Abstract: A device (1) is provided for heating up a tool holder which is part of a tool clamping chuck. Tool clamping chucks of this type have a coupling part which can be inserted into a machine tool. The tool holder, which can be removed from the chuck or is connected to it, is cylindrical or conical, for example, and has a coaxial fitting hole for receiving the shaft of an interchangeable tool, for example a drill or hob. The device (1) has a heating unit (2) to heat up and thus expand the tool holder and its fitting hole. Furthermore, the device (1) has an insert (3) with an exactly fitting internal receptacle opening (4), in particular for a planar contact with the outer side of the tool holder. This insert is made of a heat-conducting material or metal which can be heated with the heating unit (2), so that the heat energy transferred via this insert can be transferred in a uniform, quick and targeted manner, as well as in the desired dosing, to the tool holder.