Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
1“GRANMET®” Cutting Tool Materials and“MightyShield®” Coating Realize High-speed Gear
Processing

Mitsubishi Heavy Industries Machine
To o l C o . , L t d .

As one of the few manufacturers that handle both tools and machines for cutting gears,
Mitsubishi Heavy Industries Machine Tool Co., Ltd. is the first company in the world to makecomplete dry cutting of gears using a high-speed steel hob*1) and a pinion cutter*2) possible. This
both shortens machining time and extends tool life dramatically. As a result, high-speed dry gear
cutting at 200 m/min or faster has become widespread at gear mass-production sites, andmass-production cutting in the high-speed region exceeding 250 m/min has also commenced
recently.
This paper presents our GRANMET®*3) cutting tool material and MightyShield®*3) coatingthat enable high-speed gear cutting at low cost.
*1) Hob: Cylindrical cutting tool used mainly for cutting external gears*2) Pinion cutter: Disk-shaped cutting tool used mainly for cutting internal gears.
*3)“GRANMET” and “MightyShield” are registered trademarks of Mitsubishi Heavy Industries Machine
Tool Co., Ltd.

|1. Features of GRANMET®
One of the features of GRANMET® is its high hardness in high temperature environments. In
high-speed cutting of gears, the cutting heat generated by the friction between chips and the toolcauses a rapid temperature increase at the tool tip, as well as tool wear progress. Therefore, whether
the tool material can maintain high hardness even in a high temperature environment is important.
Figure 1 shows the relationship between temperature and hardness. Compared with conventional
high-speed steel, GRANMET® has less hardness degradation even in high temperatureenvironments and is expected to be highly resistant to wear during high-speed cutting.

Figure 1

Relationship between temperature and hardness

The following introduces an example of hob cutting with a GRANMET® hob. Usually, thetool wear in hob cutting is classified into two types. One is flank wear that occurs on the tool flankdue to the friction between the flank and the workpiece, and the other is crater wear that occurs on
the tool rake surface due to the friction between the rake surface and chips. In high-speed cutting
regions, crater wear tends to be dominant among the two types.
Dry hob cutting at a cutting speed of 300 m/min was performed. Table 1 lists the cutting
conditions at that time, and Figure 2 shows the amount of tool wear. The GRANMET® hob

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reduced the amount of crater wear by 50% compared with conventional high-speed steel hob at thecutting length of 60 m in 300 m/min high-speed dry hob cutting. When conventional high-speed
steel is used, tool wear progresses rapidly in high-speed cutting, so there is a problem with the toollife. However, the use of GRANMET® can suppress the progress of tool wear and realize
mass-production gear cutting at low cost and with high productivity.
Table 1 Cutting conditions
Workpiece
specifications

Hob
specifications

Cutting
conditions

Module2.25

Number of teeth

52

Pressure angle

17.5°

Helix angle

23°LH

Outer diameter

φ75

Number of starts

3

Number of gashes

16Cutting speed300m/min

Axial feed

2.4mm/t.rev

Cutting method

Dry cutting
No shift cutting

Figure 2 Relationship between cutting lengthand crater wear

|2. Features of MightyShield®
The features of MightyShield® include its hardness, which is as extremely high as HV4000,
as well as its high adhesion. Figure 3 shows the hardness of our and competitors’ coatings.
MightyShield® has overwhelmingly higher hardness than other coatings. Generally, when thehardness difference between substrate and the coating is larger, the coating easily peels off during
cutting. We developed an original technology capable of obtaining high adhesion to realize thepractical use of a high wear-resistant coating with a hardness of HV4000 and succeeded insuppressing tool wear in cutting as a result.

Figure 3

Hardness of various coatings

Another major feature of MightyShield® is that the amount of tooth profile change caused byrecoating can be suppressed. High-speed hob cutting requires recoating for recovering a worn
cutting edge. When this recoating is repeated and the tooth profile change of the hob accumulates,
the precision of produced gears is affected. Figure 4 illustrates the amount of tooth profile change
caused by recoating. By using MightyShield®, the amount of tooth profile change caused byrecoating was suppressed to 30% compared with our conventional coating, and stable-precision
gear cutting was made possible.
The following introduces an example of hob cutting using a hob coated with MightyShield®.
Table 2 lists the cutting conditions for this test, and Figure 5 shows the amount of tool wear. In
high-speed dry cutting at a cutting speed of 400 m/min, MightyShield® improved the tool life by
about 1.5 times compared with our conventional coating.

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Figure 4 Amount of tooth profile change
caused by recoating

Figure 5 Relationship between number of
cutting workpieces and tool wear

Table 2 Cutting conditions
ModuleWorkpiece
specifications

Hobspecifications

Cutting
conditions

2.25

Number of teeth

46

Pressure angle

17.5°

Helix angle

23°LH

Outer diameter

φ75

Number of starts

3

Number of gashes

16

Cutting speed

400m/minAxial feed

1.7mm/t.rev

Cutting method

Dry cutting
No shift cutting

In recent years, high-speed cutting using water-soluble coolant has also been used, so theneed for long-life tools that can be used for both high-speed dry cutting and high-speed
water-soluble coolant cutting has been increasing. MightyShield® is highly wear-resistant not only
when used for high-speed dry cutting, but also when used for high-speed water-soluble coolant
cutting, and contributes to productivity improvement and cost reduction under various cutting
environments.

|3. Future prospects
We developed GRANMET®, a tool material that can maintain high hardness even in a hightemperature environment in high-speed cutting, and MightyShield®, a versatile coating that is
highly wear-resistant when used for either dry or wet cutting environments, and succeeded indramatically extending the tool life.
We will continue to develop new next-generation materials and new coatings that realize
even higher speeds and longer life. Taking advantage of our strength in being one of the few
manufacturers in the world that handle both tools and machines for cutting gears, We will offer
new gear cutting technologies.