Nonrotatable flanges are often referred to as fixed flanges. A nonrotatable flange is constructed of a single machined piece. Once nonrotatable flanges are welded in place their fastening bolt holes are fixed in position. The industry standard for positioning fixed flange bolt holes is to have bolt holes straddle a vertical centerline. Flanges fitted with leak test grooves are welded so that the groove is positioned on that vertical centerline. A rotatable flange is constructed of two concentric or coaxial components, these are referred to as the receiver and the insert. The receiver, or outer ring, contains the fastening bolt holes while the insert is the blank or bored center piece that contains the sealing knife edge and to which a mating tube is typically welded. The term “rotatable” pertains to the flange’s outer ring or receiver prior to the insertion of fasteners. The receiver is rotatable for the purpose of aligning bolt holes with fixed, nonrotatable mating flanges. Once the holes have been aligned and fasteners inserted, the flange is no longer rotatable. Fastening two rotatable flanges is not only difficult but also increases the possibility for damage to one or both of the mating flange knife edge seals. Caution should be exercised when fastening two rotatable flanges.

Del-Seal™ CF metal seal flanges are fitted with either clearance or threaded fastening bolt holes. Clearance holes are commonly referred to as through holes and threaded holes are commonly referred to as tapped holes. Tapped holes provide threads through the entire thickness of a flange. Exceptions to this include reducer flanges and cubes which are fitted with blind tapped holes. Blind tapped holes are drilled only part way through a flange and do not break through. Blind tapped holes are typically used where drilling through a component is not possible or desirable, like the direct tapping of a chamber wall, where breaking through would provide a leak path through the vacuum walls.