and dirt particles of the air to be cleaned deposit within the spaces being formed by two adjacent folded lamellas 25 due to the force of gravity large dirt particles will precipitate from these spaces and collect at the bottom of the filter chamber respectively will gradually deposit from the bottom upwards within the spaces of the folded filters during an extended period of operation of the tool so that for a partial contamination of the main filter a sufficient filter surface for cleaning the air still remains thereby also keeping the pressure difference low moreover after unscrewing the housing cover 4 it can be visually determined by the level of dirt within the spaces whether the main filter has to be cleaned or exchanged the visual control is performed from the outside through the air flow openings 33 within the housing wall 54 without removing the chamber housing 34 due to their even distribution the air flow openings 33 allow a view of the entire filter surface of the filter body 16 in case that cleaning is necessary the chamber housing 34 is removed and deposited dirt can easily be removed from the spaces of the folded filter and from the bottom of the filter chamber 17 at the interior side of the chamber housing 34 a circumferential shoulder 57 is provided by which a step 63 is formed pointing in the direction of the upstream side of the.