The paper describes experiments using a new concept in mechatronic robot surgery: that of a robot with a force control handle moved by the surgeon. The surgeon back-drives the robot under servo-assistance, whilst feeling the force from a rotating cutter during surgery, for example, in total knee replacement. Thus, when machining the knee bones to take a prosthetic metal replacement, the surgeon can use his inherent sensing to slow down, or take a lighter cut, when cutting hard bone. The robot however, can be provided with regions of force constraint so that, say, a flat or curved plane can be cut accurately into the bone to allow the prosthetic implant to be subsequently fitted. At the same time the robot can also be programmed to prevent entry to adjacent regions, thus avoiding damage to features such as ligaments. An experimental system using this active constraint robot (or ACROBOT) for knee surgery is described.