Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
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MHI Super-Skiving System for Longer Tool Life andEnhanced Efficiency in Internal Gear Cutting

TETSUJI MONDEN*1

TOSHIMASA KIKUCHI*1

YU CHIHARA*1

YOZO NAKAMURA*2

In recent years, the automotive industry has been demanding high-precision andhigh-efficiency processing of internal gears (ring gears) used for planetary gear devices in terms ofimprovement of fuel efficiency and reduction of noise. To achieve these, manufacturers aredeveloping a skiving cutting system expected to realize high precision and high efficiency.
However, the conventional skiving cutting system using a gear shaper cutter (skiving cutting) has
not come into practical use because, in principle, its cutting load is large and, therefore, tool life isshortened and production costs are increased. Against such a background, Mitsubishi Heavy
Industries, Ltd. (MHI) has established a process technology with similar accuracy to the skiving
cutting while exceeding it in efficiency and tool life. This paper presents the process technology.

|1. Introduction
An internal gear is used mainly for planetary gear devices for automobile automatictransmissions (AT) and reducers of construction machines. Figure 1 shows the increase/decrease
prediction in domestic automobile AT production numbers by type. Production volume of internal
gears is expected to expand along with the increase of the number of AT gear stages (increase ofplanetary gear devices) and of CVT and hybrid vehicles for the purpose of fuel efficiency
improvement.

Figure 1 Prediction of increase or decrease in domestic automobile AT
production numbers (This graph indicates the increase or decrease inproduction numbers taking 2011 as the base year.)
Source: © Automotive Ltd., 2014. All rights reserved in all countries.

Figure 2 shows a typical manufacturing process for an internal gear. Manufacturing of aninternal gear often has no finishing gear grinding process after heat treatment. Thus, it is necessary
to suppress the effects of distortion caused by heat treatment as much as possible in the previous
process and provide high-precision gear cutting before heat treatment. To meet these requirements,*1

Engineering Department, Machine Tool Division, Machinery, Equipment & Infrastructure*2

Chief Staff Manager, Machine Tool Division, Engineering Department, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
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processing of internal gears requires high-efficiency and high-precision processing technology.
However, current gear cutting of internal gears is limited mainly to gear shaper cutting andbroach cutting. These cutting methods have both advantages and disadvantages in terms ofproductivity, processing accuracy, production cost, etc., so the establishment of a cutting methodthat can attain high efficiency, high precision, and low cost was an issue. The skiving cutting
previously researched is a highly efficient and highly precise method, but the tool life is too shortfor mass production machining. In recent years, manufacturers have been developing andresearching skiving cutting methods using advanced tool materials and coating technologies.
However, significant improvement of tool life has not been attained and the skiving cutting has notcome into practical use.

Figure 2

Manufacturing process of internal gear

|2. Extending tool life and improving efficiency in internal gear
cutting
2.1

Current status and issues of internal gear cutting

Figure 3 shows internal gear tooth cutting methods. Today, a gear shaper process with a gear
shaper cutter and a broach process with a helical broach are mainly used. In recent years, however,skiving cutting using a gear shaper cutter has been attracting attention.

Figure 3

Gear cutting methods of internal gears

Table 1 shows the features of these process methods including the advantages anddisadvantages. The gear shaper cutting is barely restricted by the shape of the workpiece, butrequires a long cutting time because this process method uses the reciprocating movement of thetool for cutting, and therefore, the productivity is low. The broach cutting has better productivity,but the initial cost for the facilities is high and adjustment of machining precision is difficult. Inaddition, the broach cutting cannot cut large internal gears used for construction machines andstepped internal gears. To solve these issues, practical application of the high-efficiency andhigh-precision skiving process is being promoted.
Figure 4 shows the processing principle of the skiving cutting. As shown in this figure, thetool and workpiece (internal gear) are placed so that their rotating axes form a crossed axes angle,
and are rotated synchronously to generate a sliding motion at the contact point, and thereby cutting
is performed. Contrary to the gear shaper cutting that cuts the workpiece with a reciprocating
motion, the skiving cutting uses a rotating motion to cut the workpiece, and then achieves high

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efficiency. In addition, a processing accuracy as high as ISO class 4 to 5 is realized by transferring
the tooth profile of a tool to the workpiece through continuous processing with the tool meshedwith the workpiece. Also, the accuracy of tooth trace can be adjusted for such processes ascrowning and taper cutting. However, the skiving process has issues with large cutting resistanceand short tool life because the angle θ between the rake face of the tool and the cutting face of theobject to be cut becomes significantly negative as shown in Figure 4.
Table 1 Features of internal gear cutting methods
Cutting time
Machining precision
Tool costMachine cost
Cutting restriction
AdvantageFeatures

Figure 42.2

Disadvantage

Gear shaper cutting
Broach cutting
Bad
Excellent
Moderate
Moderate
Moderate
GoodGood
Bad
Excellent
Bad- No cutting restriction. - High productivity due
All shapes of workpiece to cutting motion in
can be cut.
which the workpiece
only moves up anddown.
- Machine cost is very
- Low productivity
because of long cutting high.
- Adjustment oftime.
machining precision isdifficult.
- Large internal gears
and stepped internalgears cannot be cut.

Skiving cuttingGoodExcellent
BadModerate
Good- High productivity andhigh machining
precision.
- Tool life is short.

Cutting principle of pinion skiving cutting

MHI super skiving system

2.2.1 Super skiving cutter
To solve the issue of short tool life with regard to the skiving process, MHI focused attention
on the technology developed for the ZI20A internal gear grinding machine. As shown in Figure 5,
the ZI20A is designed so that a crossed axes angle is formed as large as possible in order toincrease the slide-cutting speed and improve processing efficiency. The threaded grinding wheel
used is barrel-shaped for preventing interference with the internal gear. MHI has developed thesuper skiving cutter using this grinding wheel shape (Figure 6).

Figure 5
Grinding method of ZI20A internal gear
grinding machineMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
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Figure 6

Barrel-shaped grinding wheel and super skiving cutter shape(a) Barrel-shaped grinding wheel used for ZI20A internal gear grinding machine(b) Addition of gash and relief angle(c) Periphery of rough cutting area is tapered so that all teeth are involved in cutting

The processing principle of the super skiving cutter is similar to the skiving process.
However, the super skiving cutter has a tool with multiple blades for cutting the tooth space of aninternal gear in contrast to the gear shaper cutter that has a tool with only one cutting blade. This isattained by adoption of the tapered rough processing area of the tool that allows all of multiple
blades to be involved in cutting. As shown in Figure 7, in the case where the skiving cutting
requires three-pass cutting for processing the tooth space, the super skiving allows three blades tobe involved in cutting, and therefore, the number of times one of the blades is involved in cutting is
one-third of that of the pinion cutter. The reduction of the number of cutting passes also shortensthe cycle time. As such, the super skiving cutter, with multiple blades and a tapered shape, can beexpected to realize longer tool life and higher efficiency with the same machining precision as theskiving cutting. Figure 8 compares the machining precision and cutting time of each cuttingmethod.

Figure 7

Cutting method of super skiving cutter(a) Skiving cutting needs three passes with different cutting(b) Super skiving cutter allows three blades to be involved in cutting in one pass

Figure 8Comparison of cutting performance

2.2.2 Cutting simulation
With development of the super skiving cutter, cutting simulation has been developed at thesame time. This simulation estimates the cutting reaction force and tool life, based on the specs of
the workpiece, the characteristics of the tool and the cutting conditions, thereby resulting in theoptimum tool specifications and cutting conditions.

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2.3

Process test

As shown in Table 2, the process test was conducted with the cycle time approximately halfof the gear shaper cutting. According to the tool wear results in Figure 9, approximately 250 pieces
can be cut until the wear limit of 0.3 mm is reached. Figure 10 compares the number of cut piecesof each process method calculated from the characteristics of workpiece. The super skiving cutter isexpected to approximately double the tool life compared to the skiving process. The actual wear of
the tool after cutting conformed well to the cutting simulation result shown in Figure 11, and thevalidity of the simulation was verified. This tool life estimation was performed with a non-tapered
cutter.
Table 2 Characteristics of workpiece andcutting conditions
Characteristics of workpiece
Module Number Pressureof teeth
angle
257
20°

Helix
angle
18°RH

Material
SCM415

Cutting conditions
Super skiving
cutting
Gear shaper
cutting

Cutting speed
130 m/min

Cycle time90s

50-90 m/min

180s

Figure 10 Comparison of cut pieces

Figure 9Tool wear amountFigure 11

Cutting simulation result|3. Future tasks(1)

Task for super skiving cutter
For further optimization of tool specifications, cutting results of the super skiving cutter will
be fed back to the design stage.
(2) Development of skiving grinding machine
A skiving grinding machine will be developed in order to utilize the performance of the superskiving cutter to the maximum extent and attain efficient volume production of internal gears.

|4. Conclusion
MHI has developed super skiving tools for internal gear processing in order to improve
efficiency and extend tool life. This time, the tool life and the cutting simulation were evaluated,and validity of the simulation and extension of tool life in comparison to the skiving cutting wereverified. In the future, MHI will continue development for longer tool life and higher efficiency
through creating additional taper shapes and optimizing the tool design. MHI will integrate thedeveloped tool, simulation, and machine into an internal gear processing proposal suitable to eachcustomer.

Reference1.

Yanase et al., The World's First Machine for Grinding Internal Gears in Planetary Gear Systems, Mitsubishi
Heavy Industries Technical Review Vol. 46 No. 3 (2009)