Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)35

High-precision Machining Technologyfor Large Machining Centers
Special Jet Lubrication and Advanced Thermal
Displacement Suppression System
TAKAYUKI KUME*1

HIDEFUMI OMOKAWA*2

NAOTAKA KOMATSU*3 SATOSHI FURUTATE*1

SHINGO NAKAMURA*4YASUNARI YAMADA*4

As the precision required for machined parts of industrial machinery and large dies ofautomobiles increases, the need for high-precision machining of large parts is increasing. To
enable high-precision machining, it is imperative to reduce the heat deflection of machine tools
used in large machining centers due to internal and external temperature changes of the tools
themselves. A new high-precision machining technology is required because the existing
technology, which compensates for heat deflection due to heat generated by the spindle, can nolonger meet the aforementioned need for high-precision machining. To overcome the problem,
Mitsubishi Heavy Industries, Ltd. (MHI) has applied an internal cooling method for the spindle tolarge machining centers that was perfected through our experiences with small precision machine
tools, in addition to a spindle heat deflection reduction technique using special jet lubrication. Thecompany also developed an advanced thermal displacement suppression (ATDS) system for theentire machine body.

|1. Introduction
As the precision required for machined parts of industrial machinery and large dies ofautomobiles increases, the need for high-precision machining of large parts is increasing. It is often
necessary to achieve machining precision of ±10 μm. To enable such high-precision machining,measures to deal with heat deflection have usually been taken, such as to install the machine in atemperature-controlled factory or to carry out the finishing process separately. However, there isincreasing need for high-precision machining to be carried out even if the machine is installed in a
general-purpose factory environment without air conditioning. Thus far, heat deflection
compensation measures have been employed to deal with heat deflection due to internal factors
such as heat generated by the spindle, while measures using heat stabilizing wall plates andthermally symmetrical structures have been implemented to deal with external factors such as room
temperature changes around the machine. However, it is no longer possible to sufficiently meetsuch needs for high-precision machining with these measures. To tackle the issue, MHI developed
MVR, a double-column, five-face milling machine, equipped with the new ATDS system thatallows high-precision machining in general-purpose factories. The following outlines the millingmachine.

|2. Problems of and challenges for existing technology
Since large parts machining by large machining centers involves large machining tools aswell as large workpieces, the heat deflection caused per degree Celsius of machine temperature canbe several times as high as processes carried out by smaller machining centers. This significantlyaffects the machining precision. Therefore, to enable high-precision machining, it is imperative to*1Manager, Engineering Department, Machine Tool

*3

Research Manager, Takasago Research & Development Center, Technology & Innovation Headquarters*2

Engineering Manager, Engineering Department, Machine Tool

*4

Engineering Department, Machine Tool

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)36

reduce the heat deflection of the machine tools used in the large machining center due to internaland external temperature changes of the tools themselves, and measures must be applied for boththe spindle and the machine body. Existing heat deflection compensation technology compensatesfor heat deflection in accordance with the amount of heat generated by the spindle. The precision ofheat deflection compensation using this method is only ±15 μm at the most. However, since theheat deflection compensation precision is lower than the required level of machining accuracy (±10μm), it is essential to improve the precision of spindle heat deflection compensation by effectively
removing the heat generated by a spindle bearing. In addition, to meet the need for high-precision
machining in a general-purpose factory environment, it is necessary to achieve stablehigh-precision machining by providing a structure that is hardly deflected by heat, even in anenvironment where the daily temperature fluctuations are significant. For that purpose, MHI
developed a technique to compensate for the heat deflection of not only the spindle, but also of theentire machine body.

|3. Spindle heat deflection reduction technique
3.1

PurposeRequirements regarding the heat deflection of the spindle include: a smaller amount ofspindle heat deflection even during high-speed rotation; a shorter period of time required tostabilize heat deflection, thus resulting in a reduced time for warm-up; and a stable amount of heat
deflection during lengthy machining processes. To meet such needs, MHI applied an internalcooling method for the spindle to large MVR machines that was perfected through our experienceswith small precision machine tools, in addition to a special jet lubrication technology for ballbearings. The following outlines the structure and effects of the special jet lubrication spindle.

3.2

Internal structure of spindle

Figure 1 illustrates the structure of the tip end of a spindle. Ball bearings, which are thesource of the heat, are supplied with lubrication oil cooled in accordance with the generated heat sothat the bearings are effectively cooled. In addition to cooling the outer housing of the spindle,
which is a method that is already employed, the cooling oil is circulated inside the rotating spindle,
thereby preventing the spindle temperature from rising.

Figure 1 Internal structure of spindle

3.3

Effects of internal cooling of spindle and special jet lubrication of bearings

With these spindle heat deflection reduction techniques, the amount of heat deflection of thespindle is suppressed to ±5 μm or less, thus achieving a level of precision that meets theaforementioned need for high-precision machining (Figure 2). In addition, with the provision of amechanism to recover the total amount of lubrication oil supplied to the bearings, oil dripping fromthe spindle can be prevented, making the spindle suitable for materials that are incompatible withoil, such as carbon and ceramics, as well as for resin processing.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)37

Figure 2

Actual measurements of spindle heat deflection|4. Advanced Thermal Displacement Suppression (ATDS) Systemfor machines
4.1

PurposeIn general, capital investment and a decrease in production efficiency pose issues when
taking measures for high-precision machining by large machining centers. To counter this issue,
MHI developed a new ATDS system designed to improve user productivity and add further value
to high-process machining, based on a concept of "a machine resistant to ambient temperaturechanges."

4.2

Designing of machine structure resistant to ambient temperature changes

Heat deflection of machining tools is caused by changes in structure temperature resulting
from ambient temperature changes in a factory. Accordingly, it is important that the structure
temperature should not be easily changed, even when the ambient temperature changes, and thatthe temperature of the entire machine should be changed uniformly so as to prevent inclination ortorsion. MHI simulated an optimum machine structure that is less likely to cause heat deflection
even when the ambient temperature changes, optimized the structural shapes of cross rails andcolumns and determined the arrangement of heat stabilizing wall plates (Figure 3).

Figure 3

4.3

Example of heat deflection analysis

Advanced Thermal Displacement Suppression (ATDS) system

The ATDS system is a technique developed by applying MHI's proprietary thermal analysis
technology, which was perfected through our experiences with power generation turbines and otherequipment, to machining tools. The system combined, in a well-balanced manner, three thermal
analysis technologies based on thermal analysis simulations: 1. machine structure; 2. coolingsystem; and 3. thermal displacement prediction/compensation technology. This significantly
reduced the impact of heat deflection and enabled a machining process that requires no warm-up
and ensures stable precision over lengthy periods.
Figure 4 illustrates an overview of the system. The system not only reduces heat deflection
through the improvement of robustness to changes in ambient temperature, but it also adopted amethod to compensate for generated heat deflection by monitoring the temperatures of various
parts of the machine. MHI also developed a technique to correct the spindle position through thereal-time measurement of column inclination due to heat deflection, as well as a technique tothree-dimensionally compensate for the deflection of detachable attachments (additional spindles).

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Thus, the system is characterized by its ability to compensate for the heat deflection of the entire
machine.

Figure 4

Schematic view of ATDS system

Figure 5 illustrates the measurements of heat deflection when using the ATDS system. The
machine using the system, even when exposed to room temperature changes in a general factory
environment (8.4 °C/40 hours), was able to ensure a stable precision of ±10 μm or less, which iscomparable with that of machines installed in temperature-controlled facilities. This allows stablehigh-precision machining for lengthy periods without much concern for room temperature changes
or changes in spindle rotational speed.

Figure 5

Measurements of heat deflection of machine equipped with ATDS system|5. Conclusions
MHI developed the special jet lubrication method, spindle heat deflection reduction
technique and machine body heat deflection compensation technology, each of which is aone-of-a-kind technology in the field of large machining centers. With these technologies, a highmachining precision can be attained even during machining that involves frequent changes inspindle rotational speed or lengthy machining of large workpieces. The technologies also made it
possible to meet the need to improve the required machining precision on large workpieces and toenable stable high-precision machining even in a general-purpose factory environment where thefactory room temperature around the machine tends to change drastically. We believe that our newsystem, which is designed from the perspective of users, is capable of meeting the demand for"higher productivity" and "higher environmental friendliness." As a pioneer in the development of
double-column machining centers, we will continue making helpful and advanced suggestions on
how to deal with heat deflection, which is a mutual problem shared with our customers' machining
processes, so that we will be able to contribute to the improvement of user productivity.