the Single-Axis Elbow [SAE] joint is the most basic part to work with, and is the basis for other, more complex joints. Learning to make this isn't so much about knowing this joint in particular so much as it is learning the concepts of creating a moving part for your future projects.

What You Will Learn

You will learn several basic principles which REALLY need to be taken into account when creating a joint system, including:

  • Setting up pieces to fit together correctly
  • Ensuring your pieces can rotate without collision
  • using proper technique to make life easier when inserting axis

Size Counts


When you have two objects to work with, you have to think about how they fit together- If you're building a good joint such as the one for the example, one piece has to fit around the other. Simple enough, right?

Well, no. While the concept is pretty basic, the carryout is something completely different. You have to understand one thing- Paper has thickness. In my experience with different experiments while researching these joints, I've found that taking 1/100th of an inch off of either side of the smaller, or "inset" section of the joint is JUST enough to fit it PERFECTLY inside the other piece while retaining a bit of friction. In fact, if you build the example joint, you can test this by fitting one piece perfectly into the other. The original piece I made was EXACTLY 1 inch, so I reduced it to 98% of it's original size to create the smaller box.

When designing your own projects, remember to consider what the joint's purpose is- if you want it tighter, maybe reduce it a little less, or if you want it free-moving, scale it down just a bit more. Either way, TEST THE PIECE before building the rest of the model. You'll be sorry if you didn't.

Physics lesson

When you play with the tutorial model, you'll understand what I'm about to explain, trust me. If you feel it necessary, please build it ahead of time.

Rotational Limitations: Immediate Problem


If your inset box has no edits to the corners, you're going to run into some problems involving the radius of the outermost point in the area that is intersecting the wall of the outer paper structure.

TL;DR, if your box has a big corner and you try rotating it, it will try tearing through the back of the joint because it's unable to do what you want it to. See below-


Rotational Limitations: What do I edit?

The corner, of course… but how do you go about it? I'll show you- remember the big picture with the fancy circles up this page just a bit? Simply enough, the green circle represents where the inner piece needs to be to be able to freely rotate and just barely brush past the wall of the outer joint. As you can figure, the orange line shows where the corner swings out to.

You can make the rounded edge if you wish, but it's a feature that is more than likely not seen well enough to put effort into. The simplest method is to make a straight line from the tangent points of the circle. Just look at how I did it if you don't understand-


Rotational Limitations: the End of the Line

When you rotate the completed tutorial piece, you'll notice one major limitation- it can be:

straight 2903073347_ac1b483c32_m.jpg
90 degrees (in one direction) 2903075949_f5d57c5377_m.jpg

This is because of two things. The first is the little edge on the inside of the elbow, seen here-


This tricky little devil is preventing the piece from reaching anything more than 90 degrees, and is simple to bypass. Just cut away a little bit of it and your elbow can bend that much farther. Be warned, though, when bending any farther than 90 degrees, you'll have to take care of the secondary corner of the inset piece, since it will start to intersect with the larger piece again.

The second problem is the very thing we just fixed: the whole corner issue. If you want your model to move in the opposite direction as well, you can remove the paper on the opposite side and mirror the side that's already allowing the bend.

The problem with the first

+Finishing up

Ok, well, let's recap this before we move on:

  • Pieces need to fit together.
  • corners are bad if you need to rotate.
  • Things can be trimmed to adjust to angular needs.

Now, move onto the next portion of this tutorial, "Building the Example"!

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