Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)
7

The World's First Machine for Grinding Internal Gears
in Planetary Gear Systems
YOSHIKOTO YANASE

MASASHI OCHIYASUHIRO NAKAMICHIYUKIHISA NISHIMURA

YUJI ASHIZAWA

There is an increasing need for high-precision gear machining to meet customer demand
for low-noise and low-vibration planetary gear systems. Mitsubishi Heavy Industries, Ltd.
(MHI) has been developing high-precision gear-cutting technologies for use before heattreatment in the production of ring (internal) gears, one of the components of planetary gearsystems. However, high-precision high-efficiency gear grinding is also essential after heat
treatment to eliminate distortion and improve gear accuracy. In this paper, we report on thedevelopment of the Mitsubishi ZI20A grinding machine, the world's first internal gear grinder
for use in volume production.

|1. Introduction
Planetary gear systems are widely used in automatic transmissions of vehicles and powertransfer systems for hybrid cars because of their compactness and high reduction ratio. There has
been an increasing demand for higher precision internal gears as well as for external gears intransmissions to suppress noise and vibration. To meet this demand, gear manufacturers are seeking
high-precision high-efficiency techniques for grinding internal gears after heat treatment.
Figure 1 shows the manufacturing flow for the volume production of ring (internal) gears.
Traditionally, ring gears have often been built into gear systems after heat treatment without being
subjected to the finishing process. However, controlling the precision of gears after heat treatment
is difficult due to the effect of heat-treatment distortion and the growing trend for thin-walled ringgears in lighter gear systems. Gear grinding is the predominant technique for finishing gears after
heat treatment. There are two popular gear-grinding methods: generating grinding and form
grinding. Generating grinders are preferable because of their high-efficiency and high-precision
machining that is required in volume production. However, traditional internal gear grinders that
use the honing or form grinding method in conjunction with electroplated grinding wheels have along cycle time and high tool cost, and are therefore not widely used in automobile volumeproduction lines.

Figure 1 Manufacturing flow for the volume production of ring (internal) gears
The position of the internal gear-grinding process in the manufacturing flow for the volume production of ring(internal) gears

This is the reason why MHI developed the ZI20A. This new generating gear grinder uses amulti-threaded grinding wheel to produce high-precision high-efficiency internal gears with low
operating cost (i.e., low tool cost). The development concepts of the ZI20A grinding machine are asfollows:Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)8(1) Realize generating grinding based on the synchronous control of the high-speed
high-precision wheel and work spindles,(2) Improve the sliding velocity (grinding speed) by means of a larger crossed-axis angle between
the wheel and work spindles and the use of barrel-shaped grinding wheels, and(3) Provide high-precision in-process dressing.
This paper presents details of the grinding method and machine construction, and gives anexample of machining using the ZI20A grinding machine.

|2. Grinding method2.1

Grinding principle

As shown in Figure 2, the multi-threaded grinding wheel and the internal ring gear aremeshing with a generating motion in grinding process. High-speed grinding is required to improve
the grinding performance (sharpness) and the grinding ratio, and therefore the wheel and work
spindles are rotated at a high speed under synchronous control. One of the main features of thisgrinding is an increased crossed-axis angle of 20° to 35°. This is advantageous because the sliding
velocity at the grinding point increases as the crossed-axis angle between the wheel and work
spindles becomes larger. Since the ability for tooth flank modification becomes higher as thegrinding performance improves, improvement of all elements of gear accuracy is possible,
including pitch accuracy, tooth profile, and lead.

Figure 2 Grinding method

Grinding internal gears using a barrel-shaped
threaded grinding wheel(a) Dressing gear

Figure 4 Disk dresser and dressing motions

Dressing the barrel-shaped threaded grinding wheel using adisk dresser(b) Barrel-shaped grinding wheel

Figure 3 Dressing gear and barrel-shaped grinding wheel
Internal gear-shaped dressing gear and barrel-shaped threaded grinding wheel after dressing

2.2

Grinding wheel shape

For cylindrical helical gears (worms), increasing the crossed-axis angle results in interferenceat each end of the grinding wheel width, necessitating a barrel-shaped threaded grinding wheel. For
this reason, we designed the optimal grinding wheel shape through a theoretical analysis of the
shape of barrel-shaped threaded grinding wheels and applicable dressing methods, in conjunction
with checks for interference that might occur on the tooth flank surfaces between the internal gear
and pinion in the mesh.

2.3Dressing method

Like other grinding wheels, barrel-shaped threaded grinding wheels require dressing torestore the correct wheel profile. The ZI20A grinding machine supports the following dressing
methods.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)9(1) Dressing gear method
The dressing gear has an identical geometry to the finished workpiece (internal gear) andis electroplated with diamonds on its tooth flanks. During the dressing cycle, the dressing gearclamped to the workpiece fixture dresses the grinding wheel in the same way as in the gear
grinding cycle. Figure 3 shows the dressing gear and the barrel-shaped grinding wheel.
(2) Disk dresser method
The disk dresser has a profile identical to the tooth profile of the finished workpiece (theline of contact with the grinding wheel) and dresses the grinding wheel one tooth after anotherduring the dressing cycle. Figure 4 shows the principle of dressing using the disk dresser,
which consists of the rotational motion of the wheel spindle, the vertical motion of the dresser,
and the swing motion of the dresser. This method offers flexibility in the relative positional
relationship between the grinding wheel and the dresser, so the tooth flank profile can be
modified relatively easily by applying an offset to each dressing motion.

2.4

Grinding wheel life

Unlike the wheel used for external gear grinding, the grinding wheel used for internal
grinding has a smaller diameter than that of the workpiece and thus requires more frequent
dressing, which entails disadvantages in terms of the cycle time, grinding wheel life, and tool cost.
To address this in the ZI20A grinding machine, we selected a vitrified cBN wheel that provides ahigher grinding ratio than ordinary grinding wheels. It is also equipped with wheel and work
spindles that rotate at high speed under synchronous control, with a minimum crossed-axis angle of20° to achieve a grinding speed of 20 m/s or higher. This has resulted in an extended dressing
interval, reduced cycle time, and lower tool cost.

|3. Machine development
The ZI20A was based on the Mitsubishi ZE15A external gear-grinding machine to offer
additional features for internal gear grinding. Table 1 lists the main specifications of the ZI20A.
Table 1 Machine specifications
No.

Item

1

Max. workpiece diameter (outside diameter)2

Normal module

3

Max. grinding wheel speed

No.

Item(mm) φ200

5Max. crossed-axis angle (deg)

40max.3

6Main spindle motor

25(min-1) 15 000

7NC system(kW)

Simultaneous
five-axis control

43.1

Max. table speed(min-1)6 000Wheel and table spindles

Synchronous control of the high-speed and high-precision wheel and work spindles isessential to achieve high-efficiency gear grinding using a threaded (gear-shaped) grinding wheel.
For this reason, the ZI20A grinding machine uses a direct-drive design with a built-in motor forboth the wheel and table spindles. The maximum speed is 15000 min-1 for the wheel spindle motorand 6 000 min-1 for the work spindle motor. Because the wheel spindle rotates at a high speed
during grinding and at a low speed during dressing, rotational accuracy and rigidity are required inboth the low- and high-speed ranges. In addition, grinding high-precision gears in a stable mannerrequires suppression of the thermal displacement that occurs when switching from low spindle
speed for dressing to high spindle speed for grinding. We applied the same technology used in our
high-speed machining centers (MC) to the design of the wheel spindle to achieve the target
performance. Figure 5 shows the structures of the wheel and table spindles.

3.2

Dressing system

No special dressing system is required because the dressing gear is clamped to the fixture inplace of the workpiece to dress the grinding wheel. However, to permit dressing using the diskdresser, the ZI20A grinding machine is equipped with a swing-type dressing unit on the side
opposite of the operator position. The dressing system is designed to place the dresser in front of
the grinding wheel and clamp it with a high-precision coupling during dressing, although it isretracted during other operations. In addition, because the barrel-shaped threaded grinding wheel
has a different shape than ordinary cylindrical gears, more complicated dressing motions areMitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)10

necessary. Therefore, we added a swing axis to the dresser incorporating a synchronous built-in
motor and a high-precision scale in it and developed a simultaneous five-axis control program.
Figure 6 shows the coordinate system diagram for the relative positions of the workpiece, grinding
wheel, and disk dresser.
In the dressing cycle, the disk dresser can be thought of as a part of the tooth flank of theworkpiece that moves along the line of contact between the grinding wheel and the imaginary toothflank of the workpiece to dress the grinding wheel. In other words, the grinding wheel is used in thesame manner as in grinding, and the dresser is swung around the work spindle (Z1-axis) and moved
vertically in the axial direction of the work spindle. To achieve this on the ZI20A grinding
machine, we placed the wheel spindle (l1-axis) in the vertical direction, obtained the position of thedresser spindle (l2-axis) and the center of the dresser (O3) relative to the wheel spindle bycoordinate conversion, and created a dressing program. Figure 7 shows the dressing system and thedressing movement axes.

Figure 5 Wheel and table spindle structures
The structures of the wheel and table spindles
required to achieve synchronous high-speed
high-precision rotation.

Figure 6 Relative positions of workpiece,
grinding wheel, and disk dresser
The coordinate system for the relative positions of theworkpiece, grinding wheel, and disk dresser

Figure 7 Dressing systemand dressing axis
motions
The axis motions of the dressingsystem and the disk dresser withsimultaneous five-axis control

3.3

Automation

The ZI20A grinding machine includes the following features to support the highly efficientvolume production of ring gears.
(1) Work changer
To reduce the workpiece change time, the ZI20A uses a high-speed cam unit similar tothat used in an automatic tool changer (ATC) for quick loading and unloading of the workpiece.
This work changer is shown in Figure 8.
(2) Tooth meshing function
Detecting the tooth spaces of the workpiece during gear grinding is necessary to match
the phase of the grinding wheel with that of workpiece. The ZI20A uses a non-contact sensor
for this purpose to detect the position of all tooth spaces of the workpiece. The ZI20A is also
equipped with an automatic tooth meshing system using an acoustic emission (AE) sensor

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)11

because it is more difficult with internal gears than it is with external gears for the operator tocheck the contact visually between the grinding wheel and the workpiece or between thegrinding wheel and the dresser when changing the machine setup for a new workpiece orgrinding wheel. This feature enables even less-skilled operators to change the machine setupeasily.

Figure 8 Work changer
High-speed work changer for automation

Figure 9 Line of contact between the grinding
wheel and workpiece
The calculated line of contact between the grinding
wheel and the workpiece|4. Machining support software
Analyzing tooth flank error factors and compensating for those errors are important in gear
grinding. In developing the ZI20A grinding machine, we developed a tooth flank profile calculation
program based on numerical analysis, and a check tool for the interference between the grinding
wheel and the workpiece. These are especially useful in the ZI20A for advance verification or
troubleshooting when determining the shape and specifications of the grinding wheel or modifying
the grinding or dressing motions in case of tooth flank profile errors. Figure 9 shows the calculatedline of contact between the grinding wheel and the workpiece.

|5. Machining example
Figures 10 and 11 show an example of grinding using the ZI20A. The workpiece in thisexample (a ring gear of module 2, pitch circle diameter of 120 mm, and face width of 30 mm) was
ground to the machining accuracy of JIS N4. The target production cycle time was 70 s. The results
demonstrated that this grinding application is effective, and all elements of the gear accuracy
improved (tooth profile error, lead error, pitch error, and tooth space runout) after grinding
although the workpiece was thin-walled and had experienced heat treatment distortion.

Figure 10 Machining accuracy(tooth profile and lead)
Machining example using the ZI20A(tooth profile and lead accuracy)

Figure 11 Machining accuracy (pitch)
Machining example using the ZI20A(pitch accuracy)

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)12|6. Conclusion
MHI developed the ZI20A grinding machine as an internal gear grinder for volume
production use. It is able to grind ring gears for automobile planetary gear systems with highprecision and high efficiency. Tests of the ZI20A’s grinding performance showed that it could
grind the workpiece after heat treatment to JIS N4-class gear accuracy and improve the accuracy ofall elements. We also solved the issues of grinding wheel life and tool cost by improving thegrinding speed and selecting a vitrified cBN grinding wheel. The ZI20A can thus be used in allvolume production applications including automobile manufacturing. In the future, MHI will
increase the precision and productivity of the ZI20A even further and improve the ease of setup, thesetting of optimal grinding conditions, and other essential features for less-skilled operators.

References
(1)
(2)
(3)

Hashitani, M. et al., ZE15A Gear Grinding Machine for economical hard gear finishing of quiet, smallgears, Mitsubishi Heavy Industries Technical Review Vol. 42 No. 2 (2005) pp. 56-59
Nishimura, Y. et al., Gear Grinding Processing Developed for High-Precision Gear Manufacturing,
Mitsubishi Heavy Industries Technical Review Vol. 45 No. 3 (2008) pp. 52-56
Yanase, Y. et al., Gear Grinding Machine for Internal Gears of Planetary Gear System, Proceedings of theInternational Conference on Motion and Power Transmission, MPT2009 (2009), p.159-162

Authors
Yoshikoto Yanase
Engineering Department,
Machine Tool Division

Masashi Ochi
Engineering Department,
Machine Tool Division

Yasuhiro Nakamichi
Engineering Department,
Machine Tool Division

Yukihisa NishimuraManager,Engineering Department,
Machine Tool Division

Yuji Ashizawa
Manager,Engineering Department,
Machine Tool Division