In a recent effort to demonstrate how design for additive manufacturing (DfAM) can make a difference when using AM processes, a team from Sandia National Laboratories has designed and built a telescope. By taking into account AM’s strengths and weaknesses, the team explored AM as a wholly new design tool, rather than just an extension of traditional manufacturing workflows.
The team behind the telescope project was the three-year Laboratory Directed Research and Development team led by mechanical engineer Ted Winrow. In its approach, the team decided to forego 3D printing precision parts and instead explored how “less precise” printed parts could be assembled into a structure using precise tools.
By taking this approach to designing the telescope, the Sandia team found a way to leverage the benefits of AM, including rapid prototyping, design freedom and more. “That’s the nuance that seems to get lost, that you have to design differently,” Winrow explained. “It doesn’t plug into a standard design process.”
The resulting structure was a ground-based telescope that was not only lighter than traditionally constructed ones, but also cheaper (it was only about a fifth of the regular cost) and a third of the time faster to produce. The structure integrates 3D printed components, modular design elements and image-correction algorithms, which allowed the team to cut back on the telescope’s optical design costs.
Winrow also explains that the DfAM process used did away with certain recurring costs. “Where every part has to be precise, to nonrecurring costs, where you’re just buying one set of tools that you can use for maybe 10 years,” he said. “So when you’re making production runs you get cost savings. You’ve got time savings because you’re not waiting for each piece to be made.”
The Sandia researchers go on to explain that there are two approaches to building precision structures. The first, which entails machining parts, is to make every piece of the structure to exact tolerances and specifications. The second approach, which is what the team pursued, is to create “rougher” parts using AM and to use a precision process to assemble them into a stable structure.
This approach, of course, begins at the design stage. Winrow elaborates: “Can we design a system that doesn’t care if your material is not as good as you expected it to be? Can you design a system that doesn’t care that your parts aren’t as dimensionally accurate? If you make yourself insensitive to the things that additive’s not very good at, you take advantage of all its good things.”
Looking at how the Sandia team constructed the telescope lens holder, you get a clearer vision of its DfAM approach. While standard camera lenses require a highly precise ledge which position the lens with accuracy, the Sandia team opted instead to design a straight cylinder with no ledges for the telescope lens. By holding the lens at a very precise position using special tooling, epoxy was injected around it keeping it place.
“We can make parts that are less precise as far as dimensions are concerned because of the epoxy in the process. It’s the tooling that’s precise,” added Winrow.
Since creating the telescope with 3D printed components and precision tools, the Sandia team has applied for a patent for a monolithic, titanium flexure (an element used as a joint between rigid components) inside the telescope’s mirror mount. The component, which has a similar function to a spring, is a “roughly cylindrical” structure with thin flexure blades measuring about two inches in length and 3/4 of an inch in diameter. In the telescope, three flexure mounts are used to attach the mirrors to a carbon fiber backbone.
In terms of the optics used, the team integrated an image-correction software to account for the distortions of the lens images. “The thought was you could have less precise optics and correct for it with software, essentially after the fact,” said Winrow. “Similar to how we designed the mechanical hardware to be insensitive to additive manufacturing shortfalls and take advantage of its benefits, [the optics of the system were optimized] so the software maintained the image properties the algorithms could not have done as good a job correcting.”
With the results of the telescope project, the Sandia team will continue to investigate its DfAM approach. “That was what the project was looking at, how these ways could make it faster and cheaper and just as good,” Winrow concluded. “If you talk about things you can give up, things you can compensate for after the fact, it opens up realms on the design side.”