Screw Sizes Simplified: Sae Specs Table For Quick Reference

SAE screw size specifications are standardised by SAE/ANSI, which are listed under the Unified Thread Standard (UTS). The UTS system uses inches to measure screw sizes, with the screw gauge representing the diameter of the outside thread, or major diameter. The smallest machine screw gauge is #0, and sizes range from #000 to #12 for screws with an outside diameter under ¼. After this, the gauge is specified according to the fractional diameter of the screw. The number of threads per inch (TPI) is also important, with fine screws typically having more threads per inch than coarse screws. For metric measurements, the ISO system is used, which is measured in millimetres.

SAE Screw Size Spec Table

Unified Thread Standard (UTS) vs ISO metric screw systems

The Unified Thread Standard (UTS) and the ISO metric screw thread system are two of the most important thread standards in the world. They are used in mechanical engineering and a wide variety of other applications.

UTS defines a standard thread form and series, along with allowances, tolerances, and designations for screw threads. It is the main standard for bolts, nuts, and other threaded fasteners in the United States and Canada. The standard is currently controlled by the American Society of Mechanical Engineers (ASME) and the American National Standards Institute (ANSI).

ISO metric screw threads, on the other hand, are the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed upon when the International Organization for Standardization (ISO) was set up in 1947.

Both UTS and ISO metric screw threads share the same 60° profile. However, the characteristic dimensions of each UTS thread (outer diameter and pitch) are chosen as an inch fraction rather than a millimeter value as in the ISO system. In UTS, the pitch value is the reciprocal of the threads-per-inch (TPI). For example, if a bolt has 20 TPI, its pitch is 1/20” or 0.05”.

The standard designation for a UTS thread includes a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in TPI. For instance, a #6-32 UNC 2B bolt has a major diameter of 0.1380 inches and a pitch of 32 TPI. In the ISO system, a metric screw thread is designated by the letter M followed by the value of the nominal diameter and the pitch, both expressed in millimetres. For example, an M8-1.25 screw has a nominal diameter of 8 millimetres and a pitch of 1.25 millimetres.

In terms of tolerance, UTS uses three classes: Class 1 for loose-fitting threads suitable for easy assembly or dirty work environments; Class 2 for free-fit threads with greater tolerance, which are the most common and maximize strength; and Class 3 for threads with closer tolerances used in high-quality work. ISO, on the other hand, defines tolerance classes in ISO 965-1, with external threads designated by lowercase letters and internal threads by uppercase letters.

Screw gauge

A screw gauge is a device used to measure the diameter of thin wires, the thickness of small sheets such as glass or plastics, and the size of screws. It is the best instrument for the precise measurement of a spherical or cylindrical object. Screw gauges can be difficult for untrained hands to use, as they require fine-tuning.

Using a Micrometer Screw Gauge

Micrometer screw gauges usually take the form of calipers (opposing ends joined by a frame). The object to be measured is placed between the spindle and the anvil. The spindle is then moved by turning the ratchet knob or thimble until it lightly touches the object. The object should be secured so that it does not move during the measurement process. The spindle can be locked into place, and the measurement is then taken.

Using an Inch Micrometer

The thimble of an inch micrometer has different scales, measuring by one-tenth of an inch (0.100) or one hundred thousand. There are also markings representing twenty-five thousandths of an inch (0.025) and one thousandth of an inch (0.001). The sleeve has whole numbers, and the last number visible represents thousandths. For example, if the last number is 6, it is represented as 0.500. To calculate the final measurement, note the number of lines after the whole number and multiply that by 0.25.

Parameters of a Screw Gauge

There are two parameters used in every screw gauge: pitch and least count. The pitch of a screw gauge is the distance moved by the spindle per revolution. The least count of a screw gauge is the distance moved by the tip of the screw when it is turned through one division of the head scale.

Threads per inch (TPI)

TPI is calculated differently in the metric system, where pitch diameter—the distance between threads on the screw—is used instead.

Equations can be used to calculate dimensions for Unified Inch threads, where dnom is nominal diameter in inches and TPI is threads per inch:

• Dm.ext = dnom − 1.299038/TPI
• Dp.ext = dnom − 0.64951905/TPI
• Dm.int = dnom − 1.08253175/TPI
• Dp.int = dnom − 0.64951905/TPI

Stress area

The stress area of a screw is an important factor in determining its suitability for specific applications. It refers to the weakest point of the bolt, which can vary depending on the type of screw and the measuring system used.

In the Unified Thread Standard (UTS) system, which is used for American screws, the stress area is measured in inches squared. Coarse screws have a higher stress area than fine screws. This is an important consideration when selecting a screw for a particular purpose. For instance, a coarse screw with a high stress area is more suitable for clamping applications than a fine screw of the same gauge.

The UTS system uses the major diameter, or screw gauge, to indicate the diameter of the outside thread. The smallest screw gauge is #0, though gauges #00 and #000 can be found for specific projects. Screws with an outside diameter under 1/4 inch are labelled from #0 to #12, and the gauge is specified according to the fractional diameter of the screw.

The ISO metric screw thread system, used internationally, differs from the UTS system in that it does not calculate the number of threads per inch. Instead, it uses the pitch diameter, which is the distance between threads on the screw. The stress area for metric screws is given in millimetres squared.

Screw length

When specifying screw length, it is essential to maintain precise tolerances to ensure optimal fastening performance. The acceptable tolerance varies depending on the overall length of the screw. For instance, up to a length of 4 inches, the tolerance is ±.005 inches, while for screws between 4 and 6 inches long, the tolerance is ±.008 inches. Screws exceeding 6 inches in length have a tolerance of ±.010 inches.

In addition to length, it is worth noting that the thread pitch, or the number of threads per inch, is another critical factor in screw selection. Fine threads, such as those in UNF (Unified Fine Thread) screws, have a higher thread count per inch compared to coarse threads, like those in UNC (Unified Coarse Thread) screws. The thread pitch influences the stress area of the screw, which is crucial in determining its suitability for specific applications.

When working with international screws, the measurements are expressed in millimetres. The label on the box typically starts with the letter "M", followed by the diameter, pitch, and length of the screw. It is important to convert these measurements to inches if you are using an American screw size chart.

By consulting screw size charts, such as the Unified Thread Standard (UTS) or ISO metric screw thread system, and considering factors like screw length, thread pitch, and stress area, you can make informed decisions about selecting the appropriate fasteners for your specific applications.

Frequently asked questions