Thread dimensions

Threads are defined by the thread standard used, the nominal diameter of the thread, and the thread pitch. The pitch of a thread refers to the distance between two consecutive thread crests along the thread's axis, indicating the distance a screw advances linearly in one revolution. The terms "standard thread" and "fine thread" are commonly used, but "coarse thread" and "fine thread" are also used interchangeably.

Threads can be created using cutting or forming methods such as thread taps or thread mills, as well as by turning, or external threads can be manufactured using high-speed steel thread dies (threading dies), although these methods are now primarily used for thread cleaning.

Threads are used for various purposes such as fastening, moving, or adjusting parts. They are most commonly found in bolts and nuts. The primary role of bolts is to hold various assembled structures together, and the bolted joint must withstand the designed load.

The most common bolt today is the hexagonal head bolt with an 8.8 strength class according to the Metric ISO system. Usually, only the strength class number is marked on the bolt head. The Metric M-thread is part of the general SI measurement system. The first number in the strength class marking on bolts indicates the nominal tensile strength in N/mm² in hundreds. For example, 8 means 800 N/mm², and the number after the point indicates ten times the ratio of the lower yield strength ReL (or yield point Rp0.2) to the nominal tensile strength, Rm. The strength classes in metric threads include 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9. With knowledge of the screw size, strength class, and other technical properties, the load it can withstand without damage and the appropriate tightening torque can be calculated. Hex socket bolts are usually made of alloyed steel, making them suitable for applications requiring higher strength. The most common strength classes for hex socket bolts are 10.9 and 12.9.

The markings on stainless steel bolts differ from those on steel bolts. The first part of the marking consists of a letter and a number indicating the steel type, such as A4, where A signifies austenitic steel, and the following number denotes the minimum tensile strength in hundreds of N/mm².

The UN thread standard, based on inch measurements, also divides into coarse and fine threads, similar to metric threads. Coarse thread is designated as UNC (Unified Coarse) and fine thread as UNF (Unified Fine). These markings are also found on the hexagonal head of the bolts. The bolt head also shows the strength class marked with radial lines. In nuts, the marking may appear as short lines along the edge or a ring on one side. The strength classes in inch-based bolts range from 1 to 8, with class 5 being the most common, marked with three radial lines on the bolt head, roughly equivalent to the strength class 8.8 of metric bolts. Occasionally, a UN bolt head may show a single S, T, or V letter, which is a British strength class marking where S corresponds to class 5.

A vernier caliper and thread gauge are typically used to identify threads. The vernier caliper measures the external or internal diameter of the thread, while the thread gauge determines the thread profile and pitch. With this information, the specific thread type can be determined. The most common measuring tools for thread manufacturing are the thread micrometer and thread vernier caliper, which measure the pitch diameter of the thread. In large series, thread gauges are used to verify whether the thread meets the upper and lower limit requirements.

The Camcut thread tables provide the pitch diameters in addition to the external and internal diameters, along with other important manufacturing dimensions for the most common thread standards, such as ISO Metric standard and fine thread (ISO 261 and SFS-ISO 965-1), MJ threads (ISO 5855), Inch UNC, UNF, and UNEF threads (ASME/ANSI B1.1), G, BSF, Ww, and NPT pipe threads (SFS-ISO 228-1 and ANSI/ASME B1.20.1), TR trapezoidal thread (DIN 103 and ISO 2904), Rd round thread (DIN 405-1 and DIN 20400, PG conduit thread (DIN 40430), BA thread, and S thread (DIN 513).