Even with composite parts being molded to net shape, a degree of finish sanding is usually required. These are all specialty sanding blocks. The yellow sander progressively sands 2¼-inch and 3⅛-inch holes for the instrument panel. Adhesive-backed sandpaper sticks relatively well to PLA if you wipe it with acetone before applying the sandpaper.
I saw my first 3D printer in a shop setting about five or six years ago. My initial impression was that it was a) a toy and b) too small to be of much use. But the guys in that shop were a bunch of smart dudes, so I figured there might be something to this 3D printing craze.
A 3D printed mockup of the instrument panel.
I’m always open to trying new technology in the shop, even if half the time it doesn’t really pan out (I’m not sure my Shaper Origin CNC manual router will ever make it into an article, I use it so little). So I did my research and settled on a Prusa MK3S+ additive printer. It was around $800 and required assembly. I liked the assembly option so I could get an understanding of how the thing worked. Working together with my intern at the time, we assembled the printer in one long day.
My 3D printed tools tend to be items like hole pattern drill jigs (left), specially shaped sanding blocks (center) and outlines for tracing shapes on complex parts (right). These jigs are simple extrusions that take just a few minutes to model in CAD.Tube drill jig (left). This jig centers the bit on a tube and prevents it from wandering. The flat base holds the tube stable on the drill press table while drilling. 3D printing is extremely accurate (my Prusa easily holds +/- 0.005 inch), so this tool is printed 1:1 for the tool ID/tube OD. This makes the tool a tight fit, which helps hold it in the right position while drilling. This is an internal sanding drum (center and right) to ensure concentricity and constant wall thickness on the intake tube of a molded carbon fiber intercooler header.
I’ll cut to the chase and tell you that 3D printing has completely (underline and bold that too!) changed the way I design and build the SR-1 race plane. While I only have a few 3D printed parts in the airplane itself, I have printed more than 1000 jigs, tools and mockup parts on the Prusa. Actually, that’s Prusas, since I had three at one point earlier this year: the original MK3S+, the upgraded MK4 and the larger volume XL. (I’ve since sold off the original MK3S+ a few months after purchasing the MK4 since the MK4 has been phenomenally reliable right out of the box and is significantly faster than the MK3S+.)
The exterior of the intake tube is perfectly round out of the mold, but the interior still needs finish sanding (left). A threaded rod attaches to the internal sanding drum and allows it to be chucked in a hand drill. The outer positioning drum is 0.010 inch oversize to the tube OD, which ensures the internal drum stays concentric to the OD as it spins. Looks complicated but it was only about 15 minutes of CAD work and made the job easy and accurate (right).
According to the stats page on the MK3S+ printer at the time of selling it earlier this year, it had printed 4.3 miles of filament with a run time of 3400 hours. I’d place reliability at better than 99%, with the only issues I experienced being a failed printhead sensor and dead print bed thermocouple, each of which were relatively cheap and easy to fix. It’s not uncommon to have both (or all three when I had the MK3S+) printers running concurrently for much of the day when I’m prototyping something new.
This is my “Tower of Hanoi” offset line drawer (left). Composite layups typically have a master template pattern, with the carbon fiber and other consumables being 1 inch, 1.5 inches, etc., larger than the mold itself. This is a good example of a tool you could make in less than 5 minutes with basic CAD skills. A leftover silicone insert (pink) from a Click Bond nut plate holds it all together when not in use. These wheels make it fast and easy to trace out the offset shape from a master template (right).
I’ll admit that while I’ve been thinking of writing this article for quite a while, with hundreds use cases I wasn’t quite sure where to start. So this article will show just a few examples of how I’ve used 3D printing in my shop. I’ll divide this into tools, molds and actual airplane parts. Note: You’ll see different colors here, but they are all the same filament, PLA or occasionally PETG. I just buy whatever color is the cheapest. Multicolor parts just mean I ran out of filament mid-print and started a new spool that happened to be a different color.
Seat belt restraints were fabricated from unidirectional carbon fiber in 3D printed press molds (left and center). This approach allowed for quick prototyping of these parts in order to achieve the desired performance, as measured with destructive testing in my test machine (right).Another huge advantage of 3D printed molds (left) is for parts that would require five-axis machining if fabricated from metal (read: expensive). I originally modeled this bellcrank support bracket as bent sheet metal, but it proved too flimsy and time-consuming to fabricate. Fabricating in a 3D printed press mold proved to be the trick, and the resulting part is extremely light and strong. Proof tested in the test machine (right).CAD modeling of parts allows the fabrication, assembly and testing processes to be integrated. Molded parts can feature inserts that are later removed (obviating the need for post-process machining) or features like threaded inserts that are molded in situ (left). Here, a dogleg used in the canopy latch mechanism is molded in a 3D printed press mold (center). A 3D printed jig holds the entire latch arm in position for final assembly bonding (right).As mentioned above, the intake and exit headers of the intercooler are molded parts. Here, a 3D printed mockup is used to check fit and integration to the throttle body hose (left). Image of the mold in SolidWorks. Once the part model exists, generating its mold is relatively straightforward (right).Because the mold lacks traditional loft, removing the part requires trimming away the printed mold (left). The finished part bonded to the intercooler (right).
Molds & Parts
Because the SR-1 is a one-off composite design with all molded parts (i.e., very few wet layups on the airplane itself), the Prusa has really shone as a mold maker. One of the great features of the design process is the ability to 3D print mockup parts, iterate these parts to a final design solution, then use the existing part model to generate a female mold in SolidWorks. In addition, because these molds are a) cheap (a few pennies of filament, or dollars at the most) and b) easily made, I consider the molds themselves consumables when necessary. By that I mean I don’t mind if I destroy a mold in the process of liberating the part from it after curing—I can always print another mold if necessary. This has huge implications for mold design, part geometry and the fabrication process.
This 3D printed assembly (left) of the SR-1’s custom intercooler was provided to the vendor as a visual reference for the welder, in addition to engineering drawings. This template (right) served to lay out the spacing for the threaded mount bungs on the intercooler, which needed to be precisely located to ensure proper fit.
Although fused deposition modeling (FDM) 3D printers that can print parts rivaling aluminum in strength exist, they are still quite expensive, as are printers that can print in metal using lasers. Therefore, there are not a lot of 3D printed parts in the SR-1 itself. However, there are a few applications that see low loads and low heat exposure and therefore lend themselves to printing.
A similar procedure was used for the custom radiator (left). Here, a template lays out the precise angle of the coolant exit tube. This yellow 3D printed bushing (right) slips inside the hollow prop shaft of the SR-1’s Edge 912STi engine. It holds a small laser pointer. The aft end of the prop shaft has another 3D printed insert that serves as an alignment aperture. This allows the centerline of the engine to be projected onto the firewall, which serves to ensure engine alignment as well as provide the origin reference point for the engine mount.
There are countless 3D printers out there these days, and it is beyond the scope of this article to get into the technology itself, or the different printers. But there’s a lot of info on the internet and YouTube reviews of printers are a great place to start. In addition, there are a lot of ready-made CAD models available on the internet (sometimes free, sometimes for a fee) that you can download and print without learning a bit of CAD.
Here’s another part for which a traditional mold would be exorbitantly expensive and/or time-consuming to fabricate. Due to tight packaging on the SR-1 firewall, a custom coolant overflow tank was fabricated (left). The cap is a common fluid cap from a plastics supply vendor that uses a standardized no-leak thread design. A model of the threads was downloaded from GrabCAD, a community CAD-sharing forum, and integrated into the tank design (center). Here’s a mold to make a mold! The 3D part represents the shape of the desired final part, to be made from pour foam. A silicone mold is cast from the 3D part. Two-part foam is poured into the silicon mold, which peels off once the foam cures (right).These air ducts have an internal geometry that avails itself of 3D printing. The carbon tube is rotated to open or close the duct (left). The OEM housing for this GPS unit did not integrate well into the avionics bay. A new housing was printed that allowed mounting the unit vertically (right).
But combining the free version of SolidWorks available through EAA with one of the many cheap 3D printers available these days is a great way to jump into this great technology, learn some new skills and maybe even make your ride a bit more trick!
These small yellow wire clips are superglued to the side of the header tank. PLA bonds quite well with cyanoacrylate if slightly roughened and then wiped with acetone.
I’m excited to see 3D printing presented here! Great examples of making molds and prototype parts.