The reckless challenge to achieve
“thermal deformation control of machine and its environment”
In order to cut with high accuracy, it is necessary to deal with various thermal deformation that occurs during machining, and to process quickly, vibration must be dealt with. However, in the long history of machine tools, the problems of “heat” and “vibration” are very difficult and have been regarded as eternal issues.
Heat is particularly troublesome, and a machine tool made mainly of iron or casting expands by a hundredth of a millimeter per meter when the temperature rises by 1°C. During cutting, the machine body expands due to heat generated from the spindle and peripheral equipment. Furthermore, in a typical factory environment, the temperature changes about 10°C as a matter of course, which alters the posture of the machine and affects the machining accuracy. Machine tool manufacturers took measures to reduce the heat of the spindle and peripheral equipment with coolant, but this was not a real solution.
Under such circumstances, around 1994, we developed a compensation system (TAS-S: Thermo Active Stabilizer — Spindle) as an elemental technology that analyzes and compensates for spindle thermal deformation, and we visited customers who were machining die/molds with TAS-S equipped machining centers to survey the results. By doing so, we found that “suppressing spindle thermal displacement was not enough. Unless it was a large company, building a constant temperature room or other facilities would be impractical. If a machine could produce stable machining accuracies of 10 µm or less in a normal environment, dimensional compensation work would be easier...” I heard a craving voice like that. And that became a great motivation for the development team. However, it was a reckless challenge to create a machine that could provide stable dimensional accuracy in a factory where the temperature changes every moment, and striving to meet that challenge was considered insane by those around us.
The breakthrough was triggered by a radical change in thinking
For several years, we analyzed the heat and structure of a wide variety of Okuma machines in an environmental test room, and from the huge amount of data we faced, we were convinced that cooling a machine that generates heat and using extensive air conditioning, even insulating, would only be complicated and expensive. It would be more logical to trace the structure of the machine to see how the generated heat and thermal deformation could be more easily managed. It was a change in the way of thinking; of not suppressing heat but accepting it and “making friends” (thermo friendly).
In this way, we arrived at the Thermo-Friendly Concept of ① simple structural designs that generate straight thermal deformation without bending, ② careful attention to heat source arrangements and temperature balance, and ③ a system that controls (compensates) thermal deformation in real time—first commercialized with our MB-46VA vertical machining center.
Many people still misunderstand, but thermal deformation control is “not a technology that measures the amount of heat generated and compensates for the amount of complex deformation,” but is “an elegant mechanical design based on a major assumption that balanced thermal deformation will be easier to control, and in this way we are able to manage thermal deformation as a technology that achieves stable machining dimensional accuracy.”
Thanks to that, the MB-46VA, which achieved a dimensional change of 8 µm over time, was greeted with surprise, and was well received with responses like “The dimensional compensation work was drastically reduced and accuracy was assured—a great relief." In addition, competitors have also said, “Changing thermal deformation, a complaint, to a profitable technology; that was amazing.”
Expanding to a wide range of models while continuing to evolve
With the response gained from the MB-V as the driving force, we expanded the installation of the Thermo-Friendly Concept to multitasking machines, 5-axis machines, and double column machining centers, but the real turning point in this evolution was the application on multitasking and our large machines (DCMC).
With multitasking machines, deformation is complicated by the increase of heat sources from milling spindles, upper/lower turret movements, and the opposing spindle. If the method of thermal displacement compensation differs per machine, support for the required application could not be assured. Therefore, we decided to establish a method that would maintain an excellent thermal deformation balance regardless of the machine model. We have made great progress by developing a technology that can simplify the control by utilizing the vast amount of measurement and data analysis into machine design.
In the development of TFC for our double columns, we placed workpieces on a DCMC 2 to 3 times the height of our VMCs and 36 times the table area and met the challenge of further innovating thermal deformation control with a machining accuracy target of 20 µm. During this development, there was no themostatic chamber that could be used to test such a large machine, and we had a hard time measuring posture changes and thermal expansion. To that extent, we thrust ourselves into re-examining machine design and control theory. In order to verify the derived theory, a new large themostatic chamber was built and evaluated, which led to the birth of the MCR-BIII. After visiting 32 customers, we confirmed that 39 units were able to satisfy their environment-related requirements. The result of know-how gained from these birth pangs was that the dimensions were stable even with large workpieces, and usability was so high that machining step errors would not occur even after long runs.
Thermo-Friendly Concept Equipped Machines and Cumulative Shipments
Developing intelligent technologies for the unmanned automatic machining era
Okuma provides five Intelligent Technologies (Thermo-Friendly Concept, Collision Avoidance System, Machining Navi, 5-Axis Auto Tuning System, and ServoNavi) as functions that contribute to the improvement of the customer’s “machining infrastructure.” In fact, when the Thermo-Friendly Concept was developed, most of these functions anticipated “The coming age of unmanned automatic machining. In that era, what functions should be prepared by the machine tool manufacturer?” That theme will be thoroughly discussed to effectively systematize elemental technology. We have in fact sequentially achieved those functions.
Taking a 5-axis machine as an example, a rotating spindle will be misaligned no matter how good the thermal deformation control is. Therefore, we developed the 5-Axis Auto Tuning System to easily adjust geometric errors. The MU-500V equipped with the Thermo-Friendly Concept for the first time on a 5-axis machine was well received, but in combination with 5-AATS, the machining accuracy and stability were significantly improved.
In addition, Machining Navi provides "turning chatter control," which is said to be difficult, to instantly guide the machine to optimum cutting conditions and help solve chatter problems, as an indispensable function for long workpieces and threading operations.
In approximately 20 years, the Thermo-Friendly Concept has become synonymous with Okuma’s “Only-One Technology,” and as of October 2019, it has been installed in 83 of 127 models, with shipments exceeding 50,000. Nevertheless, that is small compared to the number of machine tools throughout the world, and we will pursue even higher levels of Intelligent Technology by offering “faster, more accurate machining” for applications becoming increasingly sophisticated.
- Director, Technology Division Manager, Research & Development Department General Manager
Harumitsu Senda - Joined Okuma in 1987. Graduated from Kobe University Graduate School of Science and Technology, Phd (Engineering). After joining Okuma, oversaw ultra-precision machining technology as a developer of elemental technology. Volunteered to research thermal deformation, developed a thermal deformation compensation system for the main spindle. This led to development as a machine tool thermal deformation compensation system which won the 2002 Japan Society of Mechanical Engineers Award. After that, as a group leader, he started R&D on thermal deformation control, and promoted the development of Okuma Intelligent Technologies. He assumed his current position in 17 years after serving as R&D Department General Manager in 2011, Executive Officer, Technology Division AGM, Kani Technology Dep GM in 2014, and EO, Technology Div GM, R&D Dep GM in 2016. He also currently oversees Okuma’s technology and R&D activities.