controllable ball joint

This ball joint was inspired by others' projects, of course.

The notable things here:
- sized to fit 1/2 inch pvc
- 4 control holes in each piece
- new for mk4, wedges to prevent twisting

By attaching a line to each control point, and running through the corresponding hole, the joint can be fully controlled from elsewhere.

For my test piece, I used fishing line and a small #10 washer. It is sized well for this.
Guitar strings would also work well.

By attaching 4 lines to it and running those lines through the control holes, the lines can be pulled at the other end to control the motion of the joint.

The holes down the middle of the joins allow lines from further up to pass through, opening the way for controlling many such joints from a central location.

Ultimately, I hope to create a small walking robot with all the control lines leading to a central location, run off an Arduino board.

NOTE: I have updated this design to include wedges, to keep it from tu