The focus of this paper is on the design and evaluation of a robust drive mechanism intended to robotically steer a thermal ablation electrode or similar percutaneous instrument. We present the design of an improved screw-spline drive mechanism based on a profiled threaded shaft and nut that reduces the part count and simplifies manufacturing and assembly. To determine the optimal parameters for the profile shape, an analytical expression was derived that relates the tolerance between the nut and shaft to the angular backlash, which was validated using SolidWorks. We outline the forward kinematics of a steering mechanism that is based on the concept of substantially straightening a pre-curved Nitinol stylet by retracting it into a concentric outer cannula, and re-deploying it at a different position. This model was compared to data collected during targeting experiments performed in ex-vivo tissue samples where the distal tip of the stylet was repositioned in ex-vivo bovine tissue and the location of its distal tip was recorded with CT imaging. Results demonstrated that the drive mechanism operated robustly and targeting errors of less than 2mm were achieved.