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Machine Screws and Bolts - A Handy Guide | xobba.com

Machine Screws and Bolts

What To Look For In Machine Screws

Machine screws are small bolts intended for use with a matching nut or corresponding insert affixed to metal, plastic, or wood. Machine screws incorporate a standard screw head, such as Phillips or Torx, rather than an external bolt head requiring a spanner, socket, or external driver.

They cover a range of sizes, from tiny machine screws to large industrial machine bolts use by heavy industry. The difference between a machine screw and a machine bolt is largely arbitrary and based upon whether tightening is with a nut or the screw head.

Machine screws and bolts are available in a range of materials and styles, each suited to a particular task. The abundance of choice can make searching through the various options difficult, so this guide intends to highlight the differences between the various types, pointing out the advantages and disadvantages and helping you to arrive at the perfect choice.

Torx Machine ScrewMachine Screw Driver Heads

There are a number of different screwdriver head types, each carrying its own set of advantages and disadvantages.

Slotted Heads

These are the simplest variety, consisting of a slot designed to fit flat screwdrivers. Although slots heads have fallen out of fashion for most screws, they are still a popular option for machine screws, especially with fillister heads, because the user can apply force without risk of camming out.

Phillips Heads

The Phillips head is not as popular, mainly because the tendency to cam out makes them unsuitable for many applications.

External Hex Head

Technically, a machine screw requiring an external driver, such as a spanner or socket, is technically a bolt, but the terms are still used interchangeable for smaller diameters. They are extremely useful where access is difficult, because a socket wrench can be used to tighten the bolt. They are extremely popular with sheet metal workers, where self-tapping machine screws with an external hex head are perfect for using with power tools.

Internal Hex Socket

Hex head machine screws have a hex recess in the head of the bolt, which can be engaged with a standard Allen head, and the high profile, recessed hex slot allows high torques and lowers the risk of cam out.

Torx Head/Star Drive

Torx screws contain a recess shaped like a six-pointed star, and they are designed to tolerate extreme forces. They have largely replaced the Phillips head in the automotive and aerospace industries.

Machine Screw Head Types

There are a number of machine screw head types

  • Flat Head Machine Screws: Flat head machine screws have a countersunk head, which sits flush to the surface and helps center the screw, and sits flush to the surface
  • Round Head Machine Screws: These have a rounded head and a flat base, making them perfect for decorative purposes.
  • Pan Head Machine Screws: These screws have a slightly rounded head and a flattened base, ideal for high-torque applications.
  • Oval Head Machine Screws: Sometimes referred to as ‘oval countersunk heads,’ these have a tapered base and a slightly rounded head, and they are self-centering and perfect for using with washers. When combined with a washer, they are perfect for holding thin materials, such as sheet metals, in place without causing damage.
  • Fillister Head Machine Screws: These screws have a slightly smaller diameter but deeper profile than round head machine bolts, allowing a deeper slot. This is perfect for high torques, allowing the operator to exert a large amount of fore without camming out or damaging the head.
  • Binding Head Machine Screws: These have large, flat heads with an undercut to the base, allowing electricians to bind stranded electrical wire securely, without risk of chafing or fraying.
  • Truss Head Machine screws: These have a low, flat head profile, and they are designed to maximize the surface area, making them a great alternative to using washers for affixing thin substrates.

Slotted Brass Machine ScrewMachine Screw Sizes

Machine screws and bolts cover a huge range of shapes, styles, and sizes, and the lack of a universal standard dictating machine screws sizes further confuses the issue. For a start, there is the age-old conflict between metric and imperial sizing, and manufacturers adopt different conventions for comparing length, diameter, grade, and thread coarseness.

Machine Screw Diameter

The American Standard for describing machine screw diameter uses a number between #0 and #14 to describe the vast majority of machine screws. A #0 machine screw is .06 inches, and the size increments are at .013 inch intervals. After #14, sizes are given as fractions of an inch, and the general convention is to refer to these larger machine screw diameters as machine bolts. Tiny machine screws are often given the designations #00 and #000, although these are uncommon.

Metric machine screw diameters are given in millimeters, and are prefixed by the letter ‘M,’ to prevent confusion with US conventions.

Machine Screw Thread Sizes

Another important specification is the thread size, which determines the potential uses of a machine screw. Coarse threads are easy to turn, grip better in soft materials, and are less likely to suffer from cross threading, an important consideration if power tools will be used with the screw. However, where high strength is needed, or for applications where vibration is a problem, finer threads are a better choice.

The American Standard System measures threads per inch, allowing thread density to be checked with a thread gauge. This system also uses a naming convention for machine screws, nuts, and bolts, giving extra clarity.

  • UNC Coarse Machine Screws: 24 Threads Per Inch for a #10 screw
  • UNF Fine Machine Screws: 32 Threads Per Inch for a #10 screw

Some manufacturers use UC and UF, or UNRC and UNRF, but these terms are interchangeable. There are other common measurements, and the exact number of threads per inch will vary for larger bolts, but the buyer will know that a #10 UNC bolts will match a #10 UNC insert, making it easier to buy machine screws online.

Metric machine screw thread densities are defined by the distance between the threads in millimeters.

Machine Screw Thread Classes

Machine screw thread classes define the tolerance, describing how loosely or tightly the nut or machine screw insert fits the screw. These are given as a number from 1 – 3, with an ‘A’ designation for external threads on a screw or bolt, and ‘B’ for the internal thread of nuts and inserts.

The vast majority are 2A or 2B, indicating average tolerance. For high-vibration applications, tight-fitting thread class ‘3’ screws are often used, because they resist stripping and loosening through vibration, making them popular in the aerospace industry. Thread class 1 is rare, used only where corrosion or dirt could cause the nut and bolt to jam. Unless specified, most machine bolts and nuts are class 2A and 2B.

Machine Screw Length

The length of the machine screw is measured from the base of the screw head to the tip, and is given in inches or millimeters depending upon the exact standard. Inch measurements are given as fractions or as decimals: i.e. ¼” or 0.25”

Machine Screw Size Codes

There are a number of formats used to describe machine screw dimensions, and understanding these is the key to buying the right size for the job.

For American Standard Machine Screws:

#6-32 x .5-2A

This machine screw has a diameter of 6, has 32 threads per inch, is half an inch in length, and has a thread tolerance of 2 (the tolerance is often omitted for 2A/2B bolts and nuts.

Alternatively:

1/16-UNC x 1½

The external thread diameter of this screw is 1/4 “, it has a UNC coarse thread, and it is 1½ inches in length.

For Metric Machine Screws

M4-0.3 x 18

This screw has an external thread diameter of 4mm, there is a distance of 0.3 millimeters between threads, and it is 18mm long. Metric definitions sometimes include a number such as 4g6g or 4H5H. This is another way of defining thread tightness, but most machine screws are 6g, roughly corresponding to 2A, and bolt and insert tolerance of 6H corresponds to 2B. Finally, a left-handed thread will have –LH at the end of the code.

Slotted Steel Machine ScrewMetal Machine Screw Construction

Machine screws are available in a number of materials, from hardened steel and brass, to nylon.

Standard Steel Machine Screws

Standard machine screws are fabricated from steel, and they are usually galvanized or coated with an anti-corrosion layer, such as zinc. This is crucial, because rusting and corrosion can render it practically impossible to loosen the screw from the nut or insert. Standard machine screws are cheap and readily available, but they will eventually suffer from corrosion, so alternatives are required for certain applications.

Stainless Steel Machine Screws

Stainless steel machine screws are more expensive than their standard steel counterparts, but they are extremely resistant to corrosion, making them a popular choice for precision engineering and use in wet or damp environments. Stainless steel fastenings are often used for fixing guttering, in marine environments, and in wet-rooms.

Stainless steel isn’t as strong as the standard type, so you should use longer, thicker machine screws where possible, for extra load bearing capability.

Stainless steel machine bolts and screws are usually graded, ensuring that it is easy to pick the right quality for the task. Most manufacturers use a three digit SAE number, beginning with ‘3.’ For general use, 305 and 306 grade stainless steel screws are perfectly adequate, but sensitive applications may require a 316 or higher. A few manufacturers prefer to give the grade as 16/8 or 18/10, which gives the percentages of chromium and nickel in the alloy.

Brass Machine Screws

Brass is a very common material for machine screws and bolts, because the lesser strength of brass screws isn’t as much of an issue for the small diameters and sizes associated with machine screws. Corrosion is less of a problem with brass, rarely causing the screw and bolt to lock and, as a result, brass inserts are often used in circuit boards, wooden furniture, and other tapped holes,

Buy Machine Screws Online

Now that you are fully aware of some of the peculiarities and conventions surrounding machine screws, you will be able to find the perfect machine screws for the job, quickly and easily. Looking online opens up a range of options and allows you to benefit from bulk discounts, a wide selection, and next day delivery, all from the comfort of your own home.

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  • Article by Martyn Shuttleworth

    avatar Martyn Shuttleworth is a professional freelance writer living amongst the olive groves of the Peloponnese, a spear throw away from Ancient Sparta, the home of long-haired heroes. He covers a range of styles, from academic writing to sales letters, and loves writing about home improvement, freelance writing, and all things Greek. If you want to hire a master wordsmith, please feel free to visit Freelance Writer Editor
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