Ball joint for aluminum extrusions

In the course of designing a humanoid sculpture for an upcoming exhibit at the Dover Children’s Museum, I decided to try and come up with a lockable ball joint using common hardware.

I’ll get into the overall design of the sculpture in another post, but I’d already decided to build the skeleton out of aluminum extrusions; I sort of have a ’thing’ for them.  They’re cheap and versatile, strong, and look cool.  I had imagined using a through-bolted ball on the shoulder and using two smaller “stubs” of extrusion with milled pockets to hold the ball captive and exert clamping force.  When I was prepping some of the pieces, I had a better idea.

The extrusion I’d used previously was 20mm square with a simple one-hole cross section:

But in this project, I was using 30mm square extrusion and it has a very different cross-section:

Those 4 perimeter holes are sized to be tapped for M5 thread, which gave me an idea.  If I drilled 4 matching holes in an appropriately-sized retaining plate, I could use that to retain the ball.  I got a .75” ID, 2.0” OD flat washer and drilled it up in the mill.  I tapped the extrusion holes and bolted it together.  The result is below.

The amount of grip the washer exerts on the ball is significant, and adjusting it is just a matter of loosening the bolts slightly, repositioning the ball, and cinching them back up. I’ve built them with both steel and delrin balls.  Steel looks cooler and is available predrilled/tapped, which I recommend if you want metal because most balls available in this size are hardened steel bearing balls and are difficult to machine.  Delrin is cheap ($10 for qty. 10 1” balls) and provides better grip.

I sourced the extrusions from Misumi.  The steel and delrin balls came from McMaster.

I plan to use these a lot.