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A U-Joint is basically a flexible axle. They are often used in automobiles. I printed mine in PLA with fairly standard settings. The model can also be scaled up or down to your liking.
Print "universal_joint_main.stl" at 5% infill without supports or raft.
Print "universal_joint_handles.stl" at the same setting as above.
Print "universal_joint_ball.stl" at the same settings just WITH supports.
Print "universal_joint_pegs.stl" at 50% infill on high resolution.
After you print out the parts, place them in their correct orientation and position. From there, hot glue the pegs into the sphere; though the main pieces. Then, hot glue the handles on.