This Joint Is Jumpin'

A project log for Open-Source 6-Axis Robotic Arm

A completely-open, scratch-built PUMA arm with 24" reach and 1lb payload

Colin KingsburyColin Kingsbury 12/08/2014 at 18:260 Comments

After a bunch of iterations during three months of furious design/build/test, I've finally arrived at what feels like a good design for a multi-purpose joint that can work for (at least) everything from the wrist back. This version has a number of virtues:

  1. .750" through-joint hole: there's a good-size hole through the hub for running wires or other plumbing. This size was a good compromise between size and weight of the assembly, through-hole capacity, and rigidity.
  2. Large reduction gearing: Torque is our friend, so this design allows for the use of reduction from 2:1 to 6:1 with a standard 3" diametral pitch main gear. Only minor changes would be required to use with module gears.
  3. Fully-enclosed: All the gearing can be covered to prevent wayward bits from getting sucked in.
  4. Increased lightness: The housing weighs only a few ounces, thanks to more careful design and machining.
  5. Minimized parts count: The housing body and cover are two single pieces that require only basic machining. The bearings and gears are all commodity parts costing from $5-$25 each.
The arm shown in the test video is wearing about four different joint variations, so I'll be going back and remaking those as this design solidifies. The good news, though, is that with a 3:1 reduction, the main segment of the arm is now capable of moving about 3.5 pounds at the end at 31" of extension at a reasonable speed. My wrist and gripper assembly weighs just under 3 pounds, so that's giving me 8oz of payload in the gripper at 31" reach (or 225g at 78cm) versus a goal of 16oz at 24".
There's still room to pare weight in the wrist and gripper (every ounce saved there goes right to payload) and in a few other places, so when it's done I think it's very possible to hit that goal.