Screw Sizes: Finding The Nearest 3Mm Fit

Understanding screw sizes can be challenging, especially with the various screw sizings quoted on packaging and the different measurement systems used around the world. The US uses the Imperial system, while most of the world uses the Metric system. This can make it difficult to determine the closest screw size to 3mm when dealing with Imperial measurements. In the Imperial system, screw sizes are indicated by a gauge number, which refers to the diameter of the screw. The larger the gauge number, the larger the diameter. However, there is no direct link between the head size of an Imperial screw and its gauge. To find the closest Imperial screw size to 3mm, one would need to refer to conversion charts or use digital calipers to measure the screw size accurately.

The M rating of screws/bolts indicates their diameter in mm

The "M" designation for metric screws and bolts indicates the nominal outer diameter of the screw thread in millimetres. For example, an M6 screw has a nominal outer diameter of 6 millimetres and will fit a 6 mm hole. The M sizes enable engineers to swiftly identify and match threaded components, facilitating streamlined procurement and assembly processes.

The metric thread size chart below represents the dimensions of metric coarse machine screw threads as defined by ISO standards. The most common system adhered to is the ISO Metric Screw Thread, where designations like M6x1 signify an M6 screw with a major diameter of 6 mm and a pitch of 1 mm.

The major diameter, typically measured in millimetres, refers to the largest diameter of a threaded cylinder, also known as the crest of the thread. Precise measurements can be acquired using Vernier callipers or micrometers. The minor diameter, usually measured in millimetres, is the smallest diameter of a threaded element, often referred to as the root of the thread. This dimension is a crucial indicator of the quality and strength of the thread and can be measured using specialised gauges or optical methods.

The thread pitch refers to the distance between two parallel crests on a thread, typically measured in millimetres. This is usually calculated by measuring the distance between three or more thread crests and then dividing by the number of spaces between them. Pitch is essential, as it affects both the thread's mechanical properties and its compatibility with mating components.

#6, #8 etc. refer to the diameter of the screw

When selecting a screw for a project, it is important to understand the different sizing standards. The US uses the Imperial system, while most of the world uses the Metric system. This can be confusing, especially when trying to match screw sizes with pilot hole sizes. Using an incorrect screw or hole size can lead to structural failures over time.

The numbers on screws, such as #6 or #8, refer to the diameter of the screw, with higher numbers indicating larger diameters. For example, #4 screws are designed for small crafts, jewellery boxes, and attaching hinges, while #10 screws are used for general construction, heavy-duty furniture, and outdoor projects.

To measure screw size accurately, use a digital caliper to measure the diameter and length, and refer to conversion charts for equivalent sizes if needed. Additionally, when selecting fasteners for outdoor applications, consider materials that are resistant to corrosion, such as stainless steel or coated fasteners.

Conversion charts are available for matching screw sizes with pilot hole sizes

Pilot holes are necessary for most screw types, except for self-drilling screws, which can drill their own holes. For other screw types, such as tapping, thread cutting, thread forming, or thread rolling screws, a pilot hole is required to ensure successful insertion. Even sharp-pointed screws like Type A or Type AB Self-Tapping Screws can split wood or distort metal if the pilot hole is not the correct size.

The recommended pilot hole diameter varies depending on the specific screw type and the material being used. For example, Type F Thread Cutting Screws and Type 1 and Type 23 Thread Cutting Screws require the same pilot hole and drill bit sizes due to their tapered ends. The pilot hole size also depends on the material's flexural modulus, with separate recommendations for materials with a flexural modulus of up to 200,000 P.S.I. and those over 200,000 P.S.I.

Drill bits used for creating pilot holes come in a wide range of sizes, with naming conventions based on fractions, numbers, and letters. Fractions of 64ths, from 1/64th to 63/64ths, are named by their lowest fractional equivalent, such as 1/32", 1/16", or 5/64". Sizes larger than 0.2280 that are not named as a fraction of 64ths are designated by letters, starting from A (0.2340) to Z (0.4130).

The imperial system is used in the US, while most of the world uses the metric system

The closest screw size to 3mm is a 1/8-inch screw, which is slightly smaller at 2.38mm. This is an example of the difference between the imperial system of measurements, used in the US, and the metric system, used in most other countries.

The US is one of only three countries that use the imperial system, with the majority of nations favouring the metric system. The imperial system is based on units such as feet, pounds and miles, while the metric system uses units like metres, grams and kilometres. The metric system was established in France during the late 18th century, following the French Revolution. It was designed by the French Academy of Sciences, who based the unit of measurement on the length of one ten-millionth of the distance from the North Pole to the Equator as measured from Paris.

Despite the US recognising the metric system as the preferred system of measurement since 1975, the country has been slow to adopt it. This is partly due to the cost and time involved in converting to a new system, as well as resistance from industrialists and legislators. As a result, both systems are often used alongside each other, leading to confusion.

The metric system has been taught in US schools and is used in some industries, such as food labels and car speedometers. However, many products and machinery in the US are still sized in imperial units, and workers are trained to use this system. While the US continues to use both systems, it may be challenging to fully transition to the metric system.

The metric system uses the diameter in mm and the length in mm

The metric system uses the diameter in millimetres and the length in millimetres. This is in contrast to the imperial system, which uses a "gauge" figure to refer to the diameter of a screw. In the imperial system, the larger the number, the larger the diameter. For example, a number 12 screw has a larger diameter than a number 4 screw.

The metric system is considered simpler to understand as it uses millimetres to measure the diameter and length of a screw. The diameter of a metric screw is given as the (major) diameter in millimetres, with the length also measured in millimetres. This is the same measurement used in the imperial system.

On some screws and bolts, you will see an "M" followed by a number. This "M" rating indicates that the number following it is the diameter of the screw or bolt in millimetres. For example, an M6 screw has a diameter of 6mm, while an M8 screw has a diameter of 8mm.

It is worth noting that there is no direct conversion between metric and imperial screw sizes. However, as a rough guide, the gauge of an imperial screw in sixteenths of an inch multiplied by two and minus two will give you a number close to the metric diameter in millimetres. For instance, a 5/16 inch head multiplied by two equals 10, minus two equals 8. This indicates that the metric diameter is approximately 8mm.

When working with screws, it is important to use the correct size for your project. This is because the screw size can impact the load-bearing capacity, with larger diameter screws generally having higher load-bearing capacities. Additionally, matching screw sizes with appropriate pilot hole sizes is crucial to prevent splitting or damage to the material being fastened.

Frequently asked questions