-2-1. Introduction [627794]

-2-1. Introduction
At present, the machine tool industry worldwide is
enjoying unprecedented demand, and the industry’soutput is apparently even failing to satisfy currentdemand. Japan’s machine tool industry, in particular,has boasted the greatest share in the world since1982, and its share has been exceptionally high in thelast two years. In order for Japan’s machine toolindustry to maintain this share, I believe that theenterprises involved in it must remain committed notonly to expansion and advancement of their productfacilities, but also to steady research and developmentefforts. They must continue to add more value to theirproducts in order to cope with future needs andmaintain competitiveness compared to machine toolmanufacturers in other nations.
In this paper, I intend to report on current topics
about recent machine tools and trends in the researchand development commitments of the Japanesemachine tool industry, highlighting the efforts of TheInternational Academy for Production Engineering(CIRP), an organization of which I am a member.
2. High-Speed, High-Efficiency Machine Tools
It is well known that demands are mounting for
greater maximum main spindle speeds and feedspeeds—in other words, that machine tools of higherspeed and higher efficiency are much needed.
1)
Background information about high-speed machinetools and supporting technologies, and their resultantadvantages are summarized in Table 1 .In this section, I will focus on the avoidance of
chatter vibration, which is one outstanding advantageof high-speed, high-efficiency machine tools, as otherengineering topics are discussed in papers by otherresearchers.
In the period of the 1960’s and 1970’s, there were
research efforts worldwide on the chatter vibration ofmachine tools. As a result, the underlying principlesbehind so-called regenerative chatter vibration andforced chatter vibration were clarified, and basicsolutions were proposed.
Unfortunately, however, examples of further
systematic research efforts have been rare.
In recent years, avoidance of chatter vibration has
been posing a new challenge in the machining of
Dr. Toshimichi MORIWAKI
Professor Department of Mechanical Engineering KOBE University
Table 1 Background, supporting technologies and
advantages of high-speed, high-efficient machine toolsRecent trends in the machine tool technologies are surveyed from the view
points of high speed and high performance machine tools, combined multi-functional machine tools, ultraprecision machine tools and advanced andintelligent control technologies. The machine tools are bases ofmanufacturing industries and they are strategically important products forJapan. The views of the author towards the technical developments in bothhardware and software are introduced together with the world wide trends inthe relevant fields.
(Background, supporting technologies)
¡Need for highly efficient machining and decreased costs

¡Development of high-speed main spindles and high-
ɹ
speed feed means (linear motors, etc.)
¡Advances including the development of high-speed
ɹ
machining capable tools and progress in machining
ɹ
techniques
(Advantages)
¡Decreased processing times (improved efficiency)

¡Improved machining accuracy and better quality of
ɹ
finished surfaces

¡Prevention of chatter vibration
High-speed, high-efficiency machine toolsNTN TECHNICAL REVIEW No.74 ʢ2006ʣ
<
Contribution >
Trends in Recent Machine Tool Technologies

Trends in Recent Machine Tool Technologies
-3-Fig. 1 Stability chart of regenerative chatter vibration (Y. Altintas)Real Part [ Жm/N] Axial depth of cut [mm]2
1
0
-1
-2
0
5
4
3
2
1
00 2000 4000 6000 8000 10000 12000 14000 16000500 1000 1500 2000 2500 3000 3500 40002kА=Dynamic Stiffness
Ktc=Cutting Coefficient
T= n=60/T2KКʴЏ
Тc
Frequency[Hz]Gminʹ
-1
4kАʙ
Spindle speed [rev/min]Џ=3Кʴ2tan-1lm [ПʢjТcʣ]
Re [ПʢТ cʣ]
Chip
180˚
alim =
acrit = =-1
2KtcG(Т)
2Ktc Ktc-1 2k А

-1
4kАStabiliy Lobeshard-to-cut materials as well as in high-speed, high-
efficiency machining of aluminum materials foraviation purposes. Generally, chatter vibration isavoided by reducing the depths of cuts and cuttingspeeds (low-speed stability), but it is possible to avoidchatter vibration by increasing spindle speed. This factwas already known from research done in the 1960’s.Since high-speed spindles boasting this ability werenot available in those days, however, this fact wasregarded simply as a theoretical possibility. In themathematical field, it was also difficult theoretically tohandle chatter vibration in milling processes, includingend milling. Notwithstanding, Prof. Y. Altintas et. al.obtained results for stability graphs such as the oneshown in Fig. 1 . This graph shows that chatter
vibration does not occur in the region of depths cutbelow the stability lobes relative to spindle speeds onthe horizontal axis. Though detailed discussion of thisfinding
2)is omitted in this paper, the underlying
principle of chatter vibration can be understood fromthe expressions and diagrams given to the right of thisgraph. Variations in chip thickness are caused bydifferences between the roughness of the finishedsurface generated by the immediately previousrevolution of the main spindle or by an immediatelyprevious cutting edge and the roughness of thefinished surface currently generated by the currentcutting edge. This variation in chip thicknesscontributes to variation in the cutting force andcontributes to continuing vibration. If we can run themain spindle at a higher speed that is equivalent tothe vibration frequency, then the difference betweenthe phase of vibration resulting from the immediatelyprevious revolution and the phase of vibration derivingfrom the current revolution can be effectivelycontrolled, thereby eliminating variations caused bychip thickness. If such a condition is realized, chatter
vibration will not occur even with greater cut depths.
Utilizing this principle, high-speed, high-efficiency
cutting has been implemented for aircraft componentsmade of aluminum and other materials. Given this,there has been mounting interest in the dynamiccharacteristics of a main spindle system that includesa main spindle, chuck and tools. As a result, theinterrelation of bearings and other design factors withthe dynamic characteristics of the main spindle andmain spindle system has been clarified boththeoretically and experimentally, and this achievementhas been applied to the design of main spindles.Recently, various software packages are also beingused frequently for analysis and design. Thetheoretical study of main spindle designs will becomeincreasingly important.
3.Combined Multifunctional Machine Tools
In addition to high-speed, high-efficiency, cutting-
capable machine tools, research on machine tools iscurrently focused on combined multifunctionalmachine tools, including 5-axis machining centers andcombined multifunctional turning centers. Thebackground and advantages of combinedmultifunctional machine tools are summarized inTable 2 . Combined multifunctional machine tools can
be roughly categorized into turning centers (TC) thathave been developed from lathes and machiningcenters (MC) that started as milling machines.
A portion of the investigations into machining
applications with combined machine tools based onlathes is illustrated in Fig. 2. In addition to machining
of bores, outer circumferences and end faces, certainapplications are executed for slope machining and

NTN TECHNICAL REVIEW No.74 ʢ2006ʣ
hobbing. Recently, a main spindle mounted to an area
equivalent to a turret is capable of not only auxiliarycutting processes, such as end milling, but also tomore demanding milling processes. In addition, lathe-based machine tools that resemble milling machineshave been developed. One example of such acombined machine tool is shown in Fig. 3 (Mori Seiki
Co. NT5400DCG).Many different 5-axis machining centers have been
developed. In particular, in addition to orthogonal 3-axis vertical and horizontal machining centers, manysimultaneous 5-axis control machining center productsthat have work tables with two additional axes forrotation and oscillation are used widely. Most recently,some machining centers have a work table driven by aDD motor and a high-speed, high-power rotary tablecapable of high-speed indexing, and they feature thefunctions of vertical turning centers. As mentionedabove, deriving from either lathes or milling machines,combined multifunctional machining tools may evolveinto novel machine tools that incorporate features ofboth turning centers and milling machines.
Combined multifunctional machine tools have
advantages that include the following. They arecapable of machining complex forms that requiresimultaneous control of five axes. Loss in machiningaccuracy from dismounting and remounting theworkpiece is prevented because once a workpiecehas been mounted to the chuck, all machiningprocesses are executed without need for rechuckingthe workpiece. As the needs for function-intensiveparts and components increase, advanced combinedmultifunctional machine tools are capable ofmachining these workpieces at higher precision andhigher efficiency. As superior machine tools, thedemand for combined multifunctional machine toolswill increase further in the future. To meet thisdemand, the researchers and engineers in this fieldmust develop the hardware that helps realizesophisticated functions as well as the supportingsoftware (CAM) to enable advanced controltechniques and application technology.
Incidentally, within the next 2 years, the STC-M
(Scientific Technical Committee: Machines) of CIRP(The International Academy for ProductionEngineering) will issue a keynote paper that coverscurrent and future trends in combined multifunctionalmachine tool technology.
-4-
020406080External circumference
machining (milling)
External circumference
machining (drilling)s
Bore machining
(thread cutting)External circumference
machining (thread cutting)
Slope machining
(milling)Slope machining
(drilling)PolishingHobbingExternal circumference
machining (turning)
Bore machining
(turning)
Bore machining
(milling)End face machining
(drilling)End face machining
(milling)Presence/absence of
workpiece transfer
End face machining
(turning)
Fig. 2 Survey of machining examples by combined multi-axis
machine tools based on turning machines
Fig. 3 An example of combined multi-axis machine tool (Mori Seiki Co. NT5400DCG)Table 2 Background, supporting technologies and
advantages of combined multi-functional machine tools
(Background, supporting technologies)
¡Need for high-precision, high-efficiency machining to make
ɹ

more advanced (complicated) parts and components
¡Increased sophistication of supporting software (CAM)
¡Development of high-precision, high-efficiency machine
ɹ

elements (e.g.: DD-motor driven tables, etc.)
(Advantages)
¡Improved machining accuracy, reduction in machining time
ɹ

(one-chuck process)
¡Machining of complicated shapesSimultaneous 5-axis control machining center
(e.g.: three orthogonal axes and two rotational axes)
Combined machining turning center
(e.g.: lathe and 2nd main spindle, B axis, Y axis, etc.)

-5-4. Ultraprecision Machine Tools
Other than high speed and high efficiency, the most
critical requirement for machine tools is high precision.Recently, various ultraprecision machine tools havebeen developed that are significantly more evolvedthan earlier high-precision machine tools. Previously,the industrial fields that required ultraprecisionmachine tools were limited and the market scale forultraprecision machine tools was relatively small. Incontrast, needs have been increasingly mounting forultraprecision and micro-machined parts andcomponents, such as dies for optical parts andcomponents. In response to this trend, development isin progress for various ultraprecision machine tools.
The background for the needs for ultraprecision
machine tools, supporting technologies for theirrealization and their advantages are summarized inTable 3 . Progress in the component technologies for
ultraprecision machinery, such as air hydrostaticbearings and guides, is remarkable. I believe thatadvances in hardware technologies for thesemechanical elements are contributing to the highervalue of Japanese machine tools.
In the field of ultraprecision-machining and micro-
machining, typically for the machining of optical partsand components, the requirements for form accuracyand finished surface roughness have always beendemanding, and the forms of machined parts andcomponents have become increasingly complicatedas summarized in Table 4 . Also, the process for
preparing optical lenses has changed from injectionmolding with plastic materials to a hot-pressingprocess with glass. To cope with this trend, anincreasingly larger number of dies are being made ofmaterials that are extremely difficult to machine, suchas tungsten carbide and ceramics, and these diesmust undergo many machining processes includinggrinding and polishing. These techniques in diemachining processes and glass press-formingprocesses are contributing positively to themanufacture of the lenses on camera cell phones anddigital cameras, which are both increasingly common.
Looking more closely at the lenses used in these
types of digital equipment, machining processes thatare more demanding are necessary to realize theirunique optical arrangements. For example, this isneeded to create a combination of a Fresnel lens andan aspherical lens, which enhances the opticalcharacteristics of the related optical systems. At thesame time, laser printers and other optical equipmentthat utilize lasers need more sophisticated opticalelements that involve nonaxisymmetric or free-curvedsurfaces. Therefore, needs are growing increasinglyfor more advanced ultraprecision machiningtechniques. To this end, I believe that the importanceof ultraprecision machine tools needed forultraprecision-machining and micro-machining will befurther highlighted because they are indispensable inmachining highly sophisticated parts and componentswith high added value.
One example of a 3-axis FTS (Fast Tool Servo)
3)
device, which is capable of forming a free-curvedsurface with ultra high precision and higher efficiency,is illustrated schematically in Fig. 4 . This device, as
shown in the diagram, machines a workpiece fixed ona running ultraprecision rotary table while shifting inthe radial direction a tool that has its cutting depth inthree axial directions controlled at high-speeds andhigh-response frequencies. If the axial infeed iscontrolled while the tool is fed in a radial direction,then theoretically any intended shape could be formedon the surface of the workpiece.
In reality, however, it is impossible to form a step
square to the cutting direction of a machine tool evenat the maximum possible response frequency andresponse speed of the tool. Therefore, byTable 3 Background, supporting technologies and
advantages of ultraprecision machine tools
(Background, supporting technologies)
¡Growing needs for ultraprecision and micro-machined parts
ɹ

and components
(Optical parts and components, dies, etc.)
¡Sophistication of ultraprecision machine elements (main
ɹ

spindles, guideways, feed drive systems, etc.)
¡High-precision control technologies
(Advantages)
¡Development of new market segments through ultraprecision-
ɹ

machining and micro-machining (Satisfaction of needs)
ɹ

(Optical parts and components, micro parts, mechatronic parts, etc.)Ultraprecision machine tools
(ultraprecision cutting/grinding machines)
Table 4 Trends in ultraprecision micro machining
1. Form accuracy, finished surface roughness:

¡Micronsˠnanometers

2. Form (in the case of optical parts)

¡Spherical surfaces ˠaspherical surfaces ˠnonaxisymmetric
ɹ

surfacesˠfree-curved surfaces

3. Workpiece materials

¡Soft metals (aluminum, copper, etc.)

¡Hard metals (nickel, hardened steel, etc.)

¡Resin materials, anisotropic materials (lithium niobate, fluorite, etc.)

¡Brittle materials (tungsten carbide, ceramic materials, etc.)

¡Other materials (plastics, etc.)Trends in Recent Machine Tool Technologies

simultaneously controlling the tool along three axes,
the freedom of the machined forms can be increasedand the trajectory of motion of the tool being controlledcan be simplified. Use of such FTS helps greatlyreduce the necessary cutting time on free-curvedsurfaces.
5. Advanced and Intelligent Control
The increasing sophistication of machine tools is
supported by progress in not only hardware but also insoftware. The background, supporting technologiesand advantages of advanced and intelligent controltechnologies are summarized in Table 5 . Recently,
many advanced (intelligent) control techniques areavailable that reflect an understanding of machine toolcharacteristics and machining processes. Forexample, let us think of a control technique for
controlling thermal deformation, which is the mostcritical factor adversely affecting the machiningaccuracy of machine tools. A much advanced controltechnique is now commercially used in which themagnitude of the current thermal deformation isestimated in real-time based on information about themachine tool and temperatures at various spots on thetool.
4)Using this information, the motion of the
machine tool is controlled so that higher machiningaccuracy is ensured under any operating condition.
It is also possible to simulate the motion of the
machine tool in real-time based on information aboutthe motion control applied to the machine tool. Utilizing
this idea, Okuma Co. has developed an anti-crushsystem that predicts events such as the crushing of atool with a chuck and stops the machine as necessaryby simulating the motion somewhat in advance of theactual motion of the machine tool. The schematicdiagram for this system is illustrated in Fig. 5.
-6-NTN TECHNICAL REVIEW No.74 ʢ2006ʣ
Table 5 Background, supporting technologies and advantages
of advanced and intelligent control technologies
(Background, supporting technologies)


¡Need for low-cost controllers with advanced functions


¡High-speed, high-precision interpolation systems


¡Real-time thermal deformation compensation


¡Anti-crush systems that also utilizes simulations


¡More advances in computer and IT technologies

(Advantages)


¡Greater added value for machine tools


¡Evolution of knowledge into established technologies


Advanced and intelligent control
Motion
commandVirtual machine
controlVirtual machine
program settings
Any operationMachine controlFeed forward
motion command
Interference
detection
Machine stopMachine model
Jigs, tools,
materials, etc.
Full
synchronizationSimple modeling
Real-time simulation
Fig. 5 Conceptual diagram of anti-crush system (Okuma Co.)3-axis driven PZT
ToolWorkpiece
FeedZY
X
Fig. 4 Schematic illustration of ultraprecision cutting by 3-axix FTS

-7-CAD ModelCAD/CAM
SystemCL FileCL File
InterpretationLOADTL/2
FEDRAT/10000.00,MMPMRAPIDGOTO/0.000,0.000,50.000Tool Path Geometry
Trajectory
Generationy y
x xsPi+1Pi+1
PiPi
Pc
r(t), r(t), r(t)
Axis
CommandsDisplacement
Feedrate
AccelerationS
S
S
SJerk
Feed Profilingt
t
t
tAxis Motion
Tracking ControlAxis Servo Loop
Feed Drive
Servo Dynamics
Feedback
Actual Axis Position
Actual Tool Position with Servo Errors
Tracking and Contour Error Estimationʜ

Fig. 6 System architecture of virtual CNC 4)About the author
Toshimichi Moriwaki, Dr. of Engineering
Professor, Kobe University Faculty of Engineering
1966 Graduates Kyoto University Faculty of Engineering, Department
of Precision Engineering
1968 Receives Masters degree from Kyoto University Graduate
School of Engineering, Department of Precision Engineering
1974 Becomes Assistant Professor of Kobe University Faculty of
Engineering
1975 Becomes Assistant Professor of McMaster University (Canada)1985 Becomes Professor of Kobe University Faculty of Engineering
(2000ʙ2004 Dean of Faculty of Engineering)
Specialties: production systems, machine tools, ergonomic engineering
Select prizes and awards (not all listed)
International Academy for Production Engineering (CIRP) F.W.Taylor medal 1977Machine Tool Engineering Foundation Encouragement Prizes1991, 1994, 1995, 1998, 2001Iue Cultural Prize (Science and Technology Division) (Iue MemorialFoundation) 1998Japan Society for Precision Engineering Prize 2002, 2003
Hyogo Prefecture Science Prize (Hyogo Prefecture) 2004Trends in Recent Machine Tool Technologies
In an effort to further advance this idea, under the
title of Virtual Machine Tools, the previouslymentioned STC-M in CIRP is currently attempting toperform complete simulations that cover machiningprocesses, dynamic characteristics and controlcharacteristics for machine tools.
This simulation scheme is, for example, capable of
complete computer simulation of a machine and itsprocesses, allowing it to determine how variouscomponents of a machine tool will react when amotion command is given to the machine. It can alsodetermine how the tool and workpiece will interact witheach other to machine the workpiece and how theresultant cutting force will affect the machine and tool.The architecture of one example of such a simulationscheme, the virtual CNC system,
5)is schematically
shown in Fig. 6 .
In order to make virtual machine tools become a
reality, further research and study efforts need to bemade on many issues, including machiningprocesses, and the dynamics and motioncharacteristics of machine tools. I want to point outthat such research and studies are steadily makingprogress to this end.
6. Conclusion
Based on my own experience, I have described
several examples of recent trends in machine tooltechnologies. The machine tool industry constitutesthe backbone of Japan’s machinery industry, and forthis reason, endeavors to achieve higher speed,higher efficiency and ultraprecision will continue withincreasing commitment. In concluding this paper, Iwant to express my wish that the staff members ofNTN continue their efforts so that NTN technologies
and products lead their counterparts in the globalmachine tool industry in the ever-demanding
challenge to achieve the ultra-high-speed and ultra-high-precision required of the main spindle bearingsthat are critical components of many machine tools.
References
1) Y. Kakino: Latest Trend of Main Spindle for NC Machine
Tool
2) Y. Altintas, M. Weck: Chatter Stability of Metal Cutting
and Grinding, Annals of the CIRP, 53/2 (2004) p619.
3) Wada et. al.: Development of Three-axis Fast Tool
Servo for Ultraprecision Machining, Proc. 6th Int. Conf.of euspen (2006) p115.
4) H. Senda et al.: Main spindle thermal deformation
estimation with the goal of mass production (2nd report),Japan Society of Mechanical Engineers Journal SeriesC, 71-709 (2005), p. 2813. (In Japanese)
5) K. Erkorkmaz et. al. : Virtual Computer Numerical
Control system, Annals of the CIRP, 55/1 (2006) p399.