The action of the obstruction type chip former, as used on lathe turning tools, is examined and the significance of the radius of curvature of the chip, imposed by the chip former, is discussed in relation to problems of chip control. The previous estimates of the chip curvature produced in terms of chip former dimensions are briefly reviewed and a new analysis is presented which takes into account the variations of chip thickness and the length of the chip-tool contact region on the tool rake face.