The field of engineering design is constantly innovating to incorporate new technologies. One such technology is the use of contact and non-contact 3D scanners in the creation of CAD models and their use in manufacturing processes. While their use has become widely acceptable in the initial design phases, primarily for reverse engineering and prototyping processes, there continues to be research done to improve the technology and to determine the most accurate and efficient methods of their use. Industries and design processes that require exact measurements and tight tolerances; however, find the technology inaccessible to their professions due to the scanners’ expense and inherent inaccuracy in measurement which leads to discrepancies in the CAD model and ultimately imperfections in the manufactured product.
Reverse Engineering and the Scanning Process
In 3D scanning, a sensor is used to determine the geometry of a piece. This sensor, contact or non-contact, creates a point cloud of the object which serves to give an idea of the geometry of the shape. In the use of a contact sensor, the sensor touches the object as its dimensions are mapped to achieve precise measurements. In the use of non-contact sensors, controlled light or lasers are directed at the object and the reflection of light is measured and calculated to determine the geometry. The combination of the point cloud and use of a cleaning software, which clears any points at which scanning anomalies occur, in a CAD program allows an engineer to convert the physical geometry they are trying to duplicate, into a digital model. This allows 3D scanning to be utilized in context of the reverse engineering of previously developed products.
Reverse engineering, as exemplified by Sokol and Cekus in their paper on the use of 3D scanning to reproduce parts, is the use of engineering technology, such as scanners, to recreate 3D models of pre-existing parts and geometries to be used as replacements for machinery or in the prototyping and testing phases of production of a new design. As a process it is primarily considered in the early design and conception phases, which often involve testing and prototyping of a design. The use of 3D scanners makes the reproduction of parts for prototyping much quicker by allowing them to easily and quickly convert parts into 3D models. However, reverse engineering and 3D scanning is impractical for professionals who require precise final measurements such as for airplane parts or scientific equipment due to the inherent inaccuracy of the technology and the limits of precision when multiple software are used in the production of a final part.
By nature, scanning processes have a precision assigned to their measurements due to not only the limits of the technology itself but also the process of scanning used. In addition to the imprecision of the scanner, the software then used to analyze and re-manufacture the product has a limit to its precision. Mahan and his associates, as seen in their article published in ASME Magazine, call this the “stack-up” of tolerances; which leads to poorly scaled and measured parts in scan to print processing. To show how these processes change the final dimensions of a part, they first acquired a point cloud of an object through scanning, converted it to a model, cleaned the point cloud of the scan, converted it to a model, exported the design to an STL form and then sliced the resulting model as you would for a 3D printing procedure. This resulted in parts with slightly different dimensions than the initial intended geometry; however, this change in dimensions was large enough that 3D scanning is deemed inadequate for the creation of meticulously toleranced parts.
Contact Scanning Methods
Contact scanning as presented by both Quality Magazine and the works of Sokovic and Kopac is a mechanical method of scanning by which a scanner is moved along the surface of the object being reproduced as a digital 3D model. These contact sensors are much more widely accepted due to the high precision of their measurements but come at a much greater cost and require much more time to achieve a full scan of an object. There are stationary sensors, that require more training and time to use but are highly accurate, and portable sensors, which are easier to use and require less training but are slightly less accurate and less expensive than their stationary counterparts. As with all 3D scanning methods, contact scanning does not come without limits. Due to the nature by which measurements are taken, contact scanners can only create 3D models from rigid materials, soft materials will bend and deform if a contact scanner is used making the measurements inaccurate. Contact scanners also have a difficult time gaining measurements from surfaces with unknown qualities, which makes the scanning of these surfaces almost impossible.
Non-Contact Scanning Methods
Optical Scanners
Optical scanners, as discussed by Nguyen, Aprilia and their associates are less expensive than contact scanners; however, sacrifice some accuracy for the ability to scan more quickly and the ability to scan soft materials as well as rigid material. They use directed light to scan an object, calculating the position of the object by using calculations based on the light reflected from the surface of the object being measured. The use of light, however, adds new opportunities for inaccuracy to arise. These inaccuracies were due not only to the nature of the scanner not being in contact with the surface, but also due to the surface finish of the part being scanned. Dark colored surfaces, transparent surfaces, translucent surfaces, reflective surfaces, and poor access to surfaces cause inconsistencies in the 3D model generated and result in the creation of parts that can be inaccurate for industry professionals requiring a great level of precision.
Laser Scanners
Laser scanners, as examined by Ishiel, Gonnet, and their associates, use equations to triangulate the location of the surface and can take thousands of measurements and create a point cloud model of an object very quickly. Despite their efficiency, little innovation has been able to aid in the accuracy of these scanners and they remain inaccessible to many manufacturing industries due to the inaccuracy of the 3D models they create. While laser scanners can be used on soft and malleable surfaces, they are impossible to use on reflective surfaces and geometries with steep slopes due to the refraction of the laser. Additionally, this method is wrought with issues created by the environment of testing where humidity or dust will skew the measurements. These scanners, while more accurate than optical scanning, are quite expensive, and are not nearly as precise as contact scanning which comes at a much greater price point.
Mills and Lathes and Printers, Oh my! 