Formula SAE competitions challenge teams of university undergraduate and graduate students to design and fabricate a small, formula-style vehicle. Once completed after eight to twelve months of work, the vehicles are judged in a series of static and dynamic events including technical inspection, cost, presentation, and engineering design, solo performance trials, and high-performance track endurance. Space constraints within the cars are such that accurate CAD models of the engine’s external geometries are needed. Engine manufacturers will not provide design data so 3D scanners are used to create the CAD files.
For the University of Connecticut FSAE team’s entry for a Formula SAE Michigan competition, Bolton Works (East Hartford, CT) provided advanced reverse engineering to make 3D CAD models in Solidworks of a Suzuki GSX-R600 engine. The CAD models are used to check for interference and to facilitate design modification of the engine. The use of 3D scanning equipment to make CAD files is not new within the FSAE student community, but improvements in 3D scanning have streamlined the process.
To provide UConn with a comprehensive 3D model, Bolton Works determined that an industrial-grade, high-resolution Zeiss Comet scanner with high-end optics and sensors needed to be used to ensure detail and accuracy. The accuracy of critical dimensions of the CAD model should be verified with a Zeiss Contura CMM. The engine model should include all factory bolted-on accessories of the engine, and the model should be subdivided into its 25 individual components so they can be replaced by an UConn-designed one in Solidworks, as needed. Internal geometry of the complete cylinder head’s flow path should be provided.
The Suzuki GSX-R600 engine comprises four load-bearing components: die-cast aluminum lower crank case, which includes the upper transmission case; cast aluminum cylinder block with integrated upper crank case; cast aluminum cylinder head; and de-cast lower transmission case. Bolted to these four components are covers for the clutch, stator, camshafts, vents, crankcase and sprocket, as well as an external water pump assembly, the starter motor, oil filter, oil cooler, water inlet housing and several sensors. Twenty-five components have surfaces external to the engine and all were scanned and modeled to become part of the Solidworks assembly.
There are various techniques for the measurement of a three-dimensional shape. One method is “structured light projection.” As white light or blue light can be used as lighting source for the projection of the fringe patterns, these scanners are also known as “white light” or “blue light” scanners.
The operating principle is the identification of object points by a pattern projected onto the surface and observed by a camera under a different view. The camera to surface measurement is based on triangulation. Because the angles and distances between the light source and camera are fixed, and the direction of the light ray is known, the depth of the surface where the light strikes can be calculated.
The Zeiss Comet scanner used for this project is a Structured Light scanning system for applications ranging from small, precision components to large tools, dies and vehicles. The Comet produces dense, point cloud data, which permits inspection (metrology) and reverse engineering. This scanner has an adjustable measuring volume of 75 × 75 × 50 to 900 × 600 × 600 mm, and accuracy of up to ±0.005 mm. Each view or “scan” can measure up to about 11 million X-Y-Z points. With large objects or objects with complex surface geometry it is necessary to take several measurement positions to ensure that all surfaces are recorded. There is no limit to the number of views or “patches” that can be recorded per object other than computer memory. After the scan, the patches are globally aligned to form one 3D point cloud.
Before scanning, the engine was drained and thoroughly cleaned, as the resolution of the scanner is such that dirt particles, scratches and other undesirable residue will show up in the scan data. After removal of most of the internal components (crankshaft, pistons, transmission, etc.), the external visible components were re-assembled. For the scanning, the engine was placed on a high-accuracy rotary table, of which the position is tracked to enable automatic alignment of the different scan patches. For the complete assembly, 153 patches were created, resulting in a scan data set of 4 GB. This scan data set was then reduced in size by removing overlapping points and connecting the remaining points with triangles.
This triangulated file, called an STL file, is a facetted representation of the scan geometry 1.5 GB in size (30 million triangles). As parts of the bolted-on accessories cast a shadow on the engine during scanning, large areas are not captured during the scanning process. For example, the backside of the water pump is not visible to the scanner and therefore will leave a gap in the data. To ensure completeness, the engine was disassembled and each component was scanned separately. STL files were created and then matched (best fitted) to the assembly file, to ensure correct positioning in 3D space. The individual STL files were then brought into Geomagic Design X Software from 3D Systems to fill in gaps of areas still missed during the scanning process. The gap-filling process is essential because completely closed geometry is needed to create a solid for use in Solidworks.
Bringing the STL files in Solidworks was not a viable option at the time, so Nurbs surfaces were created outside Solidworks and then imported as parasolids. To create the Nurbs surfaces, Geomagic automatically generated a network of splines on top of the STL files to approximate the geometry. From this network of splines, Nurbs surfaces are created. This method is efficient and reduces the file sizes to more manageable levels. As a final step, the surface patches are “sewn” together to form a closed solid CAD model.
This solid, constructed in Nurbs Surfaces, represents the engine as is, including any flaws there might be in the engine’s surfaces when scanned. Consequently, machining inaccuracies, worn areas, dents and scratches, etc., will be part of the model unless further steps are taken. As the goal is to be able to use the CAD models as a reference, the attachment points for frame, intake manifold and exhaust manifold need to be free of defects. They are therefore modeled in separately. Where, for example, a bolt hole would exist, a cylinder would be fitted to the scans. Solidworks can recognize the cylinder and use this as reference to which other geometry can be easily built upon.
All models are exported from the Geomagic software into the neutral parasolid format, which Solidworks can read. As all models are already in the correct position they can be loaded as assembly in Solidworks. The total size of the complete GSX-R600 engine Solidworks assembly file is about 500 MB, requiring about 1.5 GB memory when loaded in Solidworks. Although the size of the file was not an issue for any of the computer workstations in use, it is possible to suppress any of the components in the subassembly to focus on, for example, the cylinder head. To verify that the dimensions of the attachment points are correct, the solid model is also used to program the Zeiss Contura CMM and re-measure these points on a second, still-assembled engine. The solid model was then updated in Solidworks to reflect these CMM values as needed.
Soon after the FSAE community understood the capabilities of Bolton Works, requests were made for other detailed engine makes and models. Ten different engine types have been modeled to date, each with increasing level of detail.