Deformation of a scratch indicates developement of subsurface residual strain in the workpiece. Titanium workpiece,60 m/min surface speed. Visible light movie is 180000 frames per second. Field of view is about 1 mm wide. For more information, goto NIST publications "Machining Process Measurements: A Titanium Machining Example" and "Tool Temperatures in Orthogonal Cutting of Alloyed Titanium."