CIRPTemplatev4.0

MaterialsinmachinetoolstructuresHans‐ChristianMöhring(2)a,*,ChristianBrecher(1)b,EberhardAbele(1)c,JürgenFleischer(1)d,FriedrichBleicher(2)eaInstituteofManufacturingTechnologyandQualityManagement(IFQ),Otto‐von‐Guericke‐UniversityMagdeburg,GermanybMachineToolLaboratory(WZL),UniversityofAachen,GermanycInstituteofProductionManagement,TechnologyandMachineTools(PTW),TechnicalUniversityofDarmstadt,GermanydInstituteofProductionScience(WBK),KarlsruheInstituteofTechnology(KIT),GermanyeInstituteforProductionEngineeringandLaserTechnology(IFT),TechnicalUniversityofVienna,Austria

Abroadvarietyofmaterialscanbefoundinmodernmachinetoolstructuresrangingfromsteelandcastirontofiberreinforcedcompositematerials.Inaddition,materialcombinationsandhybridstructuresareavailable.Furthermore,innovativeintelligentandsmartmaterialswhichincorporatesensoryand actuator functionality enable the realization of function integrated structures. Consequently,material design and application disclosesmanifolddegrees of freedom regarding a sophisticated layout and optimisation ofmachine frames and components. This keynote paper presents the currentstate‐of‐the‐art with respect to materials applied in machine tool structures and reviews the correspondent scientific literature. Thus, it gives anoverviewandinsightregardingmaterialselectionandexploitationforhighperformance,highprecisionandhighefficiencymachinetools.MachineTools,Materials,Structures,Analysis,Optimization

1.Introduction

Theframestructureofamachinetoolisanessentialfunctionalcomponent inside themachining system.Main tasksofmachinestructuresaretheassuranceofthegeometricconfigurationofthemachineelementsevenunderstatic,dynamicandthermalloads,andtheperceptionandguidingofforcesandtorques.Regardingthe accuracy of amachinedworkpiece, themachine frame alsoshould absorb any disturbing mechanical or thermal effects.Figure1 shows modern structures of machining centresconsistingofbeds,columns,horizontalandverticalslides,Tables,mainspindles,andjoiningguidesandbearings.

Figure1:Modernstructuresofmachiningcentres[DMGMoriSeiki]Both,themechanicalandthermalbehaviourofamachineframe

are based on the elementary material properties (density,Young’s modulus, shear modulus, tensile strength, materialdamping, heat conductivity and capacity, thermal expansioncoefficient, etc.), the dimensions and cross sections of thestructural components, their joining and integration into theforce flowof themachiningsystem, the foundationof thewholeframe,andtheappliedloads.

1.1RetrospectionThe use of tools and devices is nearly as old as mankind.

Figure2summarisesmaterialuseinhistory.

Figure2:Materialdevelopmentandimportanceinhistory[74]Ahistoricalreviewabouttheuseofmaterialsinearlymachine

tools can be found in [175] and [54]. The improvements anddiffusionofmachinetoolshadamajorimpactonproductivityinindustrysincetheIndustrialRevolution1775‐1830.Priortothat

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time,practicallyallmachinerywasmadeofwood.By theuseofcokeratherthancharcoal in1784 ironbecamecheapenoughtobeamajorindustrialrawmaterial.Withtheuseofironandsteelalso metalworking machinery and machine tools appeared. In1750 iron was used in machines only where wood or anothercheaperandmoreeasilywroughtmaterialsimplywouldfail.By 1830 iron was the first material considered by engineers.

The provision of iron was increased when the steam engine(1775)multipliedthe ironmaster’ssupplyofpower.Therapidlyincreasinguseofsteamenginesinturnincreasedthedemandforcast iron. The use of metal instead of wood was a kind of“breakthrough”inmachinetooltechnology.FamousexamplesarethelathesofHenryMaudslay(Figure3).

Figure3: Lathe from 1750 (left) [175] and drawing from 1841 by

James Nasmyth of a lathe with slide rest by Henry Maudslay (right)[ScienceMuseum/SSPL]In [138] some early design rules can be foundwhich already

emphasisethetargettousethematerial(castiron)aseffectivelyaspossibleandtoconsidertheprocessandforceflowsensitively.The structural layout was depending on the experience of thedesignerratherthanoncalculations[228].Foralongtimeitwasvery difficult or said to be impossible to calculate thedeformations of complex shaped structural components underload [158]. It was already understood, that closed box crosssections, welded or casted, lead to advantages with respect tostiffness and resonances (Figure4). Koenigsberger listed somemajor design aspects including material properties (tensile‐,compression‐ and impact strength, stiffness, damping, operatingcharacteristics of sliding guideways), production limits (wallthickness accuracy, residual stresses in cast iron and heattreatment),costeffectiveness,andmassreduction.

Figure4: Cellular structure of a grinding machine (Diskus Werke,Frankfurt a. M., Germany) and different types of ribbing of a lathe bed[158,247]Regarding the decision between welded or casted structure,

material costwereweighedup against labour cost of aweldingworker which led to differences of building techniques inGermanyandtheUS.Theso‐called“Peters”ribbingwasfoundto

beadvantageouswithrespect tobendingbutboxcrosssectionsleadtothehighesttorsionalstiffness.Afundamentalcomparisonofsteelandcastironregardingtherelationshipbetweenmaterialvolume, free length of a cantilever beam and bending heightunder load can be found in [247]. In 1917 Schlesinger tried tosubstitutecastironformachineframesandslidewaysbycementconcrete because of the lack of metal material due to the firstworldwar[248].Thewearof theconcreteslidewaysprohibitedthe success of this technology at that time. The competition ofwelded steel frames and cast iron structures also concernedformingmachines(Figure5).Schlesingerpointedout that if lowworkpiece surface quality is accepTable in a process (e.g. shearpresses),steelstructurescanbeusedwiththeaimtoexploittheirstrength.

Figure5:Frictionscrewpresswithsteelframe(HenryPels,Erfurt,Germany)[247]andSCHULERpress(1928)Figure6displaysdifferentbaseplategenerationsforacolumn

stand drilling machine including stepwise improved ribbings.Weldedalternatives ledtoaweightreductionof32%comparedto cast iron. Benjamin [33] and Fischer [87] showed furtherexamplesforcastedandweldedmachinetoolstructures.Besidesthestiffnessimprovementbysupportingribstheconsiderationofthe chip flow can be seen. Haas presents a variety of weldedconstructionsin[105].

Figure6:Differentgenerationsofthebaseplateofacolumnstanddrillingmachine(Raboma‐Maschinenfabrik,BerlinBorsigwalde)[247]1.2GeneraloverviewTheaspiredstructuralcharacteristics(static/dynamicstiffness,

fatigue strength, damping, thermal and long term stability, low

weight) of amachine tool depend on the physical properties ofthe used materials as well as on the layout and shape of thecomponents. Regarding the variety of available materials,basically metal, stone, ceramic, polymer concrete, porous, andreinforcedcompositematerialscanbeseeninmachinetoolsandcomponents[197].Inaddition,materialcombinationsandhybridstructures are often applied. Research approaches alsoincorporate intelligent or smart materials providing inherentsensorand/oractuatorcapability.Figure7summarizesthemajorclassesofmaterialswithrespecttodensity(specificweight)andstiffness (Young’s modulus). Regarding lightweight design,density‐specific mechanical values (such as Young’s modulusdividedbydensity)becomemoreimportant.

Figure7:Doublelogarithmicmaterialselectiondiagram[116]Schulz discussed the requirements onmachine structures and

components for high speed machining in [252]. Lightweightconstructionisdesiredformovingcomponentsandhighdampingandstabilityshouldbeprovidedbythemachinebase[277,291].Tlusty pointed out that evenwith the best strategies andmostpowerful drives a high cornering accuracy cannot be achievedwhen moving large masses. The importance of structuraloptimisation and lightweight design is evident [276]. Forprecision applications, thermal and long term stability ofstructuresbecomesessentially important [190,246].Today, thevariety of materials and material combinations is huge. Theselectionofthebestmaterialforaspecificmachinestructureisacrucialbutchallengingtask.As mentioned by Schellekens, DeBra equalizes “design for a

high stiffness”with “designing for aminimumpotential energy”[246]. Thismeans to correctly placematerial in the right shapewhileusingaslittlematerialaspossible(i.e. lightstiffdesign).Agooddesignleadstoauniformlydistributedloading.Ideally,thestress levelunder loadshouldbethesameforallmaterialused.Thecharacteristicsoftheprocesseswhichhavetobeconductedby themachinealwayshave tobe considered [39,18,317,109,70]. A structural approach to exploit materials most effectivelywithrespecttostiffness/mass–ratioconcernsparallelkinematics(PKM)[297].However,onlyfewPKMmachinetoolscanbefoundinindustrymainlyduetocostandcomplexity.Regardingprocessdynamics and chatter stability, structural stiffness and dampingarerelevant[8].Althoughextensivelystudied,materialdampingis difficult to quantify and the machine tool designer mustgenerallyrelyonempiricalresults[246].Materialdampinghighlydepends on alloy composition, frequency, stress level and type(tension or shear), temperature, and joint preload. Due tomechanisms of friction and micro‐slip, structural jointssignificantlycontributetothestructuraldamping[291,102,47].The topic of joints inmachine structures could probably fill anownkeynotepaperandisthereforeleftouthere.

Obviously, the thermal properties of materials and structureshavea significant influenceon theaccuracyandperformanceofthe machines [295]. Most important are the values of thermalconductivity, specific heat capacity and thermal expansioncoefficients (Table1). In addition, thermal properties also affectthemechanicalbehaviourofmachinetools[187].

Table1:Thermalparametersofmaterials[277]

[g/cm3]c[J/kgK]

[W/m∙K]

a[m²/h] 10‐6/K

Struct.SteelCrNi‐steelCastironConcrete,hydr.Polymerconcr.AluminiumInvarGlass

7.857.907.202.102.312.70

‐2.70

460477450780882896450830

5014.5500.91.5229

‐0.66

0.0590.01390.0530.0022

‐0.341

‐0.0012

11.0‐

9.0‐

15‐

1.4‐

:density; c: specificheat capacity; :heat conductivity; a: temperatureconductivity; :thermalexpansioncoefficientOptimal structuraldesignbecomes increasingly importantdue

to limited material availability, energy efficiency andenvironmentalimpact,andtechnologicalcompetition,allofwhichdemand lightweight, low‐cost and high‐performance structures[124,163].Anotheraspect is thecarbonefficiencyofmachinery[81,268].Themining,provisionandprocessingof thematerialshastobeconsidered[53,74](Figure8).

Figure8: Strength and specific energy consumption of various

materials[74]The followingchapters first introduce thematerialswhichare

mostly used in machine tool structures, their basic physicalproperties and exemplary applications. Subsequently, methodsfor structure layout and optimisation are briefly discussed.Finally,intelligentmaterialsandstructuresareintroduced.

2.Materials,characteristicsandapplications

2.1Steel,castironandmetalmaterialsSteel,castironandmetalmaterialsstillarethemostlyapplied

class of materials inmachine tools. Metallic structures are also

usedforavarietyofhybridandcombinedmaterialandstructuralsolutions.Metalcomponentsareatleastrequiredformechanicalinterfaces,joints,guidesandbearings.In conventional structural design, the competition between

welded steel structures, steel casting and cast iron componentscontinues.Thegeneraldesignrulewhichassignscastingstohighvolumeproductionandweldedstructurestosmallbatchorsinglepieceproductionisoftenneglectedbymachinetoolbuildersdueto technical reasons. Whereas cast iron provides beneficialmaterialdampingcharacteristics,weldedsteelallowsformaterialand mass savings due to the higher young’s modulus. Forcastings, expensive moulds have to be produced which can beavoided by welded construction. On the other hand, complexribbingsandintegralstructurescanbecastedmoreeasily.Theexistingandstill rapidlydevelopingvarietyof steelalloys

andcast ironmaterialscanhardlybesummarized inabriefbutcomprehensive way. In addition, heat treatment and hardeningstrongly influences the material properties. Figure9 depicts aroughclassificationofironmaterials.

Figure9:Classificationofironmaterials[245]

Table2:Mechanicalpropertiesofdifferenttypesofsteel[245] Tensile

strengthRm[MPa]

YieldstrengthRe[MPa]

Breakingstrain[%]

Typicalapplication

Unalloyedlow‐carbonsteelC10EC20D

640‐780380

390205

1325

SheetmetalStructural

steel

HighstrengthlowalloyedsteelS420NLS500N

500‐680

560‐740

320‐420

400

19

16

Constr.forlowtemp.

Truckframes

QuenchedandtemperedunalloyedsteelC40(1.0511)G10800(UNS)C100S(1.1274)

580800‐1310

1470‐1670

320480‐980

1275

1624‐13

6

Cranks,boltsBits,hammerKnifes,saw

blades

Quenchedandtemperedlow‐alloyedsteelG40630(UNS)30CrNiMo8(1.6580)51CrV4(1.8159)

786‐23801250‐14501100‐1300

710‐17701050

900

24‐499

Springs,toolsBearingbushingsShafts,

pistons,gears

AusteniticstainlesssteelX2CrNiMo18‐14‐13(1.4435)

500‐700 200 40 Weldedstructures

Steelconsistsofferrousmetalalloyswithlessthan2%carbon.

Dependingonthealloyingcomponentsandheattreatment,very

different physical properties (ductile, high‐strength, hardened)can be achieved (Table2). The classification of steel grades isdefined by national and international standards (e.g. DIN EN10020,DINEN10027,SAEsteelgrades,ISO4948).In general, steel sheetmetal parts are assembled bywelding.

Duetotheweldingheat,residualstresses,bendinganddistortionof the component can occur, which has to be corrected bysubsequent production steps. Sheet metal parts are alsoassembled using bolts and screws. In this case, the resultingstructural stiffness and damping behaviour depends on thesurfaces and stresses of these bolted joints.With respect to theprincipleloadsinsideasteelstructure,amaximumareamomentof inertia in the force flux shall be achieved by the sheetmetalarrangement [277, 291]. This leads to box type or ribbedstructures[20].Thesheetmetalpartscanbebreachedinordertoreducetheweightofthecomponentandtoimproveaccessibility.In [202] principle ribbing geometries for a lightweight weldedsteel slide are analysed. In Figure10 the different ribbings areshown as well as normed displacements which occur if atheoreticalbendingloadisapplied.ThefinallyassembledslideisshowninFigure11.

Figure10:Differentwallconstructionsforaslide[202]

Figure11:Weldedsteelslideconstruction[202]Several approaches use thin walled sheet metal structures in

order to achieve light but stiff components. In [257] an ultra‐precisiondiamondturningmachinewithhoneycomb‐likebondedandsandwichedsteelbedisintroduced,whichprovideslowmassand high dynamic rigidity (1st eigenmode above 1.2kHz) withdirectional orientation. In [67] a welded honeycomb steelstructurewas applied for a gantry slide in a high speedmilling

machine(Figure12).Thehoneycombtubesarewelded togetheronly with a top and bottom plate in order to utilize frictionbetweenthemetalsheetsfordampingimprovement.

Figure12:Honeycombslidestructure[67]Amachineforlasercuttingandhighspeedmillingconsistingof

lightweight sheet metal structures is introduced in [113].Sandwichdesignswithvariouscorestructures(honeycomb,tube,grid) are applied (Figure13). For the same bending stiffness,weight savings up to 40% can be achieved compared to analuminiumdesign.

Figure13:Lightweightsheetmetalstructures[113]

Welded steel structures are often used as shells for concreteandmineralcasting,or filledwithsand,oilor foams inorder toincrease structural damping. In [302] a high precision turningmachine base applying a lightweight steelweldment filledwithsynthetic granite has been built. Cast iron rails for guidewayshave been bonded onto this structure by epoxy based bondingmaterial.Asimilarapproachwasusedin[303]foralargemirrorgrindingmachine.Cast iron compared to steel is characterised by a higher

percentage of carbon. Depending on the composition and heattreatment, different types of graphite (lamellar, spheroidal,vermicular) are built inside the material (Figure14) leading todifferentmaterialproperties[3].Lamellargraphiteleadstohighmaterial damping values and compressive strength but lowtensilestrengthduetointernalnotcheffects.Spheroidalgraphiteprovides a lower material damping but higher tensile strengthandaveryhighbreakingstrain.Basicallythestrengthdependsonthe carbon content and the matrix type. Whilst for steel, theyoung’smoduluscanbeindicatedprecisely,forcastironareasofdispersal appear due towall thickness and load scenarios. Castironwith lamellar graphite shows a nonlinear relation betweenstressandstrain.Theyoung’smodulusisderivedfromtheinitialgradient of the stress‐strain‐curve. A well‐known method toenhance structural damping of cast iron components is to keepsand cores from the casting process inside the structure [277].Sun et al. presented an approach to predict the loss factors of

sand‐filledstructuresin[273].Wakasawastudiedthepackingofmachinestructureswithballs[288].

Figure14:Lamellar(left),vermicular(middle)andspheroidalgraphite

(right)incastiron[27]AclassificationofcastironcanbefoundinDINEN1560‐1564

whereas cast steel for general purposes is classified in DIN EN10293. Exemplary cast iron grades for machine tool structuresareGJL‐300(lamellar)andEN‐GJS‐400‐18(spheroidal).Castironmaterialsevenallowweldingandhardening.

Figure15: Milling machine frame (EN‐GJS‐400‐18) by conpany SHW,

Aalen‐Wasseralfingen,Germany[27]Even complex shaped and ribbed structural parts can be

produced by casting (Figure15). Some design rules have to beconsideredtoavoidmaterialaccumulation,tocreatenodalpointswithout stresses, and to achieve a good form filling and thusaccurate geometry. A critical issue are blow holes which canoccur during the casting process and which degrade thestructuralpropertiesuptotheinitiationofbreakage.

Table3:Mechanicalpropertiesofdifferenttypesofcastiron[245] Tensile

strengthRm[MPa]

YieldstrengthRe[MPa]

Breakingstrain[%]min.

CastironwithlamellargrahiteEN‐GJL‐150(0.6015)EN‐GJL‐200(0.6020)EN‐GJL‐300(0.6030)

110‐135145‐180220‐270

‐‐‐

‐‐‐

CastironwithspheroidalgrahiteEN‐GJS‐450‐10(0.7040)EN‐GJS‐700‐2(0.7070)EN‐GJS‐600‐3(0.7060)

450600‐680500‐580

310370‐410320‐360

101.0‐2.01.0‐3.0

AnnealedcastironEN‐GJMBW‐350‐10EN‐GJMBW‐450‐6

350450

200270

106

CastironwithvermiculargraphiteEN‐GJV‐300EN‐GJV‐500

250‐300400‐500

175‐210280‐350

2.00.5

Due to the casting heat and subsequent cooling, residual

stresses appear inside casted components. Sensitive annealing,“aging”orvibrationareapplied to lower these residual stressesandtoachieveabetterlongtermstability[277].

Constantly new alloys are developed which can haveadvantageous properties with respect to machine applications.Low melting alloys can be used for clamping of complexworkpieces [13]. These materials possess solid state at roomtemperaturebut canbemelted to liquid state by slight heating.Highdampingmetalmaterials (HIDAMETS) are investigated forsome decades [22] (Figure16). Vandeurzen et al. analysed highdamping materials (“Proteus”: CuZnAl, “Gentalloy”: FeCrMo,“Sonoston”: MnCu, “Nitinol”: TiNi, “Vacrosil”: Fe‐alloy) in [283].Thedynamicproperties,elasticmodulusandlossfactorappearedto be dependent on the temperature, frequency, static anddynamicstress.

Figure16: Tensile properties and specific damping capacities of

variousalloys[22]Inadditiontomechanicalproperties,thermalissueshavetobe

considered in machine design. Most important characteristicvalues of materials are the heat conductivity, specific heatcapacityandcoefficientofthermalexpansion.

Figure17: Features for thermal design optimization and thermal

displacementinX‐directionafter10hspindlerotation[198]

In[198]thelayoutofribbings,casingsandstructurewallswithrespect to thermal deformations of a lathe headstock isinvestigated.Figure17depicts thestructural features fordesignoptimization. By Taguchi method and Finite Element Analysis(FEA), the critical thermal displacement in X‐direction wasminimized. The influence of different wall thicknesses on thethermal behaviour of structural components of a large portalmillingmachineisdiscussedin[295].Inasakietal.presentedanultra‐precision polishing machine which applies low thermalexpansion (LTE) cast iron [130]. LTE cast iron is alsodiscussedbyHashimotoin[109].Besidessteelandcastiron,alsoothermetalmaterialsareused

inmachinetoolstructures.Lightmetalalloyssuchasaluminiumalloys have a significantly lower density compared to steel andcast iron. Thus, masses of movingmachine components can bereduced.Ontheotherhand,wallthicknessescanbeincreasedinorder to reduce local strainmaintaining the componentweight.Figure18showsacrossslideofahighspeedmachiningcentrebycompanyMAPmadeoflightmetalalloy.Evenintoolbodies,lightmetal alloys havebeen applied as can also be seen inFigure18[116].Reducingthemassandinertiaoffastrotatingtoolsallowsforshorterrun‐uptimesandenergysaving.

Figure18: Light metal alloy cross slide (left) [MAP

Werkzeugmaschinen GmbH, Magdeburg] and magnesium milling head(right)bycompanyW.FetteGmbH[116]In [284] aluminium is applied for the structure of a high

precision 3D‐Coordinate Measuring Machine (CMM). Thismaterial was selected because of the thermal sensitivity forgradients /, which is very low due to the high thermalconductivity and the high thermal diffusivity /cp. Theinfluenceof thermalexpansion isminimisedbya shortdistancebetween the probe and the measuring system and mechanicalthermallengthcompensation.In ultra‐precision machines and metrology frames materials

with minimum thermal expansion are required. The frequentlyappliedInvar(Fe‐Nialloyswithtypicallyabout64%Feand36%Ni,upto1%Mn,Si,orcarbon,andupto5%Co)providesaverylow or even negative thermal expansion coefficient. In [19] aframe structure of a miniature 4‐axis machine tool applyingInvar‐36 alloy combined with a granite base is introduced.Machining tests show deviations between 0.1 and 0.52µm. In[229] a metrology frame for a compact high‐accuracy CMM ismadeofInvar.Avolumetricstandarduncertaintyof19nmcouldbeachieved.Inovco(63%Fe,32%Ni,5%Co),alsocalled“SuperInvar” provides an even lower thermal expansion coefficient of0.72x10‐6K‐1comparedtoInvar‐36with1.6x10‐6K‐1[245].Thethermal expansion of a tool holder was minimised with SuperInvar by Moriwaki et al. [199] leading to significantly higherstraightnessinturninglongworkpieces.2.2 NaturalstoneandceramicsIndustrially mined hard stone is used since decades in

measuring plates, standards and etalons as well as structuralparts of measuring and high precision production machines.

Commonly used natural stone grades are Gabbro Impala, Tarn,FineBlack,BlackGalaxyor JiNanBlack(Table4) [133].Granitecan favourablybeusedasabase forprototypemachinesdue toitshighandfastavailability.

Table4:Technicalvaluesofnaturalstonematerials[133] Impala

(SouthAfrika)

BlackGalaxy(India)

Ji NanBlack(China)

Tarn(France)

Density[kg/dm³] 2.90 2.90 3.00 2.90Compressivestrength[N/mm²]

300 190 250 180

Bendingtensilestrength[N/mm²]

20 19 26 24

Young’smodulus[kN/mm²]

90 44 70 46

Thermalexpansioncoefficient[10‐6K‐1]

6.5 6.0 5.0 6.0

NaturalstonematerialsfollowHook’slawandcanbeanalysed

by linear FEA calculations. Furthermore they are antimagnetic,non‐conductive,stainlessandtheydonotgenerateburrs.Graniteframesprovidehighdamping,lowthermalconductivity(3.2W/mK),lowthermalexpansion(0.005–0.006mm/mK)andhighlongterm stability due to the absence of residual stresses [103].Granite isacrystallinehardstoneconsistingofquartz,micaandfeldspar. Its properties differ depending on the origin of thematerial. With smaller grain size, the mechanical propertiesincrease. Fine grain granite achieves Young’s moduli of 65 –113N/mm²andapressure strengthabove180N/mm².Beinganatural product, these values of granite scatter.Due to thehighhardness (850‐900HV), abrasion resistance and homogeneoussurface, granite is suiTable for aerostatic and hydrostaticbearingsandguides [85].Wegenerrealizedanaerostaticplanarguideusingagranitebase[298](Figure19).

Figure19:Planarguidewithgranitebase[298]Indesigningwithgranite,knowledgeaboutstoneprocessingis

essential.Processingpredominantlyincludessawing,drillingandgrinding.Bygrinding,astraightnessandplanarityof5µm/mcanbe achieved. Granite is often combinedwith glued steel inserts(bushings, T‐slots)which providemechanical interfaces. Thus acombinedprocessingof bothmaterials isnecessary.By lapping,dimensional allowance of IT1 can be achieved. Due to theprocessingtechnology,graniteframesconsistofprismaticblocks.Because of its microstructure, hard stone should be loaded bypressure.Thematerialproperties regarding tensileandbendingloadsaremuchsmaller.Theforceflowinsidethestructurehastobeconsideredcarefullyinthedesignprocess.Formachineswithhigh accelerations, a ratio of 1:10 between moved and fixedmasses should be respected. Since mostly a combination ofgranite with steel is applied, the thermal properties of bothmaterials have to be taken into account in structural layout inorder to avoid bending and stresses in interfaces. For theassembly of multiple granite components, screwing and

conglutination are mostly applied. Since granite can absorbmoisture,itisoftensealedwithverythinepoxyresin.

Figure20: Grinding of granite frame (left) and linear guides and

bushingsinagraniteframe(right)[103]Granite is often used for high‐precision and metrology

applicationsbecauseofitsformstability.Abdinetal.anddeBruinstudied the dimensional stability of cast iron, granite, polymerconcrete and graphite composites [1, 65]. It is pointed out thatthe long term stability even of granite is limited. There is asignificant sensitivity with respect to the influence ofmoisture.However, also due to its thermal and damping characteristics,granite is the dominant material for CMMs and ultra‐precisionmachines[76,279,284,41,43,126].InFigure21acompactfiveaxesgrindingmachinebasedonagranitestructureisshown.

Figure21:Compactfiveaxesgrindingmachine[41,43]Since thermal stability, high stiffness and lowmasses are key

issues in high and ultra‐precision machine tools, also ceramicsare used [189, 190, 289]. Table5 gives an overview aboutcharacteristicvaluesofsomeselectedceramicmaterials.

Table5:BendingstrengthandYoung’smodulusofceramics[245] Bending

strength[MPa]

Young’smodulus[GPa]

Vickershardness[GPa]

DiamondSiliconnitride(Si3N4)Zirconiumoxidea)(ZrO2)Siliconcarbide(SiC)Aluminiumoxide(Al2O3)GlassceramicsMullite(3Al2O3–2SiO2)Spinel(MgAl2O4)Magnesiumoxide(MgO)Quartzglass(SiO2)

‐250‐1000800‐1500100‐820275‐700

247185

110‐245105b)110

‐30420534539312014526022573

130,016,011,725,426,5

a) Partiallystabilisedby3%Y2O3b) Sinteredandwithapprox..5%porosity

Ceramics are inorganic materials mostly consisting of metals

andmetalloidswith ionicbutalsocovalentbondingandvarious

complexcrystalstructures[245].Ceramicspossessratherbrittlecharacteristics.Due tomicrocracks the tensilestrength ismuchlower than the compressive strength. The tensile strengthstrongly depends on the amount of internal failures and variesamong samples of the samematerial. The desired properties ofceramics are achieved by high temperature treatment (baking).Generally, ceramics possess low thermal expansion. A lot ofceramics exhibit a remainingporositydue to themanufacturingbased on powder. For aluminium oxide (Al2O3) there is a non‐linear decrease of Young’s modulus and bending strength withhighervolumeshareofporosity.Technical ceramics are commonly used in applications where

high resistance is required [246]. Due to the low density(3.16g/cm³),highYoung’smodulusandhighhardnesscomparedtobearingsteel,siliconnitride(Si3N4)isusedinhybridbearingsforhighfrequencyspindles[2].Ceramics likeB4C,SiC,Si3N4,Al2O3havehighrefractoryvalues

and extremely high resistance against wear, corrosion anderosion. Ceramic parts may show considerable non‐uniformshrinkage deformation. Therefore, uniform sheet thickness,relatively simple structuresand theabsenceof sharpedges andpointoflineforcesareessential.Vibrationdampingofceramicsispoor. A laminated build‐up of thin ceramic tiles is preferable. Afactor of 3 in mass reduction is possible compared to analuminiumorsteelplateframestructurewiththesamestiffness.Furukawa summarized the benefits and drawbacks of usingaluminium ceramics in machine structures [94]. Shinno et al.appliedaluminaceramics(Al2O3) foranumberofhighprecisionmachine components [257, 258, 259, 260, 261, 262, 263, 308].Lowthermaldeformationandalightweightbutstiffconstructioncouldbe achieved. In [262] anAl‐ceramicsTablewas combinedwith a granite frame structure in order to achievehigh thermalanddynamicstabilityofa3Dprofilescanner.Aspatialnanometerresolution was obtained with the assembled system. Yoshiokaand Shinno presented an aluminium ceramics structure for anano‐pattern generator in [308] where the bed, columns, topbeams and positioning systems consist of Al2O3 (Figure22). Aradial errorduring circularmotion in theX‐Y‐planeof less than4.5nmappeared.

Figure22:Advancednano‐patterngenerator“ANGEL”[308]Vermeulen introduced a single point diamond turning (SPDT)

machinewithceramicslideshavingsub‐µmaccuracyandmirrorsurface quality [285] (Figure23). The spindle is held in atriangular frameproviding a vertical symmetry axis for thermalexpansion.Thisframewasattachedtoaflatgranitebaseplate.Ahorizontalgranitebeamservesasguidewayforthecrossslide.Anoptimisationofstiffnesspermassfortheslideswasconductedbystructural design and material selection. Multi‐layer aluminaceramic laminate leads to an increase in specific stiffness byfactor3 compared to steel.The structural loss coefficientof theepoxy resin adhesive amounts to about 6% giving an overallstructural loss factor of 3‐4%. The ceramic laminate material

offersextensivepossibilities indesignascomparedtostructuraldesigninsolidmonolithicceramics.

Figure23:SchematicrepresentationofSPDTmachine[285]Sriyotha et al. applied aluminium ceramics for the entire

structure of a super precision machine an achieved a motionaccuracy of 1nm [266]. Company ZEISS applies an innovativesilicon carbide ceramics for the new XENOS CMM [313] whichprovides around 50% lower thermal expansion, 30% higherrigidityand20%lessweightthanwhitestandardceramics.A fine‐grain ceramicsmaterial, calledNEXCERA, is introduced

in[218].Thepoly‐crystallineoxideceramicsprovidesnearlyzerothermalexpansionbyitsanisotropicstructure.Figure24depictsacomparisonofNEXCERAwithothermaterials.

Figure24:ComparisonofNEXCERAwithothermaterials[218]In[302]acalibratedZerodurreflectivestraightedgeisusedas

optical reference for a plane mirror interferometer type lasertransducer inside a large high precision diamond turningmachine.ZerodurisanonporousLi‐Al‐siliconoxideglassceramicmaterial with very low thermal expansion and high materialhomogeneity.Thelowexpansionisachievedbyacombinationofa glass phase with positive and a crystal phase with negativeexpansion coefficient. Besides measuring devices, Zerodur isapplied in metrology frames [110]. Optical industry uses lowexpansion materials such as Invar, super Invar, fused silica,titanium silicate glass, and glass‐ceramics. Zerodur is the mostfamous low expansion glass‐ceramics in optical use. In [192]

Zerodurisusedfortheguidewaysofahighprecisionlinearslide.Nambaintroducedaspindlemadeofglass‐ceramics(Neoceram)for an ultra‐precision surface grinder in order to achieve zerothermalexpansion[203].2.3Polymerconcrete/mineralcastingA comprehensive introduction of polymer concrete, mineral

castingor reactive resin concrete is given in [251,162,72,131,132, 30, 26]. DIN 51290‐3 provides a standard for testing ofpolymerconcreteforuseinmechanicalengineering.In 1944 company BOEHRINGER built a first lathe bed with

cementconcrete.Up tonowcementconcretebottompartswithcast iron upper components are combined to achieve improvedsystemdamping. In thiscase, theconcreteparthasno influenceon the machine accuracy. Sugishita introduced a machiningcenter with Portland concrete bed and column combined withcast ironplates[269].Thestaticstiffnesswasmadecomparabletoacastironstructure.Thedynamicstiffnesscouldbeincreasedtogether with a reduction of natural frequencies. The thermalcharacteristicswere improvedby the integrationof aheatpipe.Theconcretestructureshowedahigherthermalinertiathancastiron.Forthismaterialerosionduetocuttingfluidandshrinkageduetothedryingprocessmustbeovercome.Toavoiderosionanepoxy polymer coating can be applied. In 1983, company EMILPRINZIG filled steel frameswithhydraulicbonded concrete andimproved the static, dynamic and thermal behaviour of weldedsteel constructions. New nano‐structures nowadays allow thedevelopment of ultra‐high strength concrete materials (UHPC).This material is used in machine frames without metal casing[242]. Kleiner applied UHPC for sheetmetal hydroforming dies[156].Comparedtonormalconcretemadeofcement,waterandaggregates, UHPC contains additives such asmicrosilica and/orflyash,aswellasadmixturessuchassuperplasticizers.Since 1970s, cold‐curing reaction resins are available which

enable the production of polymer bonded mineral casting orpolymer concrete for high precision machine frames. Polymerconcreteormineralcastingdescribesacompositematerialthatisobtainedbymixingafillermaterialsuchassand,marble,quartz,pearlite,glass,fibre,dolomite,steel,orcarbonfibreswitharesinsuchasunsaturatedpolyester,poly‐methylmethacrylate[215],orepoxy, and by adding a catalyst or accelerant at roomtemperaturethatallowstougheningthroughpolymerization[17].Sometimes also hydraulic bonded concrete is called mineralcasting. The type and percentage of filler material and bindingagentdiffer significantlywithrespect toapplication. Inmachinetool structures predominantly epoxy resin bonded mineralcasting is used for bed components, frames, columns andsupports. The casting process takes place at room temperatureandrequiressometimescomplexandexpensivemouldsmadeofwood or metals. Since no external heating is necessary, theproduction of mineral cast components requires 20‐40% lessprimaryenergycomparedtocastironorsteel.Beside high Young’s modulus and damping as well as low

thermalexpansion,lowresidualstresses,minimalshrinkageandhighreproducibilitycanbeachieved.Alongprocessingtimeanddischarge of heat of reaction are essential. Hardening ismostlydonewithacuringagent.Asfillermaterials inmachinebuildingmostlyinorganicmineralfillers(quartz)andstones(graniteandbasalt)areused.Insomecasesalsoaluminiumhydroxide,siliconcarbide, iron powder or glass balls are applied. The optimalmatchingof fillermaterialswithdifferentgrainsizes toagrain‐size‐distributioncurvewithhighpackingdensityallowstoreducethe interspace to aminimumand to guide the loads at the finalstructure over the carrying grainmatrix. Theoretical grain‐size‐distribution curves can only approximately be achieved inpractice.Grainsizesreachfrombelow0.1mm(rockmeal)to0,1

– 2mm (sand) and up to 16mm (flint). For achievingreproducible material properties, dosing of resin and curingagent as well as of the filler mixture (with up to ten grainfractions) is essential. Themass percentage of filler material isapprox. 90 – 93%. Optimal bonding strongly depends on thewetting of the filler material by the epoxy resin. By rockingmotion of the casted mixture, de‐aeration and a decrease ofporosity is achieved. The exothermic epoxy reaction leads totemperaturesofup to50°C.Curing takesusually12–14hours.Theeffectofmoistureonthethermalandmechanicalpropertiesandcuringprocessofpolymerconcretewasinvestigatedin[107].A significant influence could be recognized when changing themoisturecontentbetween0%and5%.Regarding natural frequencies and modes, mineral casting

corresponds to models for isotropic, homogeneous materialsfollowingtheHooke’slaw.SahmidentifiedslightlyhigherYoung’smoduliincompressivecomparedtotensiletests[243].Withhighgrainsize(16mm)aYoung’smodulusofupto50kN/mm²canbereached.Comparedtosteelandcastiron,mineralcastingprovideslower

thermalexpansioncoefficient,muchlowerheatconductivity(1–3 W/m K compared to 50 W/m K) but higher specific heatcapacity. Thus, thermally more sTable machine frames can beproduced. Due to the low heat conductivity inhomogeneoustemperature fields can occur by influence of internal heatsources. On the other hand, mineral casting allows the directintegration of cooling circuits during casting [250]. In order toenable hybrid structures avoiding stresses at interfaces, thedevelopment of mineral castings with thermal expansioncoefficients adapted to cast iron or steelwas necessary. On theother hand, mineral cast with very low thermal expansioncoefficients (7 x 10‐6 K‐1) are available. Dou investigated thethermal deformation behaviour of mineral cast – metal –compositestructures[77].Theapplicationofmineralcastingorpolymerconcretereaches

from high performance and high speed machine tools to ultra‐precision machines and metrology applications. The success ofmineralcastingispredominantlycausedbytheexcellentdynamicproperties. In [222] it was observed that the critical dampingratioofpolymerconcretecanbefourtoseventimeshigherthanthatofcastiron.Kimanalysedtheinfluenceofcompactionratio,sizes and contents of ingredients on compressive and flexuralstrength, thermal expansion, specific heat, thermal conductivity,Young’s modulus and damping factor of epoxy resin concrete[150]. Regarding precision machine applications a significantinfluenceoftheresincontentcouldbeobserved.Haddad investigated the influence of aggregates (basalt,

spodumene, fly ash, river gravel, sand and chalk) in [106]. Anoptimum composition, with the highest flexural strength andlowest thermal expansion coefficient, was found to be basalt,spodumeneandflyash.Theresinvolumefractionalsoshowedasignificanteffectonthethermalexpansioncoefficientandflexuralstrength. The final optimized compositionwas basalt, sand andflyash(filler87%andresin13%).In[182]theinfluenceofresin,sand and fly ash contents on the compressive and flexuralstrengthaswellassplittensilestrengthhasbeeninvestigated.Ithas been found that polymer concrete mortar can achievecompressive strengths in a range of 90–100 MPa. Tensilestrengthswere as high as 15MPa for vinylester based polymerconcrete.TheresultsshowthatthepolymerbasedfillermaterialsaresuiTableforbothcompressionandtensileloadingsituations.In[234,235]pure,fibrousandpolymerimpregnatedferrocementhave been investigated in prototypic lathe beds by modalanalysis. Ferrocement shows higher dynamic stability than castiron.Discretefiberswithalengthof5and10mmandupto2%byvolumeofthematrixalreadyimprovedtheperformance.Thebestresultswereobtainedwithpolymerimpregnationespecially

regarding shear loss modulus, flexural loss modulus, firstresonance and damping ratio. Thus, improved process stabilitywasassumedbycalculations. In [304]polymercastinghasbeenused for wear resistant deep drawing tools. Neugebauerpresented an approach of inherent thermal error compensationof machine bed structures with layers of different thermalexpansionmadebymineralcasting[211].Simulationsshowedasignificantly reduced thermal deformation. Company IFT(Magdeburg, Germany) developed sealed porous mineral cast,whichenablesaflowofcoolingfluid(Figure25).

Figure25:Porousmineralcastingforactivecooling[CompanyIFT]Some mineral casting materials including granite as filler are

calledepoxygraniteorsyntheticgranite[188].CompanySTUDERcreates the mineral cast GRANITAN. Guideways can directly bemoulded as part of machine frames in this material. Especiallybecauseof itsdampingproperties,syntheticgranite(GRANITANS‐100) has been used formajor structural parts of a large highprecisiondiamondturningmachinein[302].KrausseandDeydevelopedcalculationapproachesforstrength

approximation [162, 72]. The cracking mechanism can becharacterised by initial cracks between matrix and grains andsubsequent brittle cracking of the grains at higher loads. Theporosityofthematerialhasasignificantinfluence.Sahmstudiedthe creep phenomenon and other mechanical properties ofpolymerconcrete.Maximumvaluesforcompressive,tensile,andflexuralstrengthwereobtainedusing10%epoxyresinand8mmparticle size [243]. In [17] the rotational flexural fatigue ofpolymer concrete was analysed. Amuch lower fatigue strengthcomparedtometalswasobservedwhichappearstobelimitedto1‐1.6N/mm²correspondingto107 loadingcycles.Becauseof itssensitivity with respect to tensile stresses, polymer concrete isnearlynotusedinmovedmachinecomponentssofar.Thecrossslide shown in Figure26 by company EMAG constitutes anexception. The casted Mineralit green body possesses adimensional accuracy of +/‐0.1mm so that finish machining isreducedby80%comparedtocastiron[111].

Figure26:PolymerconcretecrossslidebycompanyEMAG[111]Mineral casting construction types are “massive construction”

of monolithic structures, “composite construction” of hybridstructures and “complete construction” (with integrated screw

stays,steelrailsandplates,coolingsystems,servicepipes,cablechannels, etc.) [214]. The cost of mineral casted componentspredominantly depends on the level of integration, level ofcompleteness,manufacturingprinciple,andweight.Besides the use of polymer concrete and mineral casting in

entiremachineframestructures,therearespecificapplicationsindiscrete components. Klaeger investigated the use of mineralcasting in fixtures for cutting machine tools exploiting thefavourable damping properties and the ability to directlyintegrate mechanical interfaces [155]. Rahman introduced apolymer impregnated concrete damping carriage for guideways[237]. Low‐density cellular concrete impregnated with MMApolymerexhibitedhigherstrength,elasticmodulusanddampingcapacity compared to a steel damping carriage up to 650Hz.Panzera investigated porous composites (high purity SiO2 silicamixedwith Portland cement) for aerostatic bearings [225]. Theeffectsofsilicasizeandgeometryaswellascompactionpressurewereanalysed.Thevarietyofpolymerconcreteandmineralcastingmaterials

led to specific naming of compositions by the providingcompanies, e.g. NMT Basaldur containing basalt and crystalquartz,UHPC‐materialNanodurby companyDURCRETE,UHPC‐material Epudur, mineral cast Epument and ray absorbingEpuramofcompanyEPUCRET,BaerlitofcompanyIZMPolycast,MineralitbycompanyEMAG,DuropolbycompanyMAP‐Prinzing,Hydropol by company FRAMAG and many others. Jackischconcludes threemajor fields of development ofmineral casting:the improvement of available and development of newmineralcasting materials, the increase of completion, and thecombination of mineral casting, adaptronics and microtechnology[131].2.4Metalfoams,porousandcellularmaterialsAn extensive overview about the manufacturing,

characterisation and application of cellular metals and metalfoamsisgivenin[25,99,290,66,119,217,267].Theproductionof metal foams is known in patent literature since the 1950s.Basically,meltmetallurgic (e.g. GASARE), deposition techniques(e.g.INCO,RETIMET/CELMET)andpowdermetallurgicmethodswere developed. The foam creating process can further bedistinguished by the discrete or prefabricated building of thepores. For powder metallurgic production, mostly aluminiummelt is used as initial material (e.g. ALPORAS method)whereuponviscosityincreasingadditives(Na,Ca)andafterwardsfoaming agent (TiH2) are added. Cooling of the closed‐cell foamtakesplaceinacloseddie.Byaddingstabilizingceramicparticles(SiC or Al2O3) the foam can be skimmed (CYMAT or ALCANprocess). Open‐cell foam can be produced by ERG DUOCELmethodinwhichopen‐cellplasticfoamisusedasapattern.ThemethodofBaumeister [28] is basedon foamingof a compactedmetal powder/foaming agent mixture by heating above theliquidustemperatureofthemetal.Aftercooling,closed‐cellmetalfoamwith60–85%porosityisbuilt.Ozanstudiedtheinfluenceof manufacturing parameters (NaCl ratio and particle size,compactingpressure)ontheporeconcentrationofpowderbasedAl‐foam [223]. The manufacturing of tailor‐made closed‐cellmetallicfoamsbytitaniumhydridedecompositionisinvestigatedin[83].Syntactic foams are produced by embedding gas filled hollow

spheres (1 ‐5mm diameter Al2O3,mullite or TiO2 spheres) in amatrix (e.g. magnesium, aluminium or rare‐earth alloys) usinginfiltration technique. The mechanical properties are highercompared to aluminium foam but also the achieved density ishigher[108,233].Sincesomeyearsthereareapplicationsofaluminiumfoam(Al‐

foam)inmachinetooldesign[204,205,206].Hipkeinvestigated

characteristic design parameters such as pull out strength ofjoints and thermal expansion [118]. Advantages of aluminiumfoams in machine tool applications are the low mass and highenergy absorption capability [120, 121]. Sandwiches with Al‐foamcoreincombinationwithsteelcasingsachieveanupto40timeshigherbendingstiffnesscomparedtomassequivalentsteelsheets due to a higher geometrical moment of inertia. Al‐foam(densityapprox.0.5g/cm³)allowsadecreaseofvibrationenergyby its cellular structure and small deformations of internal thinwalls aswell as friction in cracksofporewalls.The slideof thehighperformancemillingmachineshowninFigure27isrealisedby use of Al‐foam sandwiches with different cover sheetthicknesses. The mass of the slide could be reduced by 28%compared to a pure steel construction. Dynamic stiffness anddampingareimprovedsignificantly.

Figure27: MIKRON HPM 1850U with universal slide applying metal

foamconstruction[120,121]Aspecificratioofcoversheettofoamcorethicknessshouldbe

maintainedinordertoeffectivelyexploitlightweightengineeringpotential.In[207]itisshownthatbythespecialfoamingstrategyusing foamable pre‐material a higher peeling strength can beachievedcomparedtoaluminiumbondingtosteelbyanadhesivelayer. Smolik et al. presented a hybrid approach for a machinetoolcolumnconsistingofabasicsteelframe,foamcoresreplacingthe internal ribbing in the column sides and outer steel skins[264, 265]. The hybrid column provided 64% of weight,169%/83% of static stiffness in X‐/Z‐direction, and higher firstnatural frequencies compared to the original componentwithinthemachineassemly.KolardiscussedthefillingofweldedlatticesteelwallswithAl‐foam[160].Theapplicationoffoamleadstoasignificantdampingimprovementofthestructure.Zhao reviews the thermal transport in high porosity cellular

metalfoams[316].In[64]theinfluenceofcuttingmethodsonthethermal contact resistance of open‐cell Al‐foam is discussed.Aggogeri investigated sandwich structures filled with open cellmetal foamwhich was impregnated by phase changematerials[4]. The resulting structures provided high stiffness to weightratio,gooddampingpropertiestogetherwiththermalstability.The widespread use of Al‐foam is hindered by the high

productioncostsresultingfromtheexpensiveproductionprocessandstartingmaterials.Theideaofusingaluminiumchipsinsteadof atomized powder avoiding negative influences on themacroscopic foam structure has been studied in [122](Figure28). Good foaming rates could be achieved by calciumcarbonate as foaming agent for aluminium. Since the cost ofrecycled sorted aluminium chips are rather low and calciumcarbonate ismuch cheaper than titanium oxide, a distinct pricereductionforAl‐foamseemstobepossible.

Figure28:Al‐foambasedonchips(left)andpowder(right)[122]Sincethefoamstructureinfluencesthemechanicalandthermal

properties of a component, appropriate metrology forcharacterisationandqualityinspectionisneeded.In[178]theuseof computed tomography (CT) for obtaining statisticalcharacteristicsofopencell foamgeometriesispresentedaswellas the derivation of 3D volume elements for computationalpurposes.Kashihara fabricated and analysed lotus‐type porous carbon

steel with respect to machine tool applications [144, 145](Figure29).Anincreaseofspecificrigidity,weightreductionanddecreasedpowerconsumptionofthemachineispossible.

Figure29: Cross sections parallel and perpendicular to the transfer

direction (T.D.) of lotus carbon steel slabs fabricated in nitrogen andnitrogen/argongasatmosphere(t:thickness,w:width)[144]Another approach for light weight cellular structures applies

hollow sphere composites (HSCs) [31, 194, 128, 29, 26, 286].HSCsconsistofsmallhollowspheres(filledwithair,gasormetalpowder)withavolumefractionofupto80%andareactiveresinsystem.HSCmaterialsaremadefromceramics,silicates,plasticsormetals.WaagandAndersendescribe a spray coatingprocessfor creating metallic hollow spheres [287, 10]. The mechanicalpropertiesofsinteredhollowspheresteelfoamwereanalysedin[274]. They depend on the spherical hollow bodies (material,grainsizedistribution,wall thickness), thecharacteristicsof theresin,additives,andthevolumefractureanddistributionoftheseingredients. The thermal behaviour is mainly governed by theusedepoxyresin.In[30]acombinationofcorundumbased(0.5–1mm)macrohollow spheres and aluminium silicate Fillite (5 –300µm) micro hollow spheres is regarded (Figure30). Alsodifferent typesofhollowspheres (10–2,000µmdiameter,wallthickness of 10% of diameter) in combination with cold andwarm curing epoxy resinwith andwithout fibre reinforcementhave been investigated. For a comparison of mechanicalproperties of HSC with steel, glass fibre reinforced composite(GFRP)andcarbonfibrereinforcedcomposite(CFRP)theratioof

stiffness (Young’s modulus) to density is important. HSCmaterials showed a higher ratio than either steel or GFRP(Table6). HSCs show isotropic mechanical and thermalproperties and beneficial damping can be achieved. In [30]applications in a robot arm and machine tool component arepresented.

Figure30: Hollow sphere composite (HSC); (a) bulk material of

corundum,(b)interiorofFillite,(c)HSC,(d)interiorofHSC[30]Table6:ComparisonofHSC[30] Density

[g/cm³]Young’smodulusE[GPa]

3 E /

EpoxyresinHSC(1)HSC(2)HSC(3)HSC(4)SteelGFRPCFRP

1.150.950.90.651.167.82.61.78

3.57.86.84.18.721073235

13.221.421

24.618.77.616

34.5HSC(1):65vol.‐%Fillite+corundumHSC(2):78vol.‐%Fillite+corundumHSC(3):78vol.‐%FilliteHSC(4):78vol.‐%corundum(0–2mm)2.5FibrereinforcedandcompositematerialsFiber reinforced and composite materials are particularly

interesting for applications in machine tool structures due totheir ratio of mechanical strength to density [148, 230, 172](Figure31).Liebetrau gives an overview about carbon fibre reinforced

plastic materials with respect to its use in spindle casings in[177].Adeepinsight intothematerialengineeringofreinforcedcomposites is given by [91, 92, 139, 245]. Fiber reinforcedmaterials consist of the fiber (short or long) or particlereinforcementandabindingmatrix system.Reinforcementscanbe whisker (graphite, silicon carbide, silicon nitride, aluminumoxide, thin mono‐crystals with very high aspect ratio), fibres(polymers, ceramics or glass, aramid, glass, carbon, aluminumoxide,siliconcarbide,polycrystallineoramorphous)orfilaments(steel,molybdenum,tungsten).Thereareso‐calledE‐glassfibres(electric isolator), S‐glass fibres (with higher strength), C‐glassfibres (withhigherboron content and chemical resistancy), andboron‐freeECR‐glass fibres[116].Withrespecttocarbonfibres,standard high tenacity (HT), super tenacity (ST), intermediatemodulus(IM),highmodulus(HM),andultrahighmodulus(UHM)fibres canbedifferentiated. Themechanicalproperties of fibresare depicted in Figure32. Most commonly used are glass fiberreinforced plastic (GFRP) with fiber diameter 3‐20µm andcarbon fibre reinforced plastic (CFRP)with fiber diameter of 4‐10µm. Other fibermaterials are AFRP (aramid fiber reinforcedplastics,e.g.Kevlar).Thematrixconsistseitherofpolymerresin

(forGFRP,polyester,vinylester),Epoxy(causinghighercostbutprovidingbettermechanicalproperties thanpolyesterandvinylester and less sensitivity to moisture) and Polyimide (for hightemperature applications, e.g. polyetheretherketone PEEK). Inaddition, there are thermoplastic resin and thermosetting resin.Matrix materials in machinery applications are predominantlyepoxyresins.

Figure31: Comparison of mechanical and thermal properties of

structuralmaterials[FraunhoferIWU,Chemnitz,Germany]

Figure32:Propertiesofdifferentfibres[116]Responsible for the mechanical properties are the fiber

strength, volumetric content of fibers and matrix, fiberorientation and layer build‐up sequence, bounding surfaceconnection with matrix as well as critical fiber length.Unidirectional (UD) fiber reinforced composites possess thehighest mechanical performance in fiber direction. The fibredirection relative to the structure internal force flowshas to be

considered carefully during structural design [16]. In [185] theinfluenceofthefibreorientationonasmartcompositestructureisanalysed.In“hybrid”compositesmultipledifferentfibertypesare embedded in a matrix (e.g. carbon and glass fibers in onepolymermatrix).Bythis,anisotropicpropertiescanbeachieved.Withrespecttothethermalresistanceofthereinforcedmaterials,theglass transition temperaturehas tobe taken intoaccountatwhich the solid state of the matrix dissolves. In sandwichstructures, covering layers enclosing a core (e.g. aluminiumhoneycomb core) are built. Sandwich cores can also be used toimprove damping [166, 220]. Stiffnesses comparable to that ofsteelstructurescanbeachievedbyCFRPdependingontheusedfiber, combined with a mass reduction by up to 80%. With anappropriate orientation arrangement of the fibers, a thermalexpansionclosetozerocanbeprovided.Forproducingthefiberreinforcedstructuralmaterials,several

fabrication techniques are available such as draping of pre‐impregnated material (known as “prepreg”) to build laminates,filamentwinding techniques,pultrusion,placementofdry fibersor fabrics and injection or resin transfer moulding (RTM).Figure33depictsamultiaxialfabric[92].

Figure33:Schematicsofamultiaxialfabric[92]Today, the variety of machine tool applications of CFRP and

GFRPisalreadyhugeandatremendousamountofresearchworkcan be found in literature ofwhich only a few examples can bementionedhere.AbeleanalysedaverticalCFRPaxisandachievedamass reduction to60%anddecreasedenergy consumptionof70%oftheconventionaldesign(Figure34).

Figure34: Substitution of conventional by CFRP structure [PTW,

Darmstadt,Germany]In2011aCFRPramwaspresentedattheEMOfairbycompany

Roschiwal+Partner [241].Kuliseketal. compareda thick‐walledcompositespindleramwithahybridstructurecomposedoffibercomposites with cork layers and bonded steel reinforcements[164]. For an assemblywith connection interfaces, thedampingratiowas increasedby70%compared to the conventional steeldesign.AhighspeedmachiningcenterwithverticalCFRPz‐slidehasbeenpresentedby companyMAP (Magdeburg,Germany) atthe EMO fair in 2013 (Figure35). Measurements of the IFQ

showed that the same stiffness as with a comparable cast ironslidecanbeachieved.

Figure35:MachineslidemadeofCFRP[source:MAP]The integration of joints and mechanical interfaces in CFRP

structuresischallenging.Mechanicalstabilityhastobeachievedavoiding annihilation of the achieved mass reduction by heavymetalparts[153].Thethermalexpansionofconnectedelementsshouldbesimilarinordertopreventthermallyinducedstresses[307].Adesignmethodologyforjointsinlaminatedcompositesispresented in [51]. In [129] the influence of clamping effects onthedynamiccharacteristicsofcompositemachinetoolstructuresis studied and a new clamping approach using a metal core orsleeve is introduced. Peklenik studied design variations ofcomposite plate structures with trapezoid, rectangular andcircular supporting profiles for an exemplary application in aspindle box [230]. The trapezoidal showed the highest bendingandtorsionalstiffness.Heimbsetal.studiedsandwichstructureswith single and multiple textile‐reinforced composite foldcorespossessingveryhighweight‐specificstiffnessandstrength[112].Fleischer designed a CFRP slide with internal chambers whichcan be filled with liquids in order to control the dynamiccharacteristics[90,157].Liebetrau investigated the influence of laminate build‐up and

fiber type on thermal deformations in spindle casing front andback plates considering bearing power loss, heat conductivity,bearingholediameterandposition[177].

Figure36:CFRPheadstock[IWF,Berlin]HT‐CFRPappeared tobe inapplicablebecauseof inaccepTable

high temperatures in the bearing casing wall which depend onthe laminate build‐up and thermal conductivity. Displacementscouldbereduced to30%compared toasteelwall.HMfibers inan evenly distributed 0°/90°/±45°multi‐axial laminate showedsignificantly better results. The vertical spindle bearingdisplacement could be reduced by 96%and temperatureswere

slightly lower compared to steel. With a dynamic stiffnesscomparabletothesteelwall,thefirstnaturalfrequencyraisedby77%withtheHMfibreanddampingwashigherbyafactorof3.Based on these results, prototypic headstocks for a lathe havebeen realized by Uhlmann at the IWF (Berlin, Germany)(Figure36).Thepotentialsof lowthermalexpansionarealsoexploited for

thedesignofmodulardesktopmachinetools[305].Lee and Choi introduced composite spindles [168, 61] and

observed improved process stability. In order to optimize therelationship of rotational inertia, damping and naturalfrequencies in [24] the combination of a CFRP shaft with steelflanges for high speed air spindles was analyzed. The stackingangleofthecarboncompositeaswellastheadherendlengthandthicknessweretakenasdesignvariables.CentrifugalforceshavetobeconsideredinthelayoutoffastrotatingCRFPspindleshafts.In [56] the rotor of an AC induction motor was manufacturedusing magnetic powder containing epoxy composite whosedensity is half of the conventionalmetal rotor. Themotor shaftwasmadeofHMcarbon fiber epoxy composite.To enhance themagnetic flux of the composite rotor, a steel corewas inserted.Brecherpresentedacarbon fibre rotor foranaerostatic spindlein [36]. The moment of inertia was reduced to one thirdcompared to a steel rotor. The axial and radial bearing gapshrinkage isminimised since the negative thermal expansion oftheCFRPoffsetsthepositivethermalexpansionofthesteelnose.The material expansion due to centrifugal forces was reducedsignificantly.Compositematerialsarealsousedintoolingequipment,mostly

to increase damping and decrease the moment of inertia [169,171, 278]. Lee achieved 5 times larger depth of cut with agraphite epoxy composite boring bar compared to a steel barbeforetheonsetofchatter[169,171].CompaniesXPERION[306]and EMUGE recently presented CFRP tool extensions. TheexampleofEMUGEhas20%of themassof thesteelcomponentbutsamestiffnessandstrength[84].Thereducedinertialeadstoenergy savings. Heisel presented amodular CFRP reamer [114](Figure37).Theweightof the toolwasreduced toapprox.37%whereasthestiffnesswasslightlyincreased.

Figure37:ModulardesignofCFRPreamer[114]A kind of tentativeness regarding the use of composite

materialsinindustrialmachineryconcernsthelongtermstabilityofmechanicalandthermalpropertiesundertheinfluenceofchipsand coolant, as well as the complexity of internal failuremechanismswhichcanhardlyberecognizedexternallybutwhichcould affect machining performance. Therefore, covering andsealing approaches as well as damage identification techniquesare currently investigated. However, these materials have veryhighpotential,especiallyagainstthebackgroundofdevelopmentsin aerospace and automotive industries leading to a higher cost

effectiveness. Future aspects are the further development ofhybrid structures exploiting the different material potentials, afiber related functional integration, sensor and actuatorintegration, and self‐healing composites. Another aspect is theuseofnaturalfibercompositesforlowstressedcomponents.2.6MaterialcombinationsAlotofmaterialapplicationsinmachinetoolstructuresactually

employmaterialcombinationsleadingtohybridstructures.As mentioned before, polymer concrete is combined with

metallic inserts, interfaces and joints but also with metalstructurestobuildhybridmachinebedsandcolumns.Steelplatesforguidesand interfacesareconnected to theconcretematerialby anchors and metal reinforcements. By filling metal casings(withorwithoutinternalribbing)withconcretematerial,hybridstructures are built. Compared to pure mineral casting, higherforces can be handled by thinner profiles. Since castingmouldsand demoulding chamfers are not necessary, a higher flexibilityregarding part shape and design changes is given, and shorterdeliverytimesarepossible.Thecasingsprovideahighresistanceagainst abrasion and moisture. During casting of polymerconcrete, functional elements (high pressure pipes, fluid tanks,active cooling systems, chip removal channels, etc.) can beintegrated into the components. By use of lost cores, internalstructures,materialsavingsandlightcomponentscanberealized.Hybridsteel/concretestructures(calledHydropol)areprovidede.g. by company FRAMAG. The beneficial design and dampingcharacteristics of Hydropol have been exploited for prototypichigh speedmachines by Denkena et al. [67, 69, 104]. Even theeffect of a jerk decoupling of the linear motor axes wasdiminished due to the already high damping of the hybridstructure[104].A lot of research work has been carried out with respect to

hybrid metal‐composite structures. Lee manufactured a hybridcolumn for a precision grindingmachineby adhesively bondingGFRPplatestoacastironstructure[170].Thedampingcapacitywasincreasedby35%comparedtothecastironcolumn.In[55]ahybrid headstock for a precision grinding machine wasmanufactured by adhesively bonding GFRP laminate to a steelstructure.Thestiffness increasedby12%and the loss factorby212%comparedwiththesteelheadstock.Suhpresentedslidesofa large CNC machine with HM‐CFRP sandwiches joint withwelded steel structures using adhesives and bolts [271](Figure38).

Figure38:SectionviewsofverticalcolumnsoftheX‐slide[271]

Aweightreductionof26to34%andanincreaseofdampingbyfactorsof1.5to5.7wereachievedmaintainingthestiffness[173].In [272] the reliability of the adhesive joints between thecompositesandwichandthesteelstructurewasstudiedintermsofstrengthandthermalstressinducedbytheheatgenerationoflinear motors. The adhesive bonding showed acceptableperformance.In [140] a composite/aluminium hybrid beam structure with

HM carbon‐epoxy composites was developed for an inspectingmachine of LCD glass panels. The cross‐section shape, stackingsequence and thickness of compositewere optimized regardingdynamic stiffness and damping. Carbon epoxycomposite/aluminiumhybridstructureswithfrictionlayerswereusedin[154]toincreasestructuraldamping.Twotypesofhybridcolumnswereproposed.Thestaticdeflectionduetodeadweightandthefirstnaturalfrequencyasafunctionofthestackingangleand thickness of the composites were analysed by FEA.Neugebauer presented a hybrid composite/steel ball screw,stating10 times lower thermal expansion in comparisonwith aconventionalmetalballscrew[212].In[270]asteelspindlecoverwas reinforced with CFRP in order to improve the vibrationcharacteristics.Therelationshipbetweenthe loss factorand thestacking sequence was investigated. Compared to a traditionaldesign3‐5timeshigherlossfactorswereachieved.Ahybrid steel/composite cutting toolbodywasanalysedwith

respect tomaterial types (composite and foam), stacking anglesofthecomposite,adhesivebondingthickness,anddimensionsofthe cutting tool in [151].Heisel et al. investigateda functionallygraded aluminium matrix composite (consisting of coherentaluminiummatrixandceramicreinforcementsuchasSiCorAl2O3fibers or particles) regarding large aluminium milling cutters[115].Machiningtestsrevealedtheimprovedperformanceofthehybridtoolsregardingworkpieceroughness.Zatarain analysed bonded structures in [312]. By bonding a

broad variety of materials including metals, reinforcedcomposites and honeycomb structures can be combined(Figure39). The mechanical properties of the used glue at therelevant temperature level have to be considered. Structuralimprovements could be achieved with respect to shapes, wallthicknesses and materials. A mass reduction of 40–50% wasreached leading to increasednatural frequencies. In some casesdamping is increased by a factor of 10. Zatarain identified thematerialcharacteristicsoftheusedgluebybasictestsinordertoallowmodelling.

Figure39:Bondedmachinestructures[312]Combinations of fiber reinforced composites and concrete

materials have also been investigated. Sandwich structures formicro‐EDM machines are optimized by varying compositegeometries, stacking sequence, thickness and rib geometry in[152]. The structures are composed of fibre reinforcedcompositesforskinmaterialandresinconcreteandPVCfoamforcorematerials.Thesensitivityofdesignparameters like ribandcompositeskinthicknesswasexaminedandanoptimalconditionregarding structural stiffness was suggested. In [60] carbon‐epoxy composite and resin concretewere combined in a Table‐topmachine. Several components were realized and assembled

by mechanical joining and adhesive bonding. The re‐designedstructureprovided37%lessweight,16%increasedstiffnessandup to 3.64%higher loss factor. The strength in concrete can beimprovedbyreinforcementwithrecycledCFRPpieces[219].2.7ComparativestudiesMost of the developed structures and components are

compared to the particular conventional ones. However,systematic comparative analyses about material solutions canrarelybefoundinliterature.Themostinterestingstudiesassessthe influences caused by the different structures and materialapproaches,whicharerelevantfortheprocessperformanceandmachiningresults.Saljé compared grinding machine elements with polymer

concrete,combinedpolymerconcreteandcastiron,andcastironunder the influence of forces and thermal load [244].With thepolymerconcreteframe,thefirstnaturalfrequencyraisedby50‐150Hz, the dynamic compliance decreased to 30%, and thedampingwas2–6 timeshigher than that of the cast iron frame.Figure40comparesthethermalactivityofpolymerandcastiron.

Figure40:Thermaldeformationsofcastironandpolymerconcrete[244]Brecherstudiedthedynamicsoftwoeachdifferentlyfilledtest‐

components in [37, 38, 42] (Figure41). The best static stiffnesswasachievedwithaweldedsteelcomponentfilledwithmineralcasting.Advantageousdynamic stiffness is givenbyweldedandcasted components filled with cement concrete, furan resinbonded quartz sand, or sand depending on the bending ortorsionalmode. Cast ironwith sand filling showed the shortestdecaytime.

Figure41:Comparisonoffilledtest‐components[37]

Rahmancomparedthechatterstabilityoftheferrocementbedof a lathewith a cast iron bed and observed a 50%deeper cutbefore chatter vibration occurs [236]. In [238] the related toolwearbehaviourwascompared.Lowerflankwearappearedattheferrocementbedlathe.A comprehensive comparison of different material solutions

can be found in [227] where cast iron, cast iron filled withconventional concrete, epoxy polymer concrete and a hybridsteel‐concrete (Hydropol) structures have been tested(Figure42).

Figure42:Comparisonoflathebedvariants[227]The lathe bed made of Hydropol showed the shortest decay

timebuttheaveragetoollifeincuttingtestswasthehighestwiththe cast ironbed.Thevariationof theobservedvalueswith thecastironandHydropolbedwassignificantlyhigherthanwiththecement concrete bed. With the Hydropol bed the workpiecesurface roughness was slightly improved. The advantageousdecaytimeofHydropolwasalsoverifiedin[196].Munirathnamdesigned,optimisedandcomparedvariousslide

structuresandmaterialapplications[202]regardingthedynamicpositioning behaviour of machines. An aluminium structure, awelded steel design and the application of Al‐foam wereconsidered(Figure43).

Figure43:Systematiccomparisonofslidestructures[202]Improved dynamic path accuracy was achieved by the

lightweight ram structures. The structural stiffness appeared tohave a significantly lower influence regarding the speed gainvalue optimisation compared to the mass. An increase of thespeed gain by an optimized structural damping could not beverified.

Figure44:Influenceofbedmaterialontoolwearandsurfacequality[49]

Bruni et al. studied the influences of lubrication, inserttechnology and machine bed material in turning of AISI 420Bstainless‐steel and hardened 39NiCrMo3 alloy steel in [48] and[49].Theuseof thepolymerconcretebed leads toan improvedbehaviourintermsoftoolwearandsurfaceroughnesscomparedtoasimilarcastironbed(Figure44).

3.Methodsforstructurelayoutandoptimisation

For an optimised design, besides the material selection theassessment and modification of the structural performance isnecessary [296]. Nowadays machine tool development can besignificantly improved and shortened by the use of virtualprototypes and simulation methods [9, 141, 93]. A number ofdesign optimisation methods has been introduced in literature[136,100,125,317,160]mainlyconsistingof thestepsofCAD‐based design, FEM‐modelling and parameterization, firststructural optimisation including topology and parameteroptimization, first prototype realisation and experimentalanalysis, model updating by real parameter values, secondstructural optimisation applying the real parameter values, andsecond realisation inwhich the final structure is built. Multipleiterations can be carried out and for model‐based optimisationdifferentlevelsofdetailregardingthemechatronicsystemcanbeconsidered[301](Figure45).

Figure45:StructuralOptimization[301]However, up to now the essential decisions have to be

performed by the design engineer. Multi‐objective optimisationtechniquesformaterialselectionconsideringpartlycontradictorytargetsandevolutionarycomputationinstructuraldesigncanbeapplied [15, 149]. In [75] a framework for intelligent decisionsupport for structural design analysis using a finite elementmethodisdiscussed.3.1SimulationapproachesThe basic simulation approaches in machine tool design

including the material characteristics in order to calculate thestructural behaviour apply the Finite Element Analysis (FEA)[292, 291]. Altintas summarized the different simulationapproaches and FEA‐types (linear/non‐linear static, dynamic,thermal)in[9].Severalapproachesdescribetheelasticbehaviourof machine structures by mass‐spring‐damper models [52, 35],(flexible)multi‐bodymodels[32,183,161],transferfunctionsorstate‐space representations [34, 210] which can be derived bymodal reduction from numerical calculations or obtained fromexperiments in order to increase computational efficiency.Jedrzejewskietal.presentedahybridmodel including the finiteelement method (FEM) and the finite difference method for

analysing the thermal, stiffness and durability aspects ofmachines[137].Neugebaueretal.introducedanadvancedstate‐space modelling approach with the aim to describe non‐proportional material and structural damping such as given byfiberreinforcedmaterials[210].Thecomputationofcompositeorhybrid and combinedmaterials requiresmaterialmodels at themicrolevelwhichallowtoderivestructuremodelsatthemacrolevel.Forhomogenisationofcompositematerials,representativevolumetric elements (RVE) can be developed consideringmaterialpropertiesandenablingparameterisationofFE‐models[142, 143] (Figure46). For laminated composites, layer‐wiseshelltheorycanbeapplied[146].

Figure46: RVE model of randomly distributed spherical particlereinforcedcomposites[142]Alaborioustaskistoconsidervariouskinematicpositionsofa

machinestructureinordertoallowacomprehensiveassessmentof the structural characteristics. Zatarain proposed a methodusingpre‐calculated structures toobtainentiremachinemodelsat any position [311]. Brecher and Witt developed a modelreductionandcomponentextractionstrategy[40].Litwinskietal.presented a node swap extension for the well‐known Craig‐BamptonmethodformodelreductionandcomparedthistoFEAandflexiblemultibodysimulation[181].Lawetal. introducedaposition‐dependentmulti‐bodydynamicmodelofamachinetoolbasedonreducedmodelsub‐structuralsynthesisanduseditforstructureoptimisation[167].Amethodwhichallowstocombinecomponent dynamics models to entire structures is thereceptance coupling substructure analysis (RCSA) [123, 249, 6].In RCSA, experimental or analytic frequency response functionsof individual components are used to predict the dynamicresponseofthefinalassembly.Although sophisticated modelling approaches are available,

someuncertaintieshavetobetakenintoaccount,predominantlyoriginated in the unknown real stiffness and dampingcharacteristics of joints and interfaces but also resulting fromuncertaintiesregardingtherealmaterialproperties.In[255]and[216] this aspect is studiedwith respect to castedmachine toolcomponents.In order to simulate with realistic values, model updating is

necessary which minimizes the differences between calculationand measurement results by modifying the mass, stiffness anddampingparameters[200,201].Neugebaueretal.discussedtheuseofsensitivity‐basedmodelupdatingwithrespecttomachinetoolspossessingahighnumberofparametersand introducedacascaded‐likestrategy[213].Aroraetal.presentedacomparativestudy of damped FE model updating methods in [14]. In [23]direct model updating methods are applied regarding a simpledrillingmachine.Esfandiarietal.proposedafiniteelementmodelupdatingbyuseoffrequencyresponsefunctiondata[86].In[96]anupdatedfiniteelementmodelwasusedtoderiveastatespacereducedordermodelofacenterlessgrindingmachine.3.2Modalanalysis/operationalmodalanalysisInordertoidentifythedynamicparametersandcharacteristics

of machine tool structures, experimental modal analysis

techniqueshavebeendeveloped andused for a long time [281,231]. Figure47 gives an overview about the procedure forcomponenttestingandparameteridentification.

Figure47:Testsetupfordynamicanalysis[IFT,TUVienna]In[44]aninter‐laboratorycomparisontestregardingtheuseof

theexperimentalmodalanalysis forassessingthecomplianceofmachinestructuresunderprocessconditionsispresented.Itwasshown that theestablishedandcommonlyappliedexperimentalmethodscanonlygiveanapproximatequantificationof therealbehaviour due to uncertainties in the experimental proceduresandmeasuringconditions.Furthermore,non‐linearbehaviourofmaterials and joints has to be considered. A survey about non‐linear system identification in structural dynamics is given in[147].Implementing experimental modal analysis at complex and

largestructuresnecessitatesacostlyprocedure.Afastmethodforstructure dynamics analysis using a tracking interferometer ispresentedin[45].Asanalternativetomodalanalysisbyuseofanimpact hammer or controlled excitation shaker, completemachine structures can be assessed by the operational modalanalysis (OMA), inwhich theexcitationof the structure is givenbythemachiningprocess[184,310,176].3.3StructuraloptimisationThe final objective of simulation‐based and experimental

assessment methods is to provide characteristic data forstructural optimisation. Structural optimisation aims in designmodifications applied tomachine componentswhich lead to anefficient material utilization with respect to mechanicalcomponentbehaviourandperformance[293].Besidesenhancedstiffness,dampingandstrength,materialsavingsandlightweightstructures can be achieved. The FEA‐based numericaloptimisation methods which are implemented in machine tooldevelopment are summarized in [9]. An accurateparameterizationofmaterialcharacteristicvalues isanessentialprerequisiteregardingthequalityofoptimisationresults.Topologyoptimisationcanbeusedinanearlyandroughdesign

stagetocalculateanoptimummaterialdistributionwithrespectto internal structural loads.The resulting component topologiesmust be smoothed and transferred to a CAD model. In [80]topologyoptimisationisappliedforderivinganoptimumribbingstructureof largeformingtools.Kolarpresentsanapplicationoftopology optimisation in which multiple slide positions weretakenintoaccountwhencalculatingloads[160](Figure48).Fleischer et al. coupled topology optimization with hybrid

multi‐body simulation in order to consider and updatecomponent loads and inertia which occur during machineoperation[88,301,89].

Figure48:Topologyoptimization(left)andparameteroptimization

(right)ofamachinetoolcolumn[160]Dadalau et al. presented an optimality criteria and adaptive

penalizationschemefortopologyoptimizationin[63].Comparedto the commonly used SIMP (Solid Isotropic Material withPenalization) method, better results regarding computedintermediatematerialdensitiesandapplicabilitywithrespect toself‐weight problems could be achieved. In addition to usuallyconsidered boundary conditions such as stiffness specifications,some topology optimisation systems allow the formulation ofreferencestressandnaturalfrequenciesastargetandrestrictionfunctions [253]. With the increasing performance of additivelymanufactured structures, together with topology optimisationnewdegreesoffreedominstructuraldesignappear[240].Parameter optimisation aims in finding the best structural

parameters (wall thicknesses, cross sections, fiber orientation)foradetailedcomponentandmachinedesign.Kolarconductedaparametricoptimisationofamachinecolumnbyuseof10inputparameters (5 side thicknesses, 2 dimensions of ground base,variable number of internal ribs, variable sectiondimensions ofcorner bars) [160] (Figure48). The output parameters of thesimulationwere the static 3D stiffness at theTCP, the first fournatural frequencies and the weight of the structure. Since thesensitivity of the structural stiffness with respect to inner ribswas found to be low (by topology optimisation), awelded steeldesign was proposed. The column weight could be reduced to54% (full steel construction) or 59% (hybrid constructionwithaluminium foam filling), respectively.Themachine stiffnesswasincreased to 102% (106%) compared to the original structureandthe1stnaturalfrequencywasliftedto154%(166%).In structural optimisation of fiber reinforced composite or

hybrid components, the considerationof the fiberorientation isessential [294].Dependingonthe laminatebuild‐up,anisotropicmaterial properties prevail. In order to exploit the materialperformance in fiberdirectionbut to avoid structuralweaknessduetoloadsinotherdirections,adifferentialdesignwithrefinedribbinghasbeenchosenin[163].

Figure49:Bionicinspiredstructuraloptimization[314]

In recent developments bionic inspired branched structures

appeared, aiming inanoptimal loading in longitudinaldirectionof structural elements [314, 315]. In [314] amass reduction of2.24%andan increaseof the specific stiffnessby21.10%couldbeachievedforamachinetoolcolumn(Figure49).

Figure50:Bio‐inspiredlightweightstructuredesign[315].In [315] a bionic inspired crossbeam design has been

investigated (Figure50). A mass reduction of 3.31% and anincrease of specific stiffness of 23.29% compared to theconventionalstructurecouldbeshownwithadownscaledmodelcomponent.

4.Intelligentandsmartmaterialsandstructures

Aninnovativeresearchfielddealswithso‐called“intelligent”or“smart”materials and structures,which contain sensory and/oractuator functionality. Actuator functionalitymeans a change ofshape, position, frequency or other mechanical properties as afunction of a change of temperature, electric ormagnetic fields[245]. Most commonly used smart materials for actuators arepiezo‐electric ceramics, shape‐memory alloys, magnetostrictivematerials,andelectro‐ormagneto‐rheologicalfluids.Forsensors,optical fibers and piezo‐electricmaterials (also some polymers)areused.Suchmaterialscanbeappliedforprocessandmachineconditionmonitoring,active influencingof themachinedynamicand thermal behaviour as well as for improving process statesand adaptive process control. An overview aboutmaterials andbasicapplicationscanbefoundin[127,226].In[208]areviewofmechatronic and adaptronics systems in machine tools ispresentedwhichare inmanycasesbasedon smartmaterials.Adistinction can be made between components which purely orpredominantly consist of smart materials, and intelligentstructures which combine conventional structures with smartelementssuchasintegratedsensorsoractuators.4.1ShapememoryalloysShapememoryalloys(SMA)aremetalmaterials,suchasNickel‐

Titanium and Copper‐based alloys (Cu‐Zn‐Al, Cu‐Al‐Ni), which,after deformation, reassume their initial shapewith an increaseof temperature [165]. The shape‐memory effect is based on atransition between two different crystal structures (austeniticbody‐centred cubic at higher temperatures, martensitic phasewhen cooling down) (Figure60). An overview about shapememoryalloyresearchisgivenin[134].ApplicationsofSMAfortoolclampingarepresentedin[256,186].In[50]SMAactuatorsare used for pre‐stress control of ball screw drives which areactuatedbytheheatlossesofthedrivesystem.In[224]and[191]SMAareusedinlinearactuatorsforsmallmachinetools.

Figure60: Stress‐strain‐temperature data exhibiting the shape

memoryeffectforatypicalNiTi‐SMA[165]5.2PiezoelectricceramicsPiezoelectricceramicsreactonelectricpotentialbyexpansion

orcontraction.Contrary,theygenerateanelectricfieldwhentheyare deformed. Often used ceramic piezo materials are bariumtitanate (BaTiO3), lead titanate (PbTiO3), lead zirconate titanatealso called PZT (Pb(Zr, Ti)O3), and potassium niobate (KNbO3)[137, 245]. Since piezo electric ceramics are the mostly usedmaterial in mechatronic and adaptronic devices for machinetools,itishardlypossibletogiveacomprehensiveoverviewhere.An extensive review of piezo‐basedmechatronic components isgivenbyNeugebauer[208]andHesselbach[117].Applicationsofpiezoactuatorsrangefromprecisionpositioningdevices[282]toactive vibration control systems [5] and devices for adaptiveprocess optimization [7, 68]. Piezo ceramics possess a lowelongation to size ratio but high power density and stiffness.Furthermore,piezoceramicscanbeapplied inabroaddynamicbandwidth.Animportantaspectwhenapplyingthesematerialsisto consider their long‐term durability [95]. Figure61 showscharge‐straincurvesofapiezopatchtransducerspecimenundercyclicfatiguetesting.

Figure61: Charge‐strain curves of piezo patch transducer specimen

under cyclic fatigue testing at amaximum strain level of 0.3% ar roomtemperature[95]5.3MagnetostrictivematerialsMagnetostrictive materials change their shape under the

influenceofanexternalmagneticfieldduetotherotationofsmallmagnetic domainswhich causes internal strains in thematerialstructure [221]. Figure62 shows physical effects related to themagnetostrictiveeffect.

Figure62:Magnetostrictiveeffects[221]The mostly used Joule effect means the expansion of a

ferromagneticrodinrelationtoalongitudinalmagneticfield.Theelongation ratio (L/L) can be up to 4,000ppm at resonancefrequency.TheVillarieffectisbasedonachangeofmagneticfluxdensitywhenmechanicalstressisimposed.Thiscanbedetectedby a pickup coil and utilized in sensor applications. Furthersignificant effects concern the change of Young’s modulus as aresult of a magnetic field and theWiedemann effect leading toshear strains and torsional displacements. Table7 comparespiezoelectric,magnetostrictiveandSMAmaterials.

Table7 :Comparisonofsmartmaterials[221] PZT Magnetostrictive

material(Terfenol‐D)SMA

Elongation[%]Energydensity[J/m³]Bandwidth[kHz]Hysteresis[%]

0.12,50010010

0.220102

51

0.530

In [82] a giant magnetostrictive actuator (GMA) was

manufactured and used for tool positioning in an ultra‐precisemachine. Yoshioka et al. presented a rotary‐linear motionplatformapplyingGMAin[309].5.4Electro‐andmagneto‐rheologicalfluidsElectro‐ andmagneto‐rheological fluids change their viscosity

significantly when an electric or magnet field is applied. Thiseffect has been used in many systems with the aim to allow acontrolledandimproveddamping.Electro‐rheological fluids (ERF) are suspensions which build

fibrousstructureswhenanelectricfieldisapplied.Thus,thetotalshear stress of the ERF is increased. Aoyama and Shinnopresented ERF applications for machine tools in [11, 12, 258].Ramkumar used an ERF core inside a composite sandwich boxcolumnandobtainedanincreasedstiffnessandnaturalfrequencyby applying an electric field [239]. With higher ERF layerthicknessontheotherhandthefrequenciesdecrease.Magneto‐rheological fluids (MRF) behave similar to ERF but

react on magnetic instead of electric fields. Exemplaryapplications are presented in [97, 280, 73]. Weinert andBiermann developed a damping system for deep‐hole drillingusingMRF(Figure63).Anelasticandinfinitelyvariabletorsionalclampingofthedrillingtoolwasrealised,inordertodissipatethevibrationenergywithinthedampingdevice.Twocoilscontrolthemagneticfieldandhencethemechanicaltransmissionbehaviourofthedampingsystem[299,300].

Figure63: Damping of deep‐hole drilling tools by use of magneto‐

rheologicalfluid[299,300]5.5SensorandactuatorintegratedmaterialsandstructuresIn order to achieve “intelligent” structures, an integration of

sensoryandactuatorfunctionalityintothemachinecomponentsis essential. Besides assemblies of conventional structures andsmart devices, nowadays material and structure inherentapproaches are available. Brecher studied structure integratedthermally sTable CFRP rods and optical fibers for thermalelongation measurements [46]. Fiber Bragg gratings areintegrated into CFRP hybrid structures for thermal andmechanical deformation measuring in [195]. Denkena et al.developed a spindle slide with laser structured micro strainsensors[71](Figure64).

Figure64:Intelligentspindleslide[71]In[98]shapememorypolymercompositesarepresented.Piezo

integrated composite structures are developed in [179, 209].Drossel et al. investigate the integration of piezo‐ceramic fibresintoaluminiumsheetsbyformingprocesses[78,79](Figure65)

Figure65:PiezofibresinAl‐sheet[79]In [101]smartstructuremanufacturingwithpiezo integration

by incremental forming is analysed. Leibelt introduced strainsensors which are integrated in fiber fabrics by stitching [174](Figure66).

Figure66:Stitchedstrainsensors[174]A high future potential of intelligentmaterials and structures

formachinetoolscanberecognized.Manufacturingsolutionsforautomationandcostreductionarecurrentlydeveloped.

6.Conclusion

Thispapermakesvisible,howbroad the fieldof researchandtechnology regarding materials in machine tools structuresalready is. The material selection, particular materialdevelopment, material combination, and structural design offermanifolddegreesoffreedomforthemachinetoolbuilder.Alotofexperienceshavebeenmadewithmaterial solutions in thepastandsomegenerallessonscanbelearnedwithrespecttomaterialexploitation in specificmachine tool applications. However, un‐conventional and un‐expected approach can be seen.Furthermore, permanently new materials are invented andinvestigated which can provide advantageous properties formachinetoolapplications(e.g.carbonnanotubes).Simulation methods and virtual machines can nowadays be

incorporatedandutilized forstructural layoutandoptimisation.Decision support systems and multi‐criteria optimisationtechniques as well as more comprehensive and detailedsimulation models are developed and will further improve theperformanceofcomputeraidedmachinetooldesigninthefuture.Moreefficient,accurateandreliablemeasuringandparameter

identification approaches are necessary in order to provide therequiredparameters formodeling and simulation aswell as forprototype assessment and comparison. The joint analysis ofmachine andprocess behaviour consideringmaterial propertiesand structural characteristics is increasingly important andaspiredinvariousresearchinitiatives.Theintegrationof(micro)electronicdevicesandthedesignand

application of intelligent and smart materials and structuresenables a high degree of functional integration. The creation ofmaterialandstructureinherentsensorandactuatorcapabilityisahighlytopicalresearchfield leadingtosensitiveanddexterouscomponents and machines. Manufacturing solutions areinvestigated which allow for an industrial implementation ofintelligentandsmarttechnologies.However,industrialmachinetoolbuildingisalwaysamatterof

cost and delivery time, and this significantly influences also thematerial selection and structural layout. Another raising topicconcerns the energy and resource efficiency and environmentalimpact which is associated with material application.Consequently,materials inmachine tools are subject to variouslimiting factors, but on the other hand offer a very hightechnologicalpotential.Table8 summarizes somerelevant characteristicsofmaterials

formachinetoolstructures.

Table8:Summaryofsomeselectedmaterialcharacteristics[132] GCI Steel MC UHPC Granite CFRP

Dispersalinindustry

High High Middle Low Low Low

Application

Bed,column,slide,table,spindlecasing

Bed,column,slide,table

Bed,column,(slide),(table)

Bed,column,(slide),(table)

Bed,column,(slide),(table)

Slide,table,spindlecasing,(column)

Develop.potential

++ + ++ + + ++

Rawmaterial

Iron Iron Minerals(quartz,basalt,epoxyresin)

Minerals(quartz,basalt,cement)

Naturalstone

Carbonfibers,polymermatrix

Manuf.Casting Welding Casting Casting Cutting Laminate

buildingThermaltreatment

AnnealingAnnealing / tempering / /

Processingtemp.

1,350‐1,550°C

Upto1,000°C

45°C 60°C 20°C 20°C

Restperiodbeforefinishing

3‐5days 2‐5days 2weeks 2weeks 2days 0‐7days

Finishingtechnology

Milling,grinding,scraping

Milling,grinding,scraping

Milling,grinding,casting,lapping

Milling,grinding,casting,lapping

Grinding,lapping

Milling,laser

process.,grinding

Exemplarytype

GG25 St52 EPUMENT145B

Nanodur GabbroImpala

EP/CFK

Density[kg/dm³]

7.15 7.85 2.40 2.45 2.90 1.40

Compr.Strength[N/mm²]

840 800 140 125 250 800

Bendingtensilestrength[N/mm²]

340 240 35 15 20 700

Young’smodulus[kN/mm²]

115 210 45 45 65 180

Thermalexpansion[10‐6/K]

9 12 15 11 6.5 0.1

Thermalconductivity[W/mK]

47 50 2.9 2.0 3.0 2.0

Spec.heatcapacity[J/kgK]

535 360 730 750 800 1,000

Moistureabsorption[mass‐%]

0 0 0.03 1.0 0.3 0.1

Damping[log.decrement]

0.0045 0.0023 0.0352 0.0385 0.015 0.030

GCI:GreyCastIron(lamellar),MC:MineralCasting,UHPC:UltraHighStrengthConcrete

Acknowledgement

Theauthorssincerelythankallthecolleagueswhocontributedto the collection of material and literature, especially J.Jedrzejewski, S.Kläger, P.Groche,W.‐G.Drossel, C. Ihlenfeldt, P.Kolar,M.Zaeh,H.Shinno,L.Uriarte,M.Zatarain,P.Shore,H.Sato,T.Hausotte,D.Biermann,U.‐V.Jackisch,W.Geßler,D.Lehmhus.

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