Thermally induced errors were identified as a large error source during diamond turning of micrometer-size optical structures. We investigate the effects of spindle growth, application of cutting fluid mist, and temperature variation errors (TVEs), or thermal drifts, on the ability to hold tool position tolerances. It is shown how certain process variables, such as opening and closing of doors around the machine, affect the TVE. A first-order temperature model was derived to predict the TVE of the manufacturing process. The model uses the ambient temperature inside the machine enclosure because the part temperature could not be measured directly during machining. Documented long-term drift errors over 23 h were as large as 1.7 μm. Short-term drift errors from pass to pass were as large as 0.3 μm. Transient effects caused displacement shifts as large as 0.9 μm.