Screw

Background

Screws are part of a family of threaded fasteners that includes bolts and studs as well as specialized screws like carpenter's wood screws and the automotive cap screw. The threads (or grooves) can run right handed or left, tapered, straight, or parallel. There are two types of screws, machine and wood screws. Both are made of metal, however the machine screw has a constant diameter and joins with nuts while the wood screw is tapered and grips to the actual wood surface.

History

Even though the concept of the screw dates back to around 200 b.c., the actual metal screw that is known today was not developed until the Renaissance. Early screws had to be handmade, so no two screws were ever alike. The time consuming process of hand filing the threads into the screw form made mass production and use virtually impossible. In 1586, the introduction of the first screw-cutting machine by Jacques Besson, court engineer for Charles IX of France, paved the way for more innovations.

Inspired by earlier designers and makers of scientific instruments like microscopes, clockmakers and gunsmiths led the way in screw-cutting machine design. In 1760, Job and William Wyatt, two English brothers, filed a patent for the first automatic screw-cutting device. Their machine could cut 10 screws per minute and was considered one of the precursors to mass production machinery.

During the early nineteenth century, Englishman Henry Maudslay produced the method of screw manufacture still in use today. His machine was the first power-driven, screw-cutting lathe. In the United States, at the same time, David Wilkinson also built a screw-cutting lathe and was awarded the first American screw patent. New innovations followed soon after. In 1845, Stephen Finch developed a turret lathe, and soon after the Civil War, Christopher Walker invented a fully automatic lathe.

The first screw factory, Abom and Jackson, was opened in Rhode Island in 1810. By 1895 screw makers in America were forming unions and demanding a minimum wage of $1.75 per ten-hour day for a member and $1.25 for an apprentice. Smaller scale innovations continued to be made to improve efficiency. John E. Sweet devised the angular thread-cutting method to cut an entire thread from one side.

Today, machining of screws has been superseded by thread rolling. In 1836, American William Keane developed the thread rolling process, but at the time it had little success. The iron metal that was used to create the thread-rolled screws was too low grade and had the tendency to split during the die-cutting process. The eventual need to mass produce screws at a fraction of the cost of machining led to the reevaluation and establishment of the thread-rolling manufacture of screws.

Raw Materials

Screws are generally made from low to medium carbon steel wire, but other tough and inexpensive metals may be substituted, such as stainless steel, brass, nickel alloys, or aluminum alloy. Quality of the metal used is of utmost importance in order to avoid cracking. If a finish is applied to the screw, it must be of a compatible makeup. Steel may be coated or plated with zinc, cadmium, nickel, or chromium for extra protection.

Design

On a single thread screw, the lead and pitch are identical, lead is twice the pitch on a double thread model, and three times as much on a triple thread. The pitch of a screw is the distance between two threads (or grooves) from the same point on each thread. It is also more commonly known as the number of threads per inch or centimeter. The lead of the screw measures how far it is driven in for each revolution.

The Manufacturing
Process

Machining is only used on unique designs or with screws too small to be made any other way. The machining process is exact, but too time consuming, wasteful, and expensive. The bulk of all screws are mass manufactured using the thread rolling method, and that is the procedure described in further detail.

Cold heading

• 1 Wire is fed from a mechanical coil through a prestraightening machine. The straightened wire flows directly into a machine that automatically cuts the wire at a designated length and die cuts the head of the screw blank into a preprogrammed shape. The heading machine utilizes either an open or closed die that either requires one punch or two punches to create the screw head. The closed (or solid) die creates a more accurate screw blank. On average, the cold heading machine produces 100 to 550 screw blanks per minute.

Thread rolling

• 2 Once cold headed, the screw blanks are automatically fed to the thread-cutting dies from a vibrating hopper. The hopper guides the screw blanks down a chute to the dies, while making sure they are in the correct feed position.
• 3 The blank is then cut using one of three techniques. In the reciprocating die, two flat dies are used to cut the screw thread. One die is stationary, while the other moves in a reciprocating manner, and the screw blank is rolled between the two. When a centerless cylindrical die is used, the screw blank is rolled between two to three round dies in order to create the finished thread. The final method of thread rolling is the planetary rotary die process. It holds the screw blank stationary, while several die-cutting machines roll around the blank.
• All three methods create higher quality screws than the machine-cut variety. This is because the thread is not literally cut into the blank during the thread-rolling process, rather it is impressed into the blank. Thus, no metal material is lost, and weakness in the metal is avoided. The threads are also more precisely positioned. The more productive of the thread-rolling techniques is by far the planetary rotary die, which creates screws at a speed of 60 to 2,000 parts per minute.

Quality Control

The National Screw Thread Commission established a standard for screw threads in 1928 for interchangeability. This was followed by an international Declaration of Accord in 1948, adopting a Unified Screw Thread system. The standards focus on three main elements: the number of threads per inch, the designated pitch and shape of the thread, and designated diameter sizes. In 1966, the International Standards Organization (ISO) suggested a universal restriction on threads to ISO metric and inch size ranges with coarse and fine pitches. Compliance with the ISO suggested standards has been global.

Where to Learn More

Books

Brittania Company. Screws and Screw-Making. James H. Wood, 1892.

Camm, F. J. Screw Cutting. Cassell and Company Ltd., 1920.

Glover, David. Screws. Rigby Education, 1997.

Periodicals

Koepfer, Chris. "Technology Gamble Pays Off." Modern Machine Shop, February 1995, pp. 94-104.

—JenniferSwiftKramer

screw / skroō/ • n. 1. a short, slender, sharp-pointed metal pin with a raised helical thread running around it and a slotted head, used to join things together by being rotated so that it pierces wood or other material and is held tightly in place. ∎  a cylinder with a helical ridge or thread running around the outside (a male screw) that can be turned to seal an opening, apply pressure, adjust position, etc., esp. one fitting into a corresponding internally grooved or threaded piece (a female screw). ∎  (the screws) hist. an instrument of torture acting in this way. ∎  (also screw pro·pel·ler) a ship's or aircraft's propeller (considered as acting like a screw in moving through water or air). 2. an act of turning a screw or other object having a thread. ∎ Brit. a small twisted-up piece of paper, used as a container for a substance such as salt or tobacco. 3. inf. a prisoner's derogatory term for a prison guard or warden. 4. vulgar slang [in sing.] an act of sexual intercourse. ∎  a sexual partner of a specified ability. 5. Brit., archaic, inf. a mean or miserly person. 6. Brit., inf. a worn-out horse. • v. 1. [tr.] fasten or tighten with a screw or screws: screw the hinge to your new door. ∎  rotate (something) so as to fit it into or on to a surface or object by means of a spiral thread: Philip screwed the top on the flask. ∎  [intr.] (of an object) be attached or removed by being rotated in this way: a connector that screws on to the gas cylinder. ∎  (screw something around) turn one's head or body around sharply: he screwed his head around to try and find the enemy. 2. [tr.] (usu. be screwed) inf. cheat or swindle (someone), esp. by charging them too much for something: if you do what they tell you, you're screwed we ended up getting more money than what they were trying to screw us for. ∎  (screw something out of) extort or force something, esp. money, from (someone) by putting them under strong pressure: your grandmother screwed cash out of him for ten years. 3. [tr.] vulgar slang have sexual intercourse with. ∎  [intr.] (of a couple) have sexual intercourse. ∎  [in imper.] inf. used to express anger or contempt: Screw him! PHRASES: have one's head screwed on (the right way) inf. have common sense. have a screw loose inf. be slightly eccentric or mentally disturbed. put the screws on inf. exert strong psychological pressure on (someone) so as to intimidate them into doing something. a turn of the screw inf. an additional degree of pressure or hardship added to a situation that is already extremely difficult to bear. turn (or tighten) the screw (or screws) inf. exert strong pressure on someone.PHRASAL VERBS: screw around 1. vulgar slang have many different sexual partners. 2. inf. fool around. screw someone over inf. treat someone unfairly; cheat or swindle someone. screw up inf. completely mismanage or mishandle a situation: I'm sorry, Susan, I screwed up. screw someone up inf. cause someone to be emotionally or mentally disturbed: this job can really screw you up. screw something up 1. tense the muscles of one's face or around one's eyes, typically so as to register an emotion or because of bright light. 2. inf. cause something to fail or go wrong: why are you trying to screw up your life? 3. summon up one's courage: now Stephen had to screw up his courage and confess. DERIVATIVES: screw·a·ble adj. screw·er n.

screw simple machine consisting essentially of a solid cylinder, usually of metal, around which an inclined plane winds spirally, either clockwise or counterclockwise. It is used to fasten one object to another, to lift a heavy object, or to move an object by a precise amount. The ridge forming the inclined plane is called the thread; in cross section the ridge may be approximately triangular, square, or rounded. The vertical distance from any point on one thread to a corresponding point on the next successive thread is called the pitch. A thread can also be placed on the inner surface of a hollow cylinder. Two screws of the same pitch and diameter, one on the outer surface of a solid cylinder and the other on the inner surface of a hollow cylinder, can be arranged so that one may be driven spirally into the other, as in the common nut and bolt. The thread on the surface of the bolt is called the external, or male, screw; that on the inner surface of the nut, the internal, or female, screw. The common jackscrew used to lift automobiles, houses, and other heavy objects is an application of this principle. The internal screw is situated in the base, the external screw on a metal cylinder; at the top of the cylinder a lever or handle is fastened. As the handle is rotated, the external screw moves up the internal screw and the object placed on top of the jack is lifted. The mechanical advantage of the jackscrew, as of any other screw, is theoretically the ratio between the circumference through which the end of the handle moves and the pitch of the screw. Since, however, there is much friction in the operation of a screw, the amount of work put into this machine is much greater than the amount done and the efficiency is small. On the other hand, the small effort necessary to turn the handle, when compared to the enormous load raised, makes such a device of great value. The screw is often used for making delicate adjustments of tools and machines, e.g., in the micrometer screw and in the carburetor of the gasoline engine (for regulating the flow of gasoline). The self-tapping screw has notches in the first few threads that can cut female threads in a hollow cylinder. Wood and metal screws, the carpenter's and machinist's vise, the propeller of a boat or airplane, Archimedes' screw, and many other devices are applications of the screw.

screw
A. mechanical contrivance of which the operative part is a spiral groove or ridge XV; worm or boring part of a gimlet XVI; (fig.) XVII.

B. (from the vb.) act of screwing XVIII; object screwed or twisted up XIX.

C. (sl. senses of obscure development) unsound horse; wages, salary XIX. In A — OF. escroue fem. (mod. écrou m.) either (i) — WGmc. *scrūva (MHG. schrūbe (G. schraube), corr. to MDu. schrūve), or (ii) :- (the source of the Gmc. forms) L. scrōfa sow, medL. female screw.

Hence screw vb. XVII. screwed (sl.) intoxicated. XIX; also (earlier) screwy.

screw n. also screw propeller a ship's or aircraft's propeller (considered as acting like a screw in moving through water or air).

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