Threading taps are cutting tools that produce internal threads in previously formed holes. They can be used in machines such as drill presses or with hand tools. Common types of threading taps include hand taps, spiral point taps, spiral flute taps, thread forming taps, and pipe taps. Hand threading taps, also called straight flute taps, collect chips in the straight flutes of the tool. Spiral point taps, also called “gun taps,” push chips through the hole ahead of the tool. Spiral flute taps pull chips back from the tip of the tool, up and out of the hole. Thread forming taps, also called cold forming taps, create threads by pressing rather than cutting and generate no chips. Pipe taps create threads in pipes and pipe fittings.
Cutter material is an important consideration when selecting a threading tap. Carbide is harder than high-speed or cobalt steel, making it more wear resistant at high speeds. Cobalt steel has higher feed rates than high-speed steel. High-speed steel (HSS) is a common, general-purpose steel for cutting tools and offers toughness and shock resistance. Powdered-metal steel has performance approaching that of carbide tools, but with the toughness and shock resistance of high-speed steel.
Thread type designates the thread pitch of a tap and is identified by an abbreviation. Common types include the N-series (Unified Thread Standard), Metric, UNC (Unified Coarse), UNF (Unified Fine), and NPT (National Pipe Taper), among others.
Tolerance ensures that the internal threads created by the tap correspond to the external threads of a fastener. Most threading taps are designed to create a 2B class thread fit, indicating the threads are a medium fit with a 2A class fastener. Tolerances H1 through H10 apply to taps up to 1 inch in diameter, and describe the specific pitch diameter of the tap.
Thread size is measured in the U.S. inch or metric system, and conforms to common screw and bolt sizes. Thread size is identified by the external diameter, followed by the number of threads. For example, a 1”-8 tap means that it has a 1” external diameter and 8 threads per inch. An example of a threading tap title is 7/8-14UNF-2B, meaning it has 7/8” diameter, 14 threads per inch, Unified National Fine threads, Class 2 fit, and internal threads.
Some threading taps are uncoated, while others have finishes that enhance performance, prevent material build-up, and extend tool life. A finish of titanium nitride (TiN), for example, combines hardness and low friction to allow faster cutting speeds than uncoated tools, while a gold oxide finish reduces grinding stress within the tool.
Chamfers are the tapered or incomplete threads at the front of the tap. The choice of chamfer type depends on the hole being threaded. Plug chamfers, with 3 to 5 tapered threads, can be used in both through holes (extending through the workpiece) and blind holes (with only one opening). Bottoming chamfers, with 1 to 2 tapered threads, are suitable for holes where the threads must come as close to the bottom of the hole as possible. Taper chamfers, with 7 to 10 tapered threads, are the longest type of chamfers, and are used for starting threads and for through holes.