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Journal of Materials Processing Technology 166 (2005) 321329AbstractmotorsThiscompositeBoththeisK1.fordiameterdefinecriteriarate2,3pieceseemstypically46allotableplacemichael.hilleryul.ie0924-0136/$doi:10.1016/j.jmatprotec.2003.08.009Development of a high-speed CNC cutting machine using linear motorsSeamus Gordon1, Michael T. HilleryDepartmentofManufacturingandOperationsEngineering,UniversityofLimerick,Limerick,IrelandReceived 18 April 2001; received in revised form 6 December 2001; accepted 6 August 2003The traditional approach to construct CNC machine tools has been to use rotary drive motors and ballscrews to achieve table motion. Linearoffer several advantages over this approach, including low inertia, better performance, increased accuracy and reduced complexity.article describes how a low cost, high-speed cutting machine was built using linear motors in order to conduct machining tests on amaterial. The machine configuration was a gantry type X/Y motion system on which a high-speed electrospindle was mounted.were interfaced with a standalone CNC controller for which a Microsoft Windows based interface was written in C+. The machine hasusual functionality of a CNC router and is capable of table speeds of over 40 m/min and a spindle speed of 30,000 rev/min. The machineused to carry out tests on tool wear monitoring as part of an ongoing research project in the university.2004 Elsevier B.V. All rights reserved.eywords: High speed machining; CNC cutting machine; Linear motors; C+Introduction The advantages of high speed machining include increasedmaterial removal rates, reduced tool wear, improved sur-The term high speed machining (HSM) has been useda number of years to describe end milling with smalltools at high rotational speeds. The criteria used toHSM vary 1. In addition to rotational speed, varioussuch as peripheral cutting speed, material removaland feed rate may be used to define HSM. In addition,the type of cutting operation and tool used and work-material may impose limits on the speeds attainable.However, for conventional machining processes thereto be a consensus that spindle rotational speeds of10,000 rev/min to 100,0000 rev/min are involved,though rotational speeds as low as 5000 rev/min arewed for in certain milling operations 7.High rotational speeds are usually partnered with highfeed rates, with contour machining typically takingin the range of 1560 m/min 1.Corresponding author. Tel.: +353 61 202852; fax: +353 61 202913.E-mailaddresses: seamus.gordonul.ie (S. Gordon),(M.T. Hillery).1Tel.: +353 61 202888.fandspeedmoreandmachineMouldapproachmotorsinmum8composedLineardriofaccelerationsareandpledria see front matter 2004 Elsevier B.V. All rights reserved.ace finish, the ability to machine thin walled componentsthe capacity to machine hard materials 1,8. High-machine tools are expensive because they need to berigid in construction then conventional CNC machinesneed higher than normal specification CNC controls tointricate contours, which are commonly found inand Die cavity work at high speeds. The traditionalin building machine tools has been to use rotaryand drive screws to achieve motion. Limiting factorsthe performance of this type of machine are limited maxi-rotation speed, inertia, windup, backlash and hysterisis. A linear motor is simply an electromagnetic actuatorof two rigid parts supported by a linear bearing.motors offer significant advantages over traditionalve systems. Velocities in excess of 6 m/s and accelerations100 m/s2are common, 8 while speeds of 11 m/s andof 200 m/s2have been claimed 9. As thereno moving parts except the motor block, repeatabilityaccuracy are also improved 8. In addition, the sim-mechanical arrangement and relatively low cost of linearve technology makes it attractive as a basis for developingtable motion system for high-speed milling.322 Processing2.which(a)(b)(c)(d)(e)2.1.tionalspindleused,Thermalmanceessaryliquid500conturntemtologueresultingequipment.2.1.1.knoDIN69871)tresmostangleanglesignatedrangetoufrecommendedISOrotationalpronecausedvibrations.tion,changes.holdingThistranslatedtemthetactAtofsizestoareporateanding.systempermutationsmanhangThewhichk2.2.beendesignedThisthroughlayingsteppingmotorsS.Gordon,M.T.Hillery/JournalofMaterialsMachine design and constructionThe design of the machine consisted of the following tasks,were carried out in the given order:specifications of a high-speed electrospindle;calculation of payload and specifications of the linearmotor system and CNC control;fabrication of machine chassis;interfacing of the electrospindle with the CNC control;andprovision of a PC based user interface for the system.ElectrospindleSpindles are available which cover a wide range of rota-speeds and power ratings. Factors that govern thespeed and life are the type and size of bearingsthe lubrication system used and the power unit 1.growth is a significant factor affecting the perfor-of a spindle 10,11 making a cooling system nec-. The spindle chosen was a Faemat 110 CU 5.1 kWcooled electrospindle capable of speeds betweenrev/min and 30,000 rev/min controlled by a frequencyverter. The spindle was interfaced to a PLC, which inneeded to be interfaced to the CNC control. The sys-allowed the spindle speed and direction of rotationbe governed by means of 20 V digital and 05 V ana-inputs. The overall mass of the spindle was 29 kgin a total payload of 35 kg including ancillaryToolholdingCollet chucks incorporating the steep taper commonlywn as the ISO taper (ISO 297, BS1660 Part 4,have been in common use on machining cen-for many years and are available in a range of sizes, thecommon taper sizes being 30, 40, 45 and 50. The taperused is 7:24. The chuck and machine spindle taperare manufactured to varying degrees of precision des-by (ISO 1947, BS4500, DIN7178) giving rise to aof AT numbers ranging from AT1 (high tolerance)AT6 (lower tolerance). Standard machine tools are man-actured to AT3 or AT4. AT2 taper controls or better arefor high speed machining applications 12.taper chucks have some disadvantages when used at highspeeds. They have a large mass and size and areto distortion by centrifugal forces. In addition, run outby inaccuracies in the taper interface can give rise toAs the system uses a single taper to provide loca-there can be some errors in repeatability between toolThe HSK (ISO/DIN 12164-1, ISO/DIN 12164-2) toolsystem has gained popularity in HSM in recent years.designation is an acronym for hollow short taper asfrom German and first appeared in 1993. This sys-numbertric,DritubestatedsimpleItthatencircleneticaballscre2.2.1.poeredmotoreprimaryTechnology166(2005)321329uses a hollow shank and a taper of 1:10. It has halfmass of a conventional tool holder and uses two con-surfaces to provide location thus increasing repeatability.high speeds centrifugal forces act on the internal facesthe tool holder, increasing the clamping pressure. Theavailable are 50, 63, 80 and 100, which correspondISO 30, 40, 45 and 50, respectively. HSK tool holdersavailable in Forms A, B, C, D, E and F which incor-variations on flange size, drive keys, coolant tubesaccommodation for manual or automatic tool chang-However, the system has some negative aspects. Theis more sensitive to dirt 11 and the number ofavailable is sometimes perceived as being tooy. Also, the short taper can mean that a longer tool over-is needed in some cases leading to stability problems.spindle chosen was equipped with a HSK 50 Form B,possesses an oversize flange and does not have driveeys.LinearmotorsystemsThe advantages of linear motors over rotary systems haveoutlined above. Traditionally, linear motors have beenby opening out flat their rotary counterparts 8.is the same as the imaginary equivalent of cuttinga conventional motor armature and rotary stator andthem out flat. Variations of this approach include linearmotors, platen type linear motors, linear inductionand U-shaped linear brushless motors. There are aof suppliers including GE Fanuc, Mitsubishi Elec-THK America, Parker/Compumotor and others. Linearves Ltd. manufacture a motor, based on a magnetic thrustand moving thrust block carrying circular coils. Theadvantages of this design are that it is mechanicallyand does not require a precision air gap or alignment.also differs from the designs mentioned above in the senseit is not a laid out flat design. As the magnetic coilsthe stator magnets, more use is made of the mag-flux to provide better performance. When mounted onbearing rail it can be used as a drop in replacement for aw 13.MotorspecicationThe motor arrangement chosen was three Linear Driveswered motor modules in a gantry arrangement. A pow-motor module is a self contained unit consisting of atube, block, linear bearing and encoder mounted on anxtruded aluminum base and is shown in Figs. 1 and 2. Theor X-axis was 2 m in length and was driven by a dualFig. 1. Linear motor and bearing.ProcessingparallelwvdriminimizethethemotorlinearrizedPeakPeakPeakFig.40S.Gordon,M.T.Hillery/JournalofMaterialsFig. 2. Motormotor pair set 1 m apart. The Y-axis (transverse axis)as then mounted on top of these. This in turn carried theertical Z-axis slide ways and electrospindle. The Z-axis wasven by a conventional rotary servo motor and ballscrew tothe effects of gravity on the system.The motion envelope required was 1500 mm 1000 mm75 mm. To calculate the motor sizes, a payload of 50 kg onX-axis and 35 kg on the Y-axis was estimated based onspindle mass of 35 kg and a mass of 15 kg for the Y-axisand gantry structure.The motor chosen was a Linear Drives LD 3806 tubularmotor. The performance characteristics are summa-below and shown in Fig. 3.force (N) 750velocity (m/s) 4.5acceleration (m/s2) 1703. Time vs. displacement for LD3806 linear motor with payload ofkg.waccelerationableequi10by3103200WencoderspitchbyporesolutionFigs.2.3.machinetainTheofinclampingTricatedtable.Technology166(2005)321329 323detail.For the X-axis (mass = 45 kg) the following accelerationas possible:=peak forcemass= 33.33 m/s2or 3.39gThe maximum traverse velocity of 40 m/min was attain-over a distance of 3.3 mm in a time of 0.02 s.Similarly, for Y-axis maximum acceleration of 21.42 m/s2valent to 2.18 g was calculated giving a ramp distance ofmm in 0.03 s. These values are in line with those reported5,14.The motors were powered by three Linear Drives LDAbrushless servo amplifiers. Linear magneticwere incorporated in both the axes. The encoderwas 0.001 mm giving a resolution of 0.005 mm.The Z-axis was used for positioning only and was drivena Baldor BSM 63A brushless AC servo motor which waswered by a Baldor DBSC 104 amplifier. The positionalwas 0.001 mm. The machine layout is shown in4 and 5.MachinechassisThe machine chassis was fabricated in two parts. Thetable needed to have a large mass in order to main-stability of the machine under high axis accelerations.table measured 2 m 1.2 m 0.3 m and had a massapproximately 1500 kg. The table was cast in one piecesteel reinforced concrete. Airways to facilitate a vacuumsystem were cast into the top surface of the table.o provide a means for attaching the linear motors, a prefab-extruded aluminium frame was also embedded in theThis provided a flat surface and tee slots for clamping324 ProcessingthelaminatedandloconcretetoS.Gordon,M.T.Hillery/JournalofMaterialsFig. 4. Machinemotors. The base is shown in Fig. 6. A sheet of tufnolplastic material was screwed to the table surfacethen machined flat to provide a datum surface.The base of the machine was fabricated from 100 mm hol-w section steel and was bolted to the underneath of thebase. Eight feet with adjustable pads were providedfacilitate levelling.2.4.motionsystem.withcontrolFig. 5. Electrospindle andTechnology166(2005)321329layout.CNCcontrolThe control used was a Baldor Nextmove BX 3-axis servocontroller, which was supplied with the linear motorThe control was a standalone unit that communicateda PC in terminal mode via an RS-232 serial link. Thewas capable of storing and executing entire programsslide arrangement.asviaitalprodeisapplicationshasalloaxMINTtalOUTuntil2.4.1.spondingS.Gordon,M.T.Hillery/JournalofMaterialsProcessingFig. 6. Machine concrete base:well as being controlled by individual typed commandsa terminal interface. In addition, eight opto-isolated dig-inputs and outputs as well as two analogue outputs werevided, which could be used for interfacing with othervices. The programming language used was MINT, whicha structured form of BASIC designed for motion control15. In addition to the normal BASIC syntax, itadditional motion specific keywords built in to it, whichw control of axis position, speed and synchronization ofes. Table 1 shows a fragment of a typical MINT program.can also be used to raise and lower individual digi-outputs as well as test inputs. For example, the command.1=1: PAUSE IN.3 would raise output 1 and then waitinput 3 was raised.InterfacingthecontrolandspindleThe electrospindle system provided 20 V output corre-to the following functions: spindle switched toCNC;aonespeed.possibleTSampleCOMMSMODE.RAAERRREPEAHM0,1=0.00,0.00PUNTILSERSPEED0,1=100,100MOTechnology166(2005)321329 325(a) top; and (b) bottom.spindle OFF; and spindle at target speed. The inputsvailable werestart spindle and stop spindle. In additionanalogue of 05 V input was used to govern spindleOnce the spindle was wired to the controller, it wasto write MINT subroutines to start and stop theable 1MINT codetmRS232=1CTIVEINLEVEL=0xF8FFORIN 0,1,2=8,9,10TAUSE ID 0,1VEL0,1=0VOC0,1VEA 0,1=300.00,0.00:GO326 Processingspindle.automatictherefore,ning.analogueaspindle2.4.2.positionbeloopandthegral,(PIDloopdemandWherefollo(samplevvsarythedonetemoutputprofile.(iii)increaseandachievv(0.02tak2.5.thefunctionsItinterf.2.5.1.(API)controlerarchytoject.loonS.Gordon,M.T.Hillery/JournalofMaterialsThese subroutines tested whether the spindle was inmode before setting the speed and starting motion,preventing axis motion unless the spindle was run-The analogue output was governed by a 14 bit digital toconverter (DAC) built into the control, which gavemaximum resolution of 8192 increments, which resulted inspeed increments of 3.6 rev/min.MachinetuningAt the lowest level of control software, instantaneous axisdemands produced by the controller software musttranslated into motor demands. This is achieved by closedcontrol of the motor. The controller compares desiredactual positions and calculates the correct demand formotor. The torque is calculated by a proportional, inte-derivative velocity feedback and velocity feed forwardVF) algorithm. This is referred to as the servo loop. Thisis executed once every 500H9262s on the NextMove control.The equation of the loop closure algorithm is as follows:= GNeDDelta1eDelta1 KVv+ KFV + KIsummationdisplayeGN is servo loop gain; D is derivative of error; e iswing error in quadrature counts; is servo update periodtime); KV is velocity feedback gain; v is actual axiselocity; KF is velocity feed forward gain; V is demand axiselocity; and KI is integral feedback.To ensure satisfactory closed loop control it was neces-to set up the parameters for the PIDVF loop to matchsystem inertia and motor/encoder combination. This wasempirically by measuring the velocity profile of the sys-using an oscilloscope connected to an analogue positionon the control and comparing it with an ideal velocityThe most critical parameters are as follows:(i) The servo loop gain parameter (GN) controls a motorforce proportional to the following error. This causes themotor to move towards the desired position. If no otherforce is acting, the motor will tend to overshoot andoscillate about the desired position. High values of GNwill result in the motor delivering higher thrust forcesbut with an increased tendency to become unstable.(ii) The velocity feedback gain is a damping term that pro-vides a force proportional to the motor velocity and inthe opposite direction to the force governed by GN. Thisparameter is set to offset the instability caused by highgain values. Ideally a system should be critically dampedso that no overshoot or oscillation occurs.The integral feedback KI term is introduced to reducethe settling time and steady state error of the motor. TheIntegral feedback provides a force that increases as afunction of time and tends to move the motor towardsthe desired position.The approach used in tuning the system was to firstthe servo loop gain until the onset or instabilitythen increase the velocity feedback until damping wasandWprogrammingwhichfwithFig.2.5.2.controldohalttoobjectetions..adoptwsystemwFothertheTechnology166(2005)321329ved. This was done incrementally until a satisfactoryelocity profile was achieved. Finally, the integral feedbackalue was set by pushing an axis off position by a smallmm) amount when at rest and then measuring the timeen for the axis to move back into position.UserinterfaceIt was considered desirable to provide a user interface formachine that would, in as far as possible, provide the sameas found on a conventional machine tool control.was decided to incorporate the following features in theace (Fig. 7):support for ISO G-Code programs with an interactiveprogram edit screen;machine homing, manual jog and datum setting;graphical simulation of program; andpositional readout of machine axes.ProgramminglanguageSource code in C+ for an Application Program Interface16 that provided support for communication with thewas available. The source code consisted of a class hi-, which allowed an instance of a class CNextMoveBXbe declared. This can be thought of as a controller ob-The member functions of the CNextMoveBX object al-wed data transfer and other functions to be performedthe control via a serial RS-232 link between the controla host computer. It was decided to create a Microsoftindows based gr

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