Picture of K'nex Expanding Sphere
parts list.JPG
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.
elysedavene made it!2 months ago

I made it! Super fun, thanks for the instructions!

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BeforeTron1 year ago
Unreal. Incredible. Make sure hoberman sees it! ( google hoberman sphere)


This is awesome!
Sorunome1 year ago
na, nice one! :D
Kona-chan2 years ago
Congratulations for being selected as a finalist in the Toy Rods and Connectors Contest!
j9mosely (author)  sandroknexmaster2 years ago
Thanks. Congratulations to you, too. Your ball machine is fantastic!
Haha no problem and thanks :D
Kenneth18182 years ago
Thanks so much for these instructions. I was able to build this in a day. ( for some reason I can't add pictures :-( Sorry)
Kenneth18182 years ago
Thanks so much; i was working on it all day yesterday and got it complete. :-)

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?
Hiyadudez2 years ago
This is great! I wish I still had all of my Knex, I would've totally built this.

Can you supersize it by exchanging rod sizes? Or would it affect the angles and dimensions?
j9mosely (author)  Hiyadudez2 years ago
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).

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!!!
j9mosely (author)  Picturerazzi2 years ago
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.
hunter9992 years ago
Looks awesome as well as unique!
I guess it is alright, a cool idea. All the same, good job.
monty3242 years ago
Johnhall142 years ago
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.
mikeasaurus2 years ago
I love these things, and I've never seen one made from K'NEX before!
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