Integration of new programming capabilities into a CNC milling system.docx

Integration of new programming capabilities into a CNC milling system.docx

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IntegrationofnewprogrammingcapabilitiesintoaCNCmillingsystem

IntegrationofnewprogrammingcapabilitiesintoaCNC

millingsystem

SotirisL.Omirou

andAntigoniK.Barouni

DepartmentofMechanicalEngineering,FrederickInstituteofTechnology,Nicosia1303Cyprus

Availableonline24December2004.

Abstract

Thispaperproposesaseriesofmachinecodesselectedforintegratingadvancedprogrammingcapabilitiesintothecontrolofamodern

CNC

system.ThenewprogrammingcapabilitiesweredevelopedandtestedintheframeworkofaPC-basedmillingmachinecontroller.Namely,tool-motionalongspacecurves,cutteroffsettingforfree-formcurvesandtwomachiningcyclesforrevolved（externalorinternal）surfaceswithfree-formprofiles,constitutethenewcharacteristicsproposedtobeintegratedintothesystemofa

CNC

millingmachine.Basedonrecentlydevelopedalgorithmswhosemathematicaldescription,formulationandverificationareavailableinrespectivereferredpublishedarticles,thispaperdescribeshowthenewfunctionsareproperlyintegratedintoa

CNC

millingsystem.Inthisdirection,anewclassofmachinecodesforthespecificationofeachofthefunctionsisproposed,whilecertaintopicsarisedinpracticeareextensivelyfurtherdiscussed.Theselectedmachinecodes,togetherwiththeircomplementaryrequisitedata,neededtobeintroducedintheNCprogram,areexemplifiedviacertainexamplesandactualmachiningtestsarepresentedforeachofthecases.

Keywords:

CNC

interpolators;Gcodes;PC-controller

ArticleOutline

I.Introduction

2.

G-codeprogramminglanguage

3.

Motionalongspacecurves

3.1.Implementationissues

3.2.Codificationofaspacecircularmotion

4.

CutteroffsettingofBeziercurves

4.1.DesigningwithBeziercurves

4.2.CodificationoftoolmotionalongtheoffsetofaBeziercurve

5.

Machiningcyclesforrevolvedsurfaceswithfree-formprofiles

5.1.Toolpathplanning

5.2.Codificationforthemachiningcycles

6.

MachiningtestsonaPC-basedNCmillingmachine

6.1.BriefdescriptionofthePC-basedNCmillingmachine

6.2.Machiningtests

7.

Concludingremarks

References

I.Introduction

Thedevelopmentandincorporationoftoolpathgeneratorsinto

CNC

systems,basedonefficientandaccuratecurvetracingmethods,capabletosatisfytheincreasingindustrialdemandformachiningcomplex-shapepartsisanimportantgoalinthefieldofcomputer-aidedmanufacturing.

Insolidmodeling,forexample,thegeneratededgesatwhichadjacentfacesofasolidmodelintersectarethree-dimensional（3D）curves.Inthemachiningstageefficientinterpolatorsareneededtodrivethecutting-toolalongsimilarspacepaths.Tool-radiusoffsettingoffree-formcurvesisanothercaseinpoint.OffsettracingtypicallyarisesinthegenerationofCAMdatafromCAD,withthecutterradiusbeingtheoffsetdistance.Inbothcases,theCADsystemperformsapiecewiselinearapproximationofthedesiredpath（thespaceortheoffsetcurve）,whoseresolutiondependsonthedesiredaccuracy.IntheCAMstage,eachoftheresultingcurvesegmentsistracedbythecuttingtool,underthecommandofthelinearinterpolatorofthe

CNC

machine,entrustedwiththetaskofmachiningpartoralloftheCADmodel.

Anotherfrequentdemandismetinthefieldofsurfacemachining.Alotofsculpturedsurfacesasarethecasesofmolds,stampingdies,forgingtools,rollingshapes,etc.,aredefinedasrevolvedsurfaceswithfree-formprofiles.DespitetheparticularityinthedefinitionandthedesignofthesesurfacestheavailableCAMsystemsdealwiththemaswithfree-formsurfaces.Thatis,asequenceofstraightlinesisusedtoapproximatethepartsurfaceandvoluminousdatadescribingthemmustbesenttothe

CNC

machine.

ThisstandardinterfacebetweenCADandCAMhasrecentlybeencriticizedforitsshortcomings:

thegenerationbytheCADsystemandtransmissiontothe

CNC

machineofavoluminousamountofsegmentdataandtheunnecessaryconflictbetweenaccuracyandefficiencyinherentinthisprocess.Conflictarisesfromthefactthathigheraccuracyresultsinsmallersegments.Sincethesearetracedbythelinearinterpolatoronaone-at-a-timebasis,theirprocessinginducesrepeatedacceleration–decelerationcyclesonthe

CNC

machine,thusraisingmachininginaccuraciesandsubstantiallyincreasingthewholemachiningtime.

Toremedythissituation,severalauthorshaveproposed“general”interpolators,capableofconvertinganyanalyticallydefinedpathintoatimedcuttertrajectory[1],[2],[3],[4],[5],[6],[7],[8]and[9].Theemploymentofgeneralinterpolatorsin

CNC

controlsismotivatedbytheprospectofeliminatingtheneedforpiecewiselinearapproximationofthecurveattheCADlevel,withconsequentreductioninthevolumeofCAMdata[10].ThisdevelopmentwouldmaketheCAD/CAMinterfacemoreefficientsince,inthatcase,onlytheanalyticalrepresentationofthecurvewouldhavetobetransmitted.Itwouldalsoreplacetheinefficientstart-stopmodeofoperationofthe

CNC

machine,inducedbythelargenumberofsegments,byasinglesmoothmotionofthecutterwiththeprogrammedfeedrate.

Torealizethesebenefits,however,trulygeneralinterpolatorsarerequired,capableofgeneratingreal-timereferencecommandsforrealizingthecuttermotion.Inthelightofthisframe,theresearchworkdescribedin[11],[12]and[13]isanattempttomeetthepreviouslysetdemands.Whilethemathematicaldetailsofthedevelopedinterpolatorsaresufficientlydiscussedthere,thepurposeofthispaperisrathertodealwiththepracticalsiderelatedtotheimplementationissuesofthenewmachiningcharacteristics.Theprimaryaimofthispaperistopresenthowthedevelopedreal-timeinterpolators[11],[12]and[13]maybeintegratedproperlyinthecontrollerofa

CNC

millingmachine.

Actually,thepaperispatternedafterasimilarworkofFaroukietal[14].Theauthorsofthespecificwork,proposeaseriesofmachinecodesforthespecificationoftool-pathsalongafamilyoffree-formcurves[15],[16]and[17]（Pythagoreanhodographintheauthors’terminology）.Accordingly,thepresentpapercomprisesassignmentofG-codesforeachofthenewprogrammingcapabilities,descriptionofthenecessarydatawhichmustaccompanythesecodesunderspecificcodification,dealingwithandprovisionofsolutionstotheindividualdemandsforproperintegrationofeachofthecasesandplanningofthemachiningprocessinthecaseofthemachiningcycles,allgivenandexplainedthroughspecificexamplesofpartprogramming.Tosupporttheadequacyoftheinterpolatorsinrealcuttingconditions,representativetestswereimplementedonaPC-basedNCmillingmachineequippedwiththeappropriatesoftwareandphotosofthemachinedpartsareprovided.

Theplanofthepaperisasfollows:

Section2reviewsthebasictermsofG-codeprogramminglanguagewhilethenextthreesectionsdealwiththeintegrationofthenewprogrammingcapabilitiesinthe

CNC

controllerand,specifically,Section3withspacecurvemotion,Section4withcutteroffsettingforBeziercurvesandSection5withthemachiningcyclesforrevolvedsurfaces.Section6presentsthemachiningtestsperformedonthePC-basedNCmillingmachinetogetherwithabriefdescriptionofthecertainsystem.Finally,Section7summarizestheoverallcontributionofthepaper.

2.G-codeprogramminglanguage

Theoperationofa

CNC

machinetooliscontrolledbyaprogramwrittenintheG-codeprogramminglanguagecalledNCorpartprogram.AnNCprogramcontainsanorderedsequenceofblocks,beingessentiallycommandsspecifyingindetailthecuttermotionstobeexecutedandtheauxiliaryoperations（e.g.,spindleon/off,spindlespeedandfeedrate）toberealizedbythe

CNC

machineinordertomachineaspecifiedpart.Thatis,anNCprogramisexecutedsequentially,onecommandatatime.Whenaprogramisexecuted,thecontrolwillencounterthefirstcommandintheprogram,executeit,thengoontothesecondcommand.Thecontrolexecuteseachcommandinthesameorderencountered.Ablockismadeupofwords.Characters,andnumericaldigitsaretheelementsthatconstituteaword.Forexample,theblock

N10G01X200Y300Z100F200M03S800

consistsofthewords“N10”,“G01”,“X200”,“Y300”,“Z100”,“F200”,“M03”,“S800”,meaningthattheblockwithsequencenumberN10,requiresmovementofthetoolfromitscurrentpositiontoposition（200,300,100）linearly（G01）withafeedrateof200 mm/minandwiththespindlerotatingclockwise（M03）at800 rpm.

EachofthecodesotherthanGandMhasauniquefunction.TheGandMhavemanyfunctionsdependingonthefollowingtwo-digitnumber.Thesefunctionshavebeenstandardizedandarecommonlyknownas“GandMcodes”.GenerallyaG-code,namedpreparatoryfunctioncode,definesonetypeofmotionoronemodeofoperationwhileanM-code,namedmiscellaneousfunctioncode,turnsvariousoperationson/off（e.g.,coolantflow,spindle,etc）.Itmustbenotedthatnotallthepossiblecodenumbersareallocated,whichmeansthatcontrolsystemsmanufacturerscanusethoseremainingforwhateverpurposetheywish.New

CNC

programmingcapabilitiesareamongthesepurposes.Real-timesurfaceinterpolation[18]andthepreviousmentionedcaseofPythagorean-hodographcurves[14]arerepresentativeexamplesofsuchnewprogrammingcapabilities.Intheseworks,authors,besidesthepresentationofthenew

CNC

interpolators,proposeG-codificationforperformingtherespectiveinterpolation.Similarly,specificG-codificationisproposedinthefollowingsectionsforembeddingthenewcapabilitiesina

CNC

con