Growing power densities in the main spindle of machine tools caused by approaches to increase the productivity lead to deflections of the tool-center-point (TCP) and thus limit the achievable accuracy. In order to optimize the operating point of a spindle in the context of these contrary requirements, effective tools need to be developed. By modeling thermo-elastic effects using a FE approach, interactions of heat sources and sinks can be investigated deeply on the one hand and a model based correction of TCP deflections can be established on the other hand. For a practical use, short computing times as well as precise simulation results are core requirements. In the present paper, an online thermal simulation model is developed, parameterized and validated.