Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)23

New Processing Method Allowing for Grinding Internal,
External and Shoulder Type Gears in a Single Machine

MASASHI OCHI*1

YOSHIKOTO YANASE*2

YASUHIRO NAKAMICHI*1

KENICHI YAMASAKI*1

YUKIHISA NISHIMURA*3

YUJI ASHIZAWA*2

In the field of gear units, which includes automotive transmissions, there is a growing needfor higher-precision for the improvement of fuel consumption, the achievement of low-noise andlow-vibration and the reduction of transmission errors. Whilst after-heat treatment gear grinding
has been spreading and the precision of gears increasing in recent years, shapes of workpieces
have become complicated along with the reduction of weight and cost. Mitsubishi HeavyIndustries, Ltd. (MHI) has added a grinding wheel and a fixture for grinding external gears to ourZI20A, a previously-developed internal gear grinder for use in volume production, and developed amethod allowing for the highly precise and efficient grinding of external gears including
shaft-integrated gears, which were difficult to machine in the past.

|1. Introduction
In the field of gears used for automotive transmissions and reducers in robots, after-heat
treatment gear grinding has been spreading for the achievement of gear units with lower noise andhigher precision. Along with the further reduction of the weight and cost of gear units, there is a
growing number of workpieces that are difficult to machine such as ring gears (internal gears) inplanetary gear systems and multi-shoulder type gears widely used in the automatic transmissions ofvehicles and power transfer systems for hybrid cars. One of the methods for grinding internal gearsis a form grinding method that grinds tooth spaces one by one. However, this method is rarely usedin volume production because its machining efficiency is low. Therefore MHI developed theZI20A, the world's first internal gear grinder for use in volume production in 2009.
For grinding external gears, on the other hand, a continuous generation gear grinding method
using a multi-threaded grinding wheel has been widely used. MHI also has the ZE series thatemploys such a method. This method achieves highly efficient grinding by meshing amulti-threaded grinding wheel with a diameter of 200 to 300 mm and the gear to be ground. When
this method is used for grinding multi-shoulder type gears or shaft-integrated pinion gears(workpieces that turn-up at the edges), however, it is difficult to grind the tooth flank because thegrinding wheel and the workpiece interfere with each other (as shown in Figure 1).
Thus MHI has equipped the ZI20A gear grinder with an hourglass-shaped grinding wheel
and a fixture for grinding external gears to develop a method allowing for the highly precise andefficient grinding of gears that were difficult to machine in the past.
The main technologies used in the developed machine to allow for grinding external gears
are the following three:(1) A grinding wheel spindle that achieves rigid and stable rotation at low to high rotation
speeds,(2) The employment of an hourglass-shaped threaded grinding wheel and highly precise
on-machine dressing, and*1

Engineering Department, Machine Tool

*2

Engineering Manager, Engineering Department, Machine Tool

*3

Deputy General Manager, Engineering Department, Machine Tool

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
24(3) A control method for the amount of tooth flank modification (crowning) by adding
compensation to the grinding motion.
This paper describes the machining method and features of the developed machine, andpresents machining samples.

Figure 1 Example of workpiece that is difficult to grind with existing
threaded grinding wheel (e.g., multi-shoulder type gear)
It is difficult for large diameter grinding wheels to grind multi-shoulder type gears
consisting of neighboring gears with different diameters.

|2. Grinding method
2.1Grinding principle

Figure 2 shows the grinding of an internal gear of the ZI20A gear grinder.
Because a cylindrical grinding wheel interferes with the workpiece at the both edges of thegrinding wheel due to its crossed-axis angle, a barrel-shaped threaded grinding wheel is required. A
barrel-shaped threaded grinding wheel and the internal gear to be ground (workpiece) mesh witheach other performing a generating motion to grind the internal gear. For grinding external gears,
on the other hand, an hourglass-shaped threaded grinding wheel and the external gear to be ground(workpiece) mesh with each other performing the generating action shown in Figure 3.

Figure 2

Internal gear grinding of ZI20AInternal gear grinding with a barrel-shaped threaded
grinding wheel

Figure 3External gear grinding of ZI20A

External gear grinding with an hourglass-shaped
threaded grinding wheel

For both internal and external gear grinding, higher grinding speed is required to improvegrindability (i.e., lower grinding resistance and higher grinding ratio) as shown in Figure 4. Thegrinding speed is dependent on synchronous rotation between the grinding wheel spindle and theworkpiece and the sliding of the tooth flank due to crossed-axis angle. Therefore sliding velocity(grinding speed) can be enhanced by increasing the rotation speed of the grinding wheel spindle
and the workpiece and enlarging the crossed-axis angle. The developed grinding method isachieved due to the development of a grinding wheel spindle and a work-holding table spindle thatcan synchronously rotate at higher speeds, the design of an hourglass-shaped threaded grinding
wheel dependent on a crossed-axis angle and the creation of its dressing method.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)25

Figure 4

Grinding speed and grindability

Relationship between grinding speed and grindability

2.2

Dressing method of hourglass-shaped threaded grinding wheel

Like other grinding wheels, an hourglass-shaped threaded grinding wheel requires dressing
and modification of the grinding wheel profile. The developed machine supports the dressing gear
method and the disk dresser method. The dressing gear method uses a dressing gear, which is abase metal that has an identical geometry to the external gear to be ground and is electroplated withdiamonds on its tooth flanks, to dress the grinding wheel. The disk dresser method uses a disk
dresser, which has a profile identical to the cross sectional shape of a certain tooth of the workpiece
to be ground (the line of contact with the grinding wheel), to dress the threaded surface of grinding
wheel one groove after another. Figure 5 shows the dressing of a dressing gear and a disk dresser.
In contrast to the dressing gear method that transcribes the shape of the dresser by meshing with thegrinding wheel, the disk dresser method requires time to dress if the dressed grinding wheel has alarge number of threads. However, it offers flexibility in the relative positional relationshipbetween the grinding wheel and the dresser, so the tooth flank profile can be modified relatively
easily by compensating for the dressing motion.

Figure 5

Dressing method of grinding wheel

Dressing of an hourglass-shaped threaded grinding wheelwith the dressing gear method and the disk dresser method

2.3

Figure 6

Machining motion

External gear grinding (diagonal motion)

Control method of grinding motion and amount of tooth flank modification(crowning)

A tooth flank is ground by plunge shaving that cuts in the radial direction in a similar fashion
to gear shaving or by conventional grinding that feeds the grinding wheel in the direction of theaxis (axial direction) of the workpiece. Any desired crowning in the direction of the tooth flank line
can be added by using the tooth flank profile of an hourglass-shaped grinding wheel to add anappropriate diagonal angle to the feeding motion of the grinding wheel. Figure 6 shows thediagonal grinding motion for adding crowning.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)26|3. Features and performance of the machine
The developed machine allows for grinding both internal and external gears in a single
machine by adding an external gear grinding function to the ZI20A, which was initially exclusive
to internal gear grinding.

3.1

Grinding wheel and table spindles

Synchronous control of the high-speed and high-precision grinding wheel and table spindles
is essential to achieve high-efficiency gear grinding using a threaded (gear-shaped) grinding wheel.
For this reason, the developed machine uses a direct-drive design with a built-in motor for both thegrinding wheel and table spindles. The maximum rotation speed is 15,000 min-1 for the grinding
wheel spindle motor and 6,000 min-1 for the table spindle motor.
The nominal rotation speed of the grinding wheel varies widely: a high speed of 10,000 to
15,000 min-1 for grinding internal gears; 1,000 to 6,000 min-1 for grinding external gears due to thetooth number ratio between the grinding wheel and the workpiece, and 1,000 min-1 or slower for
dressing. So the spindle rotates at a wide range of rotation speeds, from low to high, and therefore
improvement of the accuracy and rigidity is required in the entire range of operational rotation
speeds.
Typically, improvement of the rigidity of a spindle requires higher preload of the bearing.
However, when the preload is higher, the bearing generates greater heat at high rotation speeds,
causing preload rise due to the temperature difference between the inner and outer rings andincreasing the risk of a seizure. Problems such as a reduction of bearing fatigue life due to hightemperature or an increase of axial thermal displacement, etc., may also occur. Grinding highly
precise gears in a stable manner requires the suppression of thermal displacement. In particular, thethermal displacement that occurs when switching from low spindle speed for dressing to highspindle speed for grinding is a challenge.
For the resolution of these problems, we applied cooling grooves in the vicinity of thebearing inner ring for the improvement of its cooling capacity to the design of the grinding wheel
spindle, and then achieved the target performance. Figure 7 shows the structure of the grinding
wheel spindle.

Figure 7

Structure of grinding wheel spindle

This shows a cross-sectional view of a grinding wheel spindle. Cooling grooves
are employed on the bearing inner ring.

We performed a rotation test of this grinding wheel spindle to examine the cooling effect andthermal displacement suppression effect of the cooling grooves on the bearing inner ring of thegrinding wheel spindle added in the development of the machine in comparison to the previousdesign.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
27

Figure 8 shows the measurement results of bearing inner and outer ring temperature forvarious spindle rotation speeds. The temperature of both the inner and outer rings rises along with
the increase of rotation speed, but the temperature rise of the grinding wheel spindle with coolinggrooves employed on the developed machine is reduced to 1/5 of that of the existing one without
cooling grooves. In addition, the temperature difference between the inner and outer rings isreduced to 1/5 of that of the existing spindle without cooling grooves. Thus the effectiveness of thecooling grooves on the inner ring in the spindle was verified.

Figure 8 Change of bearing temperature along with variationof spindle rotation speed
This shows the bearing inner and outer ring temperature for various spindle
rotation speeds.

Figure 9 shows the measurement results of axial displacement. This measurement wasperformed without warming up. The displacement of the ring with cooling grooves employed on
the developed machine is suppressed to 5 μm or less, which is 1/3 of the existing spindle without
grooves. Thus the cooling and thermal displacement reducing effects of the cooling grooves wereverified. In addition, the required spindle rigidity is ensured over a wide range of rotation speeds,
and therefore the desired machining accuracy is met as shown in the machining examples described
below.

Figure 9 Change of spindle axial displacement along withvariation of spindle rotation speed
This shows the displacement of the spindle measured in its axial direction forvarious spindle rotation speeds.

3.2

Facilitation of setup change and automation

The setup of the developed machine can be changed easily from inner gear grinding to outergear grinding simply by changing the grinding wheel and the workpiece fixture. Changes of thegrinding wheel can also be made easily because the size of the grinding wheel is 120 mm indiameter or smaller, and the grinding wheel arbor is clamped on the spindle automatically. In
addition, an acoustic emission (AE) sensor is used to detect the positions of the grinding wheel, thedresser, and the tooth spaces of the workpiece automatically after changing the grinding wheel,
which enables even less-skilled operators to change the machine setup easily.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)28|4. Machining example
Figure 10 shows an example of external gear grinding using the developed machine. The
workpiece in this example (a pinion gear with turn-up at edges of module 1.7, 10 teeth, outer
diameter of 23 mm) was ground to the machining accuracy of JIS N4 using a standard grinding
wheel. The production time was 70 seconds.
Figure 11 shows an example of grinding using diagonal machining motion to add crowningin the direction of the tooth flank line. The desired crowning was obtained by giving an appropriate
diagonal angle to the grinding motion.

Figure 10

Machining result

A machining example of the developed machine (the tooth profile, thetooth flank line, and the pitch)

Figure 11

Machining result

A machining example of thedeveloped machine (crowning addedto the tooth flank line)|5. Conclusion
MHI has developed a method allowing for the highly precise and efficient grinding of
external gears that were difficult to machine using conventional gear grinding methods with athreaded grinding wheel. This was achieved simply by attaching a grinding wheel and a fixture forexternal gear grinding to the ZI20A gear grinder, which was initially exclusive to internal geargrinding, MHI has enabled the machine to perform external gear grinding and enhanced itsversatility. We will continually meet the needs of customers by working on the further
improvement of accuracy and efficiency of machining, as well as tool life.

References
1. Yanase, Y. 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) pp. 7-12
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
3. 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) pp. 159-162