High - S p e ed High - Rigi d it y5-Face Milling Machine MVR-Dx
HIROYUKI YAMAMURA*1

MUTSUMI YOSHIKAWA*1

1

HIROYUKI TAUCHI*1

TAKAYUKI KUME*

YOSHIFUMI TANIGAWA*1

In recent years, in the field of large part machining, there has been an increasing need for versatile machines on which the user
can use both high-feedrate tools and small-diameter drills and taps in machining, in addition to those designed for more efficientmachining and those with even higher heavy cutting capabilities. To address this need with a portal 5-face milling machine,machine manufacturers must satisfy conflicting requirements: to provide the machine with a robust structure and a high-rigidity,
high-torque spindle that enable heavy cutting and, at the same time, the high-speed performance to support high-feedrate tools andthe capability to rotate the spindle at high speeds appropriate for small-diameter tools. To meet this challenge we utilized proprietary
analysis technologies and developed and released the MVR-D x, which offers more machining capabilities than conventional
machines provide.

1. Introduction
Backed by the recent increase in demand by countries,
especially the BRICs, power system, industrial machine,
constr uction machine, and other heav y industries areenhancing their facilities to boost the potential need for highefficiency machinery.
To meet this need, machine manufacturers must develop
machines applicable to heav y and deep cutting as withconventional machines, yet allowing the use of a highfeedrate cutter in heavy cutting, and providing high spindle
speeds appropriate for small-diameter tools.
Obviously, it is also essential to further improve the rapid
traverse rate in order to reduce the non-cutting time.
To realize this, technologies that satisfy the following
three requirements must be developed:• High rigidity and light weight structure.
• Spindle with high rigidity even in the low-speed range, yetalso runs at higher speeds.
• Feed system with high thrust that supports high-speed
feeds.
The following describes the development of the MVRD x , focused on t he technical approach to meet t hese
requirements.

Saddle, horizontal (Y)Ram, vertical (Z)Crossrail, vertical (W)Table, longitudinal (X)Fig. 1 Machine construction(W-axis) and the vertical movement of the ram (Z-axis) thatruns through the saddle, while the horizontal movement ofthe spindle (Y-axis) is generated by the saddle that moves infront of the crossrail in a horizontal direction.
A workpiece is loaded on the table that moves along theX-axis.
Each axis is driven accurately by the combination of a ballscrew mechanism and a linear scale. The spindle is rotated
via the gear box located in the upper portion of the ram to cut
the workpiece loaded on the table.

2. Machine construction

3. Machine specifications

As shown in Fig. 1, the table moves inside the portal
structure in a longitudinal direction (X-axis).
The portal structure consists of the column bridge and thecrossrail and, in front of the crossrail, the ram saddle withthe spindle is located. The vertical movement of the spindle
is generated by the vertical movement of the entire crossrail

In determining the specifications of the MV R-D x , weadopted a modula r desig n approach. We est abl ishedspecifications that satisfy all customer needs and dividedt hem i nto st a nda rd, opt iona l , a nd c ustomer- spec i f ic
categories to modularize and standardize components.
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Technical Review Vol. 45 No. 3 (Sep. 2008)*1 Machine Tool Division29

Table 1 Major specifications

Table working surface (mm)Width
LengthTable longitudinal

X-axis

Saddle, horizontal
Effective travel

Y-axisRam, vertical

Z-axis

Crossrail, vertical

W-axisSpindle rotational speed(min-1)

Spindle motor output(kW)MVR28/34Dx
2 000

MVR33/39Dx
2 500

MVR38/44Dx
3 000≥4 000Travel of each axis (mm)MVR43/49Dx
3 500≥5 000

Table length + 1 000
4 300
3 400

4 800
3 900

5 300
4 400

6 000
4 900

Standard 1 000/Optional 1 200
1 200−3 200(Dependent on column height)

2 200−3 200(Dependenton column height)

Spindle specifications
7−4 00037 (continuous) /45 (30-minute rating)
Feed rate of each axis (mm/min)
Table longitudinal

Rapid traverse

X-axis

Saddle, horizontal

Rapid traverse

Y-axisRam, vertical

Rapid traverse

Crossrail, vertical

Rapid traverse

Z-axisW-axis

≤24 000
15 000Standard 3 000/Optional 5 000

5 000

X Y Z 10 000W 3 000 5 000

Maximum cutting feed
Number of tools in ATC
Maximum tool length
Maximum tool mass

≤20 000
30 000••≥60Standard 500/Optional 800
30(mm)(kg)

ATT (Attachment)
Supported ATT•Standard: Dummy plate
Right angle head (30 kW),
universal head, and various other optional attachments are available

NC system

FANUC 31i

We also tried to offer the machine in various sizes to meet
diverse customer needs (table width from 2,000 to 3,500 mm,
table length from 4,000 to over 10,000 mm, loading capacity
up to 100 tons, and column height from 2,050 to 4,050), as
well as to reduce the delivery time. Table 1 shows the major
specifications of the MVR-Dx.

cast materials and utilized all our analysis technologies toimprove the dynamic rigidity.
4.2 Determination of target static and dynamic rigidity
values
The relationship between a machine's cutting capacity
and the static and dynamic rigidity values is still unsolved.
Therefore, we collected the cutting capacity and static/
dynamic rigidity data from the various types of machines we
had ever manufactured and organized their relationships into
a database so that the target static and dynamic rigidity canbe determined based on the target cutting capacity.
The target static and dynamic rigidity values for the MVRDx were determined based on this know-how.
4.3 Process for determining structure shapes that realize
high rigidity

4. Technologies for achieving high rigidity
4.1 Selection of optimal structural materials
A machine requires robust structures with high rigidity
to exhibit high cutting performance. Although “rigidity”
in this sense normally represents static rigidity, the highrigidity required for a machine also includes the resistanceto vibration, which is high dynamic rigidity.
To design a v ibration-resist ant str uct ure, the threefactors that govern vibration, “spring constant,” “dampingcoefficient,” and “mass,” must be well-balanced. Althoughconventional machines use welded structures made of steel
with a high elastic coefficient, through the use of FEM
analysis, it has become possible to minimize the increase in
the mass and provide static rigidity equivalent to that of awelded structure even with a cast structure.
For structures such as the ram, saddle, and table, we usedcast materials as before and made them as light as possible
while maintaining their rigidity to support higher spindle
speeds. However, for the crossrail, columns, and bridge
bed, we changed from conventional welded structures toConventionally, the shape of each machine structure
had been first determined by seasoned engineers who arefamiliar with where machine weak points are located basedon their experience and hunches, and then verified throughFEM analysis that it has the target rigidit y. However,using this method, it had often been impossible to attainthe target rigidity in a single attempt, which prolonged thedetermination of structure shapes.
We considered t hat t his is due to t he di f f ic ult y i nsimultaneously taking into account many conditions that
determine the rigidity and, as a solution, adopted the methodof determining the basic machine dimensions based on
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Technical Review Vol. 45 No. 3 (Sep. 2008)30

quality engineering.
The use of quality engineering enabled us to cover all
dimensional patterns that can be designed, minimize therigidity variation caused by changes in the cutting direction
or position, and derive the basic dimensions that provide
light-weight structures with high rigidity.
Next, based on these basic dimensions, we preparedmodels of each structure and conducted FEM analysis on
all structural members. We increased the wall thickness,added ribs, or used other reinforcing measures for members
subjected to high stress and decreased the wall thickness,bore cast holes, and other weight-saving measures formembers subjected to low stress. Implementing this process
for all the structures enabled us to minimize the increase in
the mass and derive shapes that provide high rigidity.
Finally, we combined the determined structures, analyzed
the entire machine to check each portion was connected
appropriately, and fine-tuned each structure to the target
specifications to accomplish the final shapes. Using thismet hod, a lt hough t he ma in st r uct ures were cha nged
from a sheet metal construction to a casting, which has a
disadvantage in rigidity, we could minimize the increasein the mass and achieve the static and dynamic rigidity
equivalent to or exceeding that of conventional machines.
4.4 Rigidity of MVR-Dx
Figure 2 shows a comparison of the actual dynamicrigidity with the value from FEM analysis.
The graph shows that both the actual and analysis values
attained the target dynamic rigidity, with the frequency andthe peak height at the proper value matched to a satisfactory
degree.
In the analysis of dynamic rigidity, because it is essential
to consider various boundary conditions, estimating thedynamic rigidity with high accuracy has been difficult.
However, the accumulation of a substantial amount of knowhow enabled us to improve the calculation accuracy and, asa result, analyze dynamic rigidity with excellent accuracy
in the calculation stage. The synergistic effects of thishigh calculation accuracy and the engineering process thatrealized high rigidity have made the MVR-Dx a machine withan excellent rigidity balance.

20Compliance (dB)

10–10–20
10

5.1 Improvements for higher spindle speeds
A machine used for heavy cutting applications requires
a high-rigidity spindle that withstands large cutting loads
generated during machining. A lthough setting a large
preload on bearings improves the rigidity of a spindle, thisis not a straightforward way to achieve high spindle speeds
because setting an excessive preload substantially increases
the heat generated from the bearings.
Although, to solve this problem, it is necessary to conduct
an accurate thermal analysis and consider introducing abetter cooling method, usually, doing so is not easy becauseof the difficulty in accurately estimating the heat generated
from bearings, the coefficient of heat transfer between
parts, etc. However, with the know-how of accurate thermal
analysis developed through experience in various products,
as shown in Fig. 3, we minimized the temperature rise inbearings by efficiently cooling the bearings and also reduced
the temperature rise in the ram body, which is a major
cause of thermal displacement. Using this technique, we
successfully developed a high-precision and high-efficiency
spindle capable of cutting at any speed in the range of 7 to4,000 min -1.

Temperature risein ram body is minimized

Heat generated
from bearings is
quickly cooled

Fig. 3 Result of thermal analysis of spindle

Target compliance value
Little difference between
実測と解析の
actual and analysis values
誤差が少ないWeak0

5. Technologies for achieving high spindle speeds

5.2 Increasing Y-axis rapid traverse rate
For the Y-axis guide, the MVR-Dx uses a roller type linear
guide with high rigidity, which is suitable for high feedrates
and can suppress yawing of the ram and saddle when theY-axis is traveling. In addition, the capacity of the Y-axis feed
motor was expanded to further improve the conventionalY-axis rapid traverse rate (24 m/min, which is already fast)
to 30 m/min while maintaining the feed thrust.
This contributes to the reduction of the non-cutting time
without impairing the advantages of high-rigidity and highaccuracy.

Actual value
FEM analysis
Rigid305070
Frequency (Hz)90110Fig. 2 Comparison of actual dynamic rigidity of MVR-Dx withcalculated value
Accurate estimation is possible with excellent calculation accuracy.

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Technical Review Vol. 45 No. 3 (Sep. 2008)31

5.3 Z-axis feed using a twin feed motor/ball-screw system
We redesigned the Z-axis to use a twin feed motor/ballscrew system by eliminating the hydraulic-cylinder based
ram balancing system, although the rapid traverse rate of
the Z-axis is kept at 15 m/min, unchanged from the previous
model. In addition to doing this to improve the axis rigidity
during travel, we also paid attention to the layout of the ballscrews so that the axis is driven at the center of gravity.
This theoretically eliminates the moment caused by theunevenness of the force generated by the balancing systemwith that generated by the drive system, which contributes
to the development of a high-precision drive. Moreover,the elimination of the balancing system contributes to thereduction of power consumption because it allows the useof a smaller hydraulic unit, which is one of the heaviest
consumers of electricity in a machine tool.

(Cutting conditions)
Tool diameter: 250 mm
Cutting width: 190 mm
Infeed: 1.5 mm
Feedrate: 7 200 mm/min
Cutting speed: 220 m/min
Cutting capacity: 2 052 cc/min
Workpiece material: FC250
Fig. 6 Milling using right angle headMilling application using a high-feedrate cutter with right angle head
and its cutting conditions.

achieves a cutting capacity of 2,000 cc/min or more withthe right angle head, which proves the high rigidity of thismachine (Fig. 6).

6. Machining example

7. Conclusion

Backed by the high-rigidity construction obtained using
the technologies as described above, the MVR-Dx has a deep
cutting capacity of 1,250 cc/min with a φ250 cutter, whichis larger than that of conventional models (φ200), and with ahelical end mill, it achieves a performance improvement from
560 cc/min to 1,050 cc/min, almost twice as large as that ofconventional models (Fig. 4 and Fig. 5)
In addition, in machining applications using a highfeedrate cutter, which is becoming popular, the MVR-D x

The MVR-Dx uses a modular design approach to support
a wide range of machine specifications. By combining ourabundant analysis know-how and technologies, it satisfies
both high-speed and high-rigidity requirements to support
more diverse machining applications than ever.
We will continue to offer cost-effective machines thatmeet wide-ranging customer needs by utilizing our knowhow and technologies acquired through the development ofthe MVR-Dx.

(Cutting conditions)
Tool diameter: 250 mm
Cutting width: 200 mm
Infeed: 10 mm
Feedrate: 625 mm/min
Cutting speed: 200 m/min
Cutting capacity: 1 250 cc/min
Workpiece material: SS400Fig. 4 Milling
Milling application using spindle and its cutting conditions.

Hiroyuki Yamamura

Mutsumi Yoshikawa

Hiroyuki Tauchi

Yoshifumi Tanigawa

Takayuki Kume

(Cutting conditions)
Tool diameter: 63 mm
Cutting width: 31 mm (Radial)
Infeed: 60 mm (Axial)
Feedrate: 565 mm/mni
Cutting speed: 90 m/minCutting capacity: 1 050 cc/min
Workpiece material: SS400
Fig. 5 End Milling
End-milling application using spindle and its cutting conditions.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 3 (Sep. 2008)32