A device for machining a gear using a rotating machining tool. The device has a motor spindle having a drive motor and motor spindle shaft drivable by the drive motor for driving the machining tool. The device has an attachment structure for releasably attaching the device to a work spindle of a tool head. The attachment structure is configured so the tool axis is parallel to the work spindle axis when the device is attached to the work spindle. A device is further disclosed to connect a machining tool to a counter bearing in a simple manner. For this purpose, the counter bearing has a hollow shaft. A mandrel having first and second clamping regions are arranged at positions along the longitudinal axis of the mandrel. The mandrel is inserted through the hollow shaft along the tool axis into a longitudinal bore of the machining tool.