A methodology to develop and validate a shared tool to be used individually by humans and robots in collaborative assembly/disassembly systems is proposed. The solution allows the upgrade of a manual electric screwdriver and its integration in a collaborative application. This enables the sharing of the device with a collaborative robotic system equipped with a standard gripper, thus improving the potential for flexible collaboration without worsening the use for the operator in terms of biomechanical loads and wellbeing. Based on a relevant case-study, the methodology encompasses the definition of design criteria to properly combine ergonomic and productivity requirements and related evaluation metrics, the reverse engineering of a manual electric screwdriver, the CAD modeling and rapid prototyping of mechanical parts to realize the shared tool, the signal integration and the robot programming. To assess the performance of the shared tool from the human and robot perspective, practical tests have been performed in a collaborative assembly workstation. From the human side, metrics for measuring biomechanical loads and perceived comfort have been defined. From the robotic side, performance has been measured to assess accuracy, repeatability, and speed. Preliminary results demonstrate satisfactory performance from both the human and the robotic side.