A robot arm includes a base, a first arm, and a second arm. The base includes a first spindle and a first motor to rotate the first spindle in a first axis. The first arm is rotatably connected to the base around the first axis. The second arm is rotatably connected to the first arm around a second axis parallel to the first axis. The second arm includes a second spindle, a second motor to rotate the second spindle, third spindle, a third motor to rotate the third spindle, a threaded rod extending along a third axis parallel to the first axis and rotated by the second spindle, a nut threaded on the threaded rod, a fourth motor fixed relative to the nut, and a fourth spindle connected to and rotated by the fourth motor. The nut moves along the threaded rod to slide the fourth spindle.