现代技术实现高精度、高效率

MitsubishiHeavyIndustriesTechnicalReviewVol.52No.3(September2015)

PAGE

45

ModernTechnologyAchievingHigh-Precision,High-EfficiencyProcessingforMachiningCenters

YOSHIKATSUSATO*1

Inrecentyears,demandforhigh-precisionprocessinghasalsobeenincreasing,inmachiningofrelatively-longpartsanddiesusedforautomobiles,semiconductormanufacturingdevices,electroniccomponents,etc.Thesizerangeoftheseworkpiecesispositionedbetweenmachiningcentersandlargemachines.Lookingattheentireindustry,thereareonlyafewmachinetoolsthattargetsuchworkpiecesizes,andthereforethedemandedaccuracyhasnotbeenfullymet.MitsubishiHeavyIndustries,Ltd.(MHI)developedandmarketstheLH250,whichisoneofthelargestmachiningcentersrealizingmachiningaccuracyequaltothatofmicromillingmachines.Thistime,MHIdevelopedadditionalnewoptionalfunctionsthatenablehigh-precisionmachiningwithoutsignificantlychangingthefactoryenvironment.

|1.Introduction

Asmanyproductsaroundus,typifiedbyautomobiles,havebeenenhancingtheirfunctionalityinrecentyears,demandforveryhigh-precisionprocessingisalsoincreasinginmachiningofrelatively-largeparts.Evenacrossallindustries,however,thereareonlyafewmachinetoolsforworkpieceswithasizerangepositionedbetweenmachiningcentersandlargemachines,andthereforethedemandedaccuracyfortheseworkpieceshasnotbeenfullymet.Thereareanumberofrelatively-longpartsthatrequirehighaccuracyincludingnotonlyautomotivecomponents,butalsosemiconductormanufacturingdevicesandpartsneededfortheproductionofelectroniccomponents.Atthemanufacturingsitesoftheseparts,itisnecessarytorepeatedlyexecutetrialanderror,andthereforethepooryieldhasbeenaproblem.

MHIdevelopedtheLH250high-precisiondoublecolumnmachiningcenter,whichisalreadyonthemarket.Table1showsthemainspecifications.Inthedevelopmentofthisproduct,fundamentaltechnologiesfortheμV1micromillingmachinedesignedforhigh-precisionmachininganddoublecolumnlargemachinemanufacturingtechniqueswerecombined.Withahigh-speedspindleasstandardequipment,theLH250attainslong-termstableandhigh-precisionmachiningequivalenttothatofmicromillingmachines.Followingthedevelopmentofthemachineitself,MHIadvancedthedevelopmentofnewoptionsinordertosatisfyfurthermarketdemand.Thispaperpresentsthesetechnologiesandtheirexamples.

Table1 LH250specifications

Tablesize

(mm)

2,500x1,000

Maximumloadingcapacity

(kg)

3,000

Axestravel(XxYxZ)

(mm)

2,500x1,000x600

Spindletaper

HSK-A63

Spindlediameter

(mm)

Φ80

Spindlespeed

(min-1)

Max20,000

Spindlemotoroutput

(kW)

22/18.5

ATCtoolstoragecapacity

(tools)

40(Opt:64)

Machinefootprint

(mm)

7,200x3,300

Machineheight

(mm)

3,593

Machinemass

(kg)

21,000

*1 Manager,EngineeringDepartment,MachineToolDivision,Machinery,Equipment&Infrastructure

|2.FeaturesofLH250high-precisiondoublecolumnmachiningcenter

High-performancespindle

Typicalmachiningcentershaveaspindleaspeedofaround10,000min-1asstandard,andahigh-speedspindleisavailableasanoption.Whenahigh-speedspindleisused,theinitialpreloadofthebearingthatsupportsthespindlehastobereducedbecauseofgreaterheatgeneration,andthereforetherigidityofthespindledecreasestothelevelwhereroughprocessingatalowspeedcannotbeperformed.Forthisreason,twospecificationsofthespindleareavailablesothatcustomerswhoneedroughprocessingandothercustomerswhoneedhigh-speedoperationonlycanselectonethatsuitstheirrespectivepurpose.

However,throughtheemploymentofcoolingbothinsideandoutsideofthespindleandspeciallubricationofthebearing,whichwasdevelopedfortheμV1,theLH250achievesahigherinitialpreloadandreductionofheatgenerationatthemaximumspeed,andthenrealizesasstandardahigh-rigidityspindlerotatingatahighspeedof20,000min-1thatcanalsoperformroughprocessingatalowspeed.Thismachinehandlesroughprocessingandhigh-speedfinishingwithitssingleunit,andthereforeimprovesmachiningefficiency.

Evenwhenthespindlerotatesatthemaximumspeedof20,000min-1fromacoldstart,theLH250limitsthespindlethermaldisplacementto2μmorlessinalloftheX,Y,andZaxesdirections(1)andthespindlevibrationto2μmP-Porlessovertheentirerotationspeedrangeusingnoelectricalcorrection.Asaresult,theLH250contributestotheimprovementofthemachiningaccuracyandthemachinedsurfacequality.

High-precisionfeedingmechanism

Alsoforthefeedaxes,theLH250employsnarrow-pitchlarge-diameterballscrewsandastrongsupportingmethodsimilartotheμV1toattainahigh-rigidityandhigh-responsefeedaxessystem.Inaddition,theLH250usestheHGP2control,whichfeaturesproprietarycontroltuning,andrealizesmachiningoperationwithoutlosingitsshapeeveninhigh-speedshapemachining.Forexample,aroundnessof1.8μm,whichisequivalenttomicromillingmachines,wasachievedforthecirclecutting(Φ218mm)performedbysynchronizedtwo-planeaxes(1).

Furthermore,coolantflowsintotheinsideoftheballscrews,nuts,supportbearings,anddrivingmotormountingflangesinasimilarmannertothespindletodrasticallyreduceheatgeneration,consequentiallyrealizingastableandaccuratefeedingmechanism.

|3.W-2thermo-stabilizer

TheLH250employsthecoolingoftheheat-generatingpartsofthemachinesuchasthespindleandthefeedaxestosuppressthermaldisplacementcausedbytheheatgenerationofthemachineitself.Ifachangeintheambienttemperaturearoundthemachineoccurs,however,thethermaldisplacementofthemachineisaninevitablephysicalphenomenon.Althoughthereisanelectricalcorrectionmethodusingtemperaturesensorsembeddedinvariouspointsofthemachine,thermaldisplacementisnotalwayssimplemotionandthemethodmayresultinanadverseeffect.

Figure1 Columndeformationcausedbytemperaturedifferencearoundmachine

Whenthereisatemperaturedifferencebetweenthefrontandrearorupperandlowerpositionsaroundthemachine,forexample,thecolumndeformsandthespindleinclines(Figure1).Inthiscase,someelectricaloffsettingmethodsintendedtocorrectthedisplacementintheXaxisdirectionsucceedinthecorrection,butotherelectricaloffsettingmethods,movingtheZaxisfor

example,resultinafurtherincreaseofdisplacement(Figure2).Squarenesscorrectionorspatialcorrectionmaybepossiblesolutions,butinsuchcases,correctionbecomesdifficultwhenusingtoolsofdifferentlengthsorwhenwarpageunlikesimpleinclinationoccurs,aswellasunderothercircumstances(Figure3).

Figure2 AdverseeffectcausedbymovingtheZaxisincorrectionofspindleinclination

Figure3 Exampleofconditionswherethespindleinclinationisdifficulttocorrect

Figure4 ConfigurationofW-2thermo-stabilizer

ThereforetheLH250doesnotuseelectricalcorrectionandemploystheoptionalW-2thermo-stabilizer,amethodtomakethecolumntemperatureuniformtosuppressinclinationandthermaldisplacementusingatemperaturecontrolfluidthatcirculatesandcontrolstemperatureinthecolumn(Figure4).Thetemperatureofthetemperaturecontrolfluidiscontrolledtosamelevelasthatofthebedbyinstallingatemperaturesensorthereon,becausethebedisplacednearthelarge-heat-capacityfloor,andthereforeisrelativelystableandrarelyresultsinatemperaturedifference.Inaddition,thetemperatureoftheaforementionedcoolantthatflowsintothespindleandthefeedaxesiscontrolledinreferencetothebedtemperature.Furthermore,whentheoptionalcuttingfluidtemperaturecontrollerisadded,temperaturesofthespindle,thefeedaxes,thecolumn,thebed,thetable,andtheworkpieceallbecomeidenticalandthereforeattitudinalchangesandthethermaldisplacementofthemachinecanbesuppressed.

Figure5showstheactualmeasurementresultsofthethermaldisplacementofthemachineagainsttheenvironmentaltemperaturechangeofthemachineperiphery.TheresultsindicatethatthethermaldisplacementofamachineequippedwiththeW-2isapproximatelyhalfofthatofamachinewithouttheW-2.

Figure5 ComparisonofmachinethermaldisplacementbetweenunitwithandwithoutW-2

|4.Large-diametertoolhandlingcapabilityofopticalimagetypetoolmeasurementsystem

Theopticalimagetypetoolmeasurementsystemisanon-machinetoolmeasurementapplicationdevelopeduniquelyasoptionalequipmentfortheμV1.Asitsadvantage,thissystemcanconfirmthatthermaldisplacementhassaturatedandstabilizedusingaCCDcameratoperformmeasurementofthetoolthatrotatesatthesamespeedasmachining,andthereforecansuppressmachiningerrorscausedbythermaldisplacementandtoolchange.

ThespindleusedfortheμV1istheHSK-E32(toolholderflangediameterof32mm),whichusesatoolwithamaximumdiameterofaround10mm,andtheopticalimagetypetoolmeasurementsystemhasspecificationsthatsuitthetoolsused.Ontheotherhand,thespindleusedfortheLH250istheHSK-A63(toolholderflangediameterof63mm),whichusesatoolwithamaximumdiameterof125mm.Theopticalimagetypetoolmeasurementsystemcanmeasurelarge-diametertoolsinprinciple,butthelensfocusingrangeinwhichaccuratemeasurementcanbemade(depthoffield)isverynarrow,andthereforetheprobabilitythatthetipoftherotatingtoolissettledwithinthedepthoffielddecreasesproportionallytothetooldiameter(Figure6).Asaresult,therewereconcernsaboutunstablemeasurementaccuracy,longmeasurementtime,etc.,inthemeasurementoflarge-diametertools.

Figure6Conceptualdiagramoflarge-diametertoolmeasurementwiththeuseofopticalimagetypetoolmeasurementsystem

Thistime,theimagingmethodwaschangedfromtheconventionaloneinwhichmultipleimagesweretakeninarandommanner,tothenewoneinwhichimagesaretakenefficientlyaccordingtothespindlespeedinordertoaccuratelymeasureevenlarge-diametertools.Figure7showstheactualmeasurementofatoolwithadiameterof125mmwiththeuseoftheopticalimagetypetoolmeasurementsystemandanexampleimage.Table2comparesrepeatedmeasurementresultsoftheconventionalrandomimagingmethodandthedevelopedmethodforfivetypesoftools.Itwasverifiedthatthedevelopedmeasurementmethodcanimprovethemeasurementaccuracyandreducethemeasurementtimegenerally,dependingonconditionssuchasthenumberoftoolflutesandthespindlespeed.

Figure7 Exampleoflarge-diametertoolmeasurementwiththeuseofopticalimagetypetoolmeasurementsystem

Table2 Toolmeasurementresultswiththeuseofopticalimagetypetoolmeasurementsystem

No.

Tooltype

TooldiameterΦ(mm)

Numberofflutes

Spindlespeed(min-1)

Timeofthree-timerepeatedmeasurement(Ratiotoconventional

methodofNo.1)

Fluctuationoften-timerepeatedmeasurement(Ratiotoconventional

methodofNo.1)

Conventionalmethod

Newmethod

Conventionalmethod

Newmethod

1

Endmilling

10

2

10,000

1

0.69

1

0.67

2

Endmilling

20

4

4,000

1.05

0.93

1.78

1.33

3

Milling

50

6

2,000

1.08

1.07

3.44

1.56

4

Milling

125

6

800

1.12

1.28

6.78

1.22

5

Boring

36

1

500

1.25

0.7