Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
5Latest Technologies for High-Precision,
High-Efficiency Gear Grinding Processing

YOSHIHIDE KITO*1TOSHIFUMI KATSUMA*2

YOSHIKOTO YANASE*2 YOSHIHIRO NOSE*3

There is increasing demand for high-precision gears for the improvement of the fuelconsumption, downsizing, and the reduction of the transmission error of gear devices such as
automobile transmissions and reduction gears. In response to this, the need for gear grinding
machines that can process high-precision gears efficiently and stably is rapidly expanding. This
paper presents the technological features of the MHI ZE series generating-type gear grinding
machine for the realization of high-precision and high-efficiency processing.

|1. Introduction
Demand for improvement in the accuracy of gears installed in widely-used gear train devices
such as automobile transmissions and reduction gears for the improvement of quietness, theenhancement of transmission efficiency, and the reduction of transmission errors has risen sharply
in recent years. Market demand for a continuous generation grinding method using a thread-shaped
grinding wheel is also showing a rapid increase among high-precision gear processing methods formass production such as honing, carbide skiving hobbing, etc. Mitsubishi Heavy Industries, Inc.
(MHI) has a product line that suits needs for mass production gear grinding consisting of the ZE
series gear grinding machine for external gears and the ZI series gear grinding machine for internalgears. We have further improved the efficiency and accuracy of the ZE series gear grinding
machine for mass production of external gears in response to the recent market demand describedabove.

|2. Features of the ZE series
2.1

Realization of high-precision and high-efficiency dressing

A gear grinding process is a method to transfer the shape of a grinding wheel that is dressedby a dressing device. For the improvement of process accuracy, it is important to dress the grinding
wheel accurately. However, the improvement of dressing accuracy cannot be attained just by theperformance enhancement of a single dressing device. It has to go with the enhancement of therigidity and accuracy of the whole machine.
For the realization of highly accurate tooth profiles, the ZE series performs on-machine
dressing using a rotary dresser located in a position almost the same as that of the workpiece, sothat the dressing of a grinding wheel can be performed in the same position and posture as theactual gear grinding. In addition, by increasing the maximum rotation speed of the rotary dresser
from the conventional 3,260 min-1 to 6,000 min-1, the rotation speed of the grinding wheel can beincreased without changing the rotating speed ratio between the dresser and the grinding wheel.
Together with high-speed and high-precision control of the grinding wheel shift axis, this results inthe reduction of the dressing time.
*1Engineering Department, Machine Tool Division, Machinery, Equipment & Infrastructure*2Chief Staff Manager, Engineering Department, Machine Tool Division, Machinery, Equipment & Infrastructure*3Manager, Engineering Department, Machine Tool Division, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)62.2

Tooth profile pressure angle correction function

To perform tooth profile pressure angle correction, in the case of competitor’s gear grindingmachines, the conventional two-rotary dresser specification needs manual fine adjustment of thedressing device mounting angle. The formed composite dresser specification cannot change theon-machine pressure angle and requires correction of the shape of composite dresser. Figure 1
shows the conventional pressure angle correction method.

Figure 1

Conventional pressure angle correction

Pressure angle correction is performed by adjusting the dresser mounting angle manually or adjusting thepressure angle of dresser.

Our ZE series has a function to simultaneously dress both sides of the grinding wheel tocorrect the pressure angle by simply inputting correction data. Due to this function, even an
unskilled operator can easily and accurately adjust the pressure angle. The tooth profile pressure
angle correction changes the relative positional relation between the dresser and the grinding wheel
in dressing. Conventionally, the grinder shape was corrected by fine adjustment of the dresser. For
this machine, the dresser is fixed and the correcting function is realized by correcting each axis of
the grinding wheel using the NC. Figure 2 shows an example result of the correction performed byinputting the required pressure angle correction values of both grinding surfaces in the NC andperforming the correction. As this figure indicates, the correction result is almost the same as theinput values. This function is essential for production sites where the number of skilled workers isdecreasing.

Figure 2

Example of correction of pressure angle

Correction amount is input based on gear measurement result

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)72.3

Quick setup change for similar workpieces

Today’s production sites are shifting from traditional small-variety mass production tomulti-variety variable production, thereby necessitating frequent setup changes. A generating-typegear grinding machine requires changes of tools including grinding wheels and dressers depending
on restrictions such as workpiece specifications, target accuracy, etc., when the machined
workpiece is changed to a similar workpiece such as one having a different number of teeth. Whenthe machined workpiece is changed, dressing tooth meshing (meshing work between the grinding
wheel and the rotary dresser) and processing tooth meshing (meshing work between the grinding
wheel and the gear workpiece) are necessary. This machine adopts an AE (acoustic emission)
sensor to realize automatic tooth meshing.
In addition, the ZE series is equipped with two sets of dressing devices with rotary dressersthat have settings different from each other, which can reduce the number of dresser changes in
response to the workpiece to be processed. Figure 3 shows an example of this reduction. When theworkpiece to be processed is changed to one for which the current mounting tools and grinding
wheel are used commonly, the dressing device can be exchanged with the other one by a simple
operation on the dialog screen, and setup changes can be made without opening the door of themachine. Furthermore, the meshing work can be omitted depending on the specifications ofchanging workpieces, resulting in the reduction of setup time by 10 minutes or more.

Figure 3

Specifications for two sets of dressing devices

The ZE series' two sets of dressing devices reduced the number ofsetup changes.

2.4

High-precision process

There are an increasing number of cases where even a non-involute gear that was formerly
ground and formed using a single grinding wheel is generation-ground using a thread-shaped
grinding wheel for high-efficiency finishing. With regard to non-involute gears, the tooth profile
accuracy and the OPM (i.e., the distance between teeth bottoms) are considered to be important.
Figure 4 shows an example where a sizing device is added to the high-rigidity and high-heat
rigidity machine in order to perform high-precision processing. In continuous processing of
non-involute gear workpieces with an external diameter of 150 mm, module 4 in the middle ofwhich the grinding wheel is dressed, this specification attained the OPM fluctuation of 10 μm orless, while that of a conventional machine was approximately 40 μm.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
8Figure 4

Mechanism of sizing device

A sizing device is added to the counter column side to correct the distance
between axes

2.5

Gear grinding process with water soluble grinding liquid

Water soluble grinding liquid is used for many grinding processes due to its superior coolingproperty in comparison with grinding oil and with the aim to reduce environmental load in disposal
and prevent fires caused by sparking during processing. It has not been used practically forgenerating-type gear grinding machines, however, because of problems with tooth surface burningand sludge. This time, MHI developed a grinding wheel specifically for water soluble liquid andwater soluble grinding liquid itself and modified the machine specifications including the coolant
tank in cooperation with Toyota Motor Hokkaido, Inc., Noritake Co., Ltd, and Yushiro ChemicalIndustry Co., Ltd. As such, a mass production gear grinding process with water soluble grinding
liquid was brought into practical use.

|3. Conclusion
There has been sharply increasing demand for higher-precision gears in recent years, and themarket requires gear grinding machines that allow even unskilled operators to perform
high-precision, high-efficiency processing and maintenance work. MHI planned to improve theaccuracy and efficiency of processing with the ZE series in response to recent needs for theenhancement of productivity, and realized the reduction of machine down time and more accurate
processing. We will henceforth seek even higher precision and shorter cycle times for processing,and aim to create production sites for our customers where environmental load and total costs are
reduced by offering high-value-added gear grinding machines with superior operability andmaintainability.

Reference
1. 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)
2. Nishimura, Y. et al., Gear Grinding Processing Developed for High-Precision Gear Manufacturing,
Mitsubishi Heavy Industries Technical Review Vol. 45 No. 3 (2008) pp.33-38