Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)45

Modern Technology Achieving High-Precision,
High-Efficiency Processing for Machining Centers

YOSHIKATSU SATO*1

In recent years, demand for high-precision processing has also been increasing, inmachining of relatively-long parts and dies used for automobiles, semiconductor manufacturing
devices, electronic components, etc. The size range of these workpieces is positioned between
machining centers and large machines. Looking at the entire industry, there are only a fewmachine tools that target such workpiece sizes, and therefore the demanded accuracy has not been
fully met. Mitsubishi Heavy Industries, Ltd. (MHI) developed and markets the LH250, which is oneof the largest machining centers realizing machining accuracy equal to that of micro millingmachines. This time, MHI developed additional new optional functions that enable high-precision
machining without significantly changing the factory environment.

|1. Introduction
As many products around us, typified by automobiles, have been enhancing theirfunctionality in recent years, demand for very high-precision processing is also increasing inmachining of relatively-large parts. Even across all industries, however, there are only a few
machine tools for workpieces with a size range positioned between machining centers and large
machines, and therefore the demanded accuracy for these workpieces has not been fully met. There
are a number of relatively-long parts that require high accuracy including not only automotivecomponents, but also semiconductor manufacturing devices and parts needed for the production ofelectronic components. At the manufacturing sites of these parts, it is necessary to repeatedly
execute trial and error, and therefore the poor yield has been a problem.
MHI developed the LH250 high-precision double column machining center, which is alreadyon the market. Table 1 shows the main specifications. In the development of this product,
fundamental technologies for the μV1 micro milling machine designed for high-precision
machining and double column large machine manufacturing techniques were combined. With ahigh-speed spindle as standard equipment, the LH250 attains long-term stable and high-precision
machining equivalent to that of micro milling machines. Following the development of the machine
itself, MHI advanced the development of new options in order to satisfy further market demand.
This paper presents these technologies and their examples.
Table 1
Table size
Maximum loading capacity
Axes travel (X x Y x Z)
Spindle taper
Spindle diameter
Spindle speed
Spindle motor output
ATC tool storage capacity
Machine foot print
Machine height
Machine mass
*1

LH250 specifications
(mm)
(kg)
(mm)
(mm)
(min-1)(kW)(tools)
(mm)
(mm)
(kg)

2,500 x 1,000
3,000
2,500 x 1,000 x 600
HSK-A63
Φ80
Max 20,000
22/18.540 (Opt: 64)
7,200 x 3,3003,593
21,000Manager, Engineering Department, Machine Tool Division, Machinery, Equipment & InfrastructureMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)46|2. Features of LH250 high-precision double column machining
center
2.1

High-performance spindle

Typical machining centers have a spindle a speed of around 10,000 min-1 as standard, and ahigh-speed spindle is available as an option. When a high-speed spindle is used, the initial preloadof the bearing that supports the spindle has to be reduced because of greater heat generation, andtherefore the rigidity of the spindle decreases to the level where rough processing at a low speed
cannot be performed. For this reason, two specifications of the spindle are available so that
customers who need rough processing and other customers who need high-speed operation only
can select one that suits their respective purpose.
However, through the employment of cooling both inside and outside of the spindle andspecial lubrication of the bearing, which was developed for the μV1, the LH250 achieves a higher
initial preload and reduction of heat generation at the maximum speed, and then realizes asstandard a high-rigidity spindle rotating at a high speed of 20,000 min-1 that can also perform roughprocessing at a low speed. This machine handles rough processing and high-speed finishing with its
single unit, and therefore improves machining efficiency.
Even when the spindle rotates at the maximum speed of 20,000 min-1 from a cold start, theLH250 limits the spindle thermal displacement to 2 μm or less in all of the X, Y, and Z axes
directions(1) and the spindle vibration to 2 μmP-P or less over the entire rotation speed range using
no electrical correction. As a result, the LH250 contributes to the improvement of the machining
accuracy and the machined surface quality.

2.2

High-precision feeding mechanism

Also for the feed axes, the LH250 employs narrow-pitch large-diameter ball screws and astrong supporting method similar to the μV1 to attain a high-rigidity and high-response feed axes
system. In addition, the LH250 uses the HGP2 control, which features proprietary control tuning,
and realizes machining operation without losing its shape even in high-speed shape machining. For
example, a roundness of 1.8 μm, which is equivalent to micro milling machines, was achieved forthe circle cutting (Φ218 mm) performed by synchronized two-plane axes(1).
Furthermore, coolant flows into the inside of the ball screws, nuts, support bearings, anddriving motor mounting flanges in a similar manner to the spindle to drastically reduce heatgeneration, consequentially realizing a stable and accurate feeding mechanism.

|3. W-2 thermo-stabilizer
The LH250 employs the cooling of the heat-generating parts of the machine such as thespindle and the feed axes to suppress thermal displacement caused by the heat generation of themachine itself. If a change in the ambient temperature around the machine occurs, however, thethermal displacement of the machine is an inevitable physical phenomenon. Although there is anelectrical correction method using temperature sensors embedded in various points of the machine,
thermal displacement is not always simple motion and the method may result in an adverse effect.

Figure 1 Column deformation caused by temperature difference around machine

When there is a temperature difference between the front and rear or upper and lower
positions around the machine, for example, the column deforms and the spindle inclines (Figure 1).
In this case, some electrical offsetting methods intended to correct the displacement in the X axis
direction succeed in the correction, but other electrical offsetting methods, moving the Z axis forMitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
47

example, result in a further increase of displacement (Figure 2). Squareness correction or spatial
correction may be possible solutions, but in such cases, correction becomes difficult when using
tools of different lengths or when warpage unlike simple inclination occurs, as well as under othercircumstances (Figure 3).

Figure 2 Adverse effect caused by moving the Z axis in correction of spindle inclination

Figure 3

Example of conditions where the spindle inclination is difficult to correct

Figure 4

Configuration of W-2 thermo-stabilizer

Therefore the LH250 does not use electrical correction and employs the optional W-2
thermo-stabilizer, a method to make the column temperature uniform to suppress inclination andthermal displacement using a temperature control fluid that circulates and controls temperature in
the column (Figure 4). The temperature of the temperature control fluid is controlled to same level
as that of the bed by installing a temperature sensor thereon, because the bed is placed near thelarge-heat-capacity floor, and therefore is relatively stable and rarely results in a temperature
difference. In addition, the temperature of the aforementioned coolant that flows into the spindle
and the feed axes is controlled in reference to the bed temperature. Furthermore, when the optional
cutting fluid temperature controller is added, temperatures of the spindle, the feed axes, the column,
the bed, the table, and the workpiece all become identical and therefore attitudinal changes and thethermal displacement of the machine can be suppressed.

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)48

Figure 5 shows the actual measurement results of the thermal displacement of the machine
against the environmental temperature change of the machine periphery. The results indicate thatthe thermal displacement of a machine equipped with the W-2 is approximately half of that of amachine without the W-2.

Figure 5 Comparison of machine thermal displacement between unit with and without W-2|4. Large-diameter tool handling capability of optical image type
tool measurement system
The optical image type tool measurement system is an on-machine tool measurement
application developed uniquely as optional equipment for the μV1. As its advantage, this system
can confirm that thermal displacement has saturated and stabilized using a CCD camera to performmeasurement of the tool that rotates at the same speed as machining, and therefore can suppressmachining errors caused by thermal displacement and tool change.
The spindle used for the μV1 is the HSK-E32 (tool holder flange diameter of 32 mm), whichuses a tool with a maximum diameter of around 10 mm, and the optical image type tool
measurement system has specifications that suit the tools used. On the other hand, the spindle used
for the LH250 is the HSK-A63 (tool holder flange diameter of 63 mm), which uses a tool with amaximum diameter of 125 mm. The optical image type tool measurement system can measurelarge-diameter tools in principle, but the lens focusing range in which accurate measurement can bemade (depth of field) is very narrow, and therefore the probability that the tip of the rotating tool issettled within the depth of field decreases proportionally to the tool diameter (Figure 6). As aresult, there were concerns about unstable measurement accuracy, long measurement time, etc., in
the measurement of large-diameter tools.

Figure 6 Conceptual diagram of large-diameter tool
measurement with the use of optical image type tool
measurement system

Mitsubishi Heavy Industries Technical Review Vol. 52 No. 3 (September 2015)
49

This time, the imaging method was changed from the conventional one in which multiple
images were taken in a random manner, to the new one in which images are taken efficiently
according to the spindle speed in order to accurately measure even large-diameter tools. Figure 7
shows the actual measurement of a tool with a diameter of 125 mm with the use of the opticalimage type tool measurement system and an example image. Table 2 compares repeatedmeasurement results of the conventional random imaging method and the developed method forfive types of tools. It was verified that the developed measurement method can improve themeasurement accuracy and reduce the measurement time generally, depending on conditions suchas the number of tool flutes and the spindle speed.

Figure 7 Example of large-diameter tool measurement with the use of opticalimage type tool measurement system
Table 2 Tool measurement results with the use of optical image type tool measurement system

No.

Tool typeTooldiameterΦ (mm)

Numberof flutes

Spindlespeed(min-1)12
3
4
5End milling
End milling
Milling
MillingBoring102050
12536

2
4
6
6
1

10,0004,0002,000800
500Time of three-time
repeated measurement(Ratio to conventional
method of No. 1)
ConventionalNew
methodmethod
10.69
1.05
0.93
1.08
1.07
1.12
1.28
1.25
0.77

Fluctuation of ten-time
repeated measurement(Ratio to conventional
method of No. 1)
ConventionalNew
methodmethod
1
0.67
1.78
1.33
3.44
1.56
6.78
1.22
2.0
1.56

In addition, this method can measure a tool flutes individually in association with the spindle
rotation angle. Therefore the detection of unevenness of the tool tip height and chipping, which wasdifficult in the past, becomes possible, and the further improvement of the machining accuracy andthe machined surface quality can be realized as a result.

|5. Conclusion
This paper presented the features of the LH250 high-precision double column machining
center and the newly developed optional functions. The LH250 contains technologies that achieve
high precision and highly-efficient processing of mid-sized workpieces, which are hard to machinewith a machining center or a large machine. The newly developed optional functions are considered
to have advantages such as the capability to eliminate poor yield without significant improvementsof the factory environment.
MHI will continue to further advance machine tool, optional functions, and machining
technologies to satisfy market needs, and contribute to the development of the manufacturingindustry.

Reference
1. Sato, Y. et al., Technology for High-speed and High-performance Milling of LH250 High-precision Double
Column Machining Center, Mitsubishi Heavy Industries Technical Review Vol. 50 No. 1 (2013) pp. 43-48