A rotatable gripper grips and turns a container cap. When locked, a collet clamp engages the cap, and when unlocked, the clamp releases the cap. The collet clamp is elongate with a first and second ends. The first end has a substantially rigid section. The second end has a gripper section with a substantially cylindrical recess that expands in width to hold the cap when the clamp is moved from locked to unlocked. Between the rigid and gripper sections is a clamping section. An outside surface of the clamping section is conical with a width increasing towards the gripper section. Sliding a ring on the conical clamping section deforms it into the locked position, constricting the recess tightly onto a container cap. When unlocked, the recess widens, permitting the cap to be removed from the recess.