Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)50

Technical Change ofSuper Dry Coating for Gear Cuttingwith Low Cost and High Efficiency

P r e c i s i o n C u t t i n g To o l &A u t o m o b i l e P a r ts S a l e s G r o u p
S a l e s D e pa r t m e n t , M a c h i n e To o l D i v i s i o n
M a c h i n e r y, E q u i p m e n t & I n f r a s t r u c t u r e

Mitsubishi Heavy Industries, Ltd. (MHI), deploying its diverse business operations, enjoys ahigh reputation at home and abroad as one of the few machine tool manufacturers that deals in bothtools and machines for gear processing and as a unique manufacturer that provides total solutions
relating to gear processing technology from gear cutting to gear grinding. In 1997, MHI enabledfull-dry processing of gears for the first time in the world, using a hob Note 1) and pinion cutter Note 2)
made of high-speed steel, and has since realized dramatic savings of processing time as well asextension of tool life. These achievements of MHI have allowed the widespread dry processing ofgears in many of the production sites where gears are mass-produced and processed.
In this paper, the super dry coating process that enabled low-cost high-efficiency gearprocessing is introduced.
Note 1) Hob: A cylindrical cutting tool mainly used to machine external gears
Note 2) Pinion cutter: A disk-shaped cutting tool mainly used to machine internal gears|1. Advantages of dry cutting
Dry cutting is advantageous in that it uses no cutting fuel at all and so never causes sootduring machining, thereby resulting in an improved work environment. In addition, gears can be
cut at a higher speed than conventional wet cutting with oil, and the life of tools also increases
dramatically, quite advantageously realizing low-cost, high-productivity gear cutting.
The realization of full-dry gear cutting was greatly contributed to by the development of amachine tool for dry cutting, as well as by the development of super dry coatings to protect tooledges from cutting heat. Dry machining by high-speed cutting causes the cutting heat generated by
the friction between the chips and the tool to rapidly increase tool-edge temperature, thus
promoting wear. The tool edge is covered by a highly heat and wear-resistant coating, which plays
the role of blocking and protecting the tool edge from cutting heat. As far as dry machining by
high-speed cutting is concerned, the coating performance greatly affects tool life.
Figure 1 exemplifies the dry cutting of gears with a super dry coating hob. Unlike a TiN
coating hob typically used in conventional wet machining with a cutting fluid, dry machining at a
cutting speed of 200m/min is possible, not only reducing the conventional gear cutting time in half,
but also increasing the number of gears cut 6.6 fold, thus proving the dramatic improvement of toollife. Previous dry machining of gears with a TiN coating hob couldn’t be used in mass production
since the tool life became too short, while super dry coating enabled stable cutting in drymachining, thus realizing the mass production of low-cost, high-productivity gear machining.

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Figure 1

Example of dry gear machining with super dry coating hob|2. Features of super dry coating
The greatest feature of super dry coating lies in its characteristics of excellent oxidation
resistance at high temperature. Figure 2 shows the initial oxidation temperature measurements ofthe super dry coatings Note 3) so far developed. In high-speed, cutting-based dry machining, thesharp increase in tool edge temperature causes the coating surface to be rapidly oxidized, hence
promoting wear, but the higher the temperature at which oxidation starts, the slower the progress ofwear, thereby resulting in a longer tool life. In Figure 2, the initial oxidation temperatures of Super
Dry III and Super Dry IV exceed 1300°C, proving their significantly high resistance to oxidation.
Note 3) State of super dry coating development: Super Dry I was first developed. Development and market
introduction have already progressed to Super Dry II to IV.

Figure 2

Initial oxidation temperature of super dry coating

Figure 3 shows the roughly estimated gear cutting speed of Super Dry I to IV applied to ahigh-speed steel hob. All of the super dry coating hobs are usable for dry machining, but the higher
the coating’s initial oxidation temperature, the higher the possible gear cutting speed. High-speed
dry machining of gears with a hob made of cemented carbide was also utilized previously, but was
not universally adopted because of an increase in the cost for tools, the hindrance of long-term
stable machining due to tool edge breakage (chipping) caused by intermittent cutting and for otherreasons. At present, the further improvement of super dry coatings has realized the stablemachining of gears at a cemented-carbide hob-equivalent cutting speed of 300m/min, even using ahob made of high-speed steel.

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Figure 3 Roughly estimated speed of gear machining using eachsuper dry coating hob(Machining speed indication for 3 or less-module carburized material (ofHB180 or less in hardness) as targeted workpiece gear)|3. Future outlook
MHI enabled full-dry processing of gears in 1997 for the first time anywhere in the world,using a hob and pinion cutter made of high-speed steel, and has since realized dramatic savings ofprocessing time, as well as the extension of tool life. This has made low-cost, high-productivity
machining for mass production possible.
MHI intends to continue to not only proceed with the development of mainly
coating-purpose tools for the realization of further savings of processing time and the extension oftool life, but also to continually provide total solutions related to gear machining technologies,
taking advantage of our strength as a developer/manufacturer of both tools and machinery, andsatisfying many customers.