Ver tical Machining Center
DCV50 for Die Mold Cutting
TOSHIO YOKOMOTO* 1
SHIGERU HONDA* 1
YOSHIKATU SATO* 1

In order to hold advantage over the cheap Chinese products the die mold cutting industry in Japan where verticalmachining centers are mainly used has come up with manufacturing complicated high-precision die molds in a shortdelivery term. The machine tool manufacturers have responded to such demands by introducing machining centers
equipped with various complicated functions. However, they have still not been able to satisfy the customers. Paying
reconsideration to the machine concept based on the current requirements for die mold cutting, Mitsubishi Heavy
Industries, Ltd. (MHI) has successfully developed a Machining Center DCV50 simple in design but capable of stable,
high-precision cutting. The outline of the machine is introduced below.

As for the machining, the micro-machining using tools
with smaller diameters is increasing, causing the machining time using 1 piece of tool to get longer.
Accordingly, higher precision of machining and stability
performance for long-term continuous operation has been
increasingly demanded.
According to the present trend, the dissatisfaction of
the mold users in terms of the quality of machined surface and work shape is mostly attributed to theinadequate stability of the machine against the precision requirement.
1.2 Present trends in machine tool
In order to respond to the aforesaid demand for longterm machining precision stability performance, eachmanufacturer has adopted correction as a countermeasure against thermal displacement, a method used alsoin MHI ∑ Series.
The correction here refers to a method of making correction of axial travel using a certain corrective formula,with the temperatures near the heat source, the maincause of thermal displacement, as a parameter. The correction, however, differs according to the factory
environment, so that independent adjustment under theenvironment of each factory is necessary. In addition,
the higher the demand for precision is, the larger thenumber of measuring points becomes, practically making the control more complicated.
There is a limit to the control of correction. The precision including spindle, thermal displacement of theunit is generally limited to +/-5 μ m, falling far shortof the actual requirement from users for machining precision: + /- 2 to 3 μ m. Further high precision
requirement is expected in the future, calling for a solid
countermeasure.

1. Introduction
In response to the upcoming of China, the mold industry and high added-value parts machining industry
in Japan are struggling for survival by manufacturing
more complicated parts of higher precision in a shorter
delivery term. However, the machining precision andshort delivery term are the key factors to the capacity ofthe machining center.
The vertical machining center M-V∑ launched in themarket by MHI under the brand name of " Σ Series" isequipped with high-precision and almost all required
functions, so that the machine falls short of meeting thehigh-end requirements of the customers including thecost performance.
MHI has therefore developed a machining center
simple in structure but capable of long-term stable, highprecision machining equivalent to VΣ by reviewing theconventional functions in order to respond to the needsof the customers. This paper describes the needs of customers and the corresponding technologies thereof.
1.1 Current demands on die mold cutting
The works in die mold cutting in Japan have generally come to be smaller in size, while the precision anddelivery requirements are becoming stricter, so that theindustry is corresponding to the demands for high precision and short delivery term through integration ofarrangements, execution of machining needing no finishing by using high-speed spindle, automation of work
exchange and work control using ID chips, and by introducing various other functions. An increasing number ofmold manufacturers are responding to short delivery
term by purchasing the rough machined products fromoverseas and carrying out only finishing machining.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)*1 Machine Tool Division1

In order to ensure stability of machining precision essentially required in long-term machining, MHI has
made a re-study on the countermeasure, returning backto the original point and starting mainly with the heatgeneration.
2.2 Objective of the new function
2.2.1 Restrained heat generation through review ofspindle cooling circuit
The stability of spindle against heat during long-term
operation is extremely important in die mold cutting wheremachining is carried out continuously at a high-speed
spindle rotation over a long time period. Further, since correction only cannot meet the recent precision requirement,
the shortening of the time up to the thermal balance without making any correction plays a key role. Hence, thestandard spindle internal cooling circuit has been reviewed
in the vertical ∑ Series.
In the Σ Series where attention is focused to theZ-axis that inflicts biggest effect on the machining precision, i.e. the restraint (prevention) of spindle elongation
due to heat generation, a constant-temperature coolant
is circulated inside the spindle in synchronization withthe machine temperature to restrain the spindle elongation caused by heat generation. However, because of theconstant-temperature control inside the cooling unit, it isunresponsive to the spindle heat generation, causing follow-up delay to occur.
The new circuit adopts a control system for cooling down
only the heat generated in the spindle to restrain the heatgeneration. The result of pre-verification by using a spindleof 30 000 min-1 has clearly shown that the elongation causedby heat generation can be restrained to below a certain
level through synchronized control of the coolant discharged
from the spindle internal cooling circuit with the main bodytemperature by using the aforesaid circuit. Further, in order to improve the trackability (follow-up property) against
temperature, the temperature controller has been replacedwith the inverter control system capable of estimating thecooling capacity from the volume of heat generated in thespindle to follow up. Fig. 1 shows the spindle cooling circuit diagram.

2. Technology for long-term, stable precision
2.1 Key points of high-precision machining
With the controller and ball screw drive system becoming highly accurate, the difference in positioning
precision of feed mechanism is becoming less conspicuous among companies, so that the heat restraining
technology to maintain the long-term high precision isconsidered to be a technology for differentiating the company from the other companies.
The technology often uses two types of correction forthermal displacement correction: (1) the correction ofchange in machine posture (position) due to temperature change in the factory, spindle speed, etc. made in
the direction of feeding axis and (2) the correction of theelongation of the spindle itself caused by the heat due torotation of the spindle.
However, with the machining precision requirementfrom the users becoming stricter, the manufacturers havestill been unable to come up with guaranteed values after thermal displacement correction or the permissible
value after the thermal displacement correction is considerably high against the requirement. Thus, anestablished technology is still to come.
As a provisional countermeasure, therefore, there aremany cases where this correction is made invalid at thetime of high-precision machining. Instead, high-precision machining is carried out after maintaining thethermal balance of the machine through a long-termwarm-up operation.
Although the warm-up operation appears a waste oftime and contradictory when viewed from the standpoint of the demand for short-term delivery of molds,
in view of long-term machining of molds and taking into
consideration the re-machining of the mold due to defective precision at the finishing process inflicting large
effect on the delivery term, it can be regarded as an
unavoidable option for the users, considering thepresent levels of technical prowess of each manufacturer. However, improvement to this regard is an
imminent need.

Present system

New system

SensorSensorSpindleTank(constant temperature)

Spindletemperature
controller

SpindleTank

Spindletemperature
controller

Fig. 1 Main spindle cooling circuit

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

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DCV50 is equipped with a 20000 min -1 spindle. The
confirmation test for verification using the actual machine shows the elongation along Z-axis due to spindle
heat generation gets shortened approximately to onethird: from 65 μ m to 20 μ m when the spindle isoperated at the maximum speed of 20000 min -1 .
Furthermore, the time needed for reaching the stateof thermal balance by using the new system has beenshortened to 20 min. against 60 min. by using thepresent control system, with the time shortened to onethird. This indicates that a high-precision machining
can be obtained by carrying out warm-up operation for
Fig. 2
only one-third as long (Fig.
2).
Further, in the case of the conventional oil cooling
unit a surge of +/-2μm occurs even after the thermalbalance because of the effect of the valve switchingsystem of the cooling unit, which is directly linked tothe elongation/contraction of the spindle, causing surge
to appear on the machined surface. In the new inverter
system, the surge has been reduced to +/-1μm or under, leading to the improved quality level of themachined surface.
2.2.2 Restraint of thermal displacement throughreview of the countermeasure against thermal
displacement of the main body
The coolant circulating inside the spindle is simultaneously made to circulate the outer jacketof thespindle, resulting in stable thermal displacement of
the main body.
If postural change of the machine main body depends largely on the heat transfer to the main bodyfrom bearing, spindle motor, etc. caused by the spindle
rotation, the effect is larger when the spindle speed ishigher. In the case of Σ Series, therefore, the temperatures at the main points were measured to carryout successive correction of the heat transferred fromthe spindle.
However, because of the longer time taken due tothe heat-transfer delay and correction of spindle thermal balance, it took about 1- 2 hours to reach the stateof high-precision thermal balance. In the new control
system, however, MHI has been successful not only in
restraining the elongation of spindle but also the generation of heat from the spindle.
Further, the thermal displacement of the main bodycauses postural change depending on environmental
change in a factory, heat generation in the machineheat generation at the time of machining, etc., so that
it is necessary to minimize the effect of heat in orderto prevent the postural change from occurring.
As for the heat generated in the machine itself, it
is shielded with insulation board to prevent the effectfrom the machine control panel at the rear (of themachine), and the machine itself was entirely covered: Present cooling system: New cooling system
10.0
0.0

0

20

40

60

80

Elongation ( m)

-10.0
-20.0
-30.0

Balancing time

-40.0
Restrained to 1/3

-50.0
-60.0
-70.0

min
Z-axis thermal displacement
at rotational speed 20 000 min-1

Fig. 2 Thermal displacement of main spindle in Z-axis direction

with guard to protect from the environmental effect of
the factory.
As a result, the thermal displacement correction of
the main body has also been simplified as shown below. Correction has so far been made by means of thecorrective formula using the parameter of temperatures obtained from 6 sensors installed to the main
body. In the new system, however, because the heatgeneration from the spindle has been restrained, an
equivalent level of correction could be made by measuring temperatures only at 2 points instead of 6 points
in the conventional system.
The result is shown in Fig. 3
3. The graph shows thechange of spindle end measured for each direction
against the table of factory environment change within
the range of 8 o C for 24 hours. In the conventional system the environmental change range in the factory was
assumed to be 4 o C, while the 8 oC change range in the
new system is equivalent to the general change levelof a factory, thus realizing high-precision machining
in general factories and ensuring enhanced robustness
against thermal displacement.
Further, it is highly useful for the end users that asufficiently high-precision machining can be carriedout in a general factory without having to make an
expensive temperature-controlled room.
Moreover, from the standpoint of correction, it isbetter to have 2-point correction than 6-point control
if the same level of precision can be obtained since theerror-causing factor gets reduced. Thus, the new system has successfully responded to the requirement of
end users by maintaining the long-term high-precision
through restraint of heat generation.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 2 (Apr. 2005)3

oRoom temperature: 8 C change

Displacement correction ( m)

10
5
0
-5
-10
10
5
0
-5
-10
10
5
0
-5
-10

30

Corrective simulation

X-axis

Before control 5
After control 2 m

0

3

5

8

Y-axis12 15Time (h)

18

21

0
24

Before control 13 m

1
After control 5 m

0

3

5

8

Z-axis12
15Time (h)

18

21

0
24
Before control 11 m

1

After control 6 m
0

3

5

8

12 15 18
Time (h)

21

24

0
27

Room temperature
o

25
20

m

1

Temperature range 8 C
0

2

4

6

8 10 12 14 16 18 20 22 24
Time (h)Fig. 3 Correction of main body thermal displacement using 2-point temperature sensor

2.2.3 Improvement of feed axis
In DCV50, improvement is made in the Y-axis feed
guide system that carries out back and forth movementof the table in feed axis. In M-V Σ Series, the oil staticpressure slide system was adopted for Y-axis feed guidesystem in order to reduce the effect of feed axis characteristics caused by the change in the weight of themachining work. This system has been replaced with an
orthodox type surface slide system. The surface slide
system with simple mechanism has been adopted because
the maximum weight of work is 700 kgf (equivalent distribution) and the effect of moving spindle characteristics
due to change in work weight is within the error range.
Further, as a countermeasure against the rising oftable (front-up phenomenon) or the slide surface oil at
the time of feeding, regarded as a problem in the conventional surface slide type, the oil pressure has beenreleased by adopting gib, reducing the oil groove width
for slide surface oil supply from half size and making oil
groove pitch smaller. Consequently, the front-up level at
maximum feeding speed = 24 m/min has been reduced
from1/5 size decreased.
2.2.4 Evaluation of new function using actual work
Next, the long-term stable machining is confirmed bymachining an actual work.
The work in Fig. 44, called a "sample of fine level-difference machining," is put to character cutting formachine name using fine-deep machining after machining the wavy sample. The character cutting of themachine name DCV50 is carried out using deep cutting
on the surface starting successively from D to the depthof 1 μ m, 2 μ m, 3 μ m, 5 μ m and 10 μ m respectively.
The cutting time required for finish cutting is 9 hours
30 minutes, during which time the aforesaid level-difference cutting has to be carried out. Hence, it is quite ahard job. The result follows up to the level-difference of
1 μ m, that the spindle elongation at continuous operation of the spindle is stable and that the main body has
stable postural change.

Vertical
2.0
Horizontal 500.0

m/cm
m/cm

1 m level difference

Enlarged photograph
Scanning direction

Vertical
2.0
Horizontal 500.0

m/cm
m/cm

10 m
Fine level-differencemachining
sample modelFig. 4 Micro-machining level-difference sample cutting

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

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Next, evaluation is made by carrying out actual precision die mold cutting. The sample in Fig. 5 is amouse-shaped sample cut by using various machines inorder to evaluate the general die mold cutting in MHI.
Since this work is subjected to continuous machining for
more than 2 hours at 20 000 min -1 for finishing machining, it can be used for the evaluation of long-term stablemachining. The measurement of rough surface after machining has been confirmed to be Ry = 4.1μm or under.
Fig. 6 shows a model of digital camera, with the copper electrode subjected to electro spark machining using
DCV50. In electro spark machining the electrode machining precision gets transcribed, indicating theroughness of the mold surface after machining to be
Ry = 4μm or under, proving that the electrode has beenmachined uniformly.
The machining of copper electrode of cellular phone
in Fig. 7 is also the same.
The machining of aforesaid 4 samples has proved thatthe newly developed DCV50 ensures a long-term stablemachine precision. The machine also sufficiently conforms to the present precision requirements.

Rv: 4.1 m or underFig. 5 Mold "Mouse" sample
Roughness of machined surface: Ry = 4.1 m or under

3. Future prospect of the machine
DCV50 ensures the high end precision requirement
simply by carrying out warm-up operation for maximum
20 minutes. It is expected that there will be increasing
demand for higher precision and shorter delivery term.
With the spindle speed likely to get faster, more technique will be needed for restraining the spindle heatgeneration and control method. The advanced cooling
control system based on the estimation made from rotational frequency instead of the simple follow-up typecontrol system and the method of restraining the heatgeneration near the spindle taper so far not being cooled
off are going to be problems.
The important point is that the market demand should
be responded by making a new proposal, not only as anextension of the present. The conventionally unnoticed
waste time of the user shortened by paying attention to
the actual method applied by the user, and the ensured
stable precision will certainly help gain the user's satisfaction. MHI is determined to pay heed to customer's
stricter demands and to responds accordingly.

Fig. 6 Digital camera copper electrode and machined surface
after electrospark machining
Smooth and continuous shape is transcribed by electrospark machining.

Fig. 7 Copper electrode of cellular phone and machined surface
after electrospark machining

4. Conclusion
Toshio Yokomoto

MHI will be highly honored if the aforesaid technologies applied in the new machine could contribute tohigher productivity of our customers.

Shigeru Honda

Yoshikatu Sato

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

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