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Screw thread
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A screw thread is a helical 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.
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Encyclopedia
A screw thread is a helical 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 pitch of a screw thread 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.
Applications
Screw threads have several applications:
- Fastening
- Fasteners such as wood screws, machine screws, nuts and bolts.
- Connecting threaded pipes and hoses to each other and to caps and fixtures.
- Gear reduction via worm drives
- Moving objects linearly by converting rotary motion to linear motion, as in a leadscrew.
- Measuring by correlating linear motion to rotary motion (and simultaneously amplifying it), as in a micrometer.
- Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew of a lathe.
In all of these applications, the screw thread has two main functions:
- It converts rotary motion into linear motion.
- It prevents linear motion without the corresponding rotation.
Basic concepts of design
Gender
Every matched pair of threads, external and internal, can be described as male and female. For example, a screw has male threads, while its matching hole (whether in nut or substrate) has female threads. This property is called gender.
Handedness
The helix of a thread can twist in two possible directions, which is known as handedness. Most 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 counterclockwise. This is known as a right-handed (RH) thread, because it follows the right hand grip rule (often called, more ambiguously, "the right-hand rule"). Threads oriented in the opposite direction are known as left-handed (LH).
By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. Left-handed threads are used:
- Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to fretting induced precession, e.g. on a left-hand bicycle pedal.
- In combination with right-handed threads in turnbuckles.
- In some gas supply connections to prevent dangerous misconnections, for example in gas welding the flammable gas supply uses left-handed threads.
- In some instances, for example early ballpoint pens, to provide a "secret" method of disassembly.
- In some applications of a leadscrew, for example the cross slide of a lathe, where it is desirable for the cross slide to move away from the operator when the leadscrew is turned clockwise.
Form
The cross-sectional shape of a thread is often called its form or threadform (also spelled thread form). It may be square, triangular, trapezoidal, or other shapes. The terms form and threadform sometimes refer to all design aspects taken together (cross-sectional shape, pitch, and diameters).
Most triangular threadforms are based on an isosceles triangle. These are usually called V-threads or vee-threads because of the shape of the letter V. For 60° V-threads, the isosceles triangle is, more specifically, equilateral. For buttress threads, the triangle is scalene.
The theoretical triangle is usually truncated to varying degrees (that is, the tip of the triangle is cut short). A V-thread in which there is no truncation (or a minuscule amount considered negligible) is called a sharp V-thread. Truncation occurs (and is codified in standards) for practical reasons:
- The thread-cutting or thread-forming tool cannot practically have a perfectly sharp point; at some level of magnification, the point is truncated, even if the truncation is very small.
- Too-small truncation is undesirable anyway, because:
- The cutting or forming tool's edge will break too easily;
- The part or fastener's thread crests will have burrs upon cutting, and will be too susceptible to additional future burring resulting from dents (nicks);
- The roots and crests of mating male and female threads need clearance to ensure that the sloped sides of the V meet properly despite (a) error in pitch diameter and (b) dirt and nick-induced burrs.
Ball screws, whose male-female pairs involve bearing balls in between, show that other variations of form are possible.
Angle
The angle characteristic of the cross-sectional shape is often called the thread angle. For most V-threads, this is standardized as 60 degrees, but any angle can be used.
Lead, pitch, and number of starts
Lead and pitch are closely related concepts. The difference between them can cause confusion, because they are equivalent for most screws. Lead is the distance along the screw's axis that is covered by one complete rotation of the screw (360°). Pitch is the distance from the crest of one thread to the next. Because the vast majority of screw threadforms are single-start threadforms, their lead and pitch are the same. "Single-start" means that there is only one "ridge" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of one ridge. "Double-start" means that there are two "ridges" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of two ridges.
Analysis of pitch and TPI for single-start threads
The following discussion assumes a single-start thread, whose pitch and lead are always equivalent.
Pitch is the distance from the crest of one thread to the next. It is analogous to the wavelength of a wave. A closely related concept is threads per inch (TPI), which is how many threads occur per inch of axial screw length. Pitch and TPI describe the same underlying physical property—merely in different terms. When units of measurement are constant, pitch is simply the inverse of TPI, and vice versa. For example, a -20 thread has 20 TPI, which means that its pitch is inch (0.050").
Another wave analogy is that pitch and TPI are inverses of each other in a similar way that period and frequency are inverses of each other.
Inch-based thread standards are usually named in terms of TPI (that is, threads per distance), whereas metric thread standards are usually named in terms of pitch (that is, distance per thread).
Coarse versus fine
Coarse threads are those with relatively larger pitch (fewer threads per axial distance), and fine threads are those with relatively smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, whereas fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a saw or file, or between coarse grit and fine grit on sandpaper.
The common V-thread standards (ISO 261 and Unified Thread Standard) include a coarse pitch and a fine pitch for each major diameter. For example, -13 belongs to the UNC series (Unified National Coarse) and -20 belongs to the UNF series (Unified National Fine).
A common misconception among people not familiar with engineering or machining is that the term coarse implies here lower quality and the term fine implies higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter. Coarse threads can be made accurately, or fine threads inaccurately.
Diameters
There are several relevant diameters for screw threads: major diameter, minor diameter, and pitch diameter.
Major diameter
Major diameter is the largest diameter of the thread. For a male thread, this means "outside diameter", but in careful usage the better term is "major diameter", since the underlying physical property being referred to is independent of the male/female context. On a female thread, the major diameter is not on the "outside". The terms "inside" and "outside" invite confusion, whereas the terms "major" and "minor" are always unambiguous.
Minor diameter
Minor diameter is the smallest diameter of the thread.
Pitch diameter
Pitch diameter, also known as mean diameter, is a diameter in between major and minor. It is the diameter at which each pitch is equally divided between the mating male and female threads. It is important to the fit between male and female threads, because a thread can be cut to various depths in between the major and minor diameters, with the roots and crests of the threadform being variously truncated, but male and female threads will only mate properly if their sloping sides are in contact, and that contact can only happen if the pitch diameters of male and female threads match closely. Another way to think of pitch diameter is "the diameter on which male and female should meet".
Thread pitch diameter is analogous to gear pitch diameter, which is related to how two mating gears should meet.
Classes of fit
The way in which male and female fit together, including play and friction, is classified (categorized) in thread standards. Achieving a certain class of fit requires the ability to work within tolerance ranges for dimension (size) and surface finish. Defining and achieving classes of fit are important for interchangeability.
Standardization and interchangeability
To achieve (a) predictably successful mating of male and female threads and (b) assured interchangeability between males and between females, standards for form, size, and finish must exist and be followed. Standardization of threads is discussed below.
Thread depth
Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, which is known as the thread depth or percentage of thread. The UTS and ISO standards codify the amount of truncation, including tolerance ranges.
A perfectly sharp 60° V-thread will have a depth of thread ("height" from root to crest) equal to 86.6% of the pitch. This fact is intrinsic to the geometry of an equilateral triangle—a direct result of the basic trigonometric functions. It is independent of measurement units (inch vs mm).
The typical depth of UTS and ISO threads with truncation included is around 75% of the pitch. Threads can be (and often are) truncated a bit more, yielding thread depths of 60% to 65%. This makes the thread-cutting easier (yielding shorter cycle times and longer tap and die life) without making any large sacrifice in thread strength. For many applications, 60% threads are strong enough for the job, and 75% threads are wasteful (in the sense that extra resources were invested in creating them, but they were not necessary). For male threads, by turning down the bar stock a bit more, the major diameter is truncated a bit farther. For female threads, by drilling with a slightly bigger tap drill, the minor diameter is truncated a bit farther. Notice that the pitch diameter is not the variable being changed here.
This sort of trade-off (truncation versus thread strength) is pervasive in the profession of engineering. Safety factors must always be built in, but one must also quantify the safety factors in order to decide rationally "how much is enough". This balance is what keeps the cost of the product affordable.
Standardization
Standardization of screw threads has evolved since the early nineteenth century to facilitate compatibility between different manufacturers and users. Many of the standards also specify corresponding bolt head and nut sizes, to facilitate compatibility between spanners and other driving tools.
ISO standard threads
The most common threads in use are the ISO metric screw threads (M) and BSP threads also called G threads for pipes.
These were fully standardized by the International Organization for Standardization in 1947. Although metric thread were mostly unified in 1898 by the International Congress for the standardization of screw threads, separate metric thread standards were used in France, Germany, and Japan, and the Swiss had a set of threads for watches.
Other current standards
In particular applications and certain regions, threads other than the ISO metric threads remain commonly used. This is largely for reasons of backwards compatibility. Non-ISO threads in common use include:
- Camera case screws, used to mount a camera on a photographic tripod:
- Royal Microscopical Society (RMS) thread, a special 0.8"-36 thread used for microscope objective lenses.
- Microphone stands:
- ?? 27 threads per inch (tpi) Unified Special thread (UNS, USA and the rest of the world)
- ¼? BSW (not common in the USA, used in the rest of the world)
- ?? BSW (not common in the USA, used in the rest of the world)
- Stage lighting suspension bolts (in some countries only; some have gone entirely metric, others such as Australia have reverted to the BSW threads, or have never fully converted):
- ?? BSW for lighter luminaires
- ½? BSW for heavier luminaires
- Panzergewinde (Pg) (also: Stahlpanzerrohr-Gewinde) is an old German 80° thread (DIN 40430) that remained in use until 2000 in some electrical installation accessories in Germany.
History of standardization
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.
A tremendous amount of engineering work was done throughout World War I and the following interwar period in pursuit of reliable interchangeability. 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). Therefore, in theory, one might expect that by the start of World War II, the problem of screw thread interchangeability would have already been completely solved. Unfortunately, this proved to be false. Intranational interchangeability was widespread, but international interchangeability was less so. 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.
Engineering drawing
In American engineering drawings, ANSI Y14.6 defines standards for indicating threaded parts. Parts are indicated by their nominal diameter (the nominal major diameter of the screw threads), pitch (number of threads per inch), and the class of fit for the thread. For example, “.750-10UNC-2A” is male (A) with a nominal major diameter of 0.750 in, 10 threads per inch, and a class-2 fit; “.500-20UNF-1B” would be female (B) with a 0.500 in nominal major diameter, 20 threads per inch, and a class-1 fit. An arrow points from this designation to the surface in question.
Generating screw threads
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.
Thread cutting
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. This type of screw applied for most plastics parts.
Thread rolling
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 dies, either flat or cylindrical. 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, because of the effect of the cold forming action on the metal's grain structure.
Process Characteristics
- Produces external rolled threads
- Material is plastically deformed and cold-worked
- Produces rolled threads with excellent strength and surface finish
- Forms major thread diameters greater than blank diameter
- Is a high production process
- It is usually three to five times faster than thread cutting.
- The threads produced are very accurate and strong because they are cold worked
Setup and Equipment
The workpiece is held in a chuck or other workpiece holding device while the thread rolling die-head traverses the workpiece. Several sizes of thread rolling heads can be used over a wide range of workpiece diameters to produce threads that are superior in strength and finish to threads produced by other machining processes.
Geometrical Possibilities
Thread types include UNC, UNF, ISO, ACME, Whit-worth, worm, buttress, ball-screw, wood-screw, tapping-crew, bag-screw, and many others. Typical diameters for rolled threads range from 0.075 in. to 2 in. Typical lenghts range from 2 in. to 10 in., but with through-feed thread forming, lenghts of 100 in., or more, can be readily produced.
Tolerances and Surface Finish
Tolerances and surface finish depend on several variables, such as type of material to be formed, quality of the workpiece material, condition and type, and the rigidity of the equipment. In general, the tolerances and surface conditions are very good. Tolerances range from +-0.001 in. to +-0.0006 in. for the die major diameter and pitch. The surface finishes range from 6 to 32 micro-inches.
Tolerance and surface finish depend upon the following:
- Dimensional accuracy of the blank
- Tolerances, surface finish, and wear of the dies
- Workpiece material
- Machine rigidity
- Thread form and size
To increase productivity, the blank diameter should equal the pitch diameter. Ends of the blank should be beveled approximately 60 degrees. When considering the blank diameter tolerance, a change in blank diameter will affect the major diameter by an approximate ratio of 3 to 1.
Tool Style
| DESCRIPTION | STYLE | APPLICATION |
|---|
| FLAT DIES | RECTANGULAR | MACHINE, TAPING AND WOOD-SCREW | | CYLINDRICAL IN-FEED 2 DIES | CYLINDRICAL | LARGE OR BALANCED SCREWS | | CYLINDRICAL IN-FEED 3 DIES | CYLINDRICAL | TUBE FITTING, SPARK PLUGS | | PLANETARY DIES | PLANETARY | HIGH VOLUME MACHINE, SHEET METAL, DRIVE SCREWS |
Effects on Work Material Properties
Cold working of the surface material during rolling increases yield stress of the work material. Thread rolling also increases resistance to fatigue failure because the surface layer of the thread is left with smooth burnished roots and flanks and is left stressed in compression, especially in the root section of the threads.
| WORK MATERIAL PROPERTIES |
|---|
MECHANICAL
- Yield and shear yield stress increased
- Fatigue resistance is increased
- Ductility is decreased
| | PHYSICALLittle effect | | CHEMICALLittle effect |
Lubrication and Cooling
The purpose of lubrication is to reduce friction and wear between the dies and workpiece, thereby improving surface finish. Because high temperatures are generated during the drastic mechanical forming of thread rolling, the cutting fluid also acts as a cooling agent. When using soluble oil as cutting fluid in flat rolls the diameter of threads go from 1/4 in. to 1-1/4 in., in cylindrical the diameter goes up to 1/2 in., and in planetary the diameter goes from 1/4 in. to 1-1/8 in. Instead if mineral oil is used, in cylindrical rolls the diameter goes from 1/2 in. to 4-1/2 in.
Production Rates
| THREAD DIAMETER (in.) | FLAT DIES (pieces/min) | CYLINDRICAL(pieces/min) | PLANETARY (pieces/min) |
|---|
| 1/8 | 40 to 500 | 75 to 300 | 450 to 2000 | | 1/4 | 40 to 400 | 60 to 150 | 250 to 1200 | | 1/2 | 25 to 90 | 50 to 100 | 100 to 400 | | 3/4 | 20 to 60 | 5 to 10 | - | | 1 | 15 to 50 | 1 to 50 | - |
Power Requirements
| MATERIAL | BLANK SIZE (in.) | THREADS per inch* | HP required |
|---|
| Brass or aluminum | 1/4
3/8
1/2 | 20
16
13 | 1.75
3.0
4.0 | | Alloy steel | 1/4
3/8
1/2 | 20
16
13 | 3.5
6.0
8.0 |
- For a 3-in. long workpiece with a standard thread feed rate of 0.0005 to 0.002 in./revolution.
Cost Elements
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. Cost elements include the following:
- Setup time
- Thread rolling time
- Positioning time
- Tool change time
- Direct labor rate
- Tool costs
- Overhead rate
- Amortization of equipment and tooling
Thread forming
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, because of the effect of the cold forming action on the metal's grain structure.
This process is more often employed in soft, ductile metals (such as aluminium) than in hard, brittle metals (such as cast iron).
Thread casting
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 mold 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 casting 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
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.
Thread lapping
Rarely, thread grinding will be followed by thread lapping in order to achieve the highest precision and surface finish achievable. This is a toolroom practice when the highest precision is required, rarely employed except for the leadscrews or ballscrews of high-end machine tools.
Examples
Examples of screw threads include:
- Fasteners:
- Wood screws.
- Nuts and bolts, machine screws, cap screws.
- C-clamps.
- Threaded pipe fittings.
- Threaded attachments used to mount equipment on stands, such as:
See also
External links
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