Lego Robot

Robotic gripper with rotating wrist AND pinching action.
Picture of wrist:
Picture of wrist on robot arm:

Some designs to make a wrist use a rarer LEGO "turntable" piece; but I modified a 40 tooth gear to do the same. The 40 tooth gear is easier to find (many LEGO sets include a 40 tooth gear which you can modify; or you can buy a modified version from me.)

40 tooth LEGO gears. My modified gear (RED arrow) on left. And a regular LEGO gear (GREEN arrow) on the right:

The modification of the 40 tooth gear is to drill out the middle with a "drill press" so an axle will spin without rotating the gear itself. This allows the gripper to close without rotating the wrist itself. (The gripper closes when the axle in the middle of the gear rotates.) I use a drillpress to drill the center hole to ensure a perfectly staight, centered, and smooth hole. You can use a drill press and a 3/16" drill bit to make the hole. Even a hobby drill press would work (they aren't too expensive to buy).

The wrist itself is connected to the 40-tooth gear's off-center holes and rotates only when the gear is turned by rotating the gear by the gear's 40 teeth (not by the center axle thru the middle of the gear).

Picture of wrist with red showing rotation of axle to open close the gripper/fingers; the green arrow shows rotation of the axle to rotate the wrist:

Picture with the 40-tooth gear removed:

Using a Drill Press ensures that the axle doesn't wobble and makes the axle stick out at exactly 90 degrees. You can try to use a hand drill but a drill press will avoid mistakes.