Mitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
97

Latest Technologies of MVR-Eχ 5-face Machining
Center for Realizing High-precision Cutting
KENJI KURA*1

KEIICHI NORIYAMA*1

HIROYASU ARUGA*2

TAKAYUKI KUME*1

KOJI IWATA*1

SHINGO NAKAMURA*3

The challenge in the high-precision cutting of large workpieces is the reduction of heat
deflection on machine tools caused by external and internal temperature changes. There is now aneed for a new high-precision cutting technology, as existing heat deflection correction technology
for dealing with the heat generation of the spindle no longer meets the need for higher precision.
Mitsubishi Heavy Industries, Ltd. (MHI) developed the "MVR-Eχ" double column machining
center. The MVR-Eχ realizes high-precision cutting with easier operation thanks to the internal
spindle cooling system and thermally stabilized columns, which were installed as standard on adomestic product for the first time. As well as developing an operator assist system and adding a
preventive maintenance system, we addressed improvements in the exterior design and made themachine take on a look that gives the impression of high precision.

|1. Introduction
As are typified by the automotive and electric appliance industries, more large die moldworkpieces are being produced overseas. There is also a move, however, to return production toJapan, and therefore there is an increased need for stable, high-quality die mold manufacturing. In
addition, the demand for machine tools that can attain the machined surface quality level that
requires no polishing has also been increasing, as has the need for improvement in the accuracy andquality of the machined surface, such as the reduction of stepping caused by thermal displacement,
etc. Against this backdrop, MHI released the MVR-Eχ large double column machining center,
which is equipped with the high-precision technology adopted for the recently released LH250
high-precision machining center. This paper presents the technological features of the MVR-Eχ,
which realizes high-precision machining, and also details a cutting case example.

|2. Latest technology for high-precision cutting
2.1

PurposeIn the manufacturing of large die molds that last up to several dozen hours, the thermalelongation 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.

2.2

Internal spindle cooling

The MVR-Eχ series uses internal spindle cooling technology 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 in high-speed rotation and the heatgeneration of the bearings to establish a structure that allows high-precision cutting without heat
deflection correction. In addition, it uses efficient spindle cooling technology to increase thepreload of the spindle bearing in low-speed rotation for the establishment of a structure thatwithstands rough cutting. In this way, the high-precision spindle achieves both low-speed heavy*1*2*3

Chief Staff Manager, Machine Tool Division, Machinery, Equipment & Infrastructure
Chief Staff Manager, Yokohama Research & Development Center, Technology & Innovation Headquarters
Machine Tool Division, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)
98

cutting and high-speed, high-precision finishing.

2.3

Thermally stabilized column

The MVR-Eχ series is equipped with a thermally stabilized 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 suchas the inclination and shrinkage of the column that cannot be corrected by thermal correction. For
example, changes in perpendicularity between the x axis and the z axis 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.

2.4

High-speed cutting contour control (FM control)

For cutting high-precision die molds, it is not enough to just reduce mechanical errors, andthe correction of errors resulting from time lags of the controls is required. MHI uses uniquehigh-speed cutting contour control (FM control) to attain high-speed and high-precision contourcontrol. It is generally thought that an NC control machine tool such as an NC milling machine
traces the exact path directed by a program, but actual operation has errors in the path caused by theservo characteristics of the feeding shaft. These errors increase when the curvature of the path islarger or the speed is faster. For the positioning or linear processing of milling, this causes noproblems, but for die mold processing containing curved processing, however, it results in thedilemma of increased cutting speed worsening the accuracy of the product shape, thereby
necessitating a decrease in the cutting speed for an accurate cutting shape. The FM system is basedon the design concept of not decreasing, but rather eliminating completely the shape error, andsolves this problem by using a method in which no cutting shape errors occur even when thecutting speed is increased.

|3. Development of Two-stage Electric Turbocharging System
3.1

Features of operation screen

Together with the performance, the operability is enhanced through the introduction of
cutting support functions for the entire cutting process including machine operation, reflecting
customer needs. The operation screen is a large 15-inch monitor, and touch screen operation ismade possible in addition to the conventional hardware switch operation. The multi-screen allows
the simultaneous display of different pieces of information including positional information andtool information. This reduces the number of screen changes and touches, reducing the burden onoperators. Operators can select and set what information is to be displayed on the multi-screen,
enabling them to check necessary information according to the use conditions on a single monitor(Figure 1).

3.2

Preventive maintenance function

The majority of inquiries about machines from customers are regarding recovery methods
and how to deal with alarms. In response to this, a maintenance screen is provided in order to allow
customers to solve problems by themselves. For the recovery of existing machines, the customer
needs to perform recovery work while following the manual. On the other hand, the MVR-Eχ
highlights the relevant points using 3D drawings and allows the operator to perform recovery justby making simple selections on a screen and operating dedicated hardware switches (Figure 2).

Figure 1 Screen with improved operability

Figure 2 Visually understandable
recovery screenMitsubishi Heavy Industries Technical Review Vol. 52 No. 1 (March 2015)99

In addition to alarm display, the MVR-Eχ indicates causes and countermeasures.
Furthermore, the locations of the components that cause an alarm are indicated visually using 3Ddrawings. Accordingly, the clarification of alarms and fast maintenance and recovery are madepossible.

|4. Cutting case example
This chapter shows the result of the machining of an A5052 test piece. This test cutting wasaimed at verifying the effects of the heat deflection suppression structure of the MVR-Eχ using thehigh-precision spindle and the thermally stabilized column. The test cutting was performed without
any electrical correction of heat deflection such as spindle thermal elongation correction, whilechanging the spindle speed from 1000 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, thecutting was performed within 30 minutes 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 a high-precision machined surface with a small amount of stepping up to 1
μm even under the conditions of the test environment where spindle thermal elongation tended tooccur (Figure 3).

Figure 3

Case example of high-precision cutting (vertical shaft cutting)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, side face cutting of an aluminum test piece was performed with a right head
(horizontal spindle) attached. This resulted in a small amount of stepping of 8 μm between twomachined surfaces of machining passes that were performed with an interval of several hours evenwhen the outside air temperature of the machine peripherals changed by 5.5°C over 18 hours.
Accordingly, the structure of the machine with which heat deflection and stepping of the machined
surface caused by changes in outside temperature are much less likely to occur was verified(Figure 4).
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. In fact, the effects of the high-precision spindle and thermallystabilized column were verified in the cutting of a die mold model sample that lasted more than 40hours. This die mold model sample cutting resulted in a small amount of stepping between 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 (Figure 5).

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Figure 4

Case example of high-precision cutting (horizontal shaft cutting)Figure 5

Effects of high-precision spindle and thermally stabilized column|5. Conclusion
This paper presented the features, performance, and a cutting case example of the MVR-Eχ
double column machining center, which realizes high-precision die mold manufacturing.
High-precision cutting is attained by suppressing the stepping of the machined surface through thesignificant reduction in heat deflection of the spindle and the mechanical components. This will
help eliminate manual finishing. By understanding the need for the accurate machining of large,
integrated die molds in the automotive and electric appliance industries, we will enhance theapplication and lineup of the machine, and meet the needs of users aiming at the advancement of
their facilities. By doing so, the MVR-Eχ and the previously-released LH250 high-precision double
column machining centers will contribute to social progress.