Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)29

Latest Gear Cutting Technologythat Improves Customer Productivity(Increase in Speed of ST Series NC Guide Gear Shaping Machine)

TOSHIMASA KIKUCHI*1

YOSHIHIRO NOSE*2

KATSUHIDE YABE*2

TAKAHIDE TOKAWA*3

Mitsubishi Heavy Industries, Ltd. (MHI) was the first company in the world to establish thedry cut system, which contributes to a clean work environment and high productivity by eliminating
the use of cutting oil for the processing of gears. We have been actively offering new technologies,
including the one mentioned above, that satisfy the needs of customers. There is a growing need forfurther improvement of productivity, including the reduction of processing time, from mass
production lines of automobiles and other products, as well as from job shops that manufacture
multiple types of gears in small quantity. In this paper, we will introduce the latest technologies for
productivity improvement of gear cutting machines, taking the ST series NC guide gear shaping
machine, one of the gear cutting machines developed by MHI, as an example.

|1. Introduction
A gear shaper is a gear cutting machine that is generally used to process internal gears andstepped gears. As shown in Figure 1, the cutter and the workpiece are rotated synchronously while
the cutter is moved back and forth for processing gears. The gear shaper features a unique function,
where a helical guide is used to move gears helically along the cutter lead for the processing ofhelical gears that are twisted along the helical path. The helical guide needs to be replaced
responding to the change of the cutter lead, but helical guides are expensive and replacement
requires a fair amount of time and a lot of skill. MHI’s ST series NC guide gear shaping machine
eliminated the need for the replacement of the guide by numerically controlling the helicalmovement at high speed and high precision. Utilizing this technology, the functions of theST40CNC model, for which the need for productivity improvement is especially high, wereimproved.

Figure 1

Gear shaper cutting method*1

Engineering Department, Machine Tool

*3

Engineering Manager, Engineering Department, Machine Tool

*2

Manager, Engineering Department, Machine Tool

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)30|2. Drive principle of the NC guide gear shaping machine
The helical movements (lead generation mechanism) of general gear shapers and the NC
guide gear shaping machine are explained below.
As shown in Figure 2 (a), the helical path is created by a pair of male and female helicalguides with a lead identical to that of the cutter in general gear shapers. The helical guide (female)is connected directly to a worm wheel and moves the circumferential feed at a certain speed for thecreation of teeth on the entire circumference of the workpiece using a motor. Under thismechanism, retooling of the helical guide in the machine is required for the use of a cutter with adifferent lead.
Meanwhile, the NC guide gear shaping machine has a pair of female and male straightguides, which move the cutter back and forth along the straight line in the axial direction, as shownin Figure 2 (b). The rotation of the cutter is synchronized to the motion to attain the intended helicalmovement, creating a path similar to those created by gear shapers incorporating general helicalguides1, 2.

Figure 2

Guide structure and helical movement generation mechanism|3. Functional enhancement for improvement of productivity
As explained above, the gear shapers of MHI’s ST series do not require the replacement ofguides, remarkably improving the tooling performance in helical gear processing. Because of thisfeature, the shapers better respond to the production of workpieces with various torsion angles inlots and have been widely used for the production of various types of gears in small quantity.
The functions of the ST series ST40CNC have been enhanced responding to recent needs forfurther improvement in productivity, as shown in Table 1. Refer to Table 2 for the major
specifications of the ST40CNC.
Table 1

Type ofproduction

Needs of job shops and (gear shaper) technologies that are requiredMassproduction line
Large volume
production oflimited kinds ofproducts

Job shopNeeds(Example)
Required technologies

Small volume
production of manykinds of products

Tooling

Not many

Many

Productivity

HighLow

Improvement intooling
performance
Improvement inproductivity

Elimination of helicalguidesQuick return/high stroke spindle
Reduction of data transfer timeMitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)31

Table 2

Primary specifications of the NC guide gear shaping machine ST40CNC
ST40CNC

Max. diameterof workpiece

Outer diameter ofexternal gear
Inner diameter ofinternal gearMax. module
Max. width of workpiece(*)
Cutter stroke width(*)
Max. helical angle(*)
Cutter stroke speed(*)
Spindle quick return mechanism
Circumferential feed
Radial feed
Main motor output

(mm)
(mm)
(mm)
(mm)
(deg)
(str/min)(mm/min)(mm/str)(kW)

φ400
φ250 + Cutter PCD(Max. φ400)
8115
0-135
±36°
60-500 (standard spec.)60-800 (option) newly established
Not mounted (standard spec.)
Mounted (option) newly established2.5-3200
0.001-0.1
30(*) Based on specifications of the processed workpiece and the cutter to be used

3.1

Control for quick return of spindle

In gear shapers, the turning motion is changed to linear reciprocating motion (vertical) by thecrank mechanism. One rotation of the crank is divided into the cutting process and the returnprocess. As shown in Figure 3, processing is performed in the motion from the top dead center tothe bottom dead center, while processing is not performed in the motion from the bottom deadcenter to the top dead center, during which chips are not discharged. The spindle is controlled toreturn quickly to reduce the cycle time; It moves quickly in the return-process in which processing
is not performed.
The speed of the piston movement of the gear shaper crank mechanism is approximated by
the sine curve shown in Figure 4. In normal processing, the cutting process and the return processcontrast, as in the curve showing the speed of vertical movement, while the peak speed of the returnprocess is designed to exceed the peak of the original sine curve in the quick return control. As thefigure indicates, the cycle time for one cycle is shortened according to the speed of quick return.

Figure 4 Curves showing changes in the speed of thevertical movement of the gear shaper cutter
Figure 3 Trace of one verticalstroke of the cutter

In incorporating the quick return control, consideration is given to the increase in load on themain motor and cutter rotary motor, as well as vibrations that could affect the gear shaper, toprevent influence on processing accuracy. In this respect, the acceleration/deceleration commandfor the quick return range is also set at a sine curve-shaped command in the ST40CNC to improve
the following capability of the motor, realizing a speed curve that smoothly connects to the sine
curve of the cutting process. In addition, measures have been taken to maintain accurate
synchronization between the cutter rotary axis and table rotary axis during the quick return process.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)32

Through these mechanisms, the optimum speeds for rough cutting and finish cutting
specified by the operator are automatically calculated, and the ratio of quick return speed (ratio ofspindle stroke counts for the return process to the cutting process) can be set at twofold at the most.
The limit for quick return speed may change depending on the processed workpiece or thespecifications of the cutter to be used.
The effects of the quick return control of the spindle, in terms of time, are explained below
using some examples. As Table 3 and Figure 5 indicate, if the quick return ratio is set at about
twofold, the cycle time is reduced by approximately 16%.
Table 3 Examples of cutting by ST40CNC
Target workpiece
ModuleNumber of gear teeth
Helical angle
Face width
Cutter lead
Material of workpiece
Cutter stroke speed at cutting
process(*)
Quick return ratio(*)

(deg)
(mm)
(mm)
-Internal gear
2100
1545
1650.793
SCM415(str/min)180/260/260-

2.0/1.6/1.6
Without quick
With quick return
return1916-16

Cycle time(min)

Time effect(%)(*) Three processes including rough processing, intermediate finishing and finishing

Figure 5 Speed command curves of cutter rotary axis under conditions
indicated in Table 3

3.2

Increase of spindle strokes

For further improvement in the productivity of the ST40CNC, an optional 800 str/minspecification is offered in addition to the 500 str/min specification for the maximum stokes of thespindle (Table 1). This was realized by changing the drive gear ratio of the spindle and adding
balance weights that reduces vibration, as shown in Figure 6. A proven vibration control
technology used in the SE series, MHI’s high-speed gear shaper, has been incorporated, optimizing
the location and weight of the balance weight.
Figure 7 is based on the measurement of vibration during the spindle operation at anarbitrary stroke width and was created by plotting vibration amplitudes in the cutting shaft
direction. This indicates that vibrations in the high stroke range are reduced in particular throughthe addition of the balance weight.
We have been promoting analyses for further improvement of processing speed and precision
by utilizing three-dimensional simulations including multibody dynamics (MBD).

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)33

Figure 6Changes made to the spindle with 800 STR specification

Figure 7

Effect of vibration control by the balance weight

3.3

Shortening of data transmission time

In the ST series, the positional data of the three shafts (spindle, cutter rotary axis, table rotaryaxis) needed for synchronous processing is calculated by the personal computer attached to the NC
at a distribution cycle of 0.5 msec. The data is transmitted to the NC, stored in the internal memoryand distributed to motors at high speed to realize intended operations.
Conventionally, all the calculated path data was transferred to the NC at once. In thatmethod, the data volume is huge, and takes a long time for transfer, wasting the time of theoperator. To improve workability, the conventional method of operating according to the memory
was discontinued and a new method was incorporated to reduce the amount of data to be handled.
As an example, we would like to explain the control of the cutter rotary axis in theST40CNC. The shaft is controlled by a combination of (1) repeated helical operation per vertical
reciprocating stroke and (2) shifting in turning direction (Circumferential feed) during processing,Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)34

as shown in Figure 8. In the new method, control is repeated for each of the processing cycle
ranges (for example, three ranges including the rough processing, intermediate processing andfinishing ranges in Figure 8) to create the point group data (1) of one cycle. The data is transferredto the NC and is repeated with high speed and precision by the NC. Furthermore, by adding thecircumferential feed shifting operation via NC program command (2), the intended command value
is created. Through these measures, the calculation volume and transferred data volume have been
reduced substantially.

Figure 8

Positional command curve for cutter rotary axis

As shown as improvements in Table 4, data size has been reduced, which has led to a
substantial reduction in data transfer time.
Table 4 Effect for reduction of transfer time
Volume of created data(in the case of 4 cuts)
Transfer time

Conventional method

New control method

(MB)

85

1(sec)

Approx. 240 sec.

Approx. 4 sec.

|4. Conclusion
Functions were improved for MHI’s ST series gear shapers (ST40CNC), which do not
require the replacement of the helical guides and feature high tooling performance, to satisfy therecent need for productivity improvement. The cycle time was shortened and operability byoperators was improved through these improvements. We are currently developing a high-value
gear shaper featuring higher tooling performance, operability and maintainability, aiming forhigh-precision processing.
We will continue to offer gear cutting machines, including gear shapers, hobbing machines
and shaving machines, of higher precision and productivity.

References
1. Okino, H. et al., Development of Mitsubishi NC Guide Gear Shaping Machine, Mitsubishi Juko Giho Vol. 34
No.2 (1997) pp. 108-111
2. Nabekura, M. et al., Gear Cutting and Grinding Machines and Precision Cutting Tools Developed for Gear
Manufacturing for Automobile Transmissions, Mitsubishi Heavy Industries Technical Review Vol. 43 No. 3(2006)