Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
61

Features of High-precision Double-column MVR-Eχ
Machining Center and Cutting Example

Equipment & Machinery Sales GroupS a l e s D e pa r t m e n t
M a c h i n e To o l D i v i s i o n
M a c h i n e r y, E q u i p m e n t & I n f r a s t r u c t u r e

Large die molds are necessary for the production of large parts used for automobiles, electric
appliances, etc., and are increasingly produced overseas. With an increased need for stable,
high-quality die mold manufacturing, however, there is also a move toward return production toJapan. In addition, for today’s machine tools for molds, higher precision in terms of machined
surface quality, such as that at the level that needs no polishing and reduces the stepping caused bythermal displacement, etc., is required.
Against this backdrop, Mitsubishi Heavy Industries, Ltd. released the MVR-Eχ large double
column machining center, which is equipped with the high-precision technology adopted for therecently released LH250 high-precision machining center (Table 1). This paper presents thetechnological features of the MVR-Eχ, which realizes high-precision machining, and also details acutting example.
Table 1Main specifications of MVR-Eχ series (standard specification machine)

Item/Model type

MVR25Eχ MVR30Eχ MVR35Eχ MVR40Eχ
Width
1,500
2,000
2,500
3,000(mm)Workarea
Table
Length3,000
3,000
4,000(mm)Loading mass (t/m)
12/3.020/3.025/4.0Distance between columns (mm)
2,050
2,550
3,250
3,750
Distance between spindle end and1,650
1,850
workpiece mounting surface (mm)Ram size (mm)350 x 350
SpindleSpindle speed (r/min)
20 to 8,000
head
Spindle motor output
22/30 (continuous/30 minutes)(kW)
Number of ATC tools
50
Machine weight33.5/3,000 33.5/3,000 51.6/4,000 59.6/4,000(tons/work area length: mm)

MVR45Eχ
3,500
6,000
35/6.04,250

98.2/6,000|1. Features of MVR-Eχ series
Because the manufacturing of large die molds can take up to several dozen hours, the thermal
elongation of the spindle caused by heat generation from its rotation and the heat deflection of themachine peripherals due to temperature changes result in the stepping of the machined surface.
This hinders the elimination of manual finishing. Although a thermally symmetrical structure orheat deflection correction technology is typically adopted for large machine tools, no correction
technology can correct the inclination of the machine or the tool. As a result, technology
improvement that does not overly depend on heat deflection correction is required in order toeliminate manual finishing. To address this, the MVR-Eχ series uses internal spindle coolingtechnology and lubrication optimizing technology that have been cultivated with the LH250
high-precision double column machining center to suppress the thermal elongation of the spindle inhigh-speed rotation and the heat generation of the bearings to establish a structure that allows

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high-precision cutting without heat deflection correction. In addition, it uses efficient spindle
cooling technology to increase the preload of the spindle bearing in low-speed rotation for theestablishment of a structure that withstands rough cutting. In this way, the high-precision spindle
achieves both low-speed heavy cutting and high-speed, high-precision finish cutting.
Also with regard to the machine body, the MVR-Eχ series is equipped with a thermallystabilized column as standard equipment for high-precision cutting without the need for heat
deflection correction. The column suppresses the heat deflection due to changes in the outside
temperature, and even reduces heat deflection such as the inclination and shrinkage of the columnthat cannot be corrected by thermal correction. For example, changes in perpendicularity between
the x axis and the z axis against a change in the ambient temperature of 7°C are suppressed to themaximum limit of 6 μm per 500 mm to attain a structure that is not subject to environmental
temperature changes in the machine installation location. Meanwhile, the rigidity of the main
structural components such as the saddle and the cross rail is enhanced to realize high-power andstable cutting even with the ram fed at 800 mm (Figure 1).

Figure 1

Example of heavy cutting|2. Cutting example
This chapter shows the result of the high-precision cutting (vertical shaft cutting) of an
A5052 test piece (Figure 2). This test cutting was aimed at verifying the effects of the heat
deflection suppression structure of the MVR-Eχ using the high-precision spindle and the thermallystabilized column. The test cutting was performed without any electrical correction of heat
deflection such as spindle thermal elongation correction, while changing the spindle speed from1000 min-1 to 8000 min-1 in order to intentionally generate spindle thermal elongation. To confirm
the stabilization of spindle thermal elongation in a short time, the cutting was performed within 30minutes of changing the spindle speed. Typically, a relative large heat deflection occurs in a change
from low speed to high speed or from high speed to low speed, but the test resulted in ahigh-precision machined surface with a small amount of stepping up to 1 μm even under theconditions of the test environment where spindle thermal elongation tended to occur.
In addition, for the confirmation of the reduction in heat deflection (in particular the steppingof the machined surface caused by column inclination) resulting from changes in machine
peripheral temperature, high-precision side face cutting (horizontal shaft cutting) of an aluminum
test piece was performed with a right angle head (horizontal spindle) attached (Figure 3). This
resulted in a small amount of stepping of 8 μm between two machined surfaces of machining
passes that were performed with an interval of several hours even when the outside air temperatureof the machine peripherals changed by 5.5°C over 18 hours. Accordingly, the structure of themachine with which heat deflection and stepping of the machined surface caused by changes inoutside temperature are much less likely to occur was verified (Figure 3). In addition, the virtual
lack of change in perpendicularity allows stable high-precision cutting, and even perpendicularity
and accurate hole pitch in the five-face machining of a general workpiece cutting can be attained.

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Figure 2

Example of high-precision cutting (vertical shaft cutting)Figure 3

Example of high-precision cutting (horizontal shaft cutting)

In fact, the effects of the high-precision spindle and thermally stabilized column wereverified in the cutting of a die mold model sample that lasted more than 40 hours (Figure 4). Thisdie mold model sample cutting resulted in a small amount of stepping between finished surfacesmachined with multiple tools having various diameters, suppressed the stepping of the connection
between finished surfaces (where stepping tends to occur) to 1.5 μm at most and realized theelimination of manual finishing due to a high-quality machined surface that had no disorder of die
mold dimensional accuracy and surface-machined shape.

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Figure 4

Effects of high-precision spindle and thermally stabilized column|3. Attachments for realization of various types of machining
For supporting the various types of cutting of customers, the MVR-Eχ can mount a wide
variety of attachments (Figure 5). In addition to a right-angle attachment for double-column
machining, an extension head for the improvement of accessibility to the workpiece and a universal
head optimized for tilted surface machining are also available. In addition, the MVR-Eχ can satisfy
the needs of various sites such as a reduction of the lead time, automation, high-precision
machining, etc., by utilizing a high-speed universal head optimized for high-speed finishing of a diemold, a 5-axis head that enables curved shape machining of aircraft parts and railway parts andother attachments.

Figure 5

Lineup of various attachments