High-speed & High-accuracy
Precision Die Milling Machine:
MVR-FM Developed for
Die & Mold Manufacturing as
Base of Mass-production
HIROYUKI YAMAMURA* 1
KOJI IWATA* 1

MUTSUMI YOSHIKAWA* 1
KEIJI TOMIMATSU* 1
KENJI TSUMURA* 2

Die milling machines, with higher performance (speed, precision, etc.) and reasonable cost, are required for reduction of milling time and polishing process. Mitsubishi Heavy Industries, Ltd. (MHI) has developed a die milling machine MVR-FM equipped with FM control system (MHI's proprietary high-speed, high-accuracy contouring controlsystem). The new milling machine is also equipped with high-precision feed system using X-axis twin-ball screw and Zaxis direct drive, and 12 000 min-1 high-speed spindle with low vibration. The new machine is designed based on portal
table moving type machining center MVR. Hopefully, the newly developed machine will contribute to the development
of die mold milling, the base of industry.

1. Introduction

2. Technologies for realization of high-speed, high-accuracy milling

There are increasingly strong demands for reduction of die manufacturing cost and delivery term. Inorder to satisfy to such demands and to win in the competition with the rival manufacturers, reducing themilling time and the polishing process are indispensable. For realization of reduced the milling time (=high-speed milling) and the polishing process (highprecision milling) MHI has developed a new die millingmachine MVR-FM with a high cost value based on
MHI's portal table moving type machining center,MVR.
This paper introduces MVR-FM, giving description
mainly of the technologies used in the machine for realization of high-speed, high-accuracy milling.

2.1 Machine configuration
As shown in the photograph above, the table travels
back and forth (along X-axis) on a longitudinally arranged bed. Further, the crossrail installed on the front
guide of the column bridge composed of two pieces of column and bridge travels up and down (along W-axis). A
saddle travels at the front of the crossrail left and right(along Y-axis), and the ram that passes through thesaddle travels up and down (along Z-axis). Each axis issubjected to precision drive by means of ball screw andlinear scale. The main spindle is located at ram end tocut the work piece mounted on the table.
2.2 X-axis twin-ball screw drive
The heavy work mounted on the table is driven by ballscrew in the portal table moving type machining center.
However, deflection or torsion occurs in the ball screw system at the time of acceleration/deceleration because of
the inertia of the work table. The deflection or torsion
causes error against positional command at acceleration/deceleration, leading to tiny damage of the die millingsurface. Further, the driving force induces vibration of
the ball screw system, causing disordered milling pick on
the die milling surface. Since a strong rigidity is required
for the X-axis drive system in die milling machine MVRFM, the table is driven by using two pieces of ball screw
and drive motor as shown in Fig. 1
1. This leads to the rigidity of the drive system to grow two times stronger.
Moreover, compared with the case where the rigidity isimproved by increasing the ball screw diameter, the new
method restrains the increase of drive system inertia, ensuring high controllability and reducing the error.

Fig. 1 X-axis twin ball screw
Indicates two pieces of ball screw installed at the rear end of the bed.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)*1 Machine Tool Division*2 MME Corporation

12.3 High-precision ram direct drive
In die mold milling of automobile outer panels suchas bonnet, door, etc., a periodic fluctuation such as meshof gear in the Z-axis drive system causes pattern on thedie mold surface. Hence, the drive motor and the ballscrew are directly connected in the MVR-FM, and theoil pressure balancer is installed to compensate the ram
weight, realizing a high-precision drive.
2.4 High-speed and low-vibration spindle ensuringhigh productivity
The MVR-FM is equipped by standard with a 12 000
min-1 high-speed spindle. Connected directly to the builtin motor, the spindle has low vibration and less torquefluctuation, contributing to the reduction of die mold finishing time and improving the quality of the die moldmilling surface.
2.5 High-speed and high accuracy control (FMcontrol)
In order to carry out high-accuracy precision die milling it is necessary to correct the errors caused by control
system lag in addition to minimize the errors of the machine system. The MVR-FM is equipped with originalhigh speed and high accuracy machining control (FMcontrol) system.
The NC controlled machine tools such as NC millingmachine, etc. are generally considered to move correctly
depending on programmed command. In actual practice,
however, some shape error exists caused by the lag of thefeed axis servo system. The error amount is becoming
larger for larger curvature of shape and higher machining speed. This makes no problem for positioning andlinear machining such as milling, drilling. But in die milling (free curved machining), the shape gets deformed whenthe machining speed becomes high and the speed has tobe slowed down to get precision-machining in terms ofshape. Thus, one has to face the dilemma.
There is a method called feed forward control that
reduces the shape error to try to solve the aforesaid problem. In the case of the MVR-FM, however, the designconcept is based on making the shape error amount zerorather than simply reducing it. The machine causes noshape error in machining even when the machining speed
is increased.
Fig. 2 shows a block diagram of the feed forward system that improves the shape error reducing the lag ofservo characteristic with speed feed forward and acceleration feed forward. The inward shape error exists infree curved machining. The inward shape error (ΔR ) is
directly proportional to the square of machining speed,
and indirectly proportional to the virtual radius ( R ) as
expressed in the equation;

The constants are;T 1: acceleration/deceleration time constant
T 2: determined positional loop gain and feed forward factor

Speed feedforwardNC data

Acceleration
feed forward
Feed motor

NC
Commandposition
Feed forward control

Feedback speed
Pulse coder

Feedback position

Servo-amp

Linear scale
Transfer function = G(s)Linear scale position = Command position

Transfer function G (s)Command position

Fig. 2 Block diagram of feed forward control

Feed motor
NC dataNC
Commandposition

High speed andhigh accuracy control

Inverse transferfunction (1/G (s))
Feedback speed

Servo-amp

Feedback position

Pulse coder
Linear scale

Transfer function = G (s)Linear scale position = Command position x Inverse transfer function 1/G (s) x Transfer function G (s) = Command position

Fig. 3 Block diagram of high speed and high accuracy control (FM control)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)2

Hence, the machining speed had to be slowed down
more when the virtual radius was smaller (i.e. the curvature was larger).
Fig. 3 shows a block diagram of the high speed andhigh accuracy control using inverse transfer function thatcancels the servo system delay and reduces the shapeerror to zero. With the transfer function of the servo system supposed to be G(s), when the command position
applied with the inverse transfer function 1/G(s) is transmitted to the servo, the result obtained by further
applying the servo transfer function G(s) becomes themachine position (= linear scale position).
After that,machine position = command position x 1/G(s) x G(s)= command position
It is possible to get free shaped machining with zero
shape error being independent of the shape curvatureor machining speed.
The basic theory is as mentioned above, but for practical use, following functions have been added to the machine.
(1) Multi-block acceleration/deceleration function
In die milling using a fine segment program (a dataobtained by approximating the free curved surface withshort segments), the speed gets slowed down before
attaining the target level in case the number of processing blocks is limited. Hence, acceleration/
deceleration has to be carried out over enough multiple
blocks in order to constantly obtain the data equivalent
to the travel distance required for acceleration.

(2) Automatic shape follow-up speed control function
When the acceleration in a corner or in a shapewith extremely large curvature (i.e. with extremelysmall virtual radius) exceeds the permissible level, it
causes vibration and deteriorates the machined surface precision. NC program is pre-read and themachining shape is identified in FM-control. The opt imu m f eed r at e is au t omat ic ally d e ter minedaccording to machining shape by detecting the cornerof machining shape and calculating the curvature
based on the allowable acceleration calculated fromthe rigidity of the machine.
(3) Smoothing function
When the NC program sampling is rough, thekinked points of the blocks may appear on the cutsurface. This function can achieve high-quality machined (smooth cut) surface and high speed machining
by interpolating the fine segment program.

3. Example of die mold milling
The portal table moving type die milling machine like
MVR-FM is often used for press dies mainly for automobiles, and especially the die of automobile outer panels
requires high-quality machined surface and high-speed
machining.
Fig. 4 shows the die for rear trunk of a hatchback car(milling at spindle speed 12 000 min -1 and feed speed 8m/min), indicating the milling surface with ordered milling pick. The reciprocating milling pick depth difference(reciprocating level difference) as index of milling surface quality has been improved by 50% as compared withconventional MHI machines and the milling time has
been improved by more than 30%.
Fig. 5 shows the die for automobile inner panels,
which does not need the milling surface quality and themilling speed as high as that of the automobile outerpanels. However, the die for automobile inner panels
consists of the complicated shapes. Various attachments(MVR-FM universal head etc.) and precision positioningof cross rails to an arbitrary place is possible, ensuringmachining freedom for realization of high productivity.

Spindle: 3 200 min-1
Cutting feed:1.6 m/min
20 carbide
ball end mill
Pick: 0.4 mm

Spindle: 12 000 min-1
Cutting feed: 8 m/min
30 ball end mill
Pick: 0.5 mm

Fig. 5 Example of the die for automobile inner panel
Indicates an example of the die for automobile outer panel
showing complicated shapes.

Fig. 4 Example of the die for automobile outer panel
Indicates an example of the die for automobile outer panel showing
high-quality milling surface with ordered milling pick.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

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Table 1 List of main specifications
MVR 25-FM

MVR 30-FM

MVR 35-FM

Table working range; width
length

(mm)
(mm)

1 500
3 000 - 5 000

2 000
3 000 - 5 000

2 500
4 000 - 5 000

Distance between columns

(mm)

2 050

2 550

3 250

Each axis travel; X: Table, longitudinal

Each axis rapid
traverse rates;

Table length + 200

Y: Saddle, crosswise
Z: Ram, vertical (mm)
W: Crossrail, vertical

2 500700800

X: Table, longitudinal
Y: Saddle, crosswise
Z: Ram, vertical (mm)
W: Crossrail

30301033 000

3 500
700
1 100222210
3Maximum cutting feed rate(m/min)

XY-axes 15, Z-axis 10Permissible mean feed rate(m/min)

6ATC Number of tools
Maximum tool length
Maximum tool weight
Tool change time(piece)
(mm)
(kg)

50
Standard: 400 (Optional: 500)25
5 sec (6 sec)

ATT Mountable ATTStandard: Dummy plate 30 kW/12 000 min-1
RH30 kW/6 000 min-1
OP: Extension
22 kW/6 000 min-1-1
OP: Universal head 15 kW/5 000 min Every 5 deg. Automatic indexing, OP: Every 1 deg

AAC rack

For 2 ATT + Additional rack

NC device

FANUC 18 i

4. Specifications
Thanks to the original FM control technology, etc., theMVR-FM can be used for milling of diverse die molds including the die for automobile panels. Machines with tablesize ranging from width: 1.5 - 2.5 m and length: 3 - 5 m
and various optional functions are available to meet thee 1.
customers' needs. Main specifications are given in Tablable

5. Conclusions
High accuracy for NC data was realized by adopting
MHI's own FM-control technology and high-accuracy,
high-rigidity feed system. The quality of milling surface(reciprocating level difference) is improved by 50% compared to the conventional MHI machines, and the diepolishing process can be reduced. The milling time forouter panels is reduced by 30% after the above-mentioned
feed system and the 12 000min -1 high-speed spindle areadopted. High productivity for die manufacturing is realized.
Hopefully, the high-speed, high-accuracy precision die
milling machine MVR-FM with excellent cost performance will contribute to the development of die and moldmanufacturing as the base of industry.

Hiroyuki Yamamura

Mutsumi Yoshikawa

Keiji Tomimatsu

Kenji Tsumura

Koji Iwata

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)

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