This patented structure is externally equipped with fast-cooling air duct, thus, high-temperature air will flow out from the outside, without any heat on machine bed, ensuring lasting durability; The ventilation system, with right and left zone, is generally controlled by butterfly valve and has intensive suction, hence, which makes smoke exhaust smooth under the support of large-caliber fast-cooling air duct on both sides; Patented butterfly valve damper performs well in airtightness and environmental protection, without any noise while opening and closing.