An apparatus (20) and method are provided for measuring loads on a rotating friction stir welding tool (14) of a friction stir welding machine (10). The apparatus (20) includes a frame (22) that can be connected to the friction stir welding machine (10), and rollers (46) rotatably connected to the frame (22) are structured to contact and rotate with the friction stir welding tool (14). Each roller (46) is adapted to adjust in a respective direction and communicate with a respective load cell (48) so that the load cells (48) detect a positional characteristic of the rollers (46) and measure loads applied to the rotating friction stir welding tool (14) during operation. The rollers (46) can- contact the rotating tool (14) directly during operation of the machine, such as while the rotating tools is being moved through a workpiece to form a friction stir weld joint, to measure the loads on the tool in the direction of movement of the tool and a direction normal to the movement, i.e., the path and path normal directions.