ISO metric screw thread
The ISO metric screw thread is a standardized system for general-purpose fastening threads, characterized by a symmetrical V-shaped profile with a 60° included flank angle, metric-based diameters ranging from 1 mm to 300 mm, and varying pitches for coarse and fine series, as defined in ISO standards for bolts, screws, nuts, and related components.[1][2] This thread form ensures interoperability in mechanical assemblies worldwide, with basic dimensions including a thread height of approximately 0.541 × pitch for the design profile and a fundamental thread angle that facilitates manufacturing and strength.[2][3] Developed under the auspices of ISO/TC 1 Screw threads, the system originates from efforts in the mid-20th century to unify metric threading practices, replacing disparate national standards with a cohesive international framework.[4] Key governing documents include ISO 68-1, which outlines the basic and design profiles; ISO 261, specifying general dimensions and thread series; and ISO 262, detailing preferred sizes for diameters from 1 mm to 100 mm in coarse (e.g., M6 × 1.0) and fine pitches.[1] Tolerances and deviations are further addressed in ISO 965 parts, ensuring compatibility between mating threads through classes like 6g for external and 6H for internal threads. Widely adopted in industries such as automotive, aerospace, and construction, ISO metric threads promote efficient production and global supply chain integration due to their precision and load-bearing capabilities, with the 60° angle optimizing shear strength and the truncated V-profile reducing stress concentrations.[5] The designation "M" followed by diameter and pitch (e.g., M10 × 1.5) allows straightforward specification, supporting both unified coarse pitches for general use and fine pitches for applications requiring higher tensile strength or adjustment precision.[5]Introduction and History
Overview
The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide, defined by its reliance on the metric system for dimensions and a symmetrical 60° thread angle that ensures consistent performance in fastening applications.[6][7] This standard provides a unified profile for external and internal threads, promoting reliability across diverse engineering contexts.[8] Key advantages of ISO metric screw threads lie in their exceptional interchangeability, allowing parts from various manufacturers to mate precisely without custom adaptations, alongside simplified manufacturing processes enabled by metric units that align with global tooling and measurement practices.[9][8] These attributes have driven their widespread adoption in critical sectors including machinery, construction, and the automotive industry, where efficient assembly and maintenance are paramount.[6] Originating from 19th-century European proposals for metric-based thread standardization, such as those discussed at the 1898 International Congress for the Standardization of Screw Threads, the system achieved international formalization through the International Organization for Standardization (ISO) in 1947, in the postwar era to support unified global engineering practices.[10][11] As of 2025, ISO metric screw threads account for the majority of non-imperial threaded fasteners globally, with specifications dominating the international manufacturing market for such components.[12][13]Development and Standardization
The origins of ISO metric screw threads trace back to 19th-century efforts in France to develop the metric system, which provided a unified framework for measurements essential to engineering and manufacturing. By the late 19th century, European nations including France, Germany, and Switzerland had developed national metric thread forms, but variations in profiles and pitches hindered interoperability. In 1898, the International Congress for the Standardization of Screw Threads achieved initial unification of metric threads, establishing a 60-degree flank angle as a common basis that influenced subsequent standards.[10] Early adoption in Germany occurred through the Deutsches Institut für Normung (DIN), founded in 1917, which promulgated DIN 13 as one of its initial standards for metric screw threads around 1919, promoting widespread use in industrial applications. These national efforts set the stage for broader international coordination, particularly as global trade expanded in the interwar period. However, disparate systems persisted until post-World War II reconstruction emphasized standardization to facilitate economic recovery and technical exchange.[14] In 1947, the International Organization for Standardization (ISO) established Technical Committee 1 (ISO/TC 1) specifically for screw threads, marking a pivotal milestone in global unification. One of the committee's earliest achievements was ISO/R 68 in 1958, which defined the basic profile for metric screw threads with a symmetrical triangular form and flat crests and roots.[4][15] This standard provided a foundational geometry that ensured compatibility across borders.[16] The 1960s saw significant harmonization in Europe, driven by ISO recommendations that merged elements from national systems, including influences from the British Whitworth thread's rounded roots for improved strength. By the mid-1960s, ISO had agreed on general-purpose metric threads, prompting countries like the United Kingdom to begin replacing numerous imperial sizes—such as 74 Whitworth variants—with a streamlined set of 13 ISO metric sizes by the 1970s. This transition enhanced manufacturing efficiency and supported the European Economic Community's integration goals.[17] Further refinement came with ISO 261 in 1973, which specified basic dimensions and preferred diameter-pitch combinations for general-purpose metric threads from 1 mm to 300 mm, building directly on ISO/R 68. In the 1990s, ISO 965 series standards introduced more precise tolerance classes, enabling finer control over thread fits for high-precision applications while maintaining backward compatibility. ISO 1502, published in 1996, standardized gauges and gauging methods to verify compliance with these profiles and dimensions.[18][19] Ongoing evolution reflects adaptations to modern needs, with amendments to core standards like ISO 965-1 in 2021 updating principles for tolerances to support advanced manufacturing processes, and further 2025 publications of ISO 965-4, 965-5, and 965-6 providing updated limits for specialized applications such as hot-dip galvanized threads.[20][21] These developments have solidified ISO metric threads as the dominant global system, adopted by over 160 countries for their reliability and interchangeability.Thread Geometry and Profile
Basic Profile
The basic profile of the ISO metric screw thread forms a symmetric V-shape in the axial plane, characterized by flanks inclined at a 60° included angle to ensure uniform load distribution and compatibility between mating components.[2] This geometry derives from an equilateral triangle with height H = \frac{\sqrt{3}}{2} p \approx 0.866 p, where p denotes the thread pitch, providing the foundational template for both external and internal threads.[3] Unlike asymmetric profiles such as the British Standard Whitworth thread, the ISO metric design features identical flank angles on both sides of the V for male and female threads, which facilitates straightforward tapping, assembly, and interchangeability without specialized adjustments.[3] The basic profile, as defined in ISO 68-1, truncates the sharp-V by 1/8 H at the crest and 1/4 H at the root, resulting in a basic thread height of 5/8 H ≈ 0.541 p for both external and internal threads. In the design profile, the external thread incorporates a flat crest and a rounded root with a radius of 0.1443p to mitigate stress concentrations and enhance fatigue resistance, while the internal thread features a flat root and a rounded crest with a radius of 0.0381p for similar durability benefits.[22] These modifications from the basic truncated profile maintain the symmetric 60° flank angle while accommodating practical manufacturing and performance needs. The minimum root radius for external threads is 0.125 p as per the 2023 revision of ISO 68-1.[3] Standard ISO drawings depict the basic profile in a longitudinal section, illustrating the parallel thread axes, symmetric V flanks converging at 30° to the perpendicular, and the specified root and crest treatments as flat segments (in basic) or rounded (in design) proportional to the pitch, emphasizing the thread's uniformity across sizes.[3] This representation underscores the profile's role as the geometric core for deriving all metric thread dimensions.[1]Dimensions and Calculations
The dimensions of ISO metric screw threads are derived from the basic 60° V-shaped profile defined in ISO 68-1, with key parameters scaled by the nominal major diameter D (equal to the nominal size in millimeters) and the pitch p (selected from ISO 262 standard tables for preferred sizes). The pitch p represents the distance between adjacent threads, and all other dimensions are calculated relative to these to ensure compatibility between external and internal threads. These basic dimensions provide the theoretical reference for design and manufacturing, before applying tolerances. The pitch diameter d2, which is the diameter of an imaginary cylinder where the width of the thread ridge and groove are equal (each P/2), is calculated for external threads as: d_2 = D - 0.6495 p This coefficient 0.6495 arises from trigonometric derivation based on the 60° thread angle. The fundamental triangle height is H = \frac{\sqrt{3}}{2} p \approx 0.8660 p, representing the height of the sharp V profile. The basic profile truncates the crest by 1/8 H (approximately 0.1083 p) and the root by 1/4 H (approximately 0.2165 p), resulting in a basic thread height of 5/8 H ≈ 0.5413 p for external threads. The pitch line lies such that the radial distance from the major diameter D to the pitch diameter is 3/8 H ≈ 0.3248 p per side, or 0.6495 p total deduction across the diameter.[2][3] The minor diameter d1 for external threads, the smallest diameter at the root of the thread in the basic profile, is given by: d_1 = D - 1.0825 p This deduction reflects the full basic thread height from the major diameter to the root, derived as 2 × (5/8 H) = (5/4) H ≈ 1.0825 p. The single flank height is 5/8 H ≈ 0.5413 p. In the design profile, rounding at the root adjusts the effective minor diameter, with the core (d3) being smaller, but the basic d1 uses the flat root reference.[2][3][23] To prevent sharp edges that could cause stress concentrations or manufacturing issues, the basic profile includes allowances for root and crest clearances through truncation. In the design profile, the root radius r for external threads is 0.1443 p (full) or minimum 0.125 p, blending the flanks to a circular arc at the root to provide clearance while maintaining strength. The crest is truncated with a flat width of p/8 ≈ 0.125 p, removing 1/8 H from the sharp crest, calculated as \Delta h = \frac{p/8}{2 \tan 30^\circ} = \frac{p}{16 \sqrt{3}} \approx 0.1083 p. These features ensure the actual thread height is the basic 0.5413 p (single side), preventing interference in mating threads. For internal threads, the root (major diameter) has a larger truncation of p/4 to accommodate the external root radius. As an example, consider an M10×1.5 external thread, where D = 10 mm and p = 1.5 mm (from ISO 262 coarse series). To compute the basic pitch diameter d2 using trigonometric derivation:- Calculate the fundamental height H = \frac{\sqrt{3}}{2} \times 1.5 \approx 0.8660 \times 1.5 = 1.2990 mm.
- The crest truncation height is \frac{1}{8} H \approx 0.1624 mm, so the distance from basic major D to pitch line is \frac{3}{8} H = 0.375 \times 1.2990 \approx 0.4871 mm per side.
- The total deduction across the diameter is $2 \times 0.4871 = 0.9742 mm (equivalent to 0.6495 p).
- Thus, basic d2 = 10 - 0.9742 ≈ 9.026 mm.
Designation and Size Selection
Thread Designation
The designation system for ISO metric screw threads provides a concise and unambiguous method to specify thread characteristics, as outlined in ISO 261, which specifies the general plan and basic dimensions, including designation rules. The standard format begins with the capital letter "M" to denote a metric thread, followed immediately by the nominal major diameter in millimeters (expressed as a whole number or with decimals if necessary), and then "x" followed by the pitch in millimeters. For instance, a thread with a 10 mm diameter and 1.5 mm pitch is designated M10×1.5. This system ensures compatibility across manufacturing and engineering applications by standardizing the sequence of elements. When the pitch corresponds to the preferred coarse series defined in ISO 261, it may be omitted from the designation to simplify notation, resulting in formats like M10 for a 10 mm diameter coarse thread (which has a standard pitch of 1.5 mm). Fine pitches, however, must always be explicitly stated after the "x" to distinguish them from the coarse default, such as M10×1.25 for a finer variant. Left-hand threads, which rotate in the opposite direction to standard right-hand threads, are indicated by appending "LH" after the pitch (or after the diameter if pitch is omitted), yielding examples like M10×1.5 LH or M10 LH. Tolerance classes, which define allowable deviations for fit and function, are added as a suffix separated by a hyphen, using symbols like 6g for medium tolerance on external threads or 6H for internal threads; a full example is M10×1.5-6g. Preferred pitches and detailed tolerance suffixes are specified in related standards such as ISO 262 and ISO 965-2.[9] For complete screw specifications, the effective thread length or overall screw length can be appended at the end when relevant, typically for fasteners, as in M10×1.5×50 to indicate a 50 mm long screw with fine pitch, per standards like ISO 1502. In technical drawings and documentation, abbreviations follow these rules strictly, often without spaces around the "x" for compactness (e.g., M8x1.25). Common errors include confusing metric designations with imperial equivalents, such as interpreting M6 as compatible with a 1/4-inch UNC thread, which it is not due to differing profiles and dimensions, potentially leading to assembly failures. Another frequent mistake is omitting the pitch for fine threads or neglecting the "LH" indicator, which can result in incorrect handedness during manufacturing or installation.[26][2]Preferred and Standard Sizes
The ISO metric screw thread system defines preferred sizes through two primary series: the coarse pitch series per ISO 261 and the fine pitch series per ISO 262. These series specify combinations of nominal diameters and pitches optimized for manufacturing efficiency, mechanical strength, and interchangeability in general-purpose applications. While ISO 261 defines the general series up to 300 mm diameter, ISO 262 selects preferred coarse and fine sizes up to 100 mm for bolts, screws, and nuts.[27] The coarse pitch series, outlined in ISO 261, covers nominal diameters from M1.6 to M68, with pitches selected to maximize thread engagement and core strength while facilitating rapid assembly. These pitches follow a progression based on the Renard R10 series for diameters, ensuring coarser threads for larger sizes to balance material removal in tapping and robustness against shear forces. For instance, common combinations include M6×1, M10×1.5, and M12×1.75, where the pitch provides sufficient holding power for structural fasteners without excessive thread length. This series is recommended for general structural and machinery uses due to its ease of production and resistance to cross-threading.[27][5]| Nominal Diameter (mm) | Coarse Pitch (mm) |
|---|---|
| M1.6 | 0.35 |
| M2 | 0.4 |
| M3 | 0.5 |
| M4 | 0.7 |
| M5 | 0.8 |
| M6 | 1 |
| M8 | 1.25 |
| M10 | 1.5 |
| M12 | 1.75 |
| M16 | 2 |
| M20 | 2.5 |
| M24 | 3 |
| M30 | 3.5 |
| M36 | 4 |
| M42 | 4.5 |
| M48 | 5.5 |
| M56 | 5.5 |
| M64 | 6 |
| M68 | 6 |
| Nominal Diameter (mm) | Fine Pitches (mm, selected) |
|---|---|
| M3 | 0.35, 0.5 |
| M4 | 0.5, 0.7 |
| M5 | 0.5, 0.8 |
| M6 | 0.75, 1 |
| M8 | 1, 1.25 |
| M10 | 0.75, 1, 1.25, 1.5 |
| M12 | 1, 1.25, 1.5, 1.75 |
| M16 | 1.5, 2 |
| M20 | 1.5, 2, 2.5 |
| M24 | 2, 3 |
| M30 | 2, 3, 3.5 |
| M36 | 3, 4 |
| M42 | 4, 4.5 |
| M48 | 4, 5.5 |
Obsolete and Non-Preferred Sizes
Pre-ISO DIN sizes, as defined in standards such as DIN 13 from its initial publication in 1919 through various revisions in the 1970s, included nominal diameters and pitch combinations that deviated from the unified ISO system adopted later. For instance, smaller thread sizes like M1.1, M2.2, and M4.5 were specified in earlier versions of DIN 13 but were deleted upon harmonization with ISO standards to streamline manufacturing and improve global interchangeability. These legacy sizes, often featuring fine pitches differing from ISO coarse series (e.g., finer options for diameters under 6 mm), are now obsolete and primarily encountered in maintenance of pre-1970s equipment from German-speaking regions. The primary reason for their obsolescence was the lack of compatibility with the international ISO 261 and ISO 262 frameworks, which prioritized standardized pitches to reduce variability in production.[31][32] Non-preferred sizes include extra-fine or special pitches outside the selected combinations in ISO 262, such as M20x1.0 or M20x1.25, which are permitted under the broader ISO 261 but not recommended for new designs due to limited availability and reduced standardization. These variants, often used in legacy automotive or precision applications, face obsolescence for the same reason as pre-ISO sizes: diminished interchangeability, as they do not align with the preferred coarse (e.g., M20x2.5) or fine (M20x1.5) series that ensure broader compatibility.[33][34] During transitions from the 1950s to 1980s, obsolete sizes were typically matched to the nearest ISO equivalents for retrofitting, such as substituting old DIN fine pitches (e.g., certain pre-ISO M5 variants) with ISO M5x0.5 fine or M5x0.8 coarse, depending on application requirements. Conversion tables from DIN to ISO often recommend selecting the closest pitch and diameter in ISO 262 to maintain functional integrity while avoiding custom manufacturing; these practices facilitated the shift to unified standards without complete redesigns in existing systems.[35][36]Tolerances and Fits
Tolerance Classes
The tolerance classes for ISO metric screw threads are specified in the ISO 965 series of standards, which define a systematic approach to controlling deviations in thread dimensions such as major diameter, pitch diameter, and minor diameter to achieve desired fits between mating components. This system ensures interchangeability and functionality by categorizing tolerances based on precision requirements, with classes expressed as a combination of a numerical grade and a letter indicating the position of the tolerance zone relative to the basic thread dimensions.[37] Tolerance grades range from 3 to 9, where a lower number denotes a tighter tolerance band, suitable for applications demanding higher accuracy, such as precision machinery; grades 3 and 4 are typically used for fine pitches and close fits, while higher grades like 8 or 9 accommodate coarser manufacturing processes.[38] Fundamental deviations, represented by letters, establish the position of the tolerance zone: for external threads, lowercase letters like g (small clearance fit) and h (zero or minimal deviation at pitch diameter) apply negative or zero offsets from the basic size; for internal threads, uppercase letters such as H (zero fundamental deviation) and G (positive allowance for clearance) position the zone at or above the basic size. Common tolerance classes include 6g/6H for medium fits in general-purpose applications, providing balanced clearance for reliable assembly without excessive play, and 4h/6H for closer fits in mechanisms requiring greater positional accuracy.[39] The limits of size are derived by applying the fundamental deviation to the basic dimension and adding the tolerance value from the selected grade; specifically, for external threads, the upper deviation ES equals the fundamental deviation (negative for clearance positions), and the lower deviation es = ES - ITgrade, while for internal threads, the lower deviation ei = fundamental deviation - ITgrade, and upper EI = fundamental deviation (zero for H).[40] As an example, the pitch diameter tolerance Td2 for grade 6 on an M10 thread (nominal diameter 10 mm) is 118 μm, establishing the allowable variation in the critical mating dimension.[41] ISO 965-1 includes tables detailing IT values for grades 4 through 8 across nominal size ranges (e.g., 1–6 mm, 6–30 mm) and pitches, enabling calculation of tolerance zones for major (Td), pitch (Td2), and minor (Td3) diameters.[42] These classes influence fit characteristics: looser specifications like 8g facilitate easy assembly with generous clearance, ideal for non-critical connections, whereas tighter ones such as 3H minimize backlash for high-precision uses like instrumentation, ensuring robust performance under load.[6]Manufacturing Considerations
The primary methods for manufacturing ISO metric screw threads include thread rolling and cutting. Thread rolling is a cold-forming process that displaces material to form threads without removal, commonly used for high-strength fasteners in mass production due to its ability to enhance surface hardness and fatigue resistance.[43] In contrast, cutting via taps, dies, or lathes removes material to create threads and is preferred for prototypes or low-volume production where custom forms are needed.[44] For achieving finer tolerance classes, such as product grade A under ISO 4759, grinding is employed to refine thread profiles after initial forming, ensuring precise dimensions for high-accuracy applications. Hobbing, involving a rotating cutter to generate threads, is utilized specifically for components like gears that incorporate ISO metric threads, allowing efficient production of helical forms while maintaining alignment.[45] Material selection significantly influences manufacturing processes and thread fit for ISO metric screws. In steel, such as grade 8.8 bolts, heat treatment like quenching and tempering alters microstructure to meet strength requirements, potentially causing minor dimensional changes (up to 0.1-0.2% contraction) that must be accounted for to preserve thread tolerances. Aluminum alloys, being softer and more ductile than steel, facilitate easier thread rolling but require adjusted parameters to avoid galling, with heat treatments like T6 tempering improving strength without the distortion risks seen in steel.[46] Quality control in ISO metric thread production addresses issues like burr formation and plating effects to ensure compliance with standards such as ISO 4759. Burrs, arising from cutting operations, are removed through methods like vibratory finishing or brushing to prevent assembly interference and maintain surface integrity.[47] Zinc plating, applied for corrosion resistance, adds a layer thickness of typically 5-15 μm, increasing the effective thread diameter by approximately four times the coating thickness (e.g., 20-60 μm total buildup), necessitating pre-plating undersizing to achieve post-plating fits within specified tolerances.[48] ISO 4759 provides guidelines for product grades A, B, and C, defining tolerance selections that guide these manufacturing adjustments for bolts, screws, and nuts.[49]Tools and Compatibility
Wrench and Tool Sizes
The hexagon heads of ISO metric bolts, as specified in ISO 4014 for product grades A and B, utilize widths across flats defined in ISO 272 to ensure compatibility with standard wrenches and sockets. These dimensions provide a consistent ratio between the bearing area under the head and the tensile stress area, facilitating safe torque application without excessive stress concentration. For coarse threads, the across-flats width (s) is nominally approximately 1.5 times the nominal diameter (D), though exact values are standardized to avoid rounding issues; for example, an M10 bolt requires a 17 mm wrench.[50] Socket and spanner sizes align directly with these across-flats dimensions, as outlined in ISO 272 for hexagon products and ISO 4032 for related nut specifications, covering nominal diameters from M1.6 to M64 with minimal adjustments for fine threads.[51] The table below summarizes representative across-flats widths (s in mm) for common coarse thread sizes, applicable to both open-end wrenches and socket tools.| Nominal Diameter (mm) | Across-Flats Width s (mm) |
|---|---|
| M3 | 5.5 |
| M4 | 7 |
| M5 | 8 |
| M6 | 10 |
| M8 | 13 |
| M10 | 17 |
| M12 | 19 |
| M16 | 24 |
| M20 | 30 |
| M24 | 36 |
| M30 | 46 |
| M36 | 55 |
| M42 | 65 |
| M48 | 75 |
| M56 | 85 |
| M64 | 95 |