Abstract: Apparatus for filtering dust particles from air has a bag head and a bag assembly which includes a filter bag and a skeletal bag frame received within the bag. A cylindrical bag frame collar secured to the upper end of the bag frame threadably engages a cylindrical bag head collar which depends from the bag head. Threads on the head collar are formed by rods tack welded to the outer surface of the collar. The bag frame collar receives the head collar therein and has two rivets and an adjustable bolt equiangularly spaced and projecting radially inwardly to engage the threads on the head collar. The bolt serves to lock the two collars in assembly and prevent relative rotation and loosening due to vibration.