TECHNICAL REPORT

Development of Process Integration Using a Gear Skiving Center—A Case Study Focusing on the Development of a Synchro-sleeve Gear—
L. ZHANG

K. NATSUDA

Due to the pursuit of automobiles that generate less noise and are more fuel efficient, there has been anincreasing demand for the gears used in automotive transmissions to have higher accuracy, higher efficiency, andbe manufactured through integrated processing. The gear production lines currently operating in the automobile
industry are comprised of many specialized machines (hobbing machines, broaching machines, etc.) which requirea high investment cost. The workpieces produced need to be transferred between different specialized machines
and even between different processes which could lead to positioning and assembly errors. This paper introduces
the Gear Skiving Center, as well as a case study of integrated processing of the synchro-sleeve gear using the Gear
Skiving Center.
In the past, the processing of synchro-sleeve gears required multiple machines such as a lathe machine andhobbing machine, however now, the Gear Skiving Center is the sole piece of equipment required to complete all thenecessary manufacturing processes, not to mention offers the added benefits of 1.5 times machining accuracy, 0.5
times processing time, and 0.25 times the number of necessary machines.
Key Words: gear skiving, high accuracy, process integration

1. Introduction

2. Skiving Method

Currently, gear shaping, broaching and hobbing are
the machining methods adopted for gears. Gear shaping
is the typical machining method used to shape gears,
and is the comparatively mature method used broadly
for mass production lines such as auto parts. Broachingoffers merits such as high efficiency, easy operation,and minimal variation in machining precision, and isconsidered indispensable to mass production lines.
Hobbing has high productivity and excellent precision,
therefore is widely adopted in the machining of externalgears. However, these machining methods are notversatile, and in actual production sites, there exist
various issues relating to productivity, production cost,1)
etc. (Table 1). In contrast, skiving is drawing attention
as a new machining method for both external and internalgears. Details of this machining method are given below.

Skiving is a tool cutting method performed using agear-shaping tool such as a gear shaper. As Fig. 1 shows,the principle of skiving is to tilt the tool and workpiece
relative to each other and simultaneously rotate themto generate relative speed, which is utilized to machinegears.
The principle of skiving was proposed in Europe
in the 1960s, however skiving could not be put into
practical application due to rigidity and control issuesof the machine tools at the time. In recent years, thepractical application of skiving has become possible dueto technological advancements relating to tools, controland machinery, therefore it is drawing attention as a newmachining method.
While gear shaping is a method whereby thereciprocating motion of a tool in the axial direction isTable 1 Characteristics of gear machining methods
Machining method

Shaping

Broaching

Hobbing

Skiving

Precision (new JIS)

Grade N7-N12

Grade N7-N12

Grade N6-N12

Grade N6-N12Tool cost

Low cost

High cost

Low cost

Low cost

Cycle timeLong

Supported workpiece External gear, internal gear

Short

Short

Short

Internal gear

External gear

External gear, internal gear

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)73

−Development of Process Integration Using a Gear Skiving Center — A Case Study Focusing on the Development of a Synchro-sleeve Gear — −applied to cut teeth, skiving involves rotating a tilted tool
and workpiece to generate cutting force and cut teeth.
Because skiving does not use reciprocating motion,
there is no need for idling and its machining time can besignificantly reduced.
As Fig. 2 shows, unlike gear shaping, skiving does not
require a recess, therefore it offers the merits of compact1)
and higher-strength products .
Intersecting angle
Workpiece

Tool

Actual cutting speed

Feed direction

Fig. 1 Skiving method

No recess

Recess3-5mm requiredGear shaping

3. Gear Skiving Centers
GS200H, GS300H, GS700H
In November 2013, JTEKT equipped a machining
center with the technologies of high-precisionsynchronized control, high-speed, high-rigidity turning
table and skiving tool to develop GS300H, a gear skiving
center capable of performing all processes from lathing
to gear machining and drilling. This made it possible tomachine gears of u300mm or less. GS300H was primarily
developed for the machining of auto parts.
Moreover, in 2015, to respond to a variety of customer
products and needs, such as construction and farming
machinery, we newly developed GS700H, a gear skiving
center. This made it possible to machine large gears suchas module 6 and those sized u700mm or less (Fig. 3).
Finally, in 2016, we developed GS200H as a gear
skiving center achieving high productivity and stable
precision in a size suitable for mass production lines ofauto parts. By adopting a machine structure and devices
capable of withstanding the high load necessary for highefficiency gear skiving, we realized high productivity byincreasing the rigidity of the machining devices and eachaxis (Fig. 4). Table 2 shows the main specifications.

Skiving
87Broaching

Skiving

Fig. 2 Skiving method merits

Module65
4Robots,
construction
machinery

3Trucks

2Moreover, compared to broaching, skiving offers
low tool and equipment costs, in addition to making it
possible to perform integrated machining of the internalgear. Also, by controlling the tool’s gear profile andmachining trajectory, it is also possible to easily adjust1)
gear profile correction, such as crowning .
Compared to hobbing, skiving offers the merits ofallowing machining of both external and internal gears,
and reduced workpiece assembly and positioning errors
through the single-chuck machining of compound gears.

10(newly developed)

Cars

100

200

300

400500

600

700

800

Product O.D., mmFig. 3 Scope of gear skiving center

Fig. 4 Gear skiving center-GS200H

74

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

−Development of Process Integration Using a Gear Skiving Center — A Case Study Focusing on the Development of a Synchro-sleeve Gear — −Table 2 GS200H specifications
ItemFeed unit
SpindleATCC axis

Speciﬁcations
Travel (X, Y, Z)

mm

470，360，620

Rapid feed rate

m/min

48

Spindle end proﬁle−

BT No. 50

Spindle RPM

min−1

12 000Tool holding capacity pcs

B

8 [OP：20]
−1

RPM

A

min

3 000

Max. workpiece dia.

mm

u220

Control

CNC−

TOYOPUC-GC70

Requiredsurface area

Width × depthmm

2 400 × 3 000

A part: Taper profile

B part: Missing teeth profile

Fig. 6 Special gear profile

4. Initiative for the Process Integration ofSynchro-sleeve Gears
T h e s y n c h r o - s l e ev e g e a r i s u s e d i n m a n u a ltransmissions and works in sync with the shift lever toswitch the gear ratio (Fig. 5).
To change gears smoothly while the gears are mutuallyrotating, the synchro-sleeve gear has a special gearprofile; namely taper and missing teeth (Fig. 6).
High-precision machining of special gear profiles isnecessary as poor precision could lead to shift change
problems and slipping.
Currently, rolling is used to create the taper profile
of synchro-sleeve gears, while broaching is used for themissing teeth profile (Fig. 7). This process/machining
method creates positional errors due to machining
precision problems and workpiece assembly errors and it
leads to the problem of a high defective rate.
Here, we will introduce an example of replacing
rolling and broaching with skiving and achieving processintegration and high precision in a single gear skiving
center, including rolling of the inner diameter and teeth
cutting.

135
246R

Machining locations
I.D.
Internal gear

Taper proﬁleDownstream
process

Downstream
process
Teethcutting

Process Turning

Missing teeth
proﬁleDownstream
process

Rolling

Broaching

4 machines

Fig. 7 Conventional machining process

4. 1 Workpiece Specifications
Table 3 below shows the gear specifications, including
size (u115×25mm) and material (chromium steel SCR420H).
Table 3 Gear specifications
u115

O.D., mm
I.D., mmu93

Moduleml. 86

No. of teeth

51

Angle of torsion, °30 (RH)

Tooth width, mm

25

Tooth depth, mm

2

Shift lever
Torque flow

Synchro-sleeve gear

When in neutral

Sleeve movement

When in 1stSleeve movement

When in 2ndFig. 5 Synchro sleeve

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)75

−Development of Process Integration Using a Gear Skiving Center — A Case Study Focusing on the Development of a Synchro-sleeve Gear — −4. 2 Target Value

Pull machining

Regarding taper angle and phase error, in order toachieve high precision, we set the target value of precisionat half or less of the conventional results.
<Target value of precision>
Taper (angle error): 0.5° or lessPhase error: 0.01° or lessGear precision: JIS grade N7Back③④Push machining

4. 3 Details of the Development
In order to machine special gear profiles using theskiving method, we divided up the individual processes
and extracted the necessary development requirements.
The following section introduces the tools and machining
method to machine the taper and missing teeth profile.
1) Machining of the taper profile
The taper portion comprises the three elements ofthe taper profile, symmetrical recesses and double-endprofiles. Figure 8 shows the machining method developedby JTEKT in order to satisfy these.
In regards to the taper angle, we adopted a machining
method whereby the taper angle is replaced with a helicalgear’s angle of torsion, and the tool and workpiece aresynchronously rotated to machine an arbitrary taper angle.
For the symmetrical profile, we devised a method tomachine the left and right tooth flank respectively. In
the case of skiving, the tool trajectory differs dependingon the rotational direction and tool torsion at the timeof machining, and the cutting zone varies, therefore
differences arise in the contact of the left and right toothflanks and there is asymmetry in the recess portion.
As such, we designed a special tool and developed amachining method to control the rotational direction.
Both ends of the workpiece are given a taper profile,
therefore we designed tooling to enable machining ina single chuck. We devised a machining method ofmounting the rake face of the tool around the opposite
way and feeding it in the reverse direction to theconventional method.
2) Missing teeth profile machining
There are three missing teeth profiles on thecircumference of the workpiece. As such, we adopted
one tool and a synchronization ratio of 1:3, as shown in
Fig. 9. The removal zone was considered as a profile andthe tool’s cutting edge was designed to suit this shape.
When a machining method with varying
synchronization ratio is used, it can be assumed thatthe meshing status of the tool and workpiece will beuneven, thus creating large load variance and resulting
in chipping. As such, we performed FEM analysis anddynamic rigidity analysis to create a high-rigidity shapewhereby there is as little displacement of the tool as
possible due to load during machining.

①②

FrontFig. 8 Taper machining process

To design the tool’s cutting edge, we developed
design simulation software of arbitrary gear profiles.
Commonly, the gear profile is designed in accordancewith an involute generation theory. As such, conventional
tool development involved generating motion of the tool
cutting edge profile, then calculation and optimization of
the workpiece gear profile. However, there is no missing
teeth generation theory that can be applied to the missing
teeth profile used on this occasion, therefore we wereunable to settle on a profile for the cutting edge.
As such, we developed a new simulation softwareto reverse calculation of the tool profile from theworkpiece’s missing teeth profile. By entering the missing
teeth profile of the workpiece and tool specifications,the optimal tool cutting edge profile is outputted, andthis makes it possible to design tools of all non-involute
profiles, such as reduction gears and pulleys (Fig. 10).
Tool

Workpiece

Tool: 1 rotation
Workpiece: 1/3 rotation(Machining of the 1st tooth)

Air swing of tool(No machining)

Tool: 2 rotations
Workpiece: 2/3 rotations(Machining of the 2nd tooth)Fig. 9 Image of missing teeth machining process

Side viewFig. 10 Tool for missing teeth

76JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

1.0

0.5

0.50

Conventional

0.02

0.02

0.3

TargetPhase error, °Taper angle error, °

1.0

0.01

0New
technology

Conventional

0.01

0.01

TargetNew
technology

Gear profile error, µm−Development of Process Integration Using a Gear Skiving Center — A Case Study Focusing on the Development of a Synchro-sleeve Gear — −1211.5

11.0

606.9

Conventional

TargetNewtechnology

Fig. 11 Processing results

4. 4 Machining Results
We performed a machining evaluation for the taper
portion using a skiving tool with 29 blades, as well as amachining evaluation of the missing teeth portion using a2-blade skiving tool with a torsion of 20°.
The machining evaluation results were a taper angle
error of 0.3°, missing teeth (profile error) angle of0.05mm and a phase error of 0.01° or less, meaningthat gear precision was equivalent to the target value ofJIS grade N7 (Fig. 11). Figure 12 is a post-processing
photograph.
Consequently, as Fig. 13 shows, a machine adopting
the skiving method of machining integrates processes toeffectively perform the tasks of four machines in one, inaddition to halving tool costs.

5. Conclusion
In regards to the machining of synchro-sleeve gears
for manual transmissions, we successfully achieved highprecision, high efficiency combined machining by using
the GS200H and adopting the skiving method. This has
enabled the reduction of the number of machines andachieved a space-saving, flexible machining line capableof swiftly responding to changes in manufacturing
processes and production volume.
Moreover, by utilizing the technologies and skiving
centers developed on this occasion, it is possible tosupport a wide-range of profiles other than gears, such as
the non-involute profiles of pulleys, tapers and chamfers.
JTEKT will continue engaging in the development ofgear machining technologies and contributing to thetechnological innovation of gear manufacturing.
*1 TOYOPUC-GC70 is a registered trademark of JTEKTCorporation.

References
1)

L. ZHANG, H. OHTANI, H. NAKANO: JSAE Chubu
Branch Research Presentation proceedings (2016). (inJapanese)

4412
10

ConventionalNew
technology

Tool cost

No. of machines

Fig. 12 Post-processing workpiece
1.0

0.5

0.50

Conventional

Newtechnology

Fig. 13 Effects

L. ZHANG**JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

K. NATSUDA*

M a c h i n e To o l s & M e c h a t ro n i c s E n g i n e e r i n g
Dept., Machine Tools & Mechatronics Operations
Headquarters

77