The Union Butterfield 1590(UNC)/1590(UNF)/1591(UNC)/1591(UNF) high-speed steel heavy-duty spiral flute tap has a round shank with a square end and a black oxide finish, and is used to create right-hand threads in a range of materials. It is available in two different ANSI thread specification standards: UNC (Unified Coarse) and UNF (Unified Fine). The 40-degree spiral flutes on this tap pull chips back from the tip of the tool up and out the hole to prevent clogging. This characteristic makes the tap suitable for blind or bottoming holes (with only one opening). The heavy-duty design (slower helix angle, large core diameter, and wider flute throat) makes it suitable for tougher materials in which average- or fast-spiral designs perform marginally. The black oxide finish of this tool will not chip. It improves lubricity, resists corrosion, and helps to prevent buildup and welding on the cutting edge of the tool.
Right-hand threads are designed to tighten when a head or nut is rotated clockwise (the most common threading direction). High-speed steel (HSS) is a common general-purpose steel for cutting tools and is compatible with a variety of materials. The round shank with square end helps to minimize rotation in the tool holder. This tool can be used in machines, such as drill presses or lathes, or with hand tools such as tap wrenches.
In the Union Butterfield 1590(UNC)/1590(UNF) series, taps with machine-screw sizes 6 and 8 have two flutes; machine-screw sizes 10 and 12 have three flutes. In the 1591(UNC)/1591(UNF) series all taps have three flutes. In both series taps are available with a variety of chamfers. This tap conforms to ANSI standards and can create threads to a depth 2 ½ times its diameter. It is suitable for the ANSI 3B tolerance class: 3B produces close-tolerance fasteners that resist loosening, as in aerospace applications.
Taps produce internal threads in previously formed holes and can be used in machines (such as drill presses) or with hand tools. Common types of taps include hand taps, spiral point taps, spiral flute taps, thread forming taps, and pipe taps. Hand taps (straight-flute taps) collect chips in the straight flutes of the tool; spiral point taps push chips through holes ahead of the tool; and spiral flute taps pull chips back from the tip of the tool, up and out the hole. Thread forming taps, also known as cold forming taps, press rather than cut to create threads in a drilled hole, and no chips are created. Pipe taps create threads in pipes and pipe fittings. Each thread type designates a thread profile and each type is identified with an abbreviation. In the U.S. and Canada, the Unified Thread Standard is the dominant thread type, including UNC (Unified Coarse), UNF (Unified Fine), and NPT (National Pipe Taper) threads, among others.