Measurements of Free Form Surfaces and Best …

1 Measurements of free form Surfaces and best -fittingMeasurements of free form Surfaces and best -fittingMexican CMM Club MeetingVOLKSWAGEN de M xico (VWM)Puebla, Pue., M xico22 October 2007 Mexican CMM Club MeetingVOLKSWAGEN de M xico (VWM)Puebla, Pue., M xico22 October 2007 Kostadin DoytchinovInstitute for National measurement Standards National Research Council DoytchinovInstitute for National measurement Standards National Research Council Technologies form SurfaceThe skin of a 3D geometric element which do not have rigid radial dimensions, unlike regular Surfaces such as planes, cylinders and conical : turbineblades, gear Surfaces , car bodies, aircraft bodies, etc. free form AreaFree form SurfaceExamples: turbineblades, gear Surfaces , car bodies, aircraft bodies, etc. The measurement ProcessCollection of data points on a partSoftware EvaluationMeasuring DeviceDeviations, Results, U(k=2)Based on these results users make decisions for acceptance, rejection, fixing, modifying, scrapping.

2 Choosing the right evaluation method is crucial to the results of the Measuring InstrumentsPrinciplesofSensorsofModernCo ordinateMeasuringSystems: tactileswitching3D tactileanalog3D optical2D camera opticalvideo auto focus opticaltriangulation laserscanners photogrametry opticalpointfocus tactile-optical opticalinterferometric lasertrackers opticaltime offlight X-ray GPS, radar, CMMsArticulating Arm SystemsROMERFAROCMMs (Non-Cartesian) Laser TrackersMajor suppliers: Leica, Faro, APIU ncertainties of measurement starting at about 10 m and up. The shape of the uncertainty zone is dominated by the lower uncertainties of the two angles. CMMs (Non-Cartesian) Video & PhotogrametryReasonable accuracy 1:100 000 (10 ppm) or worst. OGPM itutoyoOGPT raditional Video Measuring MachinesScanner conceptAdvantages:high data density, high speed -tenths of 1,000s of points/secDisadvantages:relatively low accuracy, sensitive to surface finishScanner on a CMML aser ScannersApplications:reverse engineering, sheet metal parts, free form Surfaces MetrisFringe Projection Systemscamerasprojector/ scannerFringe Projection Systems FraunhoferIPTU.

3 Neuschaefer-RubeSheet Metal ExamplesBentelerAutomotiveBentelerLaser GaugeThe optical methods are replacing traditional CMMsfor data collection when tolerances allow. Uncertainties range typically 30-100 m with measurement time down to 1-2 minutes and huge data density millions of with a Tip (Stylus)Physical Tip with a known radiusSurface Measurements Data Collection free form Surfaces require much higher point density for proper surface evaluation Parts with free form Surfaces tend to have larger tolerances attached The above facts favor the optical methods High data density Uncertainties still kept in workable proportion to the tolerance zones 1:10, 1:5 Surface Measurements Data Collection The data high density millions of points is not always an advantage The number of point overwhelms the computer capabilities and slows down the evaluation process. Data very often needs filtering to improve accuracy and reduce data noise.

4 The evaluation process requires different software capabilitiesThe Goal of the measurement Determines the Evaluation ProcessTolerance ComplianceManufacturing Process AnalysisSPC (Statistical Process Control)Data Evaluation ProcessData Collection ( measurement )Need for task specific evaluation: the analysis may lead to very different results using the same raw data!Surface Measurements Measurements of an unknown geometry nominal information Reverse engineering Measurements of a known geometry we have a drawing or a CAD model Control for compliance Troubleshooting If the real geometry deviates significantly from the theoretical one, at one point, depending on the accuracy required it should be treated as unknown .Tip Radius Compensation on a free form SurfaceThe effective tip radius should be corrected from the tip center, to the surface in the direction of the estimated normal vector at the point of contact. Poor estimation of the normal vector leads to cosine radius correction -cosine errorErrorRa= (cos())1 The cosine erroris proportional to thetip radius and to the cosine of the angle between the real and estimated spatial normal vectors Tip Radius Correction -Cosine ErrorNormal vector error in radius R (mm)Cosine error ( m)Laser tracker typical tip radiusTip Radius Correction for Geometrical FeaturesFor machined parts with regular Surfaces the cosine errors are usually negligible Attention has to be paid when measuring parts with large curvatures and castingsSurface Measurements Known Geometry Tip Radius Compensation CAD model available tip radius is compensated perpendicular to the surface Results depend on the part alignment ( best -fit).

5 Center of tip pointsTip Radius compensated pointsMeasurements of a known , , , , , from given as: X, Y, Z, i, j, kXYThe data should be given in a well defined coordinate system (set-up) otherwise not reliable. Typically, it requires datum target definition. The CMM is then programmed to measure the points in a direction opposite to the normal vector (pointing out of the material).()ijk2221++=ConditionDiscrete point informationVectoring Accuracy = + + ()()()NXXNYYNZZ222 True position(reported from the CMM)NX, NY, NZX, Y, ZThe true position contains theCMM vectoring errors (the inabilityof the CMM to drive the probe indefined direction). This can be a majorproblem since many of the CMMsdonot have a good vectoring CMM trajectoryVectoring accuracyApproach vectorThe component of the true position which is caused by the vectoring errors can be significant if higher accuracy isrequired.

6 If the tip radius is compensated after best - fitting , these errors can be greatly Accuracy0, 0, 0 Simple experiment for vectoring error estimation. Can be repeated in several orientationsOur SIP 560M CMM had vectoring errors up to 40 mOur LegexCMM has vectoring errors below to mSurface Measurements Known Geometry Tip Radius Compensation CAD model available tip radius is compensated perpendicular to the surface Results depend on the part alignment ( best -fit).Center of tip pointsTip Radius compensated pointsTip Radius CompensationX= center coordinatesUncertainty zonePlus the knowledge that this point is one tip radius away from the real surface if a tip radius is presentCompensating the tip radius during the best - fitting is the right approach, but in certain conditions this may not be accurate Radius Compensation Using CADD istance (Deviation)Uncertainty zone of the data pointXmeas, Ymeas, ZmeasReal SurfaceProbe tipXnom, Ynom, ZnomThe deviation from nominal is most often the distancefrom the measured point to the nominal surfaceFitUncertainty due to Tip Radius CompensationIn this case the rotation was degrees which will move a point metresfrom origin by 5 mm from the original of Kotem5 mm on an area with high curvature may mean a very different normal vector applied to the same point!

7 Uncertainty due to Tip Radius CompensationIn this example the maximum deviations are about 50 mm In majority of the cases better best - fitting means better tip radius compensation but incritical cases a local reverse engineering is needed to better estimate the surface normal vector and compensate the tip radiusThe Nominal DataCAD Geometry ProblemsReversed surface normal vectorsSeeing Surfaces with bright and dark colors when opening a file should be investigatedReported deviations will be with opposite sign and the results wrongInaccurate CAD GeometryCheck the CAD geometry before best -fittingLow Accuracy ModelThis nominal cylinder has a form error of mm built in! fitting measured points to this surface will be affectedCAD to CMM Data FlowGeometry Description Model AccuracyThe center position of this nominal cylinder is questionable!CAD to CMM Data FlowExamples of ProblemsPropeller CAD modelThis model visibly looks fine. Only after thorough examination one can discover problems which could cause uncertaintyOn one of the blades, some small Surfaces have a normal vectors pointing inside the material!

8 During Measurements this may cause some deviation to change sign.,With a very high magnification it is visible that part of the surface is missing and we can see through the gap inside the blade! CAD to CMM Data FlowIGESSTEPCAD systemnative formatPlanesCylindersCircles, ,i,j, softwareProgramexecutionCMMD uring the conversion from native CAD format to IGES, VDA or STEP, some of the geometrical data may be modified or lost! The result is that the CMM is working with a model different from the original! The model may end up incompleteand with many faultsin CAD systems very rarely use generic equations when describing simple elements as cylinder, cone, circle. They may all be described with NURBS (splines)! At the same time the user expects, for example, a radius and a hole center location. This forces the CMM software to calculate the nominal dimensions of elements from splinesurfaces. This a source of additional ,Elements.

9 ?CAD to CMM Data FlowModel InterpretationInterpretation errors from mm all the way down to Model HealingAcrobat DocumentCAD to CMM Data FlowExamples of ProblemsTo address the issue, numerous CAD model healing packages exist . It is important to note that some problems are reparable just fine, while others require modifications of the model. The client has no idea how the healing is done. The only thing he notices is that the file does not crash modifications are a source of way of assessing the accuracy of the data transferIGESSTEPCAD systemnative formatThe CMM software generates points with X,Y,Z,i,j,k on the critical Surfaces of the model or on the whole modelCMM software0. MaxDistGoal:The so created points are imported back into the original CAD system and a calculation is performed for the distance between the points and the original model. If the transfer is good, the distances should be negligible. What is best - fitting ?

10 best - fitting is the process of finding the best mutual relationship between Measured data and Nominal data when the part is not fully constrained while trying to satisfy a specific MetalDatum PointsBest-fittingAfter best -fitBefore best -fitConstrained Features Soft GaugesSoftware gaugeHard gaugeDrawingConstrained cylindersComplete assessmentNon-constrained cylindersComplete assessmentWeckerman, Heinrichowskiand Mordhorst, best - fitting of Shapes and Patterns We best -fit for the purpose of: Comparison Measured data to nominal (Example: to satisfy tolerances) One part to another part Assembly To show that parts fit well in an assembly To analyze an assemblyBest- fitting can help with misalignment, assembly and location problemsBest- fitting CANNOT change sizes!Measuring Device -Software IntegrationX, Y, Z measured coordinates plus tip radius when applicable best -fit and Analysis SoftwareCAD Model IGES, VDA,DXF, STLThe best - fitting EnvironmentNominals/CADM easured DataMath CriteriaWeights/ExcludeTolerancesRelease sFiltersCalculationsGroupsStatisticsResu lt 2 Result 1 Result NBest- fitting ProcessTransferring the ResultLocator #RemarksLOC1 LOCATOR AN 1 /LOCA# 1Z = = AN 2 /LOCA# 2000009Z = = AM 1 /LOCA# 4Y = = AM 2 /LOCA# 5Y = = C /LOCA# 6X = = coordinatesOriginal coordinatesORRPS Reference Point SystemBest- fitting PrinciplesPoint-to-pointPoints-to-surfac eThe measured points are not associated to a particular point.