Reverse-Engineering Toys: 3D Printed Hoberman Sphere

Inspired by the excitement my students had when I tried the Take Apart and Rebuild Toys project with my robotics students, I was excited to develop a toy-based engineering project for my design students that would reinforce the engineering design content we had learned throughout the year with a fun and challenging final project.

The assignment for students was simple: Recreate a toy from your childhood by reverse engineering it.

This activity is a great way to reinforce precision measurement skills, 3D modeling, additive manufacturing, and the iterative nature of the design process by applying all of these skills to a real-world design project your students will be excited to create.

This Instructable has two purposes. For those who are checking this out simply as a Hoberman Sphere fan, this instructable will give you all the instructions, models, etc. you need to 3D print and assemble your own functional toy. Perhaps more importantly, though, this Instructable will also give teachers a roadmap for how to use a reverse-engineering toy recreation project in the classroom.

I hope the joyful nostalgia of recreating childhood toys offers your students as much engagement and enjoyment as it did mine.

Supplies

Depending on what toy you choose to recreate, your supplies could vary. For the Hoberman Sphere, you will only need the following supplies:

• Measurement tools (calipers, precision rulers)
• 3D modeling software
• 3D printer (and filament)
• Glue

Step 1: Measure, Measure, Measure!

Once you decide what toy you want to recreate, the first step in reverse engineering that toy is to start taking measurements you can later use to generate your own version of the toy. If you are recreating a toy that can be taken apart, by all means, begin by taking it apart first to make the measurement process easier. Then, start taking measurements! Calipers are a great choice to help with thickness measurements and small pieces, but any ruler should work just fine for this step. The more measurements you can take upfront, the easier your 3D modeling will be and the less iterating you will have to do to successfully generate the pieces for your project. You will undoubtedly need to keep taking measurements as you create the 3D model of the part as you realize what extra information you need, but having a solid foundation of measurements to start creating your design with is key.

For the Hoberman Sphere, measurements were taken with the part fully assembled (...because we were scared we were going to break it and not get it back together if we tried to unsnap the pieces). After identifying how many different pieces there were in the project overall (8), each piece was measured with calipers to gather the information that would be needed to reproduce those shapes in a 3D model.

Note: Project/toy selection is key for this project. If you are planning to primarily use 3D printing to recreate your toys, make sure the toy selected fits within the scale of what you can print with your machine. Also, make sure the material you print with will work for your toy.

Step 2: Create 3D Models

Once you feel confident you have captured critical measurements, you can begin creating the 3D models of the pieces required for your toy.

As noted previously, the Hoberman Sphere toy only has 8 exclusive components and is not too difficult to model. A parts list for the complete sphere and the related 3D models are included here:

• Big Link(F): Qty: 48
• Big Link(M-L): Qty: 24
• Big Link(M-R): Qty: 24
• Small Link(F): Qty: 48
• Small Link(M-L): Qty: 24
• Small Link(M-R): Qty: 24
• Square Connectors: Qty: 36
• Triangle Connectors: Qty: 16

Step 3: Print, Test and Iterate Your Parts

While I'd like to say that your first attempt at making 3D models that accurately reflect your toy parts will be perfect, that is rarely the case. Mentally prepare yourself to 3D print a few pieces only to realize that the precision of your machine requires you to make a few changes to your initial design. Print out your initial designs, see how they work together, and iterate your 3D models and reprint as needed until your parts connect as desired.

The recreated Hoberman Sphere can be a bit of a beast to 3D print. There are some pros and cons to consider when printing the components for this toy. Pro: The parts are super small and take just a couple of minutes for each one to print. Con: There are just SO MANY parts! Feel free to print multiple parts on your printer at one time to reduce the manpower needed to keep resetting your machine.

Before you print all the parts, start by creating individual linkages and confirming their connections. It took a few tries to get the connections between pieces to be just the right size so they could snap together with a snug fit. (The final .stl and g-code files used to 3D print this design are included in this step for reference.) The attached images show how to assemble and test each linked segment of the ball. For both the big and small linkages, each segment consists of two female linkages, one left male linkage, and one right male linkage. The images above show these parts and how they are snapped together to form a completed linkage. When you attach these parts to create a complete linkage, test your system to ensure the parts freely compress and expand.

Step 4: Assemble Your Toy

Depending on what toy you are recreating, this step can vary greatly. Many toys will assemble quickly, but the Hoberman Sphere example in this Instructable takes a bit of assembly effort. Before assembling your own sphere, I recommend watching the video HERE by Maria Redondo that breaks down the expanding mechanism and how it is connected using a simple demonstration with popsicle sticks. This video shows the general assembly strategy that should be used for your Hoberman Sphere.

An outline for where the big vs. small parts will go in your sphere assembly can be seen in the attached diagrams. There is really no wrong place to begin assembling this, but I recommend creating the network of BIG linkages (created in the previous step) connected with SQUARE connectors first. Next, make the subassemblies of SMALL linkages with TRIANGLES (shown in the next diagram) before finally connecting those subassemblies into the existing square connectors of the previously created frame. For both the square and triangle connectors, once you have inserted the linking parts' connecting pegs into the connector holes, apply a small amount of glue to attach the flat "lid" to lock the parts in.

Troubleshooting suggestions: Keep in mind that this is another place where a few iterations and tweaks will likely need to occur.

• When the initial assembly for this reverse-engineered Hoberman Sphere was completed, a few of the connection points were not as snug as they needed them to be to allow the structure to move in and out without popping out. If this happens to you, applying heat to the male ends of each loose connection point with the tip of a hot glue gun and apply pressure to compress the male connector. This should slightly thicken the connection peg and ensure it will stay attached (yet still rotate freely).
• Another common problem is that if you don't remove the 3D printing support structure cleanly, you may have too much resistance between parts for the ball to open and close easily. If this happens, sand down restrictive parts until they glide together more easily.

Step 5: Enjoy Your New (Old) Toy

Once you have completed your assembly, your toy is yours to enjoy. For some projects, you may want to paint or decorate your toy before calling your project complete. For the Hoberman Sphere, try printing your parts using different color filaments to create different designs.

If you are using this as an engineering lesson, I recommend wrapping up the project by having students reflect back on the project by discussing their use of the design process and reverse engineering, pros and cons of 3D printing, and what they learned about the importance of being able to take precision measurements and use them to generate a 3D model.