Machining Center at theForefront of Production
TOSHIO YOKOMOTO** 1
KENSUKE IDE* 1
KEIJI MIZUTA*1CHIAKI YASUDA*2
TANEHIRO SHINOHARA* 2

Machining Centers are widely used for a variety of the latest machining processes. In 2002, Mitsubishi HeavyIndustries, Ltd. (MHI) released the VΣ-series and HΣ-series machining centers that provide high-accuracy andhigh-efficiency part machining for the die/mold, power systems and aircraft industries. The operations are realized incorporating innovative technology such as high-speed low-vibration spindles, thermal displacement restriction, high-speed hydrostatic guide ways and high-speed high-accuracy servo drives. Σ series are at the forefrontof advanced technologies.
chining center M-V50-FM and the horizontal machining
center M-H 60 are shown on Table 1
1. The key technologies are discussed below.
2.1 High speed yet highly rigid spindle
In the die/mold industry, high speed, highly rigid, lowvibration spindles are required in order to cut smooth
3D surfaces with higher efficiency. For the VΣ series, a30 000 min-1 low vibration spindle has been developed.
With this spindle, high efficiency die & mold machining is realized thanks to higher rigidity at all speed
ranges, obtained by optimum bearing layout and ourautomatically variable pre-load mechanism. Also,
spindle nose end vibration within 3μm P-P can be maintained for long periods by utilizing coil springs in placeof Belleville springs in the tool retention mechanism,and incorporating multi-phase dynamic balancing technology.
As a result, drilling of diameters as fine as 60 μm,
which is hitherto almost impossible, is feasible as shown
on Fig. 1
1.
2.2 High speed high precision feed system
For higher efficiency of machining, higher feed speed
is required in order to reduce both cutting time and noncutting time. Furthermore, for higher accuracy, it isessential to enhance the rigidity of the guide way system. However, simply increasing feedrates will increase
positioning error due to inertial force of the moving parts,

1. Introduction
The progress of technology requires higher accuracy
and higher efficiency for metal cutting. For example, inthe die & mold industry, IT (Information Technology)
requires smaller and much more precise machining details. Machines that can minimize the amount of handfinishing are in strong demand, to reduce cost andshorten delivery. In the power systems and aircraft industries, efficient machining of hard-to-cut materials
such as Inconel or titanium is demanded.
To meet such needs, MHI has released vertical andhorizontal machining centers called the Σ series, which
incorporate MHI's state-of-the-art advanced technology.
High efficiency machining requires high-speed rotating spindles and high-speed motion feed mechanisms. To
realize high accuracy, strong guide ways and restrictionof thermal displacement are necessities. This thesis reports how Σ series realize high speed yet highly rigidspindles, high speed and high precision feed mechanisms,
and thermal displacement restriction.

2. The latest technologies for our machining centers
The basic specifications of the Σ series vertical ma-Travel

Rapid feed speed

M -V50-FM

M -H 60

X-axis (mm)8001 050

Y-axis (mm)510890Z-axis (mm)450890(m/min)24Maximum spindle speed (min-1) 14 000/30 000*

30/50* (XY only)
6 000/3 200*/16 000*

Table size(mm)510 X 1 000

630 X 630

No. of tools(tools)

15/24 */40 *40 /80*/120 */238*

Note 1: Only representative models are shown.
M-V70, M-H80. Note 2: option

series includes M-V50,0.2 mm*

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)*1 Machine Tool Division*2 Takasago Research & Development Center, Technical Headquarters1

: Circle shows
quadrants
error areas.

True position

201510Y-axis direction

YX

50-5-10-15-20-1001020

X-axis direction
Fig. 3 True position of dowel pin holes

mal expansion and contraction of structural parts is compensated by use of temperature sensors and algorithms
which correct for thermal displacement. As a result, forthe machining accuracy for dowel pins, which is very
important for automotive parts, 20 μm diameter true
position can be obtained as shown in Fig. 33.
2.4 Innovation of design technique
The technologies described above are not only basedon our experience and know how accumulated throughdecades as a machine tool manufacturer, but also are aby-product of research and development at the Aerospaceand Power Systems Divisions of MHI. This engineering
support system gives us a competitive advantage in advancing state-of-the-art technology in our machine tooldesigns.
The Σ series had been totally designed by using three
dimensional CAD, with computer simulations developed
through our engineering support system, such as structural analysis, spindle rotor dynamics analysis,
characteristics analysis of non-linear element feed systems, thermal distortion analysis of machine structure,and cooling effect simulations by thermal fluid mechanics analysis.
To support these computer simulations, actual testing using full-scale models is frequently performed.
Thus, reliable products are developed by a combinationof digital models and real models.

Fig. 2 Quadrant error in circular motion

and simply increasing rigidity of the guide ways will increase friction, thereby further increasing positioning
error. The hydrostatic guide way technology was employed for the Σ series in order to satisfy these mutuallycontradictory demands. Rigid box ways combined witha thin film of incompressible hydrostatic oil results intremendous dynamic rigidity yet extremely low frictionof linear motion, with no stick-slip. As a result, whenmachining a circular motion at 50 m/min, which is themaximum feedrate of the class, the machining error occurring when reversing the moving direction, known as
quadrant error, is held within 3 μm as shown in Fig. 22,
and accurate circle machining is realized. Moreover,owing to the high rigidity of the guide ways, cutting performance for difficult-to-cut materials such asheat-resistant alloys like Inconel 718 is greatly improved,and chipping of tools experienced with conventional
machines is eliminated, extending tool life by more than
two times.
2.3 Measures against thermal distortion
Σ series machining centers have effective ways toeliminate thermal distortion, which is another major
factor influencing machining accuracy. Basically, thermal distortion occurs when temperatures of parts are
not uniform. For our Σ series, heat insulating covers
are used to isolate the machine from environmental temperature fluctuations and major internal heat sources
are cooled by an oil circulation system. Additionally, fanscirculate air through ducts in the machine castings tomaintain uniform temperature throughout the entiremachine. This eliminates thermal deflection, which isdifficult to compensate. In addition, the uniform ther-3. Conclusions
MHI is determined to further advance of the latest
technologies and will continue to introduce newly improved machining center designs which address theneeds of our customers in meeting the increasing demands of a global economy.

Toshio Yokomoto

Kensuke Ide

Keiji Mizuta

Chiaki Yasuda

Tanehiro Shinohara

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)2