What’s eight feet across, has 90 teeth, and makes beautiful designs? Not a shark with an art degree, but this giant version of the classic Spirograph drawing toy. Chalk is the medium, and streets and sidewalks are the canvas.
The idea for this project came about at a craft show in December when a friend of mine had a little trouble with a laser-cut Spirograph we found. I teased her about her apparent lack of fine motor skills, then had the idea to make a Spirograph that only required gross motor skills. How big could I make a Spirograph?
Continue reading for the full walkthrough of the build, or jump to the finished Spirograph.
The first step was to create a template for all of the gears. I used Inkscape because it has a plugin for generating them, then exported the file to Adobe Illustrator to scale them up and make other edits (I’m more comfortable working in AI than in Inkscape). I nested the gear patterns together to save printing costs.
Once the files were ready it was time to head to the copy shop and use their large format printer. Each sheet of paper is 3′ x 3′. My helper was eager to see the results.
I added a 1″ square on each to ensure that they were printed to the same size and weren’t accidentally scaled differently by the printer. The project would fail if the inner gears didn’t fit perfectly with the outer gear. I could measure the squares or simply hold them against each other in front of a light.
With spray adhesive, I glued the pattern for the outer ring segment to a piece of MDF, then cut along the lines with my jigsaw. This would serve as a router template so I could cut all six segments exactly the same more easily than doing it all by hand.
When the template was finished, I drilled two holes through it, then through the 3/4″ plywood. A pair of bolts through the holes kept the template fastened to the plywood while I ran a router with a pattern bit around the edge to cut the segment. Repeat six times. The size of the outer gear was dictated by how many of these segments I could fit on a single 4′x8′ sheet of plywood. I ended up burning out the old router in this picture and had to get a new one halfway through. I also broke three bits.
With the outer ring done, I still had to make an inner gear. I spray-mounted the smallest gear template to a sheet of 1/2″ plywood and cut it out with the jigsaw. No need for the router this time, because I was only making one copy of each gear. Still, this was one of the most tedious parts because every tooth segment had to be cut in four passes: once on each side, going across to the corner, then squaring the bottom from corner to corner. I’ve never wanted a CNC router more than I did during this work.
The gears would ride on caster wheels that allowed them to rotate. It wouldn’t work to attach the base of the casters directly on the gear, because then the gear would be 2″ above the teeth of the outer gear. Instead the wheels would need to be mounted on a platform to raise them up. It’ll make more sense in a later picture when I show it put together.
I made a pattern on the computer showing where to drill for the mounting holes, glued it to some 1/2″ plywood, then cut it out on the bandsaw.
I used a punch to mark where the holes will go. Doing this keeps the drill bit from wandering.
5/16″ bolts will go through the outer holes to mount the wheel platform to the gear, and 1/4″ bolts will go through the inner holes (not drilled yet in the photo) to mount the caster base to the wheel platform.
1/4″ carriage bolts (so that it’s smoothly finished on the visible top surface) hold the caster wheels to the platform. I had to use jam nuts instead of regular nuts so that they wouldn’t interfere with the swiveling wheel.
Using the wheel support as a guide,
I drilled the holes for the bolts that hold them up. You can also see marked where I plan to put the holes for the chalk.
After getting the gear assembled and adjusting the mounting nuts to get the wheels offset the correct distance, it was time for the first test. And it worked! The gear teeth meshed without a problem. I was so excited after all the work so far that I probably spent 20 minutes rolling the gear back and forth across this one segment and giggling like a fool.
The gear rolled now, but it did not yet draw. My plan was to have the chalk held in the gear by a piece of 1″ PVC pipe inserted into the artist’s choice of locations, each location with a pipe coupler to accept the pipe. The pipe couplers I had were about 1.6″ in diameter — not a standard size for a drill bit — so I had to use an adjustable bit. Keep your hands away from that giant spinning mass of metal (a lesson my knuckles learned the hard way!).
I only needed one end of each pipe coupler, so I cut them in half just below the stop in the middle. I smoothed the rough cuts on a belt sander.
Would it work? All the chalk-holding, rolling gears wouldn’t mean much if it didn’t make a good design. It was time for a street test. And, success! Not to mention baffled neighbors when they found the pattern on the street later in the day, with no clue how it had been done.
I took it all apart (probably the third time doing so — getting a little tired of it) and painted it. Bright colors are called for with a toy like this, wouldn’t you say?
This shows how the wheel assembly is held together. Three 5/16″ carriage bolts go through the gear’s base from the bottom and each is held tight by a washer and a nut. Then a nyloc nut (regular nuts shook loose with all the vibration) is threaded onto the bolt, then a washer, then the wheel support, another washer, and finally an acorn nut. The gear is now able to ride only 1/4″ above the ground, so that it meshes perfectly with the outer ring resting directly on the ground.
With the concept and design proven, I put together two more gears and painted them. On these gears I wised up and went to three different hardware stores in order to find pipe couplers exactly 1.5″, allowing me to use a standard 1.5″ Forstner bit instead of the adjustable bit I used earlier.
Everything is done! It’s time for a public unveiling! The six segments of the outer ring are assembled first, ideally on as smooth and level of a surface as possible. The outer ring is just under 8′ in diameter, so it would take a pretty wide sidewalk.
The segments fit together like puzzle pieces.
Moving a gear around the outer ring is easiest with a team. Each person pulls it around where they can reach, then rolls it along to the next person. The gears have enough mass that they can roll at least halfway around the circle if you really shove.
The wheel supports work well as handles to move the gear around the outer ring.
Here’s a closeup of the chalk holder. A 1″ PVC pipe holds the chalk, and it can be moved to different locations (the pipe couplers in the background) to change the patterns. A dowel goes on top to put a little downward pressure on the chalk, and also to serve as a gauge: when it gets down to the same height as the pipe, you’re just about out of chalk. The hole in the dowel is because I originally intended to put elastic through it for more pressure, but that ended up not being necessary.
Changing chalk partway through the design makes some interesting effects.
So far I’ve built three gears: a 22 tooth, a 35 tooth, and a 44 tooth one.
Here’s a family portrait of the entire set:
Here are a few photos of some patterns I’ve made. I need to find smoother asphalt.
I plan on cleaning up the template files I used and posting them here for anyone to make their own giant Spirograph. Check back soon.