Understanding Screw Sizes: A Comprehensive Guide To Screw Indexing

Screw sizes are standardised using two systems: the Unified Thread Standard (UTS) and the ISO metric screw thread system. The UTS is used in the United States, with measurements in inches, while the ISO system is used in the rest of the world and is measured in millimetres. Both systems contain information on the screw gauge, which refers to the diameter of the outside thread, the smallest of which is #0. The number of threads per inch (TPI) is also important, with fine screws having more TPI than coarse screws. The ISO system, however, uses the pitch diameter (the distance between threads) instead of TPI.

Screw Size Index Characteristics and Values Table

Screw gauge

A screw gauge is a tool used to measure the dimensions of very small items, often up to 0.01mm but sometimes down to 0.001mm. It is particularly useful for measuring spherical or cylindrical objects.

The screw gauge is U-shaped, with a hollow cylinder attached to one side of the frame. Grooves are carved on the inside surface of the cylinder, through which a screw travels. A millimetre scale, or pitch scale, is marked on the cylinder, parallel to the screw's axis. One end of the screw has an arm attached, with a head scale that divides the head of the arm into 100 segments. The other end of the screw has a flat surface, which is stationary and referred to as the 'anvil'. The screw head has a ratchet configuration to prevent the user from applying too much pressure.

To use a screw gauge, the object to be measured is placed between the screw and the anvil. The ratchet counter is rotated clockwise to tighten the screw until it clicks three times. The thimble lock can then be used to secure this position. The measurement can be read from the pitch scale and the head scale.

The pitch of the screw gauge is defined as the distance moved by the screw per revolution. The least count of the screw gauge is the distance moved by the tip of the screw when it is turned through one division of the head scale.

Screw dimensions

When measuring screw dimensions, it is important to use a digital caliper to accurately measure the diameter and length. There are also screw thread size gauges available that can help identify different types of screws, including machine screws, wood screws, and sheet metal screws. These tools can identify the diameter, size, pitch, and length of a screw.

In the United States, screw sizes are typically measured in inches, following the American Standard Machine Screw (ASME/ANSI) sizes listed under the Unified Thread Standard (UTS). The smallest machine screw gauge is #0, and screws with an outside diameter under 1/4" are labelled from #0 to #12. After this, the gauge is specified according to the fractional diameter of the screw.

For the rest of the world, screw sizes are measured in millimetres following the ISO metric screw thread system. The first number after the letter "M" indicates the screw's diameter, the second number is the screw pitch, and the final number is the screw length.

Screw types

There are many different types of screws, each meticulously engineered for use with different materials and surfaces. Using the wrong type of screw for a project could be disastrous.

Wood Screws

Wood screws are perhaps the most commonly recognised type of screw. They have a smooth shank and tapered point that make them ideal for penetrating wood. They are designed with large threads that cut into the wood to provide a strong and secure connection. The most common type is the Phillips drive, which has an X-shaped head.

Machine Screws

Machine screws differ from other screws as they do not drill through materials themselves. Instead, they are used with pre-drilled and tapped holes, which have threads cut into them to accommodate the machine screw. When a machine screw is inserted into a tapped hole and secured with a nut, the resulting tension causes the screw to stretch, forming a strong and stable bond. They are used in situations where precision and strength are required, such as in the assembly of machinery, electronics, appliances, and vehicles.

Sheet Metal Screws

Sheet metal screws are fully threaded and pointed. They are specialised fasteners designed for connecting thin metal sheets or other materials like plastic, wood, and aluminium. They are categorised by their head styles and three numerical values that represent the screw's diameter, length, and thread count. The most common sizes range from 6 to 14.

Drywall Screws

Drywall screws are specifically created to install drywall panels. They have bugle heads that protect against tearing the paper surface and a sharp point for easy penetration. They can be found with either coarse threads suited for wood studs or fine threads made for metal studs.

Self-Drilling Screws

Self-drilling screws, also known as TEK screws, have a modified design that allows them to create their own pilot hole in one simple step, eliminating the need for pre-drilling. They are perfect for any application that requires speed and efficiency. They come in several head styles, including hex, pan, and flat head shapes.

Other Types of Screws

There are many other types of screws, including:

• Euro screws: Designed for use with European-style cabinet hinges and drawer slides.
• Lag screws: Heavy-duty fasteners used for connecting heavy lumber or other materials requiring a strong connection.
• Set screws: Used to secure one object within another, such as a gear on a shaft. They have no head and are tightened with an Allen wrench.
• Grub screws: Similar to set screws but used in applications where space is limited or where a protruding head is undesirable.
• Tamper-resistant screws: Designed to deter unauthorised access with unique head designs that require specialised tools for installation and removal.
• Metal roofing screws: Designed to securely fasten metal panels onto a building's underlying structure, with an incredibly sharp self-piercing point.
• Self-tapping screws: Used to join two pieces of material together, tapping into the material as they are driven in and creating their own mating thread.

Screw pitch

Pitch is commonly confused with lead, which is the linear travel the nut makes per one screw revolution. For a single-start thread, the lead is equal to the pitch. However, for multiple-start screws, the lead is the pitch multiplied by the number of starts.

The pitch of a screw can be determined using a screw pitch gauge, which is a useful tool for mechanics. These gauges consist of a steel case with folding leaves at both ends, with each leaf having teeth corresponding to a specific pitch.

The pitch of a screw is an important specification to consider when choosing the right screw for a project, as it affects the screw's travel distance during rotation. Screws with wide threads will have a higher pitch than those with narrow threads.

Screw length

The nominal length of a screw is the distance from under the head to the tip. The head of the screw is not included in the nominal length. The length of a screw is determined by the shaft size, which is the portion of the screw that is inserted into the material. This rule generally applies to non-countersinking screws such as round, hex, button, truss, round, and pan-head screws.

The method for measuring screw length depends on the type of screw head:

• Screws with flat bottoms (non-countersunk): Measure from the flat bottom of the head to the tip.
• Screws with flat tops (countersunk): Measure from the flat top of the head to the tip.
• Oval head screws: Measure from the widest point of the head to the tip.
• Self-drilling screws: Measure to the end of the drill point tip.

The McFeelys 2/3 Rule for Screw Length suggests that about 2/3 of the screw should be threaded into the "secondary" piece. However, there is no definitive rule for screw length, and the optimal length will depend on the construction details.

Frequently asked questions