Stability assessment while dissecting a simple mixed reality model. a) represents a mixed reality model with HD2, otic drill and gripper assembly. Note the simple 3-layer rectangular printed model. b) shows the vertical path of the drill through the model with labels at interfaces between real and virtual modes of operation. At position A, the drill is in free space, with only gravity compensation forces generated by the haptic device. This results in a steady 2.5 N z-axis force. The user moves the drill down to interact with the printed model surface at location B. From B to C, the user encounters real forces from drilling in addition to the constant gravity compensation forces. From C to D the drill is between real bone model layers and a virtual soft tissue force is generated by the haptic device, increasing with depth of penetration. From D to E, the drill is engaged with the second real bone model layer and the virtual force is off. At E, the drill enters the free space below the second bony layer where only gravity compensation forces are present. c) shows a recording of the drill’s z-axis position in metres (upper axis) and force in Newtons (lower axis) generated by the haptic device. The system remains stable throughout, without high-frequency or underdamped oscillations at boundaries between models.