Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
1

ZE16C/26C Gear Grinder - Supporting Gear
Manufacturing with High Precision and Efficiency

Mitsubishi Heavy Industries Machine
To o l C o . , L t d .

In recent years, the need for high-precision and low-cost gear manufacturing has been
growing against the backdrop of increasing demand for the electrification of automobiles, low fuelconsumption and low noise, as well as the miniaturization of reduction gears for robots. In themanufacture of high-precision gears, gear grinding is commonly performed to improve thedeterioration of shape accuracy after heat treatment. Accordingly, there is an increasing demand
for gear grinders that make the stable mass production of high-precision gears possible.
Mitsubishi Heavy Industries Machine Tool Co., Ltd. has developed the ZE16C that adopt stracture and the ZE26C, a series of gear grinders on the basis of gear processing technology
cultivated until now, to meet the needs for high-precision, highly-efficient gear manufacturing. This
paper presents these products with our efforts for high-precision manufacturing and our uniquewater-soluble coolant that meets the needs for environmental load reduction and energy
conservation.

|1. Features of ZE16C/26C gear grinders
1.1

High-precision gear grinding

Mainstream gear grinders for mass production are continuous generation types that grind
tooth flanks using a threaded grinding wheel. In case of this method, the threaded grinding wheel
and the gear to be ground rotate synchronously at high speed in accordance with the ratio of thenumber of teeth, making high-efficiency grinding possible. However, to realize high-precision
grinding, it is necessary to suppress the vibration and error components during grinding. Thegrinding accuracy of a gear is usually evaluated based on the error of the tooth profile and tooth
helix and pitch error. However, since even a very small amount of undulation of a tooth flank at a
micron level influences the vibration and noise at the meshing of the gears, minute undulation is
also evaluated for high-precision gears. Accordingly, this machine adopts a revised-structure
spindle to achieve the enhancement of the static rigidity by 50% and the dynamic rigidity by 35%
compared with our conventional machine, and thereby realizes the improvement of the grinding
accuracy and tooth flank undulation.

1.2

Improvement of productivity

Gear grinding requires non-grinding time for clamping, unclamping and replacing of the gear
to be ground, as well as tooth meshing between the grinding wheel and the gear to be ground,dressing of the grinding wheel, etc., so shortening this non-grinding time is important for
productivity improvement. Accordingly, the ZE16C/26C has a non-contact sensor for toothmeshing on the counter column side close to the gear to be ground, as well as an increased tablerotation speed at the detection time to shorten the tooth meshing time (Figure 1).
In addition, the ZE16C adopted our proprietary gantry-type counter column structure toattain high-speed workpiece changing operation and tooth meshing operation (Figure 2). As aresult, in the case of a ring type workpiece, for example, the time for changing the workpiece andtooth meshing is reduced from the conventional 17 seconds to 6 seconds, or less than half. By

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
2adopting the gantry-type counter column structure, the adoption of a high-speed workpiece changer
is made possible and the grinding wheel dressing device can have a structure for moving up anddown on the counter column side face. This structure enabled a 20% reduction of the machine bed
length compared with the conventional machine and realized space saving.

Figure 1

Tooth meshing sensor and gear to be groundFigure 2 Machine structure (ZE16C)

By combining the reduction of non-machining time described above, the increase in therotation speed of the grinding wheel axis and the extension of the grinding wheel life by enlarging
its width, productivity can be improved by 20% or more in comparison with the conventionalmachine. In addition, the maintainability and reliability of the machine are improved by analyzing
the sections that cause machine failure based on past results and reviewing the structure thereof.
Table 1 lists the machine specifications of the developed ZE16C and ZE26C gear grinders. TheZE16C can grind gears up to φ160 mm, and the ZE26C can grind a wide range of gears up to φ260 mm with high precision and efficiency.
Table 1 Machine specifications of ZE16C/26C gear grinders
Specifications

Unit

ZE16C

ZE26C

Workpiece diametermm

φ20-φ160

φ20-φ260

1-41-6Module1.3

Workpiece length

mm

200

350

Workpiece rotation speed

min-1

Up to 3,000

Up to 2,000

Grinding wheel diametermm

φ208-φ300

Grinding wheel widthmm

125/160

Grinding wheel rotation speed

min-1

Up to 8,000

Grinding example

Figure 3 shows a gear ground by the ZE16C gear grinder. A gear with a module of 3, 31
teeth, a helix angle of 20°, an outer diameter of 105 mm and a tooth width of 40 mm was ground ina cycle time of 49.5 seconds, and the grinding accuracy of JIS N1 class (JIS B 1702: 1998) wasattained. This indicates that this machine is capable of the mass production of high-precision gears.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
3Figure 3

Grinding example (ground gear)|2. Efforts for high-precision grinding
2.1

Wide grinding wheel width for polish grinding

Surface finishing (polish grinding) after normal gear grinding for the manufacture ofhigh-precision gears is being studied. The most common method is to attach a grinding wheel fornormal grinding and a grinding wheel for polish grinding in the same phase to the grinding wheel
axis (Figure 4). Polish grinding uses a resin-bonded elastic grinding wheel with a small particle
size to remove the bumps from the tooth flank to improve the surface roughness. This machine
supports simultaneous attachment of a grinding wheel for normal grinding and a grinding wheel forpolish grinding to enable polish grinding. The surface roughness Ra of a normally-ground toothflank is about 0.4 μm, but this machine achieves a surface roughness Ra of 0.1 μm or less by polishgrinding (Figure 5). As a result, it is expected that the transmission efficiency and quietness ofgears will be improved and that pitching will be prevented, which facilitates improved durability.
In addition, it was confirmed that installing polish-ground gears on an automobile transmission
enables a reduction in the vibration noise, or so-called gear noise, that occurs when the gears meshwith each other.

Figure 4 Grinding wheels for normal grinding and polish grinding

Figure 5

Grinding results of normal grinding and polish grindingMitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
42.2

Simultaneous bias modification of both tooth flanks

In the field of gear systems such as automobile transmissions and reduction gears for robots,efforts to design, manufacture and verify complex tooth flank shapes are being made for thepurpose of the downsizing and improvement of quietness. Among these efforts, bias modificationthat controls the twist of a tooth flank (so-called bias tooth flank) that occurs when crowning isapplied to a helical gear in the tooth lead has become an essential technology in the electrificationof automobiles in recent years (Figure 6). Generally, bias modification uses a grinding wheel
formed by a dressing operation different from normal operation. Our conventional machines
perform bias modification in which the tooth flanks of a gear are ground one by one. However, thismachine uses a newly-developed rotary dresser swivel mechanism to enable simultaneous bias
modification of both tooth flanks. As a result, gears with complicated tooth flank shapes, whichcannot be made using normal processes, can be manufactured in a short time.

Figure 6

Crowned tooth flank shape (bias tooth flank) and bias-modified tooth flank shape|3. Mass production of gears using water-soluble coolant
Generally, the mass production of gears using generation type gear grinding often uses
oil-based coolant. We have succeeded in the mass production of gears using generation type gear
grinding with water-soluble coolant for the first time in the world, to be able to meet the increasingneed for environmental load reduction and energy conservation. For the realization of the stablemass production of gears with water-soluble coolants, an optimal grinding wheel, coolant andcoolant device are indispensable, so we worked with manufacturers of such items to conduct
development including the verification of actual equipment (Figure 7). Using water-soluble
coolant with high cooling performance enables the reduction of the amount of coolant usedcompared to oil-based coolant, resulting in a reduction in coolant cost of roughly 80%. In addition,
the risk of fire can be eliminated, so there is no need to provide fire extinguishing systems, and theextension of the life of the dresser can be expected due to the use of a dedicated grinding wheel.
Furthermore, by reviewing all the component parts used in the equipment and replacing them withparts compatible with water-soluble coolant to prevent failures caused by the use of water-soluble
coolant, a mean time between failures (MTBF) equivalent to that when oil-based coolant is used isachieved. By these efforts, the grinder of the environmental consideration type suitable for futureage was realized.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 3 (September 2020)
5Figure 7 Paperless and environmentally-friendly coolant device dedicated for use withwater-soluble coolant|4. Future prospects
It is considered that demand for electrification, low fuel consumption and low noise ofautomobiles will continue to increase in the future, as will the trend toward higher precision andhigher efficiency in gear manufacturing. While further accelerating efforts for high-precision
grinding and the reduction of environmental load presented in this paper, and working to improve
efficiency including the reduction of non-processing time, we will continue to develop productsthat meet customer needs such as the enhancement of process support applications, machine statusmonitoring using IoT technology, preventive maintenance, etc.