This article shows how parts can be replicated by using a combination of 3D printing and 3D X-ray inspection. Initially, an aluminum 3 Jaw chuck was scanned to reproduce a plastic (AM) replica.
In order to achieve this, the metal chuck had to be disassembled first, and the pieces were scanned. All of the components were then reconstructed and surfaced.
The parts were produced with stl. polygon data in the AM printer. After printing, all the AM (plastic) components were scanned, and the stl. data was produced. The stl. polygon data produced was used in Volume Graphics to obtain dimensional verification.
Figures 1 and 2 show the 3 Jaw chuck fixture that was scanned. The chuck was disassembled before the scanning. The assemblies can be scanned together, and the pieces separated in the scan data. However, this procedure is more time consuming, provided the penetration for the surface data is good.
Figure 1. Aluminum 3 Jaw chuck
Figure 2. Disassembled 3 Jaw chuck
Guidelines for Developing a Quality Technique
The following guidelines must be followed while developing a quality technique.
- The images must contain sufficient contrast for the internal structures to be visable, such as passageways, holes etc.
- An suitable resolution that would suit the size of the feature to be captured must be chosen.
- The whole sample must be exposed to X-ray penetration from all angles, while avoiding over exposure of the detector (Figure 3).
Figure 3. The entire sample must be exposed to X-ray penetration from all angles
Voxel data of the scanned samples can be obtained and exported in many file formats including ply., .stl, obj.,. The dimensions can be verified by measuring the surface data (Figure 4).
Figure 4. Voxel data can be verified by measuring the surface data.
A point cloud is created by the software from the voxel data at a specific iso (gray) value. Depending on the neighboring voxel gray values, points are produced at sub-voxel positions (Figure 5). As the point cloud files obtained from CT scan data are large, they may have to be decimated before being loaded into printer software.
Figure 5. The software creates a point cloud from the voxel data at a specific iso (gray) value.
One of the jaws contained 4.3 million points, making a 415 MB stl file. Therefore, the file was decimated down to 5 MB for printing purposes.
After loading the stl files into the printer software, the chuck was printed. Figures 6 and 7 show the printed 3 Jaw chuck.
Figure 6. Printed components of the 3 Jaw chuck
Figure 7. Printed 3 Jaw chuck after assembling
The dimensional accuracy of the printed samples can be verified by running a CT scan. Figure 8 show the CT scan results of the printed samples.
Figure 8. CT scan of the printed samples
After scanning the printed sample, it can be measured to verify dimensions (Figure 9).
Figure 9. Dimensions of the printed samples can be verified through measurement
The printed sample must possess surface determination to perform measurements (Figure 10). The surfaces (spheres, planes, cylinders etc.) are fitted with geometric features and measurements are made based on these features. Information about the geometry of the sample can be obtained from the fit points of the feature.
Figure 10. The sample must have a surface determination to preform measurements
Deviations, if any, can be found by comparing the .scan data to the stl file or CAD model (Figure 11).
Figure 11. Scan data of the sample’s components
Point cloud data or a CAD model can be loaded into the CT software, which can then perform a best fit with reference objects, or a 3-2-1 alignment of the scan data to the CAD/point cloud data.
A surface of the scanned data in 3D is compared with the CAD/point cloud data, and colors are assigned depending on discrepancies (Figure 12). Specific details regarding the local area can be given by placing annotations on the 3D model.
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Figure 12. A surface of the scanned data in 3D is compared with the CAD/point cloud data, and colors are assigned based on deviation.
This information has been sourced, reviewed and adapted from materials provided by North Star Imaging, Inc.
For more information on this source, please visit North Star Imaging, Inc.