American standards organizations increasingly seek to find more common ground between the imperial and metric systems of measurement. These differences between units of measure have created two separate sets of hardware dimensions that possess different bolt designation systems. For this reason, the American National Standards Institute (ANSI) is working with the American Society of Mechanical Engineers (ASME) to develop unit measurement standards that ensure consistency across tooling and machinery.
Metric vs. Standard Bolts
Both metric and standard bolts measure head size according to the distance across the flat parts of the head. For standard bolts, head size is measured in inches or fractions of inches; however, metric bolt head sizes are measured in millimeters, which can cause confusion if engineers attempt to apply metric bolts to standard systems, and vice versa.
Standard Bolts
Standard bolts primarily originate in the United States, and they operate based on the imperial measurement system. This means that both the bolt and the tools used with it are measured in inches or fractions of inches. The two basic types of standard bolts in use today are the Unified National fine thread bolt and the Unified National coarse thread bolt.
For standard bolts, the pitch of the thread is expressed in threads per inch, or TPI. A standard fine thread bolt is only a little finer than a standard coarse thread metric bolt.
Metric Bolts
Metric bolts are designed in accordance with the metric system of measurement. Even when they’re designed in the U.S., fasteners like screws, bolts, and nuts may use the metric system because it’s easier to scale up and down than the imperial system. Additionally, international manufacturers much more frequently use metric bolts, so American manufacturers must design their systems with the metric system in mind if they’re to sell their products abroad.
If a metric bolt is more than 6 mm in diameter, the International Standards Organization (ISO) embosses the bolt’s head with the term “ISO M” or “M” to certify that the bolt complies with ISO standards. Manufacturers also sometimes mark metric bolts with single-digit numbers to indicate their strength. Some manufacturers also emboss the “M” and strength grade on the flats of hexagonal bolts.
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