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Journal of Materials Processing Technology 179 2006 117 123 Development of a low cost Touch Trigger Probe for CNC Lathes Marcelo Del Guerraa Reginaldo Teixeira Coelhob aEscola de Engenharia de Sao Carlos EESC USP Av Trabalhador Sancarlense 400 CEP 13566 590 Sao Carlos SP Brazil bEscola de Engenharia de Sao Carlos EESC USP Av Trabalhador Sancarlense 400 CEP 13566 590 Sao Carlos SP Brazil Abstract The use of Touch Trigger Probes for CNC Machine Tools has become a world standard mainly due to the reduction of machine setup time and the ability to promote workpiece accuracy on machine measurements However some barriers still need to be transposed like the measurement routines programming diffi culties high costs of these equipments and the low number of technical literature about this subject This work presents a review on the applications of Touch Trigger Probes on shop fl oor discussing nowadays technologies used to generate the trigger signal A new Touch Trigger Probe model based on a simple electrical contact is proposed with the needed characteristics and low cost to be used for CNC Lathes The tests of the probe developed in the laboratory shown a repeatability of 0 003mm 3s or 99 73 confi dence interval Those characteristics 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 Lathes 1 Introduction One of the nowadays most diffi cult challenges on manu facturing system developments is to achieve total dimensional control of parts produced creating statistical data analyses and part to part control This kind of integration may be reached by using devices commonly called Touch Trigger Probes attached toCNCMachineTools Theseequipmentsareusedformeasure ments and calibrations based on projects specifi cations closed loopprocesscontrol dimensionalcontrolofpartsmanufactured toolcompensation fastsetup processstatisticaldataacquisition and more It is very important in current manufacturing systems trends tomaketheuseofthesefunctions makingpossibleapart to part inspection control if necessary which assures that dimensional errors are immediately detected reducing scraps avoiding that a full batch of pieces are scraped and reducing the costs due to parts reworks Toassistdimensionalcontrolonturningoperations thiswork startsintroducingtheTouchTriggerProbescharacteristics tech nologies used to generate the trigger signal After that this paper presents a simple and low cost wireless Touch Trigger Corresponding author E mail addresses mdguerra sc usp br M Del Guerra rtcoelho sc usp br R T Coelho Probe based on a direct electrical contact principle A proto type is presented together with the repeatability and accuracy studies 2 Touch Trigger Probes earlies The fi rst Touch Trigger Probe was invented to solve a spe cifi cinspectionrequirement butwentontotransformthedesign and 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 Renishaw sTMprobes Later due to CNC advances including measurement func tions and linear encoder on its axes the use of such devices spread to machine tools where they became a vital component on automated production systems allowing companies from automotive to aerospace to deliver precise components reduc ing scraps and maximizing productivity David McMurtry developed the Touch Trigger Probes tech nologyin1972 TheproblemaroseatRolls Royceplc wherehe was Assistant Chief of Engine Design At that time he was fac ing the problem of measuring some complex pipe runs of only 6mmdiameter fortheOlympusenginesusedonConcorde The pipeshadtofi taccuratelybetweensolidmountings butthediffi cultycameinmeasuringthemoncetheyhadbeenmanufactured The solution found was the Touch Trigger Probe TTP which usesasimpleprincipletoidentifythecontactbetweentheprobe 0924 0136 see front matter 2006 Elsevier B V All rights reserved doi 10 1016 j jmatprotec 2006 03 093 118M Del Guerra R T Coelho Journal of Materials Processing Technology 179 2006 117 123 Fig 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 leading provider of metrology and spectroscopy solutions to manufac turers and researchers around the world Renishaw 1 Due to the patented mechanism used by the company other technolo gies needed to be developed by other companies that intended to join on this market 3 Touch Trigger Probes main characteristics Independently of the technology used the principle is based on the generation or interruption of an electrical signal at the moment the probe s tip touches the part to be measured This signal generallycalledtriggersignal issenttotheinterfacethat converts it to be read by a CNC or a CMM At the moment that the machine receives the trigger signal it instantly freezes and save the axis values for calculations and reports generation see example in Fig 1 The trigger signal transmission from the TTP to the probe interface can be established using basically three different com municationtechnologies infrared radiofrequenciesorasimple wire cable Infraredcommunicationisusedforconventionalapplications for standard lathes and milling machines this technology pro vides high reliability high transmission speed rates low noise interferences and low implementation costs It is close to a fool proof system except when probe and receiver are out of visual contact as when the probe is deep within a bore or other depres sion within the workpiece For these cases and applications a radiofrequencysystemisrecommended Tooling Production 2 Wire cable communication is used for tool setter probes which are mounted and fi xed on the machine tables 4 Probing cycles and programming AccordingtoModernMachineShopOnLine 3 fewprofes sionalswilldisputethemeritsofprobingforspeedingpartsetup settingtooloffsetsandperformingin processinspection atleast in theory In practice however probe systems often have been viewed as diffi cult to use and expensive to buy As a result they have become tools used primarily by experienced programmers in high production environments To make probing effective and more affordable to a wide range of users some of the main leading CNCs manufactures such as Fanuc Heidenhain Okuma Cincinnati Milacron and Siemens are including in its CNC s software graphical user friendly interfaces specially developed for probing Heidenhain TNC control series for example are able to automatically or manually carry through three dimensional measurements and enables check functions such as checking the dimensional accuracy of holes determining feeds for fi nishing checking workpiecegeometryandidentifyingdeviationsfromtolerances identifyingworkpiecespriortomachiningandscanning3Dsur faces Also probing cycles are being widely developed and can be activated by a single line of information Zhou et al 4 Today sstandardmachinetoolsdeliveraccuracyandrepeata bility approaching levels formerly available only on CMMs In addition technologyadvancesaremakingthesemachineseasier tomaintain Testandcalibrationtechnologyarenowavailableto enable shops to ensure the accuracy and health of their machine tools Telescoping ballbars are readily affordable by virtually anyshop Awell stockedtoolboxshouldalsocontaineitherelec troniclevelsoragoodsetofprecisionmachinelevels Plantsand large shops increasingly maintain their own laser interferome tersandelectroniclevels whilerentalequipmentanddiagnostics services are commercially available to small shops from various sources Modern Machine Shop On Line 5 Also according to Modern Machine Shop On Line 6 CNC machines 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 the CNCinspectionprocess thefactorsthatleadtointerdependence betweenmeasurementerrorandcuttingerrorcanbeminimized 5 Technologies used to generate the trigger signal According to Shen and Moon 7 there is no trigger sig nal generated when the probe tip touches the workpiece The probe will continue to move toward and the force between the probe tip and the workpiece will increase causing a physical quantity e g resistance to reach a threshold setting A trigger signal is generated when the physical quantity exceeds a thresh old limit in the sensing system The travel distance between the touch instant and the trigger instant is known as probe pre travel Mainly caused by bending defl ection 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 the piezoelectric effect 5 1 Kinematic contact Currently this is the most used Touch Trigger Probe widespread 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 seated on six other cylinders spheres by the pressure caused by the M Del Guerra R T Coelho Journal of Materials Processing Technology 179 2006 117 123119 Fig 2 Kinematic Touch Trigger Probe s working principle Tooling Produc tion 2 spring action restricting its degrees of freedom until the stylus is brought into contact with the workpiece An electrical current normallyfl owsthroughthethreeseatsinseries Whentheprobe tip moves away from its stable position one or more of the seat contacts breaks and the increase in resistance is immediately detected electronically Mayer et al 8 and Reid 9 After the measurement is done when the probe tip moves back from the piece the probe spring forces the mechanism back to the initial stable position re establishing the standard resistance 5 1 1 System advantages From the main advantages presented by this system it is distinguished the simplicity of the system robustness of the electro mechanicalassembly widespreadandstudiedprinciple 5 1 2 System disadvantages Due to the electro mechanical assembly kinematic probes are susceptible to the consuming and corrosion of its electric contacts Besides the main typical characteristic of this kind of probe is related to the pre travel variation Since stylus bending prior to trigger is the major constituent of probe pre travel this char acteristic varies according to changes in touch directions Reid 10 This behavior happens due to the fact that the internal mechanismseatsinthree V Blocks separatedbya120 angle causing the force necessary to generate the trigger signal to vary according to the approaching direction Kim and Chung 11 Miguel et al 12 also say that the dynamic force necessary to open one of the electric contacts depends on the approaching velocity AccordingtoShenandMoon 7 sameTouchTriggerProbes models may show different pre travel variation behavior due to manufacture and small changes during the assembly process Cumulative errors happen when using stylus longer than 30mm due to the fact that the touching force necessary to gen erate the trigger signal bends the stylus and this defl ection is proportional to its length The acquisition cost of these equipments is relatively high if compared to other systems that provide similar benefi ts This equipment is very well known and the brand that manufactures them is associated with very high quality 5 2 Strain gauges Strain gauges probes deals with analogical information deriving from the electrical resistance variation produced when the stylus touch the part to be measured To use this technique as a digital switch it is necessary an electrical signal conditioning and treatment establishing the limits from which the trigger signal is generated 5 2 1 Advantages One of the main advantages of this technique is that the pre travel variation is much lower than on kinematic probes because the stylus deformation itself is responsible for trigging the 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 providinglonglifeand low wear Based on analog signal calibration these systems are able to detect contact forces of 0 2N 5 2 2 Disadvantages The disadvantages associated with these probes rely on the highacquisitioncost extremelyadvancedelectronicsthatmakes itverydelicateequipmentandthefactthatthereareveryfewlit erature and information specifi cally approaching these systems 5 3 Laser These probes operate with an optical switch as sensor A lens system collimates the light generated from an LED and focuses it onto a differential photocell When the stylus is defl ected the differential photocell produces a trigger signal Dr Johannes Heidenhain 13 5 3 1 Advantages One of the main advantages of these equipments is related to its cost relatively low if compared to other equipments that provide similar performance 5 3 2 Disadvantages To have a change on the differential photocell point where the collimated light happens and generates the trigger signal fi rst of all a force between the stylus tip and the part to be measuredwillbeneededtodefl ecttheinternalmechanism sim ilarly to what happens with kinematic probes regarding to the opening of its electrical contacts For this reason both sys temspresentsimilarpre travelvariationcharacteristics showing 120M Del Guerra R T Coelho Journal of Materials Processing Technology 179 2006 117 123 Fig 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 than 30mm 5 4 Piezoelectric sensor Piezoelectric probes are based on the principle for which an electric potential difference is generated between the two faces of a piezoelectric crystal when it is pressed Using this physical principle it was possible to design a probe able to generate the trigger signal from the electric potential generated at the moment the probe stylus reaches the part 5 4 1 Advantages They are extremely accurate and repetitive systems Like strain gauge probes pre travel variation is much lower than on kinematic probes and it is possible to calibrate the system to be sensitive to forces in the order of 0 2N 5 4 2 Disadvantages Theymaybesensitivetosomelowfrequencynoiseslikecol lisions startofmotors highspindleaccelerations decelerations machine vibration etc 6 Proposal of a model based on a simple electric contact This work aims to use basic and well known technologies to design and manufacture a low cost robust and accurate TouchTriggerProbeabletomeasureexternaldiametersonCNC Lathes For that precise mechanical machined parts and assem bly were designed and applied to develop a system able to be used on the major Touch Trigger Probe s applications 6 1 The system Theworkingprincipleoftheproposedequipmentconsistson closing an electrical contact between the stylus grounded and theinternaltarget PositivePole Atthisparticularmoment the trigger signal is generated and sent by radio wireless to the probe s interface that will be responsible for sending a 24V dc to an appropriate input of the CNC Due to the fact that the carriage will continue to move by inertia after the trigger signal happens the target should have a drawback mechanism able to adsorb this movement assuring its surface integrity Fig 4 Consequently the repeatability of the developed Touch Trig ger Probe is directly associated with the ability of the drawback system to precisely seats back to its original position after a trigger event occurs If the drawback system does not work properly there will be a change on the probe s length preset after a measurement and the diameter of the next part measured will be showed as different than the real For this reason the development of the drawback system was done very carefully to 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 an INDEX TRAUB model GU600 22kW power The CNC was Fig 4 Dynamic behavior of the drawback mechanism during the measurement process M Del Guerra R T Coelho Journal of Materials Processing Technology 179 2006 117 123121 Fig 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 functions needed 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 010mmfromthepart diameter 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 Probe developed probing 60 consecutive times at the same point Also a manual measurement was made at that point using a regular micrometer The data were saved on the CNC and downloaded to a PC The turning operation and consecu tive measurements procedure was repeated fi ve times obtain ing fi ve data samples relative to each one of the diameters Fig 5 According to QS9000 14 one of the most effective meth odstostatisticallyanalyzeprocessandequipmentsisthemethod for computing the repeatability and reproducibility R R The range and average method to calculate R R computes the total measurement system variability and allows the total mea surement system variability to be separated into repeatability rep