ZE15A Gear Grinding Machinefor economical hard g ear
finishing of quiet, small gears
MICHIAKI HASHITANI*1
KOUICHI MASUO*1
TOSHIFUMI KATSUMA*1
YOSHIKOTO YANASE*1
KAZUTAKA MARUYAMA*1

In recent years the demands for precision machining of gears in automobile transmissions, for low noise andvibrations, has been ever increasing. Historically, conventional finish machining of gears was a pre-heat treatment
operation typically by shaving. However, the requirement for higher precision forced a shift toward a post heattreatment operation using a generating process, which eliminates thermal distortion, thus enabling high quality andprecision. The Mitsubishi ZE15A gear-grinding machine was duly developed for the high production line applications and launched into a domestic market typically dominated by European machines.
(3) Ease of use for the user achieved via interactive dialog functions and full CNC control of all axes.
This paper describes the principle of grinding andcontrol for the ZE15A before presenting some machining examples.

1. Introduction
Gear grinding can generally be divided into two types;
Generating and Form grinding. Generating grinding uses
similar principles to gear hobbing but utilizes a threaded
worm grinding wheel to realize high precision and efficiency for the mass production user. Because the domestic
market for generating gear-grinding machines has, inthe most part, been monopolized by European suppliers, an urgent call for a competitive domestic
gear-grinding machine has been recognized.
Mitsubishi Heavy Industries (MHI) has developed theZE15A gear-grinding machine, adopting the generatingprocess, in order to respond to the needs of a rapidlyexpanding market. The main specifications are given in
Table 11.
(1) High precision grinding due, in part, to on-machine,
automatic dressing of the vitrified grinding worm.
(2) High efficiency grinding using multiple threaded vitrified grinding worms.

2. Machining principle
2.1 The grinding principle
As shown in Fig 1 (a) generating gear grinding uses
a threaded worm type wheel. Similar to hobbing, thegrinding worm is held in continuous synchronizedmesh with the pre-cut gear (workpiece) as it moves
in the feed direction, thus enabling involute generation. Compared to form grinding the generatingprocess has a higher tooth-to-tooth pitch precision,and combined with easy automation a faster cycle
time is achieved.

Thread-shapedworm

1

Maximum diameter of work

2

Machinable module

3

Number of (grindable) teeth

4

Worm dia. x length(mm)

Outer diameter 300
x length 125

5

Maximum worm rpm(min-1)4 500

6Maximum number of threads
Maximum table rpm

8Main spindle (continuous rated)

9NC system

10 Machine weight

Main spindle

Specification

Item

7Threadcutting
operation

Grinding strokeaxial feeding

Table 1 Main specifications of the ZE15A gear grinder
NoMain spindle

150(mm)Work

1-4
5-1 000

7

Tangential
feeding

Tangential
feeding
Dresser(Dressing machine)

Max. pitch 32 mm-1(min )1 500(kW)17

Table spindle

Fanuc 160 iMB(kg)

Directdrive

(a)

(b)

Fig. 1 Machining and dressing operation
Indicates the traveling direction and rotating direction at machining
and dressing.

10 000Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)*1 Machine Tool Division12.2 High efficiency grinding using multiple threadedgrinding worms
The high rotational speed of the table spindle, allows
the use of multiple threaded worms thus ensuring highefficiency grinding. In the work example shown in section 5, grinding time is 150 seconds with a single thread
worm. However, when a 3-start worm is used the tablespindle RPM is increased 3 times resulting in a reduced
cycle time of 50.5 seconds.
2.3 Integrated dressing of the grinding worm
As in the case for form gear grinding the threaded
generating grinding worm also requires dressing to maintain its shape and sharpness. Setting the worms shiftaxis in tangential feed mode and synchronizing it withthe worm's rotational spindle allows dressing of thegrinding worm to commence.
Without removing the grinding worm from the machine the NC controlled dressing device is automaticallyintroduced to the grinding worm and the diamond dressing discs passed tangentially along the worm's lengthfor smooth continuous dressing. Owing to the fully automatic nature of the operation dressing times are kept toa minimum.
2.4 Diagonal grinding and wheel life
In order to ensure high-efficiency machining, diagonal grinding is adopted in the ZE15A. Combining
synchronous feed motions of the axial and tangential axes
as shown in Fig 1 (a) diagonal grinding is performed.
With this method, heavy duty grinding is possible, because new cutting edges on the grinding worm areconstantly introduced throughout the grinding operation.
Normally the diagonal method employs the entire
length of the grinding worm before it is subjected to automatic dressing. Typically, for the example workpieceshown in Table 22, dressing frequency occurs between 30and 40 parts.
Average wear per dress, of the worm's diameter is0.4mm. Since the grinding worm has an outside diameter between 220mm and 300mm its life can be calculated
up to 8000 workpieces.

With a grinding worm cost approximately 100,000 yen
per piece the tool cost per piece is 12 to 13 yen, substantially satisfying customer demands.

3. Machine development
3.1 Main grinding/table spindle design and highly
precise synchronized control
In order to realize a high efficiency gear grinder using multiple threaded grinding worms, synchronized
control of the grinding and table spindles to the highest
precision is required. Using conventional kinematics
between the drive motor and the spindle was not an option owing to errors produced by non-linear elements
such as backlash etc. thus making it impossible to achieve
the desired synchronization accuracy. In addition a gear
train in itself causes the workpiece to generate noise,
which defeats the object of producing quiet gears.
The ZE15A therefore adopts an integrated direct drive
design as shown in Fig 22.

Main spindle

EncoderTable spindle

Thread-shapedworm
Synchronized built-in motor

EncoderFig. 2 Structural diagram of main spindle and tablespindleIndicates Direct-drive structure using built-in motor.

Table 2 General items of work and grinding conditions
General items of work for grinding

General items of worm

SetFinish(min-1)

2 850

3 800

Grinding speed (rate) (m/sec)

4560

Axial feeding(mm/rev)

0.80.4

Material SCM 415
Surface hardness HRC 60

Radial depth(mm)0.250.08

Outer diameter

105 mmFeed direction

Tooth width40 mm

Coolant

Module3Gear31

Pressure angle

20ooTorsional angle(direction)

20
RightOuter diameter

Grinding conditions

Worm width

300 mmWorm rotating speed

125 mmNumber of threads

3

Worm used:
CXY120I8V104

Climb(grinding) Conveyor
Water insoluble

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)2

3.3 High speed tooth meshing system.
For gear grinding it is necessary to detect the tooth
space of the incoming workpiece in order to achieve meshing between the grinding worm and the workpiece teeth.
A non-contact sensor is used which detects the teeth overthe entire part circumference and sets meshing position.
All this is controlled by a high speed processing circuit
in approximately 1 second.

The grinding and table spindles have a maximum
speed of 4500 and 1500RPM respectively. The direct drive
technology employed for both spindles ensures high control performance within the aforementioned RPM range.
When grinding using threaded worm-grinding wheels,load fluctuations are experienced owing to the lower precision of the workpiece coming from rough machining
and heat treatment operations. These errors are mainly
due to cumulative pitch errors that cause fluctuation of
the work rotation thereby having a large effect on theprecision of the post machined gear.
In light of this special attention has been given to theconstant periodic nature of the load fluctuation. A "feedforward control system" (1) (a system where control input amount is successively obtained from the periodical
error amount for control) employed in MHI's gear
shapers, has been applied to reduce the periodic error
rate approximately 15%, drastically improving the precision requirement of the machine.
3.2 Mechanism of the dressing unit. (Patentpending)In order to save space the dressing unit of the ZE15A
is incorporated into the ringloader as shown in Fig. 3
3.
Positioned in front of the grinding worm during dressing, the dressing unit is clamped with high precisionusing a Hirth coupling.
When viewed from the grinding worm the dressing
unit is arranged at the workpiece side so that factors
causing shape errors of the grinding worm, such as main
spindle run out error etc. can be reduced. Further, since
dressing utilizes the grinders NC axes no additional exclusive axes are required for the dresser.
During workpiece grinding the dressing unit is shifted
to its park position as shown in Fig. 3. Loading and unloading of workpieces takes place via the ringloader
simply by indexing over the dressers park position.

Thread-shaped
worm

4. Supporting software for machining
4.1 Machining simulation
An important tool in gear grinding is the machining
simulation for the post-machined tooth profile and lead.
It is especially advantageous when complex tooth profiles and leads are required over simple tooth shapes.
This is especially true when detailed modifications are
required to produce gears with low noise and vibration
under load.
As an example, the machining simulation for acrowned lead of a helical gear is described below.
Introducing a curvilinear continuous displacement of
the machines radial axis as the grinding worm feeds
axially through the workpiece is the normal way to carryout lead adjustment. The result of this process simulation is shown in Fig. 44.

Dresser,dressing position

Ring loader
Dresser parkposition

Dresserswinging range(at grinding)
Table spindle
Fig. 3 Dressing unit
Dresser was installed on the ring loader.

Fig. 4 Results of simulation
The machining simulation for a crowned lead of a helical gear.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

3The result shows that target crowning has beenachieved by means of the corrective motion. It also
shows that although the tooth profile at the center ofthe face width does not show any change, the profile
angle at either ends of the tooth face width, where thegrinding worm enters and exits the cut, does showchange.
In addition the machining simulation allows comparison and analysis of the tooth shape difference if there isa synchronization error. This is an effective tool for predetection and troubleshooting.
4.2 Ease of use machining
The conventional method of pressure angle adjustment on a gear-grinding machine, when using a threaded
worm, is shown in Fig. 5
5. With this method the skill of

the operator was required in using a scale or gauge inorder to set the rotating angle of the dresser to the desired pressure angle.
Thus special skills and time were needed to achieve
the desired corrections.
For the ZE15A, a new method of automatic correction
has been developed. The desired profile correction is simply achieved by entering the pressure angle correction
amount into the machine control. MHI established themethod of modifying each pressure angle for left andright flanks. The dressing and machining operation formodification of the pressure angle is analyzed without
adjusting the setting angle of the dressing unit manually. The results of this modification method are shownin Fig. 66.

WormYX
Dresser swingingdirection

DresserFig. 5 Conventional pressure angle correction method
Dresser installation angle is manually adjusted.

Measuring position
ROOT

Measuring position
TIP

ROOT

UpUp

MiddleMiddle

Down

Down

Bias

Bias

TIP

Right tooth face

Measuring position
ROOT

Measuring position
ROOT

TIPUp

Up

MiddleMiddle

Down

Down

Left tooth face

Bias

TIP

Bias
10 m

Before correction

10 m
After correction of pressure angle

Fig. 6 Result of automatic correction of pressure angle(Right flank: -10 m / left flank + 10 m)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

4Result of pitch precision measurement

Lead shape and tooth shape precision measurement result
Right tooth face
ROOT

Toothshapeerror

10 m

Left tooth face
TIP

ROOTTIP
Left
toothfaceFa (Profile form deviation)-1 m
JIS 1
Right tooth face
BOTTOM

1 m

BOTTOM(Helix slope deviation)-1 m
JIS 0

1.6m

m

10
FPT-L

1.420m

30
JIS 1

2030

Acumulative
pitch error

200m

200m

FTU-R

0.8

200-20200-20m

10FPT-R

20
1.0 m

30
JIS 0

102030Pitch variation

10 m
TOP

Righttoothface

LeaderrorfH

FTU-L

10FPK-LSinglepitch error
Acumulative
pitch error

1 m
JIS 1

Runout error

Note: The lead error for corrected tooth shape indicates the value
excluding the crowing rate.

2007.3

10JIS 0Left tooth face
TOPmSingle
pitch error

200200Pitch variation

mFPK-R

20

6.2

10mfr

JIS 330mm
20

4.7
10m

m
20

JIS 230
JIS 230

Fig. 8 Pitch precision
Indicates the pitch precision of the work after grinding.

Fig. 7 Lead shape and tooth shape precision
Indicates the lead shape and tooth shape precision of the work
after grinding.

5. Machining example
A machining example using the ZE15A is shown in
8 with a machining time ofTabl
e 2 , Fig. 7 and Fig. 8,able
50.5 seconds (excluding tooth meshing and part loading), and workpiece gear tooth quality to JIS class-3.
This example, which is a typical automotive gear, is considered to satisfy the requirements of noise, durabilityand process cost.

6. Conclusion
A high precision, high efficiency gear-grinding machine has been developed in response to the needs of themass production industry. The large expansion anticipated in this machine tool segment, will cater for thefuture realization of:(1) On-machine dressing functions for multiple threaded
grinding worms and(2) Ease of use grinding through the adoption of full CNC
and interactive input functions.

Michiaki Hashitani

Kouichi Masuo

Yoshikoto Yanase

Kazutaka Maruyama

Toshifumi Katsuma

Reference(1) Okino, F. et al., Development of Mitsubishi NC Guide Gear
Shaving Machine, Mitsubishi Juko Giho Vol. 34 No. 2 (1997)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

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