CK6140車床主軸傳動系統(tǒng)設(shè)計-主傳動設(shè)計

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This work presentsa review on the applications of Touch Trigger Probes on shop floor, discussing nowadays technologies used to generate the trigger signal. Anew Touch Trigger Probe model based on a simple electrical contact is proposed, with the needed characteristics and low cost, to be used forCNC Lathes. The tests of the probe developed in the laboratory shown a repeatability of 0.003mm (3s or 99.73% confidence interval). Thosecharacteristics seem to be highly comparable to most needs for lathe applications in industries. 2006 Elsevier B.V. All rights reserved.Keywords: Dimensional quality monitoring and control; Touch Trigger Probes; On-machine measuring systems; CNC Lathes1. IntroductionOne of the, nowadays, most difficult challenges on manu-facturing system developments is to achieve total dimensionalcontrol of parts produced, creating statistical data analyses andpart-to-part control. This kind of integration may be reached byusing devices commonly called Touch Trigger Probes attachedtoCNCMachineTools.Theseequipmentsareusedformeasure-ments and calibrations based on projects specifications, closed-loopprocesscontrol,dimensionalcontrolofpartsmanufactured,toolcompensation,fastsetup,processstatisticaldataacquisitionand more.It is very important in current manufacturing systems trendstomaketheuseofthesefunctions,makingpossibleapart-to-partinspection control if necessary, which assures that dimensionalerrors are immediately detected, reducing scraps, avoiding thata full batch of pieces are scraped and reducing the costs due toparts reworks.Toassistdimensionalcontrolonturningoperations,thisworkstartsintroducingtheTouchTriggerProbescharacteristics,tech-nologies used to generate the “trigger signal”. After that, thispaper presents a simple and low cost wireless Touch TriggerCorresponding author.E-mail addresses: mdguerrasc.usp.br (M. Del Guerra),rtcoelhosc.usp.br (R.T. Coelho).Probe, based on a direct electrical contact principle. A proto-type is presented together with the repeatability and accuracystudies.2. Touch Trigger Probes earliesThe first Touch Trigger Probe was invented to solve a spe-cificinspectionrequirement,butwentontotransformthedesignand revolutionize the use of coordinate measuring machines(CMMs) for post-process inspection of manufactured compo-nents. In the end of 1970s, almost all the major CMM manufac-turers in the world were using RenishawsTMprobes.Later, due to CNC advances including measurement func-tions and linear encoder on its axes, the use of such devicesspread to machine tools, where they became a vital componenton automated production systems, allowing companies, fromautomotive to aerospace, to deliver precise components, reduc-ing scraps and maximizing productivity.David McMurtry developed the Touch Trigger Probes tech-nologyin1972.TheproblemaroseatRolls-Royceplc,wherehewas Assistant Chief of Engine Design. At that time, he was fac-ing the problem of measuring some complex pipe runs, of only6mmdiameter,fortheOlympusenginesusedonConcorde.Thepipeshadtofitaccuratelybetweensolidmountings,butthediffi-cultycameinmeasuringthemoncetheyhadbeenmanufactured.The solution found was the Touch Trigger Probe (TTP), whichusesasimpleprincipletoidentifythecontactbetweentheprobe0924-0136/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2006.03.093118M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123Fig. 1. A measurement system attached to a CNC Machine Tool.and the part to be measured. In 1973, David McMurtry regis-tered a company called Renishaw that early become the leadingprovider of metrology and spectroscopy solutions to manufac-turers and researchers around the world (Renishaw 1). Due tothe patented mechanism used by the company, other technolo-gies needed to be developed by other companies that intendedto join on this market.3. Touch Trigger Probes main characteristicsIndependently of the technology used, the principle is basedon the generation (or interruption) of an electrical signal at themoment the probes tip touches the part to be measured. Thissignal,generallycalledtriggersignal,issenttotheinterfacethatconverts it to be read by a CNC or a CMM. At the moment thatthe machine receives the trigger signal, it instantly freezes andsave the axis values for calculations and reports generation (seeexample in Fig. 1).The trigger signal transmission from the TTP to the probeinterface can be established using basically three different com-municationtechnologies:infrared,radiofrequenciesorasimplewire cable.Infraredcommunicationisusedforconventionalapplicationsfor standard lathes and milling machines; this technology pro-vides high reliability, high transmission speed rates, low noiseinterferences and low implementation costs. It is close to a fool-proof system, except when probe and receiver are out of visualcontact, as when the probe is deep within a bore or other depres-sion within the workpiece. For these cases and applications, aradiofrequencysystemisrecommended,(Tooling&Production2). Wire cable communication is used for tool setter probeswhich are mounted and fixed on the machine tables.4. Probing cycles and programmingAccordingtoModernMachineShopOnLine3,fewprofes-sionalswilldisputethemeritsofprobingforspeedingpartsetup,settingtooloffsetsandperformingin-processinspection,atleastin theory. In practice, however, probe systems often have beenviewed as difficult to use and expensive to buy. As a result, theyhave become tools used primarily by experienced programmersin high-production environments.To make probing effective and more affordable to a widerange of users, some of the main leading CNCs manufactures,such as Fanuc, Heidenhain, Okuma, Cincinnati Milacron andSiemens, are including, in its CNCs software, graphical user-friendly interfaces specially developed for probing. HeidenhainTNC control series, for example, are able to automatically ormanually carry through three-dimensional measurements andenables check functions, such as checking the dimensionalaccuracy of holes, determining feeds for finishing, checkingworkpiecegeometryandidentifyingdeviationsfromtolerances,identifyingworkpiecespriortomachiningandscanning3Dsur-faces. Also, probing cycles are being widely developed and canbe activated by a single line of information (Zhou et al. 4).Todaysstandardmachinetoolsdeliveraccuracyandrepeata-bility approaching levels formerly available only on CMMs. Inaddition,technologyadvancesaremakingthesemachineseasiertomaintain.Testandcalibrationtechnologyarenowavailabletoenable shops to ensure the accuracy and health of their machinetools. Telescoping ballbars are readily affordable by virtuallyanyshop.Awell-stockedtoolboxshouldalsocontaineitherelec-troniclevelsoragoodsetofprecisionmachinelevels.Plantsandlarge shops increasingly maintain their own laser interferome-tersandelectroniclevels,whilerentalequipmentanddiagnosticsservices are commercially available to small shops from varioussources (Modern Machine Shop On Line 5).Also, according to Modern Machine Shop On Line 6, CNCmachines are, in general, very accurate and extremely repeat-able. They compare favorably with the accuracy and repeata-bility of CMMs of similar size. With some care during theCNCinspectionprocess,thefactorsthatleadtointerdependencebetweenmeasurementerrorandcuttingerrorcanbeminimized.5. Technologies used to generate the trigger signalAccording to Shen and Moon 7, there is no trigger sig-nal generated when the probe tip touches the workpiece. Theprobe will continue to move toward and the force between theprobe tip and the workpiece will increase, causing a physicalquantity (e.g., resistance) to reach a threshold setting. A triggersignal is generated when the physical quantity exceeds a thresh-old limit in the sensing system. The travel distance betweenthe touch instant and the trigger instant is known as probe pre-travel. Mainly caused by bending deflection of the stylus shaft,it accounts for the majority of probe errors.Basically there are four different principles used to gener-ate the trigger signal: electrical contact (kinematic contact),light (laser interferometry), deformation (strain-gauges) and thepiezoelectric effect.5.1. Kinematic contactCurrently, this is the most used Touch Trigger Probewidespread. Developed by David McMurtry, this principle rev-olutionized the use of the coordinate measurement machines.According to Fig. 2, it is a kinematic re-seating mecha-nism based on three small cylindrical pieces that stay seatedon six other cylinders/spheres by the pressure caused by theM. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123119Fig. 2. Kinematic Touch Trigger Probes working principle (Tooling & Produc-tion 2).spring action, restricting its degrees of freedom until the stylusis brought into contact with the workpiece. An electrical currentnormallyflowsthroughthethreeseatsinseries.Whentheprobetip moves away from its stable position, one or more of the seatcontacts breaks and the increase in resistance is immediatelydetected electronically (Mayer et al. 8 and Reid 9). After themeasurement is done, when the probe tip moves back from thepiece, the probe spring forces the mechanism back to the initialstable position, re-establishing the standard resistance.5.1.1. System advantagesFrom the main advantages presented by this system, it isdistinguished: the simplicity of the system, robustness of theelectro-mechanicalassembly,widespreadandstudiedprinciple.5.1.2. System disadvantagesDue to the electro-mechanical assembly, kinematic probesare susceptible to the consuming and corrosion of its electriccontacts.Besides, the main typical characteristic of this kind of probeis related to the pre-travel variation. Since stylus bending priorto trigger is the major constituent of probe pre-travel, this char-acteristic varies according to changes in touch directions (Reid10). This behavior happens due to the fact that the internalmechanismseatsinthree“V-Blocks”separatedbya120angle,causing the force necessary to generate the “trigger signal” tovary according to the approaching direction (Kim and Chung11).Miguel et al. 12 also say that the dynamic force necessaryto open one of the electric contacts depends on the approachingvelocity.AccordingtoShenandMoon7,sameTouchTriggerProbesmodels may show different pre-travel variation behavior due tomanufacture and small changes during the assembly process.Cumulative errors happen when using stylus longer than30mm due to the fact that the touching force necessary to gen-erate the “trigger signal” bends the stylus and this deflection isproportional to its length.The acquisition cost of these equipments is relatively highif compared to other systems that provide similar benefits. Thisequipment is very well known and the brand that manufacturesthem is associated with very high quality.5.2. Strain-gaugesStrain-gauges probes deals with analogical informationderiving from the electrical resistance variation produced whenthe stylus touch the part to be measured.To use this technique as a digital switch, it is necessary anelectrical signal conditioning and treatment, establishing thelimits from which the “trigger signal” is generated.5.2.1. AdvantagesOne of the main advantages of this technique is that thepre-travel variation is much lower than on kinematic probesbecause the stylus deformation itself is responsible for triggingthe event. For this reason, this kind of probe is extremely accu-rate and makes possible the use of stylus as long as 200mm(Fig. 3).Theprobeusessolidstateelectronics,providinglonglifeandlow wear. Based on analog signal calibration, these systems areable to detect contact forces of 0.2N.5.2.2. DisadvantagesThe disadvantages associated with these probes rely on thehighacquisitioncost,extremelyadvancedelectronicsthatmakesitverydelicateequipmentandthefactthatthereareveryfewlit-erature and information specifically approaching these systems.5.3. LaserThese probes operate with an optical switch as sensor. A lenssystem collimates the light generated from an LED and focusesit onto a differential photocell. When the stylus is deflected, thedifferential photocell produces a trigger signal, (Dr. JohannesHeidenhain 13).5.3.1. AdvantagesOne of the main advantages of these equipments is relatedto its cost, relatively low if compared to other equipments thatprovide similar performance.5.3.2. DisadvantagesTo have a change on the differential photocell point wherethe collimated light happens and generates the “trigger signal”,first of all, a force between the stylus tip and the part to bemeasuredwillbeneededtodeflecttheinternalmechanism,sim-ilarly to what happens with “kinematic probes” regarding tothe opening of its electrical contacts. For this reason, both sys-temspresentsimilarpre-travelvariationcharacteristics,showing120M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123Fig. 3. Pr e-travel variation: comparing the performance of kinematic probes (TP20) with strain-gauge probes (TP800) (Renishaw 1).accuracydependenceondirectionapproachtotheparttobemea-sured and both systems are not indicate to use stylus longer than30mm.5.4. Piezoelectric sensorPiezoelectric probes are based on the principle for which anelectric potential difference is generated between the two facesof a piezoelectric crystal when it is pressed. Using this physicalprinciple, it was possible to design a probe able to generatethe “trigger signal” from the electric potential generated at themoment the probe stylus reaches the part.5.4.1. AdvantagesThey are extremely accurate and repetitive systems. Likestrain-gauge probes, pre-travel variation is much lower than onkinematic probes and it is possible to calibrate the system to besensitive to forces in the order of 0.2N.5.4.2. DisadvantagesTheymaybesensitivetosomelowfrequencynoiseslikecol-lisions,startofmotors,highspindleaccelerations/decelerations,machine vibration, etc.6. Proposal of a model based on a simple electric contactThis work aims to use basic and well-known technologiesto design and manufacture a low cost, robust and accurateTouchTriggerProbeabletomeasureexternaldiametersonCNCLathes. For that, precise mechanical machined parts and assem-bly were designed and applied to develop a system able to beused on the major Touch Trigger Probes applications.6.1. The systemTheworkingprincipleoftheproposedequipmentconsistsonclosing an electrical contact between the stylus (grounded) andtheinternaltarget(PositivePole).Atthisparticularmoment,the“trigger signal” is generated and sent by radio (wireless) to theprobes interface that will be responsible for sending a 24V dcto an appropriate input of the CNC.Due to the fact that the carriage will continue to move byinertia after the “trigger signal” happens, the target should havea drawback mechanism able to adsorb this movement, assuringits surface integrity (Fig. 4).Consequently, the repeatability of the developed Touch Trig-ger Probe is directly associated with the ability of the drawbacksystem to precisely seats back to its original position after atrigger event occurs. If the drawback system does not workproperly, there will be a change on the probes length presetafter a measurement and the diameter of the next part measuredwill be showed as different than the real. For this reason, thedevelopment of the drawback system was done very carefullyto guarantee a good performance for the system.The probe was designed to be used on VDI Toolholders,according to DIN 69880 and VDI 3425 standards.6.2. System repeatability and reproducibility (R&R)The CNC Lathe used to test the Touch Trigger Probe was anINDEX-TRAUB, model GU600, 22kW power. The CNC wasFig. 4. Dynamic behavior of the drawback mechanism during the measurementprocess.M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123121Fig. 5. Illustration of the experiment procedure used to determine the probe R&R and a photo of the probe inspecting the part.a Siemens 810D, able to provide the measurement functionsneeded for the experiment.To determine the total system R&R (probe+machine tool),the experiment was accomplished taking a part of which diam-eter was previously machined and normalized.After that, a turning operation was made using a carbide cut-tingtoolVBMT160404UC6010,cutting0.010mmfromthepartdiameter, taking all the necessary care with the cutting parame-ters to reduce the tool wear during the process.The part was then inspected using the Touch Trigger Probedeveloped, probing 60 consecutive times at the same point.Also a manual measurement was made at that point usinga regular micrometer. The data were saved on the CNC anddownloaded to a PC. The turning operation and consecu-tive measurements procedure was repeated five times, obtain-ing five data samples relative to each one of the diameters(Fig. 5).According to QS9000 14, one of the most effective meth-odstostatisticallyanalyzeprocessandequipmentsisthemethodfor computing the repeatability and reproducibility (R&R). Therange and average method to calculate R&R computes thetotal measurement system variability, and allows the total mea-surement system variability to be separated into repeatability,reproducibility and part variation. The repeatability of the mea-surementsystemmaybeestimatedfromtheaverageofthemeanscollectedbythesameoperator,whilethereproducibilitymaybeestimated from the range of variation of the average results col-lected by different operators.Table 1Probe data collect to simulate the effect of three operators measuring five different partsOP1Diameter 1Diameter 2Diameter 3Diameter 4Diameter 5Measurement 139.3316239.3254539.3133439.3067239.29861Measurement 239.3316239.3251739.3133439.3074439.29783Measurement 339.3316239.3257439.3135739.3070739.29744Measurement 439.3312939.3251739.3144339.3063839.29783Measurement 539.3312939.324939.3144339.3060539.29744R0.0003320.000840.0010940.0013870.001171x/39.3314839.3252939.3138239.3067339.29783OP2Diameter 1Diameter 2Diameter 3Diameter 4Diameter 5Measurement 139.3312939.3254539.3133439.3067239.29744Measurement 239.3312939.3257439.3137939.3074439.2982Measurement 339.3312939.3257439.3133439.3074439.29861Measurement 439.3319539.3257439.31439.3078339.2982Measurement 539.3312939.324939.3133439.3074439.29861R0.0006640.000840.0006640.0011130.001171x/39.3314239.3255139.3135639.3073739.29821OP3Diameter 1Diameter 2Diameter 3Diameter 4Diameter 5Measurement 139.3316239.3257439.3133439.3067239.2982Measurement 239.3312939.3254539.3137939.3070739.29744Measurement 339.3325639.3260539.3133439.3067239.2982Measurement 439.3312939.324939.3133439.3063839.299Measurement 539.3312939.3251739.3133439.3067239.29783R0.001270.011530.0004490.0006830.001562x/39.3316139.3254639.3134339.3067239.29813122M. Del Guerra, R.T. Coelho / Journal of Materials Processing Technology 179 (2006) 117123Tosimulatetheeffectofdifferentoperators,themeasurementroutine was taken 3 consecutive times for each diameter (3 dataset containing 60 measurements each were generated for eachdiameter).OP1 (Operator 1) represents the first data set containing 60measurementstookfor