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Temperature drifts, tool deterioration, unknown vibrations, as well as spindle play are major effects which decrease the achievable precision of CNC lathes and lead to shape deviations of the processed workpieces. Since currently no measurement system exists for precise, in situ, 3D shape monitoring, much effort is required to simulate and compensate these effects. In this article, we propose an optical measurement principle for first part quality manufacturing. The absolute shape, meaning the diameter and surface profile, of the workpiece is determined from its tangential velocity and the surface distance. In order to allow keyhole access, both measurands are determined simultaneously with a single sensor. Measurements inside a metal working lathe show that the standard uncertainty for the absolute shape measurement is below 1 μm. The measurement uncertainty is nearly independent of the lateral surface velocity and roughness. The in situ measurements are compared to measurements with a tactile coordinate measurement machine for reference. [ABSTRACT FROM AUTHOR]