An emergency braking system for a machine tool for abruptly braking a revolving shaft includes at least one brake drum and at least one brake shoe which are engaged with one another in order to brake the shaft. The braking intervention between the brake drum and the brake shoe takes place under the influence of a centrifugal force resulting from the rotation of a shaft as soon as a locking device is released. Also provided in the braking system is a diagnostic system which allows the functionality of the locking device to be checked.