This instructable shows how to make an expanding and contracting sphere out of K'nex construction toys.  The design is similar to that of the toy sold as the "Hoberman Sphere".  (Note: the original US patent on the Hoberman sphere was issued in 1991 and is now expired.)  In addition to being fun to build and fun to play with, it offers interesting lessons in mathematics.

Watch the K'nex sphere in action:

To make the sphere, you will need the following K'nex parts:
  • 96 grey 2-slotted connectors
  • 96 red 3-slotted connectors
  • 96 green 4-slotted connectors
  • 192 yellow rods
  • 96 tan clilps
  • 96 short white rods
  • 96 blue spacer rings
  • 48 short green rods
  • 12 white 8-slotted hubs
Yep, that's a lot of K'nex.  If you don't have everything on the list you can make some substitutions.  

You can use any color rods instead of the yellow rods, just as long as they're all the same length.  I've made the model using blue rods -- it's smaller and not as pretty but it's sturdier because it weighs less.  I wouldn't recommend longer rods, though -- it might fall apart under the extra weight.

You can also substitute red connectors for some of the grey ones by ignoring the extra slot, but you have to be careful how you orient them or they can limit the range of motion of the model.  

The white connectors and green rods can be left out, too, but you'll have to do something else to stabilize the "hubs" in Step 4 -- like inserting a blue rod as a diagonal brace (you'll see when you get there).

The blue spacers are also optional, but they really help keep the model moving smoothly and I strongly recommend using them if you can.

Step 1: Making Rhombuses

Remember rhombuses from math class?  A rhombus is a quadrilateral with four equal length sides.  Use all the yellow rods and the grey and green connectors to build 48 of them.  The acute (sharp) angle in these rhombuses is 45 degrees and the obtuse (shallow) angle is 135 degrees.

After you've made all the rhombuses (Latin snobs call them "rhombi"), link them together in pairs by adding a clip to the end of a white rod, slipping the rod through the center of a green hub, add a blue spacer, then another rhombus (through the green hub again) and topping it off with another clip.
It's awesome! Congrats for idea and thanks for sharing!
<p>I made it! Super fun, thanks for the instructions!</p>
<p>AWESOME!!!!!!!!!!!!!! I made it!super cool but a little bit unstable. Anyway,it's awesome! So cool!</p>
I made it, thanks for your instruction!
Woah this is great!
Unreal. Incredible. Make sure hoberman sees it! ( google hoberman sphere)
This is awesome!
na, nice one! :D
Congratulations for being selected as a finalist in the Toy Rods and Connectors Contest!
Thanks. Congratulations to you, too. Your ball machine is fantastic!
Haha no problem and thanks :D
Thanks so much for these instructions. I was able to build this in a day. ( for some reason I can't add pictures :-( Sorry)
Thanks so much; i was working on it all day yesterday and got it complete. :-)<br> <br>
Your instructions are great! I was actually able to make it.
That's great! I'm so glad you were successful. Can you share a picture?
This is great! I wish I still had all of my Knex, I would've totally built this. <br> <br>Can you supersize it by exchanging rod sizes? Or would it affect the angles and dimensions?
Angles are not affected by changing rod sizes. Longer and shorter rods will work, but it will weigh more with longer rods and the extra weight might cause some joints to buckle if they are not reinforced in some way. I can't really be more specific because I haven't tried it (I don't have enough longer rods). <br> <br>If you don't have enough rods for this model, try a simpler version of it. Instead of making 12 double scissor joints, as in Step 2, just make 4. Then link them into a ring as in Step 5, but leave out the hubs -- just the link the joints directly to each other. This makes a single ring that expands and contracts in a plane.
I remember making these wi my K'nex! Not as large though! Very cool!!!
Was your design different? Do you have any pictures?
I was a child. Lol no pictures but it did look different. It wasn't as big!
can you use red connectors instead of the two slot ones
Yes, you can. The first time I made this I didn't have enough gray 2 slot connectors and I used a lot of red ones. They limit the range of motion of the sphere a little, but not much. If you notice the red connectors bumping into each other as it flexes you can usually reconfigure which slots the connectors are in to reduce the interference.
Looks awesome as well as unique!
I guess it is alright, a cool idea. All the same, good job.
K'nexpanding. <br>
Looks neat =D
That's very nice, a beautiful design and it looks sturdy. Good job!
Haha, I was working to build the first one on this site! This is the 4th knex item! Dang...
This is just awesome :[). You've got my vote. Thank you so much for sharing.
I love these things, and I've never seen one made from K'NEX before! <br />The video demo was perfect, thanks for including it.
This is really cool, and being a math kind of guy, I love how it's based off a geometrical theory. I voted. :D
Please embed the video into the intro instead of just putting the link there. Would help a ton

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