Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
9Development ofGE10A Highly-efficient Dry-cut Hobbing Machine
Targeting the Automotive Industry

KAZUYUKI ISHIZU*1

YOKO HIRONO*2

HIROHISA ICHIHATA*1

MASARU UENO*1

YOSHIHIRO NOSE*3

With the growing number of small diameter pinion gears and sun gears for the planetary
gear reduction mechanism used for automatic transmissions (AT), the improvement of machining
efficiency is an issue. Mitsubishi Heavy Industries, Ltd. (MHI) achieved highly-productive
machining for the newly developed GE10A hobbing machine by reducing the time needed for
cutting and set-up/tool changing, as well as by reviewing tool specifications. In addition, theworking environment was further improved through countermeasures for oil mist discharge andmachining noise.

|1. Introduction
In today’s automotive industry, automatic transmissions (hereinafter referred to as AT) tend
to be increasingly multi-staged. As a result, the production volume of small diameter pinion gears
and sun gears for the planetary gear reduction mechanism used for AT is increasing. Figure 1
shows the prediction of the increase/decrease in AT production in Japan. For the machining ofpinion gears and sun gears, a dry cutting technology that uses a specially coated hobbing cutter has
been widely used. In light of the machining cycle time of pinion gears and sun gears being
traditionally short, it is difficult to improve the machining efficiency just by reducing the cutting
time, and the proposal of a total solution for gear machining is required.

Figure 1 Prediction of increase/decrease in Japanese AT production (showingincrease/decrease of production compared with 2011)
Cited from "©LMC Automotive Ltd., 2014. All rights reserved in all countries"*1

Chief Staff Manager, Engineering Department, Machine Tool Division, Machinery, Equipment & Infrastructure*2

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)10|2. Improvement of efficiency of gear machining on hobbing machine
2.1

Current issues

The mainstream method of gear machining on hobbing machines is dry cutting that uses
blown air, which is superior in cutting conditions than wet cutting that uses coolant to dry cuttingthat uses blown air for the improvement of cutting conditions. The enhancement of gear machining
on hobbing machines only through the improvement of the gear machine itself has its limits. MHI
produces not just machines, but also tools, and took advantage of this technological expertise toexamine the following three issues for hobbing machines.
[1] Reduction of cutting time through increase of spindle speed[2] Reduction of non-machining time for exchanging and clamping workpieces[3] Reduction of machine downtime (set-up/tool changing time)
Our approaches to these three issues are described below.

2.2

Reduction of cutting time through increase of spindle speed

For the reduction of cutting time, it is necessary to enhance the speed of the spindle (hobbingaxis) and to develop tools that can be used on the high-speed spindle. Typical dry-cut hobbing
machines use a hobbing cutter dedicated to dry cutting that has a special coated high-speed steel
hob. In addition, there are tools that can be used for further higher-speed machining such as a
carbide hob, a cermet hob, etc. Accordingly, it is necessary for the machine body to feature
workability based on tools with various specifications. With an existing high-speed steel hob, themainstream machining trend was to use a cutting speed of 300 m/min or lower and high-axial
cutting feed. However, the need for increased spindle speed is a recent trend, as exemplified by an
example of high-speed machining using a carbide hob.
In the development of the GE10A hobbing machine, MHI adopted a built-in motor for thespindle and utilized spindle cooling technology and design expertise cultivated in our machining
centers to develop a new spindle that supports a tool from both sides, and this results in a spindle
rotation speed about twice as high than that of an existing machine. Through the use of thishigh-speed spindle, the machining of pinion gears targeting high accuracy resulted in DIN 5 classaccuracy, and machining focused on the cycle time resulted in DIN 8 class accuracy.
Figure 2 shows a photograph of the hob head spindle, Figure 3 shows a DIN 5 classmachining accuracy chart of pinion gear machining, and Figure 4 shows a DIN 8 class machining
accuracy chart of pinion gear machining.

Figure 2

Photograph of the hob head spindle

Figure 3

Machining chart (pinion gear DIN 5)

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

2.3

Machining chart (pinion gear DIN 8)

Reduction of non-machining time for exchanging and clamping workpieces

Next in the development of this hobbing machine, MHI sought to reduce non-machining time
including workpiece exchanging and clamping time to the target time of two seconds, which is lessthan half that of existing machines. To reduce non-machining time, we analyzed the hobbing
operation and reviewed the target setting time and the action sequence of the machine operation.
In the case of existing machines, the workpiece exchanging time could not be reduced
because the revolution of the workpiece changer and its vertical travel for the insertion of theworkpiece into the fixture were performed sequentially for workpiece exchange. To overcome theissue, we adopted a cam unit that allows for the inter-connected operation of the workpiece rotating
action and the vertical traveling action for the insertion of the workpiece into the fixture into theworkpiece changer, and resulted in a reduction of the workpiece exchanging time by half incomparison to existing machines without compromising the positioning accuracy in conveying theworkpiece.
On the other hand, existing machines typically use a pressing-type fixture that clamps aworkpiece by pressing its end face using the tailstock. However, this method requires a highly
accurate workpiece end face against the workpiece inner diameter reference, causing a problem
where the machining accuracy deteriorated depending on the end face accuracy of the workpiece tobe machined. As such, we adopted a fixture that uses the tailstock side pressing mechanism tomake the collet(Note) fix the inner diameter of the workpiece. This method can clamp a workpieceusing the inner diameter reference even when the workpiece end face is not accurate, while
maintaining the clamping time equal to a pressing-type fixture.
(Note) A collet is a tube with a bore that is processed according to the shape of the workpiece, and has slitsthat radiate from the bore center. The workpiece is inserted in the collet and then held by tightening
from the outside.

There are two requirements for the adoption of this type of fixture using the collet.
[1] The workpiece can be conveyed with accuracy for insertion into the fixture.
[2] The tail stock pressing force necessary for the activation of the collet can be made.
These issues have been satisfied through the following methods.
[1] The design standards of the accuracy for the insertion of a workpiece into the fixture werereviewed in consideration of the collet holding travel and the gap between the workpieceinner diameter and the fixture.
[2] An NC tailstock system that can change the pressing force using variable motor torque wasadopted.
These methods resulted in a significant reduction of workpiece exchanging and clamping
time in comparison to existing machines and the attainment of the target non-machining time oftwo seconds or less during the machining of pinion gears.
Figure 5 shows an external view of the workpiece changer. Figure 6 shows a cycle diagramwith the reduced workpiece exchanging time.

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Figure 5 Workpiece changer

Figure 6 Cycle diagram (Reduction of workpiece changing time: comparison
to existing machine)

2.4

Reduction of set-up/tool changing time of the machine

Possible methods to improve efficiency of gear processing include the reduction of toolchanging time and set-up changing time when changing the workpiece model to be machined inaddition to enhancement of machining efficiency.

Figure 7 Hob cutter with longer cutting blade(Long hob)(1)

Figure 8 Relation between cutting blade
length and tool lifecycle

Reduction of tool changing time
The GE10A hobbing machine adopts a hob cutter that has a handle with a longer cutting
blade (long hob), as shown in Figure 7, than that of existing cutters, and thereby the toollifecycle was extended by 90% compared with existing tools and the downtime was reduced.
This aims at the extension of the tool lifecycle by 90% compared with existing tools throughthe use of a cutter with a long cutting blade for the reduction of downtime. Figure 8 shows therelation between the cutting blade length and the tool lifecycle. This hob cutter with a handle
into which the separate parts of the hob cutter and hob arbor (the holder necessary for mounting
a cutter on the machine tool) are integrated eliminates the work to set the hob cutter into thearbor, and reduces the centering time. In addition, this integration of the hob cutter and hobMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)13arbor enables the reduction of the spindle vibration level during operation using a cutter to
one-third of the conventional hob arbor system, and therefore tool lifecycle extension can beexpected. Figure 9 compares spindle rotation frequency vibration levels.

Figure 9(2)Comparison of spindle rotation frequency vibration level

Minimizing tool changing time through the use of ATC
A hob cutter that has a handle with a long effective cutting blade can extend the tool
lifecycle in comparison to an existing cutter. But its weight, up to approximately 9 kg for a tool
diameter of 65 mm and 20 kg for 100 mm, is heavier than an existing hob cutter. For thisreason, cutter exchanges by the operator become less frequent, but the burden on the operator interms of tool exchange work becomes greater due to the heavy weight. To relieve this burden,
this machine is equipped with an optional automatic tool changer (ATC) in order to reduce toolchanging time.
Figure 10 shows the ATC tool changing position and the state where the top cover isopened in conjunction with the front door. Even when machining is ongoing, the operator can
set a hob with a handle into the exchange stand safely at the tool changing position on the side
opposite to the operator where is shielded from the machining chamber. When a hob with ahandle reaches its life end during machining, the automatic tool changer ejects the hob with ahandle to the tool changing position on the side opposite to the operator and then sets a new hobwith a handle into the spindle. In addition, usability is considered such that a hub with a handle
can be conveyed to the machine not by using the ATC, but by lifting it with a crane through thetop cover that opens inter-connectedly with the front door.

Figure 10 ATC tool changing position (left) and the state where
the front door is opened (right)(3)

Reduction of set-up changing time when changing the workpiece model to be machined
The GE10A hobbing machine has a workpiece conveying grip that can be exchanged
without a tool when the set-up needs to be changed whenever the workpiece to be machined ischanged. Figure 11 shows an external view of the conveying grip. The centering of the grip canbe performed easily and repeatedly by pressing its mounting reference surface into theconveying arm when installing. This results in a significant reduction of the centering time incomparison to the existing grip, and enhances maintainability, which can reduce the set-upchanging time when changing the workpiece to be machined.

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Figure 11

Conveying grip

|3. Human-friendly machining environment
There is concern that improvement in efficiency of gear machines causes environmental
degradation for the operator who operates the machine. The dry-cut hobbing machine does not use
coolant and provides a human-friendly environment with no adverse effects on the human bodycaused by the coolant or the cleaning of the inside of the machine. In addition, the GE10A hobbing
machine suppresses oil mist discharge by minimizing the amount of oil used for intermittent
lubrication by reducing the number of intermittent lubricating points and increasing the number ofgrease sealing points in comparison to conventional machines.
There is also concern that the higher-speed machining capability of the GE10A hobbing
machine in comparison to existing machines causes an increase in machining noise. Particularly in
the North American market, where the noise requirements are more severe than Japan, a machinethat causes environmental degradation of the operator is considered to be a product with major
quality problems and will not be accepted by customers even if it attains high efficiency. For thisreason, MHI targeted the machining noise of the GE10A at 77 dB TWA (time weighted average),which is required in the North American market, while also improving the hobbing efficiency.
To achieve this goal, it is first necessary to clarify the cause of noise generation and takeappropriate measures to reduce noise during machining. As such, we made a machining noise
analysis model of the hobbing machine and verified the analysis accuracy to establish noise
reduction measures and analytically evaluate their effects in advance. Based on the results and inconsideration of feasibility, a damping material to be attached to the cover parts was adopted. The
specifications of the damping material were selected by comparing the damping performance,chip-resistance, appearance, cost and other characteristics that conflict with each other. We
performed a machining noise test using the selected damping material and verified that thismeasure attained a reduction of the machining noise to 77 dB TWA (time weighted average) orless, the requirement for the North American market.

|4. Conclusion
In developing the GE10A hobbing machine, which can cut pinion gears and sun gears
efficiently, we reviewed the tool specifications, and enhanced the machining speed and reduced thenon-machining time and downtime. In addition, we succeeded in the improvement of the working
environment through countermeasures for oil mist discharge from the machine and machining
noise.
Henceforth, we will seek to expand the market for the GE10A hobbing machine, and offer
optimum gear machining solutions mainly focusing on customers in the automotive industry.