Inch-Sized Screws: Numbered Sizes For Precision Fit

Understanding screw sizes can be confusing, especially when it comes to the difference between numbered screw sizes and inch-based measurements. Numbered screw sizes, such as #6, #8, etc., refer to the diameter of the screw, with larger numbers indicating larger diameters. This system is commonly used for wood screws, which are designed for connecting items to wooden bases. On the other hand, inch-based measurements are used in the imperial system, which is followed in the United States. The imperial system uses inches to specify the screw gauge, which is the diameter of the outside thread. For example, a #0 screw has an outside diameter of 0.06 inches, while a #12 screw has an outside diameter of 0.216 inches. Converting between numbered screw sizes and inch-based measurements can be challenging, and it's important to use the appropriate screw size to prevent damage to the material being fastened.

Understanding screw sizes: metric vs imperial

Screw sizes can be confusing, especially when you're trying to select the right one for a project. There are two main systems for screw sizing: metric and imperial. The metric system is used in most parts of the world, while the imperial system is used in only three countries: the United States, Canada, and the United Kingdom.

Imperial Screw Sizes

Imperial screw sizes are based on the Unified Thread Standard (UTS) and are measured in inches. A wood screw, for example, is sized by two numbers: the gauge of the screw, which refers to its diameter, and the length. The larger the gauge number, the larger the diameter. So, a number 12 screw has a larger diameter than a number 4 screw. It's important to note that the length given for an imperial screw is the portion that is buried in the material and does not include the head of the screw.

Metric Screw Sizes

Metric screw sizes are a bit simpler to understand. Instead of using a "gauge" figure, the metric system uses the (major) diameter in millimetres. The length is also measured in millimetres, just like in the imperial system. So, a metric screw labelled M6 has a diameter of 6mm, and the length will also be given in mm.

Converting Between Metric and Imperial

When purchasing screws, it's helpful to be able to convert between metric and imperial sizes. For example, M4 x 60 is approximately equivalent to #8 x 2 1/2 in imperial measurements. Most manufacturers provide both metric and imperial sizes on their packaging, but when buying online, some retailers may only provide one or the other. Understanding the conversion between the two systems can ensure you get the right size for your project.

Screw Callouts

In addition to the size, there are other specifications on a screw's packaging that are important to understand. For example, in the US, a screw callout may look like #3-48 UNC-2B-LH x .5. Here's what each part means:

• #3 refers to the gauge size (0.099 inches in diameter)
• 48 is the number of threads per inch
• UNC means the screw is coarse
• 2B specifies the tolerance class
• LH indicates that it is a left-handed screw
• .5 means the screw length is 1/2 inch

Understanding screw sizes and specifications is crucial to ensure you select the right screw for your project. Whether you're using the metric or imperial system, always refer to conversion charts and sizing guides to get the perfect fit.

Converting metric to imperial

Firstly, find the right conversion number. This can be done by referring to a metric-imperial conversion chart. For example, 1 meter = 1.0936 yards.

Secondly, multiply the metric number by the conversion number. For example, to convert 2000 meters into yards, you would multiply 2000 by 1.0936, which equals 2187.

Finally, you may need to adjust the answer to make it more manageable. For instance, 2187 yards can be expressed as 1.24 miles, or 1 mile and 427 yards.

So, to summarise, converting from metric to imperial involves finding the right conversion number, multiplying your metric figure by that number, and then potentially adjusting the answer to a more manageable imperial unit.

Reading a UTS screw chart

Understanding the Basics of UTS

The Unified Thread Standard (UTS) is used for bolts, nuts, and other threaded fasteners in the United States and Canada. It defines the standard thread form, series, allowances, tolerances, and designations for screw threads. The UTS is controlled by the American Society of Mechanical Engineers (ASME) and the American National Standards Institute (ANSI) and is listed under ASME/ANSI standard machine screw sizes. All measurements in the UTS system are in inches.

Key Components of a UTS Screw Chart

A typical UTS screw chart includes the following information:

• Screw Gauge or Major Diameter: This refers to the diameter of the outside thread in inches. It is often labelled as "#" followed by a number. For example, "#4" indicates a diameter of 0.112 inches. Anything less than 1/4" will be labelled from #0 to #12. After #12, the gauge size is labelled according to the outside diameter in inches, such as 1/4", 5/16", etc.
• Threads Per Inch (TPI): TPI refers to the number of threads per inch. It is usually specified right after the screw gauge. For instance, a #8-32 UNC screw has 32 threads in a one-inch section of a coarse screw.
• Thread Standard: There are two main thread standards under UTS: course (UNC) and fine (UNF). There is also an extra-fine screw thread standard (UNEF), but it is less common. The thread standard provides information about the coarseness or fineness of the threads.
• Screw Length: The screw length is typically specified at the end of the callout on the screw box. It represents the length of the shaft, which is the part of the screw that goes into the surface. It is usually measured in inches, such as 0.5 for half an inch.
• Tolerance Class: The tolerance class indicates the type of holes or nuts the screw can fit into. There are five tolerance classes, from Class 1 to Class 5. Class 1 screws have the most space between their threads and fit more loosely, while Class 3 screws have a tighter fit and are used for high-quality work.
• Right-Handed or Left-Handed: Left-handed screws are labelled with the symbol "LH" after the tolerance class. They are threaded in reverse, so they loosen when turned clockwise and tighten when turned counter-clockwise. Most screws are right-handed.

Example of Reading a UTS Screw Callout

Let's take an example of a UTS screw callout: #4-40 UNC-3A x .5. Here's how to interpret this callout:

• Diameter: #4 indicates a diameter of 0.112 inches.
• Threads Per Inch: 40 threads per inch.
• Thread Standard: UNC means Unified National Coarse, indicating a coarse screw.
• Tolerance Class: 3A indicates a tight fit screw.
• Screw Length: .5 indicates a length of 0.5 inches.

By understanding these key components and using the example as a guide, you should now be able to confidently read and interpret a UTS screw chart.

Reading a metric screw chart

The first thing to know about reading a metric screw chart is that metric callouts look different from imperial ones. Metric callouts always start with the letter "M" followed by a number. This number is the screw diameter, which is the outside diameter of the screw or the diameter of the screw shank. The screw shank is the part of the screw that lies beneath the screw's head. It can be threaded all the way to the tip, partially threaded, or not threaded at all.

The second number in a metric callout is the screw pitch, which represents the distance between each thread on the screw in millimetres. The screw pitch is not included in imperial callouts. Instead, you'll find the number of threads per inch (TPI).

The third and final number in a metric callout is the screw length, which is measured differently depending on the type of screw head. For countersinking screws, the length is measured from the top of their heads to their tips since they are designed to be installed flat or flush with the surface. For non-countersinking screws, the length is measured from beneath the head to the tip, as their heads are designed to remain above the surface.

Here's an example of how to read a metric screw callout:

M6 x 1.00 x 25

• M designates the screw is a metric size
• 6 is the nominal diameter in millimetres
• 1.0 is the thread pitch or distance between threads, in millimetres
• 25 is the fastener's length, in millimetres

Now you can confidently read a metric screw chart and purchase the right size of screws for your project!

Screw acronyms and their meanings

Screws are sized using a number (#) and measurements in inches or the metric system. The number on a screw (e.g. #6, #8) refers to its diameter, with higher numbers indicating a larger diameter.

There are various acronyms and abbreviations related to screws and their types:

• FH – Flat-Head or Countersunk screws feature a flat head tapered on the side and rest flush with the surface of the material they are installed in.
• BH – Button Head screws have a dome-shaped head and protrude from the material they are installed in.
• MS – Machine Screws are short, heavy-duty screws with uniform threading, used to hold machinery parts together.
• PH – Phillips Head screws feature a cross-shaped cutout and require a specific bit to install.
• FT – Fully Threaded screws have threading covering the entire shank, from the head to the tip.
• SH – Socket Head screws have a hexagonal recess and are installed using a hex key or socket bit.
• STS – Self-Tapping Screws have a tip similar to a drill bit, allowing them to tap their own holes.
• SS – This can refer to either Stainless Steel screws or Set Screws.

Other common abbreviations related to fasteners include:

• GR – Grade, used for US fasteners.
• GV – Galvanized.
• PH – Phillips.
• RH – Round Head or Right-Hand Thread.
• UNC – National Coarse Thread.
• UNF – National Fine Thread.
• UNEF – National Extra-Fine Thread.
• UTS – Unified Thread Standard.

Frequently asked questions