A six-axis robot arm sitting on your desk used to mean five figures and a service contract. Chris Annin's AR4 quietly tore that idea up — and with the brand-new Mark 5 revision, he's calling the hardware officially finished.

The AR4 is an open-source, six-degrees-of-freedom robot arm you build yourself from CNC-cut aluminum, 3D-printed parts, and off-the-shelf motors and electronics. It's the latest in a lineage that started with the AR2 and has been refined release after release. The Mark 5 isn't a dramatic redesign so much as a final polish: Annin says it's the last item on his hardware to-do list, with future effort going into software and tutorials instead.

What changed in the Mark 5

The headline tweak is sensing. Joints one, two, and three now use Hall effect sensors for their calibration limit switches instead of mechanical microswitches, which meant reworking a few mounting points on the aluminum parts. Joints four, five, and six keep the small microswitches. Annin has also shipped a fresh build manual and published the arm's modified Denavit-Hartenberg parameters — the math that describes how each joint moves — as fully worked-out spreadsheets, so the kinematics aren't a mystery you have to reverse-engineer.