First off: Why six?

6 degrees of freedom is the minimum needed to reach a volume of space from every angle. The longer the arm, the greater the volume that can be reached. More than 6 joints and the robot becomes kinematically redundant – it can reach the same spot at the same angle in more than one way. For example, you can pinch your nose and wiggle your elbow at the same time.

In a human arm there are at least six degrees of freedom. If your robot arm can do all of these motions then your arm can do pretty much any job a human can. So here’s a list of what the six degrees are and what they are not. Video after the break.

I just stumbled across this example that’s nearly a year old. Maybe I’m rediscovering it? It’s so short and so sweet.

Axis 1: Let your arm hang down. Raise it in front of you, turning only at the shoulder.

Axis 2: Let your arm hang down. Raise it out away from your body like you’re doing jumping jacks.

Axis 3: Bend your elbow.

Axis 4: Wave like the Queen of England by twisting at the wrist. It’s a twist between the wrist and the elbow.

Axis 5: Make a fist, then make it nod up and down. Bonus points if you draw eyes on the first knuckle of your index finger.

Axis 6: Imagine holding a big dial in your fingers. Turn the dial without moving the rest of your hand.

The robot’s tool – the claw, the gripper, the pen, the hand – is NOT a degree of freedom. Most hobby robot makers out there are incorrectly listing the number of degrees of freedom in their arms.

Intuition says each axis added is that much more complicated, so it’s probably the last axis they leave out. Oddly enough, it’s not Axis 6 that most makers omit – it’s Axis 4. Robot arms vary depending on the manufacturer, but the effect is the same. In the video I posted it’s the orange belt you see right after the elbow. In some models like this FANUC arm,

the twist is closer to the wrist. If Axis 4 is close to the elbow then motors for 5 & 6 can be closer to the wrist. If the 4/5/6 motors are behind the elbow like in the FANUC arm then the linkages between the motors and their moving parts get really interesting. Try and draw it on paper, it’s rods inside tubes inside other tubes.

Here’s another short but sweet video of the arm in the above video.



If you want to build a robot arm, I’d love to talk to you. Let’s collaborate! I’ve got the 6 axis RUMBA board which should take care of the “brain” and all the laser cutting we could ever want.

PS: I probably won’t have a post until late Sunday because I’ll be at the VHS leading 3D printer day. We meet every 2 weeks to talk, learn, and hack 3D printers.

There are 7DOF and even 8DOF robotic arms on the market:

Edit: Special thanks to Reddit user CollieMonster for pointing out a few errors and omissions.