use of digital checking fixtures and scanning techniques for reverse ...

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The following steps are used in order to perform thefixture technique: Generate digital X,Y and Z sections of the object Locate critical datum features Generate parametric primary datum Generate parametric secondary datum Place datums in CAD assembly Overlay digital sections on assembly Place part to be deformed in fixture Deform component to match datums Verify final product for accuracy using scan dataSession 3Bincomplete or obstructed from the scanners view bysurrounding components of the truck. The lack ofinformation made direct duplication of the deflected springsdirectly from the scan data impossible. There was no longeraccess to the truck itself to allow for rescanning of thedesired geometry. Therefore, a method was developed toreproduce the deflected spring using its basic geometry andthen modified to meet known dimensions of its deflectedcounter parts. A digital picture taken of the non deflectedspring can be seen below in Figure 1 along with its reverseengineered CAD model.CASE STUDYIn this case study the overall project involved the reverseengineering of a production model Peterbuilt truck. Thetruck’s geometry was captured using digital white lightscanning technology and then reproduced into a CADmodel. The model would then be used in a dynamicsimulation program called ADAMS-car. Once in ADAMS-Car roll over analysis of the truck could be conducted andthen analyzed. The resulting data would then be use by thePeterbuilt Company, which produces the truck.The research was conducted at Western MichiganUniversities Center for Advanced Vehicle Design andSimulation (CAViDS) laboratory. The project was tasked toreverse engineer components, verses simply retrieving priorCAD files from the company, in order to factor inmanufacturing differences and variances which the idealCAD files neglect or cannot account for. Those variancesmade our simulations as accurate as possible to the realmanufactured trucks. [2]Towards the end of the project, several componentsneeded to be clarified or re-created prior to simulation. Oneof the components which needed to be worked on was thetruck’s leaf spring assembly. Three scans of the springassembly were taken for the project. From these scans threemodels would be generated. One model was of the overallspring assembly in a non-deflected state, resting on theground, while fully assembled. The second model was of thespring assembly in place on the truck. This second assemblywould be deflected but only under assembly conditions. Thetruck was lifted into the air so that no other weight wasplaced onto the spring. The third and final model would beof the spring in its deflected resting state, with the truck onthe ground and the weight of an unloaded truck bearingdown upon it. The three models would be used to helpcalculate and verify the computer generated spring’s motionand deflection as well as to create the initial spring CADmodel.The challenge of creating the computer models wouldbe in the lack of provided information. Scans of the leafspring assembly were conducted months prior to its CADduplication and before the need for multiple models wouldbe required or even requested. Complete scans of the leafspring assembly, in its not deflected state, were conducted.However scans of the mounted spring were eitherFigure 1Non Deflected Spring (top) RE CAD Model (bottom)Though the scans did not allow for direct duplication,they did however capture key areas where the spring was tobe mounted relative to other components in its deflectedstate. Also some geometry of the spring was captured thatallowed for referencing based upon mounting points. Inother words the scanner revealed datum points and key crosssections of the spring. Enough key points existed that amodel of the non deflected spring could be modified andaligned with these points to reproduce its deflectedcounterpart.To modify the non deflected model, a form of digitalchecking fixture was created using the datum pointscaptured in the scan. Like a real life measuring fixture, thedatum points would hold the work piece in place while themain cross sections of the spring were bent and manipulatedto match the known cross-sections which were captured inthe partial scan. The non deformed spring would bemanipulated till it would align with the known sections ofthe deformed spring. Once finalized the deformed springwas then re-inserted into the known scanned geometry to beverified and to calculate percentage error. This techniqueallows for accurate reproduction because the softwarechanges the model according to how the metal woulddeform under loading conditions.Pittsburgh, PA March 26 - 27, 2010ASEE North Central Sectional Conference3B-2
Technique StepsFor this project an ATOS II white light scanner was used toscan part geometry. [3] Once scanned, the surface geometryof a part is stored inside the attached software. The ATOSsoftware allows a user to divide up the scanned surface intosections. Too accurately model components in this way,sections along the X, Y and Z axis were made at fivemillimeter increments. An example of the spring sectionsalong the X-axis can be seen in Figure 2. In this figure thesections are overlaid on the part. When these sections areoverlaid on top of one another the entire part can be seen inthree dimensions. This is the starting point to this reversetechnique.Figure 2Scan Sections Overlaid on ModelThe second step in creating the digital fixture was tolocate and identify the main datum reference points, then tocreate a three dimensional solid primitive to act in its place.In the case of the leaf spring, these points were the two endsof the spring and the top assembly mounts. All three ofthese primary points are bolt locations and regardless of theamount of deflection, these points will always be bolted inthese locations. An example of the described location canbe seen highlighted in Figure 3.Session 3Bboth a known size and had their geometry captured in allthree scans. An example of the finished first step can beseen in Figure4.Figure 4Primitive Cylinders with Spring for ReverenceThe third step in creating the fixture was to locate andrepresent secondary datums. In the springs case thesecondary datum would be where surfaces of the springwould come in contact with known surfaces of the truck.For example the leaf spring included a mounting platewhich not only reinforced the bolt for the center axle mountbut also came in full contact with the axle itself. Also, twomounting plates, which are perpendicular to their mountingbolts at either end of the spring, were located. These arepoints which, like the bolts from the first step, will neverchange being in contact. However they are not as critical asthe bolt locations because there have larger variances. Thesesurfaces can shift and slide depending on the amount ofdeflection and how they were assembled. For example theends of the spring will bolt to the frame of the vehicle andprevent the spring from translation along the X, Y, and Zaxis. The surface of the vehicle frame, that the spring isbrought up against, prevents translation in only the Z axisand prevents rotation. Because the surface allows for alarger degree of translational movement and is not as criticalto the overall design intent, as the bolts, the surfaces are asecondary reference. After locating these points, they wererepresented in the CAD model by solid surfaces which thespring primitive would be joined against using matecommands. An example of the third step can be seen belowin Figure 5.Figure 3Primary Datum LocationsThese points represented the primary datum points. Theprimitives where made as cylinders, which represented thebolts that affix the spring in reality. All three of the bolts areFigure 5Primary and Secondary Datums with Spring for ReferenceThe final step in creating the fixture was to import thescanned sections of the deflected leaf spring into the fixturemodel. The imported sections were transferred into themodel as an IGES format. The points were then overlaidPittsburgh, PA March 26 - 27, 2010ASEE North Central Sectional Conference3B-3
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