of tool chipping had not been solved. Then, thanks to improvements in surface-treatment technologies, high-speed steel coated hobs and dry-cut hobbing machines have been introduced, resulting in the rapid development of dry hobbing for gears. (Table 1)
The Advantages of Dry Cutting
It’s a fact that coolant produces oil dirt in factories and adversely affects the environment as well. Dry cutting not only solves such environmental issues, but it is also effective in improving cutting efficiency and reducing the life-cycle cost due to its low running costs.
The benefits of using dry hobbing from an environmental standpoint have encouraged Mitsubishi’s research into dry cutting using high-speed steel hobs, which are more stable than cemented carbide hobs. Our tests began at the same speed as that used for ordinary wet hobbing with high-speed steel hobs. The results were amazing, and it seemed like the ideal cutting method, because increasing the cutting speed to twice as fast as the ordinary cutting speed produced no abnormal wear or temperature rise on parts. However, the absence of coolant also posed major problems that prevent the practical use of dry cutting. The problems included flying chips and the heat generated by chips, which drove us to develop a new dry hobbing machine. The following discusses the functionality and tools required for dry cutting, taking our most recent machine as an example.
Main motor capacity is an important issue. In wet hobbing, parts (carburized steel) are normally hobbed at cutting speeds of 80 to 120 m/min using high-speed steel hobs. However, dry hobbing uses cutting speeds of 120 to 250 m/min, which are 1.5 to 2 times as fast as those of wet cutting. Since the cutting power is directly proportional to the cutting force and the spindle speed, dry-cut hobbing machines require a main motor capacity that is 1.5 to 2 times as large as that of conventional wet hobbing machines. In other words, converting conventional hobbing machines to accomplish dry hobbing may be possible, but it does not contribute to an increase in cutting performance in many cases because the cutting conditions are restricted by the main motor capacity.
Measures can also be taken against flying and accumulating chips. In wet hobbing, coolant serves to clean the cutting points, cool down the part, tool, and fixture, and discharge chips to the outside of the machine. By comparison, in dry hobbing, these functions are carried out by air and guard covers. Figure 1 and Figure 2 show the latest hobbing machine that implements these measures.
One strategy involves a fully sealed cutting area. Cutting chips with kinematic energy bounce around in the cutting area and fly through clearances as small as only a few millimeters, escaping the hobbing area to accumulate on machine parts. Therefore, to prevent the deterioration of machine performance and malfunction, the cutting area must be fully sealed.
On the column side, because the hob head makes vertical and swivel movements, a dual-cover design–one mounted on the hob-saddle and the other mounted on the column–is used to prevent chips from flying out. Moreover, all hoses entering the machine are connected via a relay block that separates the inside and outside of the cutting area. Of course, the portions of cables and hoses placed inside the cutting area are protected against the high-temperature cutting chips.
On the tailstock side, the openings of the partitioning cover that is provided to allow swiveling of the loader are sealed by shutter covers to prevent chips from flying out during hobbing.
A bed-top cover can also be used. As shown in Figure 3, the bed top is covered with steeply sloped stainless-steel covers with a low friction coefficient, allowing the collection of fallen chips into the chip conveyor installed at the bottom of the bed, preventing them from flying out of the cutting area and accumulating in the machine.
Then there’s the cutter-head cover. The stainless-steel cutter head cover prevents flying chips from accumulating on the cutter head or entering the main motor or clearances between machine parts.
Air blow-out of chips has also been incorporated. Dry hobbing tends to cause the catching and adhesion of waste chips on cutting tools because the washing capability of coolant is not present, resulting in flaws and chip deposition on the tooth flanks. The causes of these failures are: catching of cutout chips on tooth flanks; recatching of flying chips on tooth flanks, and; catching of chips adhered or deposited on the hob on tooth flanks after one rotation
As a countermeasure against these problems, this machine uses a flat air-blowing nozzle to produce an airflow to remove waste chips (see Figure 4). To control the catching of waste chips on the tooth flanks of the part, a general rule of thumb is that varying the hob mounting angle slightly from the normal angle is effective. There are also examples where chip catching is either eliminated or improved by theoretically analyzing the states of chips and the clearance between the cutting edge and each part tooth. Guard-cover mounting angles must take into account the rebound of chips and inclined fixture surfaces to prevent chips from entering the cutting area. In addition, air blow is used to remove chips that have accumulated on the top of the part, and a baffle plate is installed near the outer periphery of the hob to prevent flying chips from being caught and turned with the hob. However, these countermeasures will be improved as more experience with the process is gained.
Measures have been taken to guard against cutting heat. In dry cutting, these measures are essential because the majority of heat generated by dry cutting is transferred to the cutting chips. In addition, the heat generated by the machine must be controlled in other manners since coolant is not involved in this process. These measures include:
- Internal covers: to prevent the transfer of heat to the machine body, stainless-steel covers with a low thermal conductivity are installed via a layer of air for heat insulation.
- Dust collector for releasing cutting heat: to prevent the transfer of heat from cutting chips to the machine body via convection, the machine uses a dust collector that catches tiny chips produced during dry hobbing to provide an airflow that carries the cutting heat out of the machine.
- Protection of hoses and cables: A wire-braided hose is employed for hydraulic and other connections because high-temperature flying chips may cause oil or air leakage. To prevent wire breakage by cutting chips, the wiring cables are protected in heat-resistant plastic conduits.Controlling the heat generated from the machine body involves the use of non-contact oil seals. The replacement of oil seals used on the main spindle that generates a great deal of heat with non-contact seals resulted in a substantial reduction of generated heat.
Oil-mist lubrication is used for the bearings. At high-speed rotation, applying a large volume of lubricant to the bearings to let out heat also generates some heat due to the loss of agitation. However, applying too little lubricant is not effective for heat removal. Therefore, we used the oil-mist lubrication method that ensures minimal generation of heat from bearings. This method also prevents the entry of foreign materials into the seals, thanks to the effect of differential air pressure.
Characteristics of Dry Hobbing Using a High-Speed Hob
How is dry hobbing possible in the first place? As shown in Figure 5 dry hobbing a gear using a conventional TiN-coated hob causes abnormal wear, but a TiAIN-coated hob causes almost no wear.This is due to the following. First, using a coating film with excellent abrasion resistance. In dry cutting, the hob’s teeth are exposed to an extremely high temperature because of the lack of coolant. Figure 6 shows the transformation of the coating film composition at a high temperature. In TiN coating, the Ti composition of the coating film is oxidized and transformed into a brittle composition of TiO2, which prevents maintaining its original abrasion-resistance characteristics. On the other hand, in the case of the TiAIN film, AI is selectively oxidized at the depth of approximately 0.5 µm from the surface and is considered to be a rigid film. It has been found that the TiAIN coating has high abrasion-resistance characteristics thanks to the effect of this film.
Secondly, a protective film is produced by the deposition of hobbing chips. In wet hobbing, an extreme-pressure additive contained in coolant prevents the deposition of hobbing chips. In dry hobbing, however, hobbing chips are easily deposited to protect the tool surface, reducing the amount of wear. Figure 7 shows the composition image of a cutting edge after hobbing. The portion that appears in white is a hobbing chip (Fe), which is seen deposited on the cutting face of the hob.
In making a comparison between the different types of hobs, Table 2 shows the hobs used in various hobbing methods. Carbide hobs are easily chipped because of their low toughness, although they can be used in extremely high-speed hobbing. Dry cutting using a high-speed hob provides a high productivity and tool stability and thus is the most advantageous hobbing method.
Important Notes on Using Dry Hobbing
As we have discussed, an increase in the main motor capacity is required. If the cutting speed is increased from 100m/min to 200m/min through the introduction of dry cutting, the required cutting power will be doubled plus the increased no-load power of the machine, and so the main motor capacity must be increased.
Measures can also be taken to minimize chip deposition. One method involves recoating to reduce the affinity between the tool and workpiece, preventing the adhesion of hobbing chips to the cutting edges of the tool. Appropriate air blow should also be incorporated to remove the chips that are adhered to the cutting edges and to eliminate flying chips from the point of cutting.
Heat accumulation can also present a challenge. The longer the cycle time, the greater the accumulation of heat in the workpiece. In other words, the longer the contact time between the cutter and workpiece, the higher the temperature of the workpiece. Increasing the cutting speed or feed rate can lower the workpiece temperature. Therefore, it is required to set optimal hobbing conditions based on the relationship with the workpiece accuracy, tool life, etc.
The maximum speed of high-speed steel dry hobbing used to be 200 m/min. However, it has been increased to 250 m/min thanks to the development of a new coating film having better high-temperature oxidation resistant characteristics than TiAIN, as shown in Figure 8. Figure 9 shows the wear condition of the cutting edge after hobbing a workpiece (m2.25, 52T, 23†LH, B35 mm) using a hob (3 threads, 14 flutes) under specific hobbing conditions (cutting speed 250m/min axial feed 2.4 mm, without hob shift). The wear of the new coating hob is half of the TiAIN hob.
In dry hobbing, even higher cutting speeds will be pursued in the future. As for cutting tools, to withstand high-temperature hobbing, it is required to develop base materials that have excellent heat resistance and coating materials with excellent oxidation resistance.
Although the development of dry hobbing machines is ongoing, gear shapers are steadily making progress toward dry cutting. Furthermore, there is great need for a gear cutting line consisting of dry-cut machines alone, and so the development of dry gear shaving machines is to be expected.