Building Brackets around the C-LEG prosthetic

This is Part Three. See: Part One, Part Two

Started out with some photogrammetry to capture the geometry of the C-LEG, which will hopefully allow me to 3D print a bracket that fits the contours of the the C-LEG precisely.

This first scan was enough to play around with, but ultimately the glossiness and the bright sunlight caused enough gaps and distortions that I had to do a photoshoot later that night using our CNC machine as a light box. The even lighting from the LED rope was just the trick.

The next step was selecting a portion of the C-LEG’s surface to extrude into a form fitting shell. Blender was used to create a mirror image of the scan, and MeshLab was used to align the two sides and fill in the holes so I had a reconstruction of the entire CLEG (Agisoft was only able to reconstruct one side of it — I could of went back and tried another photoshoot, but decided it would be faster to just duplicate the half that worked). In the video you can see the mesh of the whole C-LEG next to the original scan.

Blender and MeshLab were used back and forth here: Blender allowed me to select a portion of the mesh freehand and export as a separate STL. MeshLab allowed me to offset this surface using ‘Uniform Mesh Resampling’ and then construct a volume around the surface using Uniform Mesh Resampling with ‘Absolute Distance’ checked off. This created an excessive and messy edge, however, so I brought it into Blender to perform a boolean intersection, extruding the surface that I selected earlier outward to overlap with the portion of the new mesh that I wanted to keep. With that cut performed, I used MeshLab one last time to perform a ‘Surface Reconstruction: Poisson” to smooth the corners. To cut a slit in the back of the model I used Tinkercad, because it’s quicker to align and subtract a cube, knowing what I know.

And it actually clipped on the way I had hoped, wrapping around the edges — but there was a considerable gap. The inner diameter of the print was 60mm, while the C-LEG is 55mm wide, so I uploaded the STL to tinkercad at 91% of the original size to continue to prototype #2:

I used some cylinder, cube, and hexagon shapes to throw together clamps that I can add nuts and bolts to for this print, to see if I can really clamp down on the C-LEG enough to hang some weight off of it.

Ended up printing copies at 93% and 96% of original size. It is not a perfect fit, but once tightened down with bolts, holds on pretty well. This one cracked due to the nut turning against the plastic — the white ABS must have shrunk more than the grey ABS, which had holes big enough for the nuts to sink into without forcing it.

Designing brackets for cLeg cosmetic fairings

A continuation of: Blending and Bending Prosthetic Leg Fairings

I wanted to design a 3D printable file that would allow me to clamp my acrylic fairings to the cLeg. Luckily Kay and Gabe created a pretty solid model of the cLeg itself so I could use its contours in my design. Again, I had to cut the model down manually in Blender because netfabb does an awful job of exporting STL (hadn’t yet investigated if this is an actual or a free-demo-imposed limitation of the software).

I also used Blender to multiply the scale by 1,000 before importing into Tinkercad.

I like Tinkercad because it makes it super easy to perform boolean functions on complex stuff like our leg here. If you’re a tinkercadder, try ungrouping the model so you can see how I made a cross section of the leg: Tinker This

And I went on to improvise a way to screw two parts together — this is very much a test fit, to see if the idea makes any sense, to see if screwing two plastic pieces on with pressure is enough to keep it from falling off.

I went back and forth about 3D printing holes for nuts and bolts, but decided I would drill it out of the finished plastic — “machining” is way more accurate than printing for this kind of thing.

OK, the original Kinect scan was not very dimensionally accurate…I guess I could have compared measurements of the model with measurements of the object, that’s what I get for trusting technology -.- I can still use the blocks of plastic for practice fitting, though. They don’t clamp around the c-Leg, but I tested screwing the pieces together and drilling into the plastic to insert a brass threaded insert, and with a few taps of a hammer that turned our really nice.

Of course, the chunky tinkercad design isn’t so fashionable, but it served its purpose to confirm that I need to press the shape of the leg more accurately (the front fairing doesn’t sit flush with the printed model at all) and that countersunk M5 screws will do a nice job of securing the fairing to the bracket, though I’ll have to play with melting a countersink so the head of the screw doesn’t stick out the back.

Continued here: Building Brackets around the C-LEG prosthetic 

Burlap-Resin Composite Boat

Searching youtube for how-tos on composites, there’s lots of videos on boat-making. So I decided, what the heck, why not get started with my boat making career. I made a really rough boat-like shape in Blender and applied a subsurface division modifier to smooth it down. The only foam I had on hand was 3/4″ thick, so I scaled the model and split it into 3 layers, set them side by side in Blender, and import the whole thing into MeshCAM.

MeshCAM generatered the gcode for our 1′ by 3′ CNC machine using a 1/4″ ballnose. Milling through insulation foam with a 1/4″ bit was a real treat — I cranked the speed up to the max and watched it cut like butter. After freeing the individual slices from the foam, I just hot glued them together.

I stacked 4 layers of SuperSap soaked burlap over the foam boat form, wrapped it up in release plastic, layered felt on top of that, and stuck the whole thing in a vacuum bag. We tried using our shopvac and then our smoke sucker, which sucked the bag tight against the form, but didn’t put any pressure on it. So, I ended up setting it in the sink under a trashbag filled with water, which visibly put a lot of pressure on the form.

After unpacking the form, and ripping out the foam (it did not go without a fight), I optimistically set it in the sink full of water, where it promptly sank. It’s totally permeable to water. Gabe’s frisbee floated a lot better. I’ll probably coat it with another layer of resin and let it cure without putting pressure on it. I figure the resin would be more solid with a two part mold, filled with resin (not squeezing the resin out of the burlap). Next time!

Update, a couple of weeks later: after pouring super-sap resin over the hole-ridden burlap boat with multiple applications and slowly spinning the boat upside down, rightside up, to keep the resin moving for about a half hour and cured in the air, the boat is sea-worthy! Unfortunately, the excess resin did slowly seep down to edge of the composite and pooled there, leaving me with heavy chunks of resin that’s just weighing the boat down. I can’t think of a good way to remove this except for a hacksaw.

I’ve also figured out that I’m not a great boat designer. The shape of my hull gives very little displacement, and the boat sits pretty low in the water without any weight (except for the aforementioned excess resin). I’ll look forward to designing a boat worth sticking a motor or sail into later on in life. In the meantime, here’s my burlap boat floating in Lake FabLab (our perpetually flooded parking lot).

Original Blender file with subdivision surface.

STL Export, scaled up to millimeter units.

Printing New Frames for Old Lenses

I think it is not an uncommon experience for people who are bit by the 3D Printing bug to look at each object in their daily life and wonder “Could I print that?” It’s also fun to look at objects and think of how you would manipulate a cube, a sphere, or a cylinder (common ‘primitive’ shapes in 3D design).

So I had been practicing Blender and noted that the lenses of my eyeglasses just pop in and out of my super-cheap plastic frames — no screws required. And of course I thought “I could print that.” It wasn’t until a newcomer to Makerspace Urbana wandered in with the same idea that I made progress.

The second design iteration

Basically, I knew how to trace the 2D oval shape of the lens (take a picture, draw a bezier curve in Inkscape on top of the photograph, import that bezier curve into Blender, convert it to a mesh, and start building the frame outward from that. But the lenses aren’t flat, and I didn’t have a clue how to measure and mimic that curvature in Blender. I wish I could remember the guy’s name (this was a couple years ago) but an optician walks into Makerspace with he same idea, except he knows about these lookup tables where you can find the curvature of particular prescriptions. It’s a pretty straight forward set of numbers: a sphere of a certain radius a certain distance from the lens. That information made it trivial to take my bezier curve in Blender deform into the actual curvature of my lens using the “ShrinkWrap” modifier. Position the curve at the right distance from the sphere, and ‘shrinkwrap’ the curve to the surface of the sphere.

Modelling the outline and curvature of my prescription lens.

The lens has a bezel top and bottom which I could measure the height of, but the angle of the bezel was just eyeballed. I simply extruded this ring outward to create the first test fit (printed at MakerLab). Plastic is now one of my favorite materials because of how forgiving it can be: it stretches and snaps and even tho I did very rough measurements, the lens snapped right into place.

From there, I printed some pince-nez style frames that I wore around for a while before sitting back down to model the earpieces in Blender. The first iteration was printed in one solid piece, face down, earpieces being build straight into the air with no supports. This worked great for a few inches, but I learned that the plastic being extruded applies a significant amount of pressure on the plastic beneath it, so that as the towering earpieces grew, they began being pushed side to side as the nozzle worked on the next layer such that subsequent layers were not stacking up straight. The night that I was determined to finish a pair (I had by now broken my manufactured frames from popping the lenses in and out so much), I wanted to be able to see on the bike ride home, so I had to finish these. Our of desperation, I braced myself against the frame of the printer and held onto the earpieces to stabilise them by hand for the last 20 minutes of the print. They still turned out looking like a dog had gnawed on the ends of the frame (the misaligned top of the print) but that sits behind my ears and I told myself it gave it a more homemade touch.

To get the lenses to fit into these frames took a few more tries (I used the same file, but the original print was done in PLA on a Makerbot, and these were now in ABS, and between the expansion and contraction of plastic and the slightly differing calibration of printers, well, point is these things aren’t always consistent.)

I got a lot of mileage out of this design, printing a few alternative colors, including glow in the dark. They looked a bit toyish, which is a style of its own. The real trick though is that they were printed with the earpieces straight up and down at a width that’s just a bit skinnier than my head: so when stretched over my face, they actually pinch just enough that they never fell off (but were still suitably comfortable.)

The next iteration (pictured at the top) was designed with hinges, but without a lot of thought into how to prevent the hinge from swinging both ways. After printing the pieces separately, I drilled a 1/32″ hole through the pivot point, stuck 20 gauge copper wire into the hole, snipped off the remainder, and filled the hole top and bottom with superglue, pivoting the hinge as it dried so that it only adhered to the top and bottom.

I liked the look of these (that PETT plastic carries light like fiber optic, so they kind of sparkle), but they did fall off my face time to time owing to the backwards bending hinges, so when I accidentally stepped on them in my morning stupor, I just went out and got contacts.

The Twenty Dollar 3D Scanner and Cloning Cacti

October 19, 2013

With a newfound ability to take digital design and make physical objects, it’s only natural to find a way to go the other direction. 3D Scanning is the technique that closes the gap in the promise of 3D printers being replicator machines. In fact, the ‘Maker Replicator’ has a companion: the $1400 “Makerbot Digitizer.” Essentially, it’s a motorized turntable, two lasers illuminating either side of the object-to-be-digitized, and a camera with a live feed to the fine tuned software that gives you a 3D model ready to print inside of ten minutes.

Makerspace Urbana has a mission of technology proliferation to people of all classes and creeds, and at $1400, the Makerbot Digitizer is another piece of new technology that’s priced out of reach of the general population. So I was very impressed to see James, a fellow member of Makerspace Urbana, playing with a different set of hardware to achieve the same result — a simple handheld laser and an old webcam (specifically a Playstation Eyetoy — talk about repurposing).

This blue laser isn’t something many people would have sitting around, but can be ordered online for about $10 before shipping. It’s 5mw and 405nm wavelength. A simple filter that can be ordered along with it turns the dot into a sharp straight line. However, any old red laser would work as well (though it would require very dim lights, higher power lasers will work much better), and can be converted from a red dot to a red line using a small plastic cylinder, for instance: a lego lightsaber!

So the components for a 3D scanner can be hacked together (perhaps you have a busted cd or bluray player that could have its laser beam harvested) — but how about the software? James had been using the free trial of DAVID3 laser scanning software. It offered a very intuitive scanning workflow, but would only allow you to save one side of the object at a time unless you pony up hundreds of dollars for a license.

This strategy of digitizing an object works by using a calibration pattern that the software recognizes (that’s the piece of paper with dots printed on it) to determine how far away the background is from the camera. When a laser-line is projected across the object, the line takes the shape of the object. The software compares the form-fitting shape of the laser line with the straight line that hits the background, and creates a “point cloud” representing the one side of the object the camera can see.

The Makerbot Digitizer has the turntable wired up to the computer, so it can scan the object while it rotates. But without this integrated turntable, we have to scan the object one side at a time — and manually piece the point clouds together after the fact. The DAVID3 software automatically aligns these point clouds, but saving the result is a privilege of the paid version.

From left to right: aligning two point clouds, the completed 8-sided point cloud and all its associated noise, and the surface reconstruction ready to print.

But no matter, the free and open source “MeshLab” allows you to align point clouds semi-automatically. For each of the 8 angles captured, you have to give MeshLab some hints as to how they line up, and it uses its fancy algorithms to piece the two together. Here is (someone else’s) video tutorial that shows the whole process.

I used that technique to piece together 8 scans of my cactus, and was able to create a ‘watertight’ mesh, a continuous volume without any holes — using a MeshLab filter called “Surface Reconstruction: Poisson.”

The generated mesh is solid, seamless and ready to print. While the general likeness was captured, I’m not so satisfied with the detail. Perhaps using a higher resolution camera would help, but I think most of the detail was lost in the noise resulting from the laser’s light-scatter — a result of the material bending and blurring the laser-line. So whatever laser scanner you use, the detail you capture will be reliant on how sharply the object reflects the laser.

After trying this method out on a few different objects, I came across 123D Catch : a cloud service that generates meshes from photographs. I’ve found it vastly more practical than setting up and calibrating lasers and cameras — even with a couple dozen pictures from my camera phone I can get very detailed meshes, with the photographic data applied to the surface. You can download the results to use how you please (under a non-commercial agreement), but it is a free service by Autodesk that they can pull anytime. Since then I’ve learned to use Agisoft — equally powerful photostitching software, for $60 with education discount. At least it’s something you can own and run on your own computer!