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67-615: The Unified Thread Standard ( UTS ) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada . It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread , but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than

134-473: A millimeter value. The UTS is currently controlled by ASME / ANSI in the United States. Each thread in the series is characterized by its major diameter D maj and its pitch, P . UTS threads consist of a symmetric V-shaped thread. In any plane containing the thread axis, the flanks of the V have an angle of 60° to each other. The outermost 1 ⁄ 8 and the innermost 1 ⁄ 4 of

201-538: 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, 1 ⁄ 2 -13 belongs to the UNC series (Unified National Coarse) and 1 ⁄ 2 -20 belongs to the UNF series (Unified National Fine). Similarly, M10 (10 mm nominal outer diameter) as per ISO 261 has

268-579: A #10 screw is 10 × 0.013 in + 0.060 in = 0.190 in. To calculate the major diameter of "aught" size screws count the number of extra zeroes and multiply this number by 0.013 in and subtract from 0.060 in. For example, the major diameter of a #0000 screw is 0.060 in − (3 × 0.013 in) = 0.060 in − 0.039 in = 0.021 in. The number series of machine screws has been extended downward to include #00-90 (0.047 in = 0.060 in − 0.013 in) and #000-120 (0.034 in = 0.060 in − 2 × 0.013 in) screws; however,

335-404: A UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch . For diameters smaller than ⁠ 1 / 4 ⁠ inch, the diameter is indicated by an integer number defined in the standard; for all other diameters, the inch figure is given. This number pair is optionally followed by the letters UNC, UNF or UNEF (Unified) if

402-408: A bit more, yielding thread depths of 60% to 75% of the 0.65 p value. For example, a 75% thread sacrifices only a small amount of strength in exchange for a significant reduction in the force required to cut the thread. The result is that tap and die wear is reduced, the likelihood of breakage is lessened and higher cutting speeds can often be employed. This additional truncation is achieved by using

469-409: A bolt and a nut of the same pitch would fit together: the same requirement must separately be made for the minor and pitch diameters of the threads. Besides providing for a clearance between the crest of the bolt threads and the root of the nut threads, one must also ensure that the clearances are not so excessive as to cause the fasteners to fail. The minor diameter is the lower extreme diameter of

536-594: A bolt), the major diameter D maj and the minor diameter D min define maximum dimensions of the thread. This means that the external thread must end flat at D maj , but can be rounded out below the minor diameter D min . Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at D min but may be rounded out beyond D maj . These provisions are to prevent any interferences. The minor diameter D min and effective pitch diameter D p are derived from

603-422: A coarse thread version at 1.5 mm pitch and a fine thread version at 1.25 mm pitch. The term coarse here does not mean lower quality, nor does the term fine imply 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

670-479: A cross-sectional view is taken in a plane containing the axis of the threads. For a screw, this is its outside diameter (OD). The major diameter of a nut cannot be directly measured (as it is obstructed by the threads themselves) but it may be tested with go/no-go gauges. The major diameter of external threads is normally smaller than the major diameter of the internal threads, if the threads are designed to fit together. But this requirement alone does not guarantee that

737-453: A direct result of the basic trigonometric functions . It is independent of measurement units (inch vs mm). However, UTS and ISO threads are not sharp threads. The major and minor diameters delimit truncations on either side of the sharp V. The nominal diameter of Metric (e.g. M8) and Unified (e.g. 5 ⁄ 16  in) threads is the theoretical major diameter of the male thread, which is truncated (diametrically) by 0.866 ⁄ 4 of

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804-478: 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 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

871-510: A liquid or paste pipe sealant such as pipe dope . The screw thread concept seems to have occurred first to Archimedes , who briefly wrote on spirals as well as designed several simple devices applying the screw principle. Leonardo da Vinci understood the screw principle, and left drawings showing how threads could be cut by machine. In the 1500s, screws appeared in German watches, and were used to fasten suits of armor. In 1569, Besson invented

938-469: A looser fit than say an H2 tap. Metric uses D or DU limits which is the same system as imperial, but uses D or DU designators for over and undersized respectively, and goes by units of 0.013 mm (0.51 mils). Generally taps come in the range of H1 to H5 and rarely L1. The pitch diameter of a thread is measured where the radial cross section of a single thread equals half the pitch, for example: 16 pitch thread = 1 ⁄ 16   in = 0.0625   in

1005-415: A particular thread, internal or external, is the diameter of a cylindrical surface, axially concentric to the thread, which intersects the thread flanks at equidistant points. When viewed in a cross-sectional plane containing the axis of the thread, the distance between these points being exactly one half the pitch distance. Equivalently, a line running parallel to the axis and a distance D 2 away from it,

1072-471: A perfectly sharp point, and truncation is desirable anyway, because otherwise: In ball screws , the male-female pairs have bearing balls in between. Roller screws use conventional thread forms and threaded rollers instead of balls. The included 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. The cross section to measure this angle lies on

1139-414: A plane which includes the axis of the cylinder or cone on which the thread is produced. Lead ( / ˈ l iː d / ) and pitch are closely related concepts. They can be confused because they are the same for most screws. Lead is the distance along the screw's axis that is covered by one complete rotation of the screw thread (360°). Pitch is the distance from the crest of one thread to the next one at

1206-527: A practical commodity. During the next 40 years, standardization continued to occur on the intra- and inter-company levels. No doubt many mechanics of the era participated in this zeitgeist; Joseph Clement was one of those whom history has noted. In 1841, Joseph Whitworth created a design that, through its adoption by many British railway companies, became a standard for the United Kingdom and British Empire called British Standard Whitworth . During

1273-511: A set of threads for watches. In particular applications and certain regions, threads other than the ISO metric screw threads remain commonly used, sometimes because of special application requirements, but mostly for reasons of backward compatibility : The first historically important intra-company standardization of screw threads began with Henry Maudslay around 1800, when the modern screw-cutting lathe made interchangeable V-thread machine screws

1340-449: A similar way that period and frequency are inverses of each other. Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, where fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on

1407-445: A slightly larger tap drill in the case of female threads, or by slightly reducing the diameter of the threaded area of workpiece in the case of male threads, the latter effectively reducing the thread's major diameter . In the case of female threads, tap drill charts typically specify sizes that will produce an approximate 75% thread. A 60% thread may be appropriate in cases where high tensile loading will not be expected. In both cases,

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1474-512: A threaded product and the gauge(s) which shall be used when inspecting those characteristics. Currently this gauging for UTS is controlled by: These standards provide essential specifications and dimensions for the gauges used on Unified inch screw threads (UN, UNR, UNJ thread form) on externally and internally threaded products. It also covers the specifications and dimensions for the thread gauges and measuring equipment. The basic purpose and use of each gauge are also described. It also establishes

1541-449: Is a standard used for classifying the tolerance of the thread pitch diameter for taps . For imperial, H or L limits are used which designate how many units of 0.0005 inch over or undersized the pitch diameter is from its basic value, respectively. Thus a tap designated with an H limit of 3, denoted H3 , would have a pitch diameter 0.0005 × 3 = 0.0015 inch larger than base pitch diameter and would thus result in cutting an internal thread with

1608-446: Is by way of a go/no-go gauge . Screw thread The mechanical advantage of a screw thread depends on its lead , which is the linear distance the screw travels in one revolution. In most applications, the lead 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, as long as no external rotational force

1675-408: Is known as handedness . Most threads are oriented so that the threaded item, when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a clockwise direction, and moves towards the viewer when it is turned counterclockwise. This is known as a right-handed ( RH ) thread, because it follows the right-hand grip rule . Threads oriented in

1742-541: Is less common to see machine screws larger than #14, or odd number sizes other than #1, #3 and #5. Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12. Sometimes "special" diameter and pitch combinations (UNS) are used, for example a 0.619 in (15.7 mm) major diameter with 20 threads per inch. UNS threads are rarely used for bolts, but rather on nuts, tapped holes, and threaded ODs. Because of this UNS taps are readily available. Most UNS threads have more threads per inch than

1809-439: Is less than caliper measurement of the male major diameter (outside diameter, OD). For example, tables of caliper measurements show 0.69 female ID and 0.75 male OD for the standards of "3/4 SAE J512" threads and "3/4-14 UNF JIS SAE-J514 ISO 8434-2". Note the female threads are identified by the corresponding male major diameter (3/4 inch), not by the actual measurement of the female threads. The pitch diameter (PD, or D 2 ) of

1876-475: Is present. This characteristic is essential to the vast majority of its uses. 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 elastic deformation . Screw threads have several applications: In all of these applications, the screw thread has two main functions: Every matched pair of threads, external and internal , can be described as male and female . Generally speaking,

1943-454: Is provided by a class, but the maximum PD of the external thread is specified to be the same as the minimum PD of the internal thread, within specified tolerances, ensuring that the two can be assembled, with some looseness of fit still possible due to the margin of tolerance. A class called interference fit may even provide for negative allowances, where the PD of the screw is greater than the PD of

2010-419: Is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a 1 ⁄ 4 -20 thread has 20 TPI, which means that its pitch is 1 ⁄ 20 inch (0.050 in or 1.27 mm). As the distance from the crest of one thread to the next, pitch can be compared to the wavelength of a wave . Another wave analogy is that pitch and TPI are inverses of each other in

2077-661: The ISO metric screw threads (M) for most purposes, and BSP threads (R, G) for pipes. These were standardized by the International Organization for Standardization (ISO) in 1947. Although metric threads 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

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2144-402: The pitch diameter is not affected. The balancing of truncation versus thread strength is similar to many engineering decisions involving the strength, weight and cost of material, as well as the cost to machine it. Tapered threads are used on fasteners and pipe. A common example of a fastener with a tapered thread is a wood screw . The threaded pipes used in some plumbing installations for

2211-534: The screw-cutting lathe , but the method did not gain traction and screws continued to be made largely by hand for another 150 years. In the 1800s, screw manufacturing began in England during the Industrial Revolution . In these times, there was no such thing as standardization. The bolts made by one manufacturer would not fit the nuts of another. Standardization of screw threads has evolved since

2278-524: The "PD line," slices the sharp-V form of the thread, having flanks coincident with the flanks of the thread under test, at exactly 50% of its height. We have assumed that the flanks have the proper shape, angle, and pitch for the specified thread standard. It is generally unrelated to the major ( D ) and minor ( D 1 ) diameters, especially if the crest and root truncations of the sharp-V form at these diameters are unknown. Everything else being ideal, D 2 , D , & D 1 , together, would fully describe

2345-639: The 1840s through 1860s, this standard was often used in the United States 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 US' poorly standardized screw thread practice. Sellers simplified the Whitworth design by adopting a thread profile of 60° and

2412-412: The amount of the theoretical sharp V which is cut off at the minor diameter by 10% from 0.25 H to ⁠ 7 / 8 ⁠ − ⁠ 0.52 / cos 30° ⁠ ≈ 0.27456 H . The number series of machine screws once included more odd numbers and went up to #16 or more. Standardization efforts in the late 19th and the early part of the 20th century reduced the range of sizes considerably. Now, it

2479-414: The correlating UNF or UNEF standard; therefore they are often the strongest thread available. Because of this they are often used in applications where high stresses are encountered, such as machine tool spindles or automotive spindles . A screw thread gauging system comprises a list of screw thread characteristics that must be inspected to establish the dimensional acceptability of the screw threads on

2546-409: The crest or root), but instead are truncated, yielding a final thread depth that can be expressed as a fraction of the pitch value. 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 0.866 of the pitch. This fact is intrinsic to the geometry of an equilateral triangle —

2613-518: The criteria for screw thread acceptance when a gauging system is used. A classification system exists for ease of manufacture and interchangeability of fabricated threaded items. Most (but certainly not all) threaded items are made to a classification standard called the Unified Screw Thread Standard Series. This system is analogous to the fits used with assembled parts. The letter suffix "A" or "B" denotes whether

2680-404: 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. Another way to express this is that lead and pitch are parametrically related, and the parameter that relates them, the number of starts, very often has a value of 1, in which case their relationship becomes equality. In general, lead is equal to pitch times

2747-447: The delivery of fluids under pressure have a threaded section that is slightly conical . Examples are the NPT and BSP series. The seal provided by a threaded pipe joint is created when a tapered externally threaded end is tightened into an end with internal threads. For most pipe joints, a good seal requires the application of a separate sealant into the joint, such as thread seal tape , or

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2814-414: The diameter-pitch combination is from the coarse , fine , or extra fine series, and may also be followed by a tolerance class. Example: #6-32 UNC 2B (major diameter: 0.1380 inch, pitch: 32 tpi) The following formula is used to calculate the major diameter of a numbered screw greater than or equal to 0: Major diameter = Screw # × 0.013 in + 0.060 in. For example, the major diameter of

2881-667: The early nineteenth century to facilitate compatibility between different manufacturers and users. The standardization process is still ongoing; in particular there are still (otherwise identical) competing metric and inch-sized thread standards widely used. Standard threads are commonly identified by short letter codes (M, UNC, etc.) which also form the prefix of the standardized designations of individual threads. Additional product standards identify preferred thread sizes for screws and nuts, as well as corresponding bolt head and nut sizes, to facilitate compatibility between spanners (wrenches) and other tools. The most common threads in use are

2948-577: The galling of the threads. For this reason, some allowance , or minimum difference, between the PDs of the internal and external threads has to generally be provided for, to eliminate the possibility of deviations from the ideal thread form causing interference and to expedite hand assembly up to the length of engagement. Such allowances, or fundamental deviations , as ISO standards call them, are provided for in various degrees in corresponding classes of fit for ranges of thread sizes. At one extreme, no allowance

3015-454: The height H of the V-shape are cut off from the profile. The major diameter D maj is the diameter of the screw measured from the outer edge of the threads. The minor diameter D min (also known as the root diameter) is the diameter of the screw measured from the inner edge of the threads. The major diameter may be slightly different from the shank diameter, which is the diameter of

3082-420: The main standard for screws smaller than #0 is ANSI/ASME standard B1.10 Unified Miniature Screw Threads. This defines a series of metric screws named after their major diameters in millimetres, from 0.30 UNM to 1.40 UNM. Preferred sizes are 0.3, 0.4, 0.5, 0.6, 0.8, 1.0 and 1.2 mm, with additional defined sizes halfway between. The standard thread pitch is approximately ⁠ 1 / 4 ⁠ of

3149-1035: The major diameter and pitch as: D min = D maj − 2 ⋅ 5 8 ⋅ H = D maj − 5 3 8 ⋅ P ≈ D maj − 1.082532 ⋅ P D p = D maj − 2 ⋅ 3 8 ⋅ H = D maj − 3 3 8 ⋅ P ≈ D maj − 0.649519 ⋅ P . {\displaystyle {\begin{aligned}D_{\text{min}}&=D_{\text{maj}}-2\cdot {\frac {5}{8}}\cdot H=D_{\text{maj}}-{\frac {5{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-1.082532\cdot P\\D_{\text{p}}&=D_{\text{maj}}-2\cdot {\frac {3}{8}}\cdot H=D_{\text{maj}}-{\frac {3{\sqrt {3}}}{8}}\cdot P\approx D_{\text{maj}}-0.649519\cdot P.\end{aligned}}} The standard designation for

3216-413: The major diameter. The thread form is slightly modified to increase the minor diameter, and thus the strength of screws and taps. The major diameter still extends to within ⁠ 1 / 8 ⁠ H of the theoretical sharp V , but the total depth of the thread is reduced 4% from ⁠ 5 / 8 ⁠ H = ⁠ 5 / 8 ⁠  cos(30°)  P ≈ 0.541 P to 0.52 P . This increases

3283-401: The maximum limits for internal ( nut ), thread sizes are there to ensure that threads do not strip at the tensile strength limits for the parent material. The minimum limits for internal, and maximum limits for external, threads are there to ensure that the threads fit together. The major diameter of threads is the larger of two extreme diameters delimiting the height of the thread profile, as

3350-404: The number of starts. Whereas metric threads are usually defined by their pitch, that is, how much distance per thread, inch-based standards usually use the reverse logic, that is, how many threads occur per a given distance. Thus, inch-based threads are defined in terms of threads per inch (TPI). Pitch and TPI describe the same underlying physical property—merely in different terms. When the inch

3417-619: The nut by at least the amount of the allowance. The pitch diameter of external threads is measured by various methods: 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 . Classes include 1, 2, 3 (loose to tight); A (external) and B (internal); and various systems such as H and D limits. Thread limit or pitch diameter limit

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3484-509: The opposing threads, and everything else is ideal, if the pitch diameters of a screw and nut are exactly matched, there should be no play at all between the two as assembled, even in the presence of positive root-crest clearances. This is the case when the flanks of the threads come into intimate contact with one another, before the roots and crests do, if at all. However, this ideal condition would in practice only be approximated and would generally require wrench-assisted assembly, possibly causing

3551-591: 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 thread applications include: 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), but commonly refer to

3618-437: The pitch actual pitch diameter of the thread is measured at the radial cross section measures 0.03125   in. To achieve a predictably successful mating of male and female threads and assured interchangeability between males and between females, standards for form, size, and finish must exist and be followed. Standardization of threads is discussed below. Screw threads are almost never made perfectly sharp (no truncation at

3685-431: The pitch from the dimension over the tips of the "fundamental" (sharp cornered) triangles. The resulting flats on the crests of the male thread are theoretically one eighth of the pitch wide (expressed with the notation 1 ⁄ 8 p or 0.125 p ), although the actual geometry definition has more variables than that. A full (100%) UTS or ISO thread has a height of around 0.65 p . Threads can be (and often are) truncated

3752-978: The pitch parameter; instead a parameter known as threads per inch (TPI) is used, which is the reciprocal of the pitch. The relationship between the height H and the pitch P is found using the following equation where θ {\displaystyle \theta } is half the included angle of the thread, in this case 30 degrees: H = 1 2 tan ⁡ θ ⋅ P = 3 2 ⋅ P ≈ 0.866025 ⋅ P {\displaystyle H={\frac {1}{2\tan \theta }}\cdot P={\frac {\sqrt {3}}{2}}\cdot P\approx 0.866025\cdot P} or P = 2 tan ⁡ θ ⋅ H = 2 3 ⋅ H ≈ 1.154701 ⋅ H . {\displaystyle P=2\tan \theta \cdot H={\frac {2}{\sqrt {3}}}\cdot H\approx 1.154701\cdot H.} In an external (male) thread (e.g., on

3819-402: The same point. 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

3886-790: The screw diameter. Coarse threads are more resistant to stripping and cross threading because they have greater flank engagement. Coarse threads install much faster as they require fewer turns per unit length. Finer threads are stronger as they have a larger stress area for the same diameter thread. Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment. Finer threads develop greater preload with less tightening torque. There are three characteristic diameters ( ⌀ ) of threads: major diameter , minor diameter , and pitch diameter : Industry standards specify minimum (min.) and maximum (max.) limits for each of these, for all recognized thread sizes. The minimum limits for external (or bolt , in ISO terminology), and

3953-400: The standardized geometry used by the screw. Major categories of threads include machine threads, material threads, and power threads. 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 ,

4020-399: The thread form. Knowledge of PD determines the position of the sharp-V thread form, the sides of which coincide with the straight sides of the thread flanks: e.g., the crest of the external thread would truncate these sides a radial displacement D  −  D 2 away from the position of the PD line. Provided that there are moderate non-negative clearances between the root and crest of

4087-615: The thread. Major diameter minus minor diameter, divided by two, equals the height of the thread. The minor diameter of a nut is its inside diameter. The minor diameter of a bolt can be measured with go/no-go gauges or, directly, with an optical comparator . As shown in the figure at right, threads of equal pitch and angle that have matching minor diameters, with differing major and pitch diameters, may appear to fit snugly, but only do so radially; threads that have only major diameters matching (not shown) could also be visualized as not allowing radial movement. The reduced material condition , due to

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4154-407: The threads are external or internal, respectively. Classes 1A, 2A, 3A apply to external threads; Classes 1B, 2B, 3B apply to internal threads. Thread class refers to the acceptable range of pitch diameter for any given thread. The pitch diameter is indicated as Dp in the figure shown above. There are several methods that are used to measure the pitch diameter. The most common method used in production

4221-416: The threads on an external surface are considered male, while the ones on an internal surface are considered 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 . Assembling a male-threaded fastener to a female-threaded one is called mating . The helix of a thread can twist in two possible directions, which

4288-400: 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

4355-443: The unthreaded part of the screw. The diameters are sometimes given approximately in fractions of an inch (e.g. the major diameter of a #6 screw is 0.1380 in, approximately 9 ⁄ 64 in = 0.140625 in ). The pitch P is the distance between thread peaks. For UTS threads, which are single-start threads, it is equal to the lead , the axial distance that the screw advances during a 360° rotation. UTS threads do not usually use

4422-406: The unused spaces between the threads, must be minimized so as not to overly weaken the fasteners. In order to fit a male thread into the corresponding female thread, the female major and minor diameters must be slightly larger than the male major and minor diameters. However this excess does not usually appear in tables of sizes. Calipers measure the female minor diameter (inside diameter, ID), which

4489-625: Was also adopted as a standard by highly influential railroad industry corporations such as the Baldwin Locomotive Works and the Pennsylvania Railroad . Other firms adopted it, and it soon became a national standard for the U.S., later becoming generally known as the United States Standard thread (USS thread). Over the next 30 years the standard was further defined and extended and evolved into

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