Machine Screw Sizes: Understanding The Standards

Machine screws are used in a wide range of applications, from construction to electronics. They are a type of fastener that joins parts together through threaded holes. The small size and unique threading design of machine screws make them different from other types of screws. To find the right replacement screw, you need to know the diameter, length, and thread pitch. The Unified Screw Thread System and ISO Metric Screw Thread System are the two major systems used to classify machine screws, based on inches and millimetres, respectively.

Characteristics and Values of Machine Screw Sizes

Machine screw vs bolt

Machine screws and bolts may appear similar, but there are some key differences between the two.

Machine screws are uniformly threaded with a nominal diameter of 1/4 inch or less. They are designed to be inserted into a pre-cut hole and connect the components of a machine. They are fully threaded to the head and are usually installed into a tapped hole with internal threading. Machine screws are commonly used in home furniture hardware and materials such as electronics and appliances. They are also found in engines, industrial lathes, and milling machines.

Bolts, on the other hand, typically have a plain portion and are not threaded all the way along their shank. They are designed to be installed with a nut and are pushed through a hole, with the nut tightening at the back of the material being fastened. Bolts are generally longer than the width of the material they are used on, as they need to protrude to screw into the nut. The unthreaded portion of the bolt adds strength and makes it more resistant to shear forces. Bolts are usually tightened using a spanner or a wrench and often require the use of a washer for installation.

Another distinction is in the head type. Machine screws typically have a slotted head and can be installed using a screwdriver, such as a Philips head or flat-head screwdriver. In contrast, bolts often have a hex head and are installed using a hex socket.

In terms of size, machine screws are typically smaller than bolts. Most machine screws are very tiny, usually less than one inch in length, while bolts can be larger, sometimes almost 2 inches long.

To summarise, the main physical difference is that bolts have unthreaded shanks, while screws are threaded all the way through. Additionally, screws are typically used with tapped holes, while bolts are used with nuts.

How to measure a machine screw

Machine screws are smaller than most other types of screws, with diameters up to 0.75 inches. They feature uniform threading and fit into a nut or a tapped hole.

Diameter

The diameter of a screw is the distance between the outer threads on either side. This is called the major diameter. You can also measure the minor diameter, which is the distance between the bottom cavity of the threads on either side.

Length

The length of a screw is the distance from the flat part of the head to the tip. The way you measure this depends on the type of screw:

• For most machine screws, measure from below the head to the tip.
• For countersunk screws, measure from the top of the head to the tip.
• For flat head, countersunk screws, measure from the top of the flat head.
• For round-headed countersunk screws, measure from the bottom of the rounded top.
• For screws with a hexagonal head, measure from the bottom of the head.
• For oval head screws, measure from the widest point of the head to the tip.
• For self-drilling screws, measure to the end of the drill point tip.

Thread Pitch

The thread pitch is the distance between the threads of a screw. It can be expressed as the number of threads per inch. You can measure this with a thread gauge, which will have multiple metal leaves with serrated edges. Hold the screw up to the light and find the gauge leaf that fits the thread pitch—no light should pass between the screw's threads and the serrated edge of the gauge.

Weight

To get an accurate weight for a screw, it is best to weigh multiple screws and calculate an average, rather than weighing a single screw.

Screw thread pitch

Machine screw sizes can be measured using the Unified Thread Standard (UTS) and the ISO metric screw thread system. The UTS system is used in the United States, while the rest of the world follows the ISO metric system.

Unified Thread Standard (UTS)

In the UTS system, the screw gauge, also known as the "major diameter", refers to the diameter of the outside thread. The smallest machine screw gauge is #0, although gauges #00 and #000 can be found for specific projects. Machine screws with an outside diameter under 1/4" are labelled from gauge #0 to #12. After this, the gauge is specified according to the fractional diameter of the screw.

The number of threads per inch (TPI) is also important in the UTS system. TPI is calculated by counting the number of threads in a one-inch section of the screw. Fine screws typically have more threads per inch than coarse screws. The stress area of a machine screw, which is the weakest point of the bolt, is lower in coarse screws than in fine screws.

ISO Metric Screw Thread System

The ISO metric screw thread system is used internationally and is measured in millimetres. This system uses the pitch diameter, which is the distance between the threads on the screw, instead of calculating the number of threads per inch. The pitch diameter is also known as the thread pitch. The metric system also uses the minor diameter, which is the smallest point of the thread on a machine screw.

To measure the thread pitch of a metric fastener, use a caliper to measure the distance in millimetres from the peak of one thread to the next.

Screw head types

Machine screws come in a variety of head types, each with its own unique features and applications. Here is an overview of some common machine screw head types:

Pan Head: This type of screw head has a wide, circular head with a flat, bearing surface that provides a secure hold. The head can be either flat or slightly domed. Pan head screws typically feature slotted or Phillips drives but are also available in other drive types. They are commonly used in machinery, automotive parts, and accessories such as license plates and headlights.

Flat Head: Flat head screws have a flat disc top and a flat bearing surface. They offer accuracy and excellent driving capacity. These screws are often used in appliances, automotive, and electrical component fixing. The Phillips-drive version is commonly used in automotive lights.

Fillister Head: Fillister head screws have a convex top and a flat bearing surface. They feature an oversized head similar to pan and cheese head screws but with greater height. Fillister head screws are used for counterbored holes and are commonly found in precision instruments and machine assemblies. They are also known as countersunk screws.

Oval Head: This type of screw head has a rounded top surface and a conical bearing surface. Oval head screws are used when a flush finish is required, such as in switch covers and construction applications where a decorative finish is needed. They provide extra grip during adjustment and have an attractive appearance.

Round Head: Round head screws have a rounded top and are used when a smooth surface is desired. They are commonly used in machinery and applications where there is a need to avoid snagging skin or clothing, such as handrails, furniture, and lighting fixtures.

Binding Head Undercut: This type of screw head has a wide, flat bearing surface and is designed to provide a secure hold. Binding head undercut screws are commonly used in applications where materials need to be bound or secured together, such as swatches and large manuals. They are also suitable for electrical applications, providing space for wire connections.

The choice of screw head type depends on the specific application, with each type offering unique advantages and suitability for different materials and industries.

Screw materials

Machine screws are made from a variety of materials, including:

Steel

Steel is an alloy made primarily of iron and carbon. It is known for its high tensile strength, making it suitable for use in a wide range of industries, including transportation and defence.

Stainless Steel

Stainless steel is a steel alloy with a mass composition of at least 10.5% chromium. It is one of the strongest steel alloys, with high corrosion and stain resistance, and is easy to sterilise. Stainless steel is commonly used in construction, medical devices, parts manufacturing, machinery, and household goods.

Brass

Brass is an alloy composed mainly of copper and zinc, with a bright gold tint. It offers low friction, good workability, durability, and a non-sparking nature. It is also antimicrobial, making it suitable for use in plumbing, decorative applications, and architecture.

Aluminium

Aluminium is a naturally occurring element with the atomic number 13. It is ductile, non-magnetic, corrosion-resistant, and has a low density. Aluminium screws are commonly used in the construction, architecture, aerospace, and transportation industries.

Titanium

Titanium (Ti) is a strong transition metal known for its low density, lightweight, and extreme corrosion resistance. It is commonly used in the aerospace, automotive, sanitary, and medical industries.

Other materials used for screw machine parts include plastics such as Delrin, Acetal, and Nylon.

Frequently asked questions