Abstract: A portable power grinder has a housing, a rotating output shaft with a grinding wheel mounting device, a grinding wheel safety guard adjustable between angular positions defined by apertures in the safety guard, and a manually operable safety guard arresting device engageable with an aperture. The arresting device has a lock slide movable on the housing and operable by an articulated lever connected at one end to the housing and at another end to the lock slide and being shifted by folding and unfolding, thereby moving the lock slide between a safety guard arresting position and a safety guard releasing position and locking the lock slide in the arresting position. The lock slide includes a spring exerting a force onto the safety guard in the arrested position. A lock spindle displaceably supported on the lock slide moves between a retracted safety guard locking position and an advanced position to lock the output shaft.