A screw thread is a helical or tapered structure used to convert between rotational and linear movement or force.
A screw thread may be thought of as an inclined plane wrapped around a cylinder or cone. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight plastic deformation.
In most applications, the thread pitch of a screw is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses.
Screw threads have several applications:
In all of these applications, the screw thread has two main functions:
Standards for machine screw threads have evolved since the early nineteenth century to facilitate compatibility between different manufacturers and users. Many of these standards also specified corresponding bolt head and nut sizes, to facilitate compatibility between spanners and other driving tools.
Nearly all threads are oriented so that a bolt or nut, seen from above, is tightened (the item turned moves away from the viewer) by turning it in a clockwise direction, and loosened (the item moves towards the viewer) by turning anticlockwise. This is known as a right-handed thread, since the natural screwing motion for a right-handed person is clockwise, and is the default because most people are right-handed. Threads oriented in the opposite direction are known as left-handed. There are also self-tapping screw threads where no nut is required.
Left-handed threads are used:
Unless stated otherwise, all standards below specify right-handed threads.
These were standardized by the International Organization for Standardization in 1947. Before that, there were separate metric thread standards used in France, Germany, and Japan, and the Swiss had a set of threads for watches.
The first historically important intra-company standardization of screw threads began with Henry Maudslay around 1800, when the modern screw-cutting lathe made interchangeable screws a practical commodity. During the next 40 years, standardization continued to occur on the intra-company and inter-company level. In 1841, Joseph Whitworth created a design that, through its adoption by many British railroad companies, became a national standard for the United Kingdom called British Standard Whitworth. During the 1840s through 1860s, this standard was often used in the United States and Canada as well, in addition to myriad intra- and inter-company standards. In April 1864, William Sellers presented a paper to the Franklin Institute in Philadelphia, proposing a new standard to replace the U.S.'s poorly standardized screw thread practice. Sellers simplified the Whitworth design by adopting a thread profile of 60° and a flattened tip (in contrast to Whitworth's 55° angle and rounded tip). The 60° angle was already in common use in America, but Sellers's system promised to make it and all other details of threadform consistent.
The Sellers thread, easier for ordinary machinists to produce, became an important standard in the U.S. during the late 1860s and early 1870s, when it was chosen as a standard for work done under U.S. government contracts, and it was also adopted as a standard by highly influential railroad industry corporations such as the Baldwin Locomotive Works and the Pennsylvania Railroad. Other corporations adopted it, and it soon became a national standard for the U.S., later becoming generally known as the United States Standard. Over the next 30 years the standard was further defined and extended and evolved into a set of standards including National Coarse (NC), National Fine (NF), and National Pipe Taper (NPT).
During this era, in continental Europe, the British and American threadforms were well known, but also various metric thread standards were evolving, which usually employed 60° profiles. Some of these evolved into national or quasi-national standards. They were mostly unified in 1898 by the International Congress for the standardization of screw threads at Zurich, which defined the new international metric thread standards as having the same profile as the Sellers thread, but with metric sizes. Efforts were made in the early 20th century to convince the governments of the U.S., UK, and Canada to adopt these international thread standards and the metric system in general, but they were defeated with arguments that the capital cost of the necessary retooling would damage corporations and hamper the economy. (The mixed use of dualling inch and metric standards has since cost much, much more, but the bearing of these costs has been more distributed across national and global economies rather than being borne up front by particular governments or corporations, which helps explain the lobbying efforts.)
During the late 19th and early 20th centuries, engineers found that ensuring the reliable interchangeability of screw threads was a multi-faceted and challenging task that was not as simple as just standardizing the major diameter and pitch for a certain thread. It was during this era that more complicated analyses made clear the importance of variables such as pitch diameter and surface finish. Classes of fit were standardized, and new ways of generating and inspecting screw threads were developed (such as production thread-grinding machines and optical comparators).
Problems with lack of interchangeability among American, Canadian, and British parts during World War II led to an effort to unify the inch-based standards among these closely allied nations, and the Unified Thread Standard was adopted by the Screw Thread Standardization Committees of Canada, the United Kingdom, and the United States on November 18, 1949 in Washington, D.C., with the hope that they would be adopted universally. (The original UTS standard may be found in ASA (now ANSI) publication, Vol. 1, 1949.) UTS consists of Unified Coarse (UNC), Unified Fine (UNF), Unified Extra Fine (UNEF) and Unified Special (UNS). The standard was not widely taken up in the UK, where many companies continued to use the UK's own British Association (BA) standard.
However, internationally, the metric system was eclipsing inch-based measurement units. In 1947, the International Organization for Standardization (interlingually known as ISO) had been founded; and in 1960, the metric-based International System of Units (abbreviated SI from the French Système international) was created. With continental Europe and much of the rest of the world turning to SI and the ISO metric screw thread, the UK gradually leaned in the same direction. The ISO metric screw thread is now the standard that has been adopted worldwide and has mostly displaced all former standards, including UTS. In the U.S., where UTS is still prevalent, over 40% of products contain at least some ISO metric screw threads. The UK has completely abandoned its commitment to UTS in favour of the ISO metric threads, and Canada is in between. Globalization of industries produces market pressure in favor of phasing out minority standards. A good example is the automotive industry; U.S. auto parts factories long ago developed the ability to conform to the ISO standards, and today very few parts for new cars retain inch-based sizes, regardless of being made in the U.S.
There are various methods for generating screw threads. The method chosen for any one application is chosen based on constraints—time; money; degree of precision needed (or not needed); what equipment is already available; what equipment purchases could be justified based on resulting unit price of the threaded part (which depends on how many parts are planned); etc.
In general, certain thread-generating processes tend to fall along certain portions of the spectrum from toolroom-made parts to mass-produced parts, although there can be considerable overlap. For example, thread lapping following thread grinding would fall only on the extreme toolroom end of the spectrum, while thread rolling is a large and diverse area of practice that is used for everything from microlathe leadscrews (somewhat pricey and very precise) to the cheapest deck screws (very affordable and with precision to spare).
The various methods are summarized below.
The excess material is cut away, with taps and dies for most smaller diameters, or with single-point thread-cutting on a lathe for larger ones (or smaller ones needing very high concentricity). With the widespread adoption of affordable, fast, precise CNC milling machines, internal threads may also be cut with a rotating cutter moving in a helical path rather than with a tap, improving speed and avoiding the problem of broken taps.
In this process the material is extruded into a male thread through mechanical pressure as the screw blank is rolled between a matched pair of flat dies. (See Cold forming.) Thread rolling is better suited to high-volume production, and produces threads of diameters typically smaller than one inch. Also, materials with good deformation characteristics are necessary for rolling; these materials include softer (more ductile) metals and exclude brittle materials, such as cast iron. A rolled thread can often be easily recognized because the thread has a larger diameter than the blank rod from which it has been made. (However, necks and shoulders can be cut or rolled to different diameters, so this in itself is not a forensic give-away.) Also, the end of the screw usually looks a bit different from the end of a cut-thread screw. Rolled male threads tend to be slightly stronger than cut male threads. This can be due to several factors, such as (a) the resultant grain structure of the metal, and (b) the fact that the root of the thread may be formed to a small radius rather than a sharp point as with a die. Thread rolling is a very economical way of producing large quantities with good dimensional accuracy. The cost of thread rolling depends on the quantity; the more parts made, the cheaper the unit cost, as the cost of setup is amortized over larger production runs.
This is the female-thread analogue of the male-thread-rolling process described above. The material is extruded into a thread through mechanical pressure by a tap that is similar to a cutting tap except that it has no flutes. Instead of cutting, the tap squeezes the material out of its way. Formed female threads tend to be slightly stronger than cut female threads.
This process is more often employed in soft, ductile metals (such as aluminum) than in hard, brittle metals (such as cast iron).
The threads take the shape of whatever mold or die that the (liquid or gas) material is poured into. When the material freezes into a solid, it retains the shape. Material is either heated to a liquid (or rarely a gas), or mixed with a liquid that will either dry or cure (such as plaster or cement). Alternately, the material may be forced into a mould as a powder and compressed into a solid, as with graphite.
Cast threads in metal parts may be finished by machining, or may be left in the as-cast state. (The same can be said of cast gear teeth.) Whether or not to bother with the additional expense of a machining operation depends on the application. For parts where the extra precision and surface finish is not strictly necessary (although it might be nice), the machining is forgone in order to achieve a lower cost. With sand cast parts this means a rather rough finish; but with molded plastic or die-cast metal, the threads can be very nice indeed straight from the mold or die.
Thread grinding is done on cylindrical grinders using specially dressed wheels matching the shape of the threads. Although expensive, threads produced by grinding are highly accurate and have a very fine surface finish with applications such as ball screw mechanisms used for precise movement of machine components.
Technically, thread grinding is a subset of thread cutting, as grinding is a true metalcutting process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). However, among people who work in the machining fields, the term cutting is understood to refer to the macroscopic cutting operations, and grinding is mentally categorized as a "separate" process. This is why the terms are usually used in contradistinction in shop-floor practice, even though technically grinding is a subset of cutting.
Rarely, thread grinding will be followed by thread lapping in order to achieve the highest precision and surface finish achievable. This is an ultra-deluxe toolroom practice, rarely employed except for the leadscrews or ballscrews of high-end machine tools.
Examples of screw threads include:
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