Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
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Development of MSS300 Super Skiving Machine- Realization of High-Precision, High-Efficiency
Gear Cutting Method TOSHIMASA KIKUCHI*1 YOSHIHIRO NOSE*2

JUNJI USUDE*3

KEISUKE YOSHIKAWA*1

YU CHIHARA*1

In recent years, the automotive industry has been demanding high-efficiency processing ofgears in terms of the trend in the increase in the number of transmissions shipped and the numberof stages for multistage automatic transmissions. This trend is prevailing over not only theautomotive industry, but also manufacturers in all fields including construction machinery andvarious reduction gears. With such a background, skiving cutting that mainly uses an internal gearhas been attracting significant attention recently. Mitsubishi Heavy Industries Machine Tool Co.,
Ltd. also developed the MSS300 super skiving machine, which has both the optimum machine
rigidity and control technology for skiving cutting. In addition, we developed a super skiving
cutter, which is advantageous for tool life, which tends to be a problem in skiving cutting. This
paper presents these gear cutting systems.

|1. Introduction
Skiving cutting has a long history and is one of the gear cutting methods that was invented
more than 100 years ago. However, it was difficult for skiving cutting to be steadily used at actual
manufacturing sites because the requirements necessary for skiving cutting including machine
rigidity, control technology and tool life are not sufficiently satisfactory.
In recent years, as NC technologies of machine tools and tool manufacturing technologies
such as coating improve, skiving cutting has been attracting attention once again, and many
machine tool manufacturers are working on the development of cutting machines and tools. We arealso developing a super skiving system that consists of the following three pillars.
(1) Development of dedicated skiving machines that are superior in machine rigidity andsynchronization accuracy.
(2) Development of super skiving cutter with an unparalleled tool life(3) Development of cutting simulation software dedicated to skiving
This paper describes the developed MSS300 super skiving machine, with a focus on themachine specifications, structure, and case examples of actual cutting.

|2. Skiving cuttingFirst, internal gear processing is explained as an example of skiving cutting. As shown in
Figure 1, skiving cutting is one of the so-called rough cutting methods before heat treatment.
Conventional main gear cutting methods consist of gear shaper cutting and broach cutting, but
skiving cutting has been attracting attention in recent years.
Next, each cutting method is explained. Figure 2 shows the cutting methods and tools used.
Skiving cutting typically uses a pinion cutter. The developed super skiving machine can use not
only a pinion cutter, but also our unique super skiving cutter described below. The user can freely
use either of these cutters depending on the shape of the workpiece to be cut.
*1

Gear Machine Designing Section, Mitsubishi Heavy Industries Machine Tool Co., Ltd.

*2Manager, Gear Machine Designing Section, Mitsubishi Heavy Industries Machine Tool Co., Ltd.

*3

Chief Staff Manager, Gear Machine Designing Section, Mitsubishi Heavy Industries Machine Tool Co., Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)
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Figure 1 Manufacturing process of internal gearFigure 2

Gear cutting method of internal gear

Gear shaper cutting can cut a stepped gear with a minimum processing clearance andvirtually without restriction in the shape of the workpiece. However, the cutting efficiency is notgood and the productivity is low because chips are generated only in outward travel of the tool
reciprocating motion.
Broach cutting is a mass production cutting method that is superior in productivity, but
requires a high initial cost in terms of both the facilities and tools, and it is difficult to adjust its
cutting accuracy.
Skiving cutting has a higher cutting accuracy than that of gear shaper cutting and broachcutting, and can attain a productivity three to five times higher than that of gear shaper cutting. Inaddition, skiving cutting can cut special tooth leads including crowning and tapering. The mostimportant issue is that the tool life is short.
Figure 3 shows the principle of skiving cutting. Skiving cutting is a method to cut a gearusing the slide generated on the contact point of the tool axis and the workpiece axis by rotating thetwo axes that are arranged so that the angle between them becomes a certain crossed axes angle.

Figure 3

Principle of skiving cuttingThis method can cut a workpiece to a machining accuracy of as high as ISO 4 to 5 because
the tool axis and the workpiece axis use direct drive motors allowing high synchronization
accuracy, and cutting with the tool meshed continuously with the workpiece makes the transfer ofMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)57

the tooth profile accuracy of a tool to the tooth profile accuracy of the workpiece easy. As shown in
Figure 3, however, skiving cutting has issues with large cutting resistance and significant heatgeneration at the tool blade tip resulting in short tool life, because the angle θ between the rake faceof the tool and the cutting face of the workpiece becomes significantly negative during cutting.

|3. Super skiving
Currently, the mainstream type of skiving cutting is pinion skiving, which uses a pinion
cutter. However, there are still many cases where the tool life becomes a problem.
Therefore, we developed a super skiving cutter ahead of the rest of the world(1).
We focused attention on the barrel-shaped grinding wheel developed for our ZI20A internalgear grinding machine. The ZI20A is designed so that a large crossed axes angle is formed in order
to increase the slide-cutting speed and improve cutting efficiency. The external shape of thegrinding wheel used is barrel-shaped to prevent interference with the internal gear, which is theobject to be cut (Figure 4(a))(2). We developed the super skiving cutter by adding a gash and arelief angle to the barrel-shaped grinding wheel to form a multiple-blade tool (Figure 4(b)). Inaddition, the super skiving cutter has a tapered external shape that allows all of the multiple blades
to be involved in cutting and the tool wear to be distributed (Figure 4(c)). In this manner, the toollife, which was considered to be an issue of skiving cutting, can be expected to be extended.

Figure 4

Barrel-shaped grinding wheel and super skiving cutter|4. Development of machineThis machine was developed based on the main concept that a high rigidity enough to deal
with heavy cutting is achieved and a high positioning accuracy is ensured in order to derive theperformance of the super skiving cutter. Table 1 shows the main specifications of the developed
MSS300 super skiving machine.
Table 1 Machine specifications
No.
12
3

4.1

Item
Specification No.
Maximum(mm)300
4workpiece diameter
Maximum cutting
4.0
5moduleMaximum spindle(min-1)6000
6rotating speed

Item
Maximum tablerotating speed
Maximum crossed
axes angle
Spindle motor

Specification-1(min )3000(deg)±30(kW)

33

BedA sliding guide surface that is superior in terms of vibration damping and rigidity is adopted
for the X axis (the feeding axis in the direction of cutting during processing). Although this is asliding guide, good positioning accuracy is secured and highly accurate cutting – also for special
tooth leads including crowning as described below – is attained.
For skiving cutting that requires high productivity, the chip discharge method is also animportant factor. The developed machine adopts a center trough structure, which is a method thatdischarges chips from the just under the cutting point directly onto the chip conveyor. As a result, it
suppresses the thermal deformation of the bed to the utmost limit, resulting in stable cutting withlittle dimensional fluctuation (Figure 5).

4.2

Column and saddle

The column and the saddle have nearly symmetrical shapes and form a well-balanced
structure that takes into consideration the heat flow and the force flow. In particular, the Z axis (forfeeding in the workpiece tooth lead direction during cutting) eliminates the hydraulic cylinder fromthe conventional combination of a ball screw and a hydraulic cylinder, and adopts a twin ball screwMitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (December 2016)58

structure instead.
The hydraulic cylinder was used for holding the weight balance of moving objects of the Z
axis (including the saddle and the spindle), but always caused energy loss during machine
operation. The elimination of the hydraulic cylinder also contributes to energy saving. Naturally,
the balance and the rigidity of the feeding system are significantly improved in comparison toexisting technology (Figure 6).

Figure 5 Center trough structure

4.3

Figure 6 Tandem control of feedaxis in tooth lead direction

Spindle and table axis

This machine adopted a built-in high-torque synchronous motor for the tool axis and thetable axis in order to realize heavy cutting. Accurate synchronous control of the tool axis and thetable axis is essential to achieve high-efficiency, high-accuracy skiving cutting. For this reason, thedeveloped machine uses a high-accuracy encoder that has good results with our gear grinding
machines to attain highly accurate synchronous control. In addition, we applied the same
technology used in our high-speed machining centers for cooling of the spindle center in order tocool the spindle body and the front and back bearings, which enabled stable high-speed rotation(Figure 7).

Figure 7

Structure of spindle and table axis

|5. Cutting example
Figure 8 shows an example of cutting with the use of the developed cutting machine. The
workpiece in this example (a ring gear (internal gear) of module 1, number of teeth: 100, helix
angle of 15 degrees and face width of 25 mm) was cut to a machining accuracy of ISO 3 for thetooth profile accuracy and ISO 4 for the tooth lead accuracy in a cycle time of 60 seconds using asuper skiving cutter (Figure 9).
In addition, a workpiece of module 1.5 (number of teeth: 70, helix angle of 20 degrees andface width of 35 mm) was cut by skiving cutting using a pinion cutter. In this case, crowningcutting (Figure 10) and tapering cutting (Figure 11), which are cutting of special tooth leads,
resulted in the tooth leads as planned.
In this manner, high-efficiency, high-accuracy skiving cutting was realized.
When a super skiving cutter was used, 700 workpieces of module 1.5, which was same as theabove-described one, could be cut per one time of edging. This means that the tool life isapproximately four times longer in comparison to that of a pinion cutter.
Even after cutting of 700 workpieces, the maximum tool wear was still within the target

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range. Therefore the test is being conducted continuously toward the final cutting target of 1,000
workpieces.

Figure 9 Superskiving cutter

Figure 8 Cutting example

Figure 10

Tooth lead accuracy in crowning cutting

Figure 11

Tooth lead accuracy in tapering cutting|6. Conclusion
We developed the MSS300 super skiving machine, which is superior in machine rigidity andsynchronization accuracy, with targeting high-accuracy and high-efficiency gear cutting, andconfirmed through actual machining that the targeted cutting accuracy was successfully attained
and special tooth lead cuttings such as crowning and tapering could be performed. In addition to themachine itself, we also developed a super skiving cutter that is a unique tool optimum for skiving
cutting. In the future, we will continuously work on the development of new technologies such as
further extending the service life of skiving tools and making the simulation software
multifunctional, in order to offer the best solutions for machinery and tools to customers.

References
1.
2.

Monden, T. et al., MHI Super-Skiving System for Longer Tool Life and Enhanced Efficiency in Internal Gear Cutting,
Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (2015) pp.101-105
Yanase, Y. et al., The World's First Machine for Grinding Internal Gears in Planetary Gear Systems, Mitsubishi HeavyIndustries Technical Review Vol. 46 No. 3 (2009) pp. 7-12