Taps are tools used to cut internal threads in a drilled hole, allowing it to accept a screw or bolt. They are essential in manufacturing, repair, and prototyping when creating threaded holes in materials such as metal or plastic. While many taps cut threads by removing material, not all taps operate the same way—different designs and classifications support different materials, thread types, and applications.
There are two primary categories of taps: cutting taps and forming taps. Cutting taps, as the name implies, remove material from the walls of a hole to create threads. Within this category, there are several designs suited for different cutting conditions. A spiral point tap (often called a “gun tap”) pushes chips forward and is especially effective in through-holes. A spiral flute tap, with its helical groove, pulls chips back out of the hole, making it better suited for blind holes where chip removal is more difficult. These spiral designs help reduce the risk of tap breakage by controlling chip flow.
Thread forming taps (sometimes called roll taps or fluteless taps) do not remove material at all. Instead, they cold-form the internal threads by displacing material plastically. This method produces no chips and often results in stronger threads because the grain structure of the material is not cut but rather compressed and realigned. Thread forming taps are particularly effective in ductile materials like aluminum, brass, or mild steel but are not suitable for brittle materials, as those may crack instead of deforming.
Taper, plug, and bottoming taps refer to different chamfer styles found on cutting taps, each designed for a specific depth or threading condition. A taper tap has a long lead-in with about 7 to 10 threads gradually increasing in diameter. This gradual entry makes it the easiest to start by hand and is typically used to begin threads in a hole. A plug tap has a shorter lead, usually with 3 to 5 tapered threads, making it more suitable for general-purpose tapping, especially in through-holes or when the tap can easily continue past the bottom of the hole. The bottoming tap, sometimes called a “finishing tap,” has just 1 to 2 tapered threads at the tip and is used to thread all the way to the bottom of a blind hole after the initial threads have been established with a taper or plug tap. While bottoming taps are essential for maximizing thread depth, they can be difficult to start on their own and are usually used as a second or third step in the tapping process.
Taps are also classified by their tolerance limits, denoted by letters and numbers. In the Unified inch system, this is most commonly seen with H limits. The “H” stands for “high” and indicates that the tap is slightly oversized relative to the basic thread size. For example, an H3 tap is larger than an H2 tap. Each “H” number represents a 0.0005 inch increment above the basic pitch diameter. This oversize is often necessary to account for coating thickness, minor inaccuracies in tap manufacture, or desired thread class. While less common, L limits (standing for “low”) indicate undersized taps, used when a tighter fit is needed or when threads will be plated after tapping. The H and L system allows machinists to choose taps that match specific thread class fits, such as Class 2B or Class 3B, ensuring that the resulting internal threads meet engineering and performance requirements. (See ANSI/ASME B94.9)