Buy Machine Screws Online: Screw Sizes

A machine screw is a small bolt designed to take a matching nut or fit into an insert fixed into wood, plastic or metal. Machine screws range from the very small to a ¼ inch in diameter, where they are generally referred to as machine bolts, although the defining line between the two is arbitrary.

Unlike bolts, machine screws usually have a standard screw head, such as slotted head or Phillips head, which is used to tighten the assembly. By contrast, bolts tightened via a nut, using an external driver or spanner.

Machine screws are available in a number of materials, including mild steel, stainless steel, brass, and nylon, with each type having its own unique set of advantages and disadvantages, suiting specific tasks and uses.

Machine Screws and Sizing

Machine screws are available in an enormous range of sizes, shapes, and styles. Despite standards ensuring that products made by different manufacturers will mate, there are still many differences, especially between metric machine screws and Imperial machine screws, with different terminologies used to describe thickness, length, grade, and thread coarseness.

Machine Screw Diameter

According to the American Standard, the diameter of the machine screw is given as a number from 0 to 10, with even screws gauges much more common than odd numbers. 12 and 14 are also available, but less common, and sizes greater than gauge 10 are often expressed as fractions of an inch.

Metric measurements of diameter are given in millimeters, and this is usually prefixed by an ‘M,’ to prevent confusion with the American Standard.

Buy Machine Screws Online 2: Thread Coarseness

Machine Screw Thread Coarseness

Machine screws are also judged by the coarseness of their thread, which dictates the potential uses of a machine screw. Finer threads offer a much firmer grip than coarser threads, and they are less likely to work themselves lose under the force of vibration. Coarser threads are much easier to engage, requiring fewer turns of the screwdriver, and they are far less likely to cross-thread, an important factor if you plan to use power tools to engage the screw.

Under the American Standard system, the thread density is measured as threads per inch, a figure that can be checked with a thread gauge, a useful device for measuring thread pitch. To simplify things, the American Standard designates codes to machine screws, nuts, and bolts; coarse (UNC); fine (UNF), and extra fine (UNEF). UNC screws have 24 threads per inch; UNF screws have 32 threads per inch. These useful designations make it much easier to buy machine screws online.

The metric system is different and measures the thread pitch according to the distance between threads, in millimeters.

Machine Screw Length

The final measurement is the length of the machine screw, and this is measured from the base of the head to the tip of the bolt, in inches or millimeters. Inch measurements can be in fractions of an inch or the decimal equivalent, such as ½” or 0.5”
Machine Screw Size Codes

For American Standard machine screws, the dimensions are usually given in the form:

#8-20 x .75

This machine screw is 8 Gauge diameter, has 20 threads per inch, and is three quarters of an inch long.

¼-UNC x ½

The external thread diameter of this screw is 1/4 “, it has a UNC coarse thread, of 24 threads per inch, and it is ½” in length.

For Metric Machine Screws

M3-0.4 x 12

This screw has an external thread diameter of 3mm, there is a distance of 0.4 millimeters between threads, and it is 12mm long.

Machine Screw Thread Classes

Occasionally, you will see the thread class of the screw or nut, which allows you to determine how tightly the nut and screw tighten. The most common thread class for machine screws is 2A – the ‘2’ shows that it is a medium fit, with the much rarer ‘1’ indicating a very tight fit, and ‘3’ indicating a looser mating. The ‘A’ shows that this is an external thread measurement – a ‘B’ is the internal thread measurement, and is reserved for nuts, inserts, and tapped holes.

Buy Machine Screws Online 3: Screw Construction

Metal Machine Screw Construction

There are a number of materials used for constructing machine screws, including steel, plated steel, brass, and nylon.

Standard Machine Screws

Standard Machine Screws are made from steel, usually galvanized with zinc for an anti-corrosion coating, extremely important in machine screws where, if they are used with nuts, rusting will soon make it almost impossible to loosen the nut. These are cheap, easily available, and suited for many tasks, but they are still prone to rusting, over time

Stainless Steel Machine Screws

Stainless steel machine screws are more expensive, but they are almost immune to corrosion, making them popular for external use and for precision parts. They are commonly used for fixing guttering, boatbuilding, and precision engineering, where their anti-rusting properties come to the forefront.

Stainless steel is not as strong as mild steel, so try to use a slightly longer and thicker machine screw wherever load-bearing capability is a genuine concern.

Stainless steel machine screws are often graded, generally with a three digit SAE Stainless Steel Grade number beginning with ‘3.’ For most uses, 305 or 306 stainless steel machine screws are adequate, but 316-grade stainless steel is common for marine engineering and precision manufacturing. Other standards for stainless steel grading are given by numbers such as 16/10 or 18/0, which give the percentages of chromium and nickel in the alloy.

Brass Machine Screws

Brass is a much more common material for machine screws than for larger bolts or wood screws. This is partly because, at smaller scales, such as machine screws, the lesser strength of brass machine screws is much less of a problem, and it has the added advantage that it does not rust as intensely as steel, so there is much less of an issue with the nut permanently rusting to the nut.

Brass is often used as inlays for wood, circuit boards, and other tapped holes, so only brass machine screws should be used with those.

Buy Machine Screws Online 4: Screw Heads

Machine Screw Heads

As with wood screws, there are a number of potential head types

• Flat Head: Flat screws, with a countersunk head, which helps the machine screw to center, and sits flush to the surface
• Round Screws: These have a flat base and rounded head, often used for decorative purposes. These have largely been superseded by other variations.
• Pan Head: Pan heads have a flat base and slightly rounded surface, providing a large diameter head to maximize torque.
• Oval Head: These screws, often called ‘oval countersunk,’ have a slightly rounded head and tapered base. These are designed for countersunk holes, where they have a better profile than round machine screw heads, and have excellent self-centering properties. They are commonly used with washers for holding thin materials in place without damage.
• Fillister Head: The oval fillister head has a slightly smaller diameter than the corresponding round head, to allow a deeper slot that suits high torque applications and increases the amount of pressure.
• Binding Head: These large, flat heads have an undercut base, which allows electricians to bind stranded electrical wire without fraying or chafing.
• Truss Head: Truss head machine screws have a low, flat head, designed to cover a large area, making them a good alternative to washers for holding thin sheet metal with a large amount of force.

Machine Screw Drivers

As with most screws, there are a number of different driver types, each with their own advantages.

Slotted Machine Screws

These contain a simple slot, requiring a standard slotted screwdriver head. Because of the risks of ‘camming out’ associated with Phillips screws, slotted head machine screws are still very popular, for the large amount of force they can handle.

Phillips Machine Screws

The Phillips head is still used on machine screws, but it is not as popular, due to the fact that they are more difficult to tighten to the degree expected for most machine bolt applications.

External Hex Head

It is debatable whether a machine screw requiring an external driver is technically a screw rather than bolt, but they are very common in medium grades. They are useful where access is limited, because a socket wrench can be used to tighten them, and they are excellent for high force applications.

Hex Socket

These machine screws contain an internal hex socket, and they are designed to be tightened with an Allen key. They have very little risk of camming out, and the recessed hex slot in a high profile, cylindrical head, can handle a large amount of force.

Star Drive/Torx Drive

These machine screws, which contain a recessed, six-pointed star, are engaged with a Torx driver head. They tolerate extremely high torque, which makes Torx machine screws popular with the automotive industry.

Square Drive

Square drive machine screws, also known as Robertson screws, have a square recess in the head of the screw. They carry little risk of camming out, and they allow the user to use fairly high torque for tightening. They are more commonly used for woodscrews, but some machine bolts carry a Robertson head.

Buy Machine Screws Online

Hopefully, this guide has given you a little insight into the world of screws, helping you to understand some of the subtle nuances and terminology used. Small Parts, using Amazon’s experience with customer service, quality, and delivery, provides a huge range of options if you wish to save money and buy machine screws online.

How To Buy Brass Machine Screws Online

Machine screws are used for a variety of applications, from cabinet making to computer hardware manufacture, and they are available in a huge array of sizes and types, each suited to a different task. For many use, brass machine screws are the best option, with their resistance to corrosion and decorative finish making them perfect for decorative features.

Machine screws are usually used with matching nuts, brass inserts, or tapped holes, and they are ideal for use with soft or brittle materials, where a standard screw would not find enough purchase to make a solid joint. Machine screws are available in a range of sizes, from the tiny screws used in electronic components to larger screws used for construction.

The Advantages of Brass Machine Screws

Brass is rustproof and resistant to corrosion, ensuring that you rarely suffer the problem of machine screw and bolt corroding together, making removal practically impossible. Brass usually consists of 70% copper to 30% zinc and, as a material, it is very easy to work with; brass is easily cut, soldered and braised. Higher zinc contents equates to higher strength, although brass machine screws do not have the load nearing capability of steel, so they are rarely used in critical structural elements.

Buying Brass Machine screws is exceptionally easy online, as long as you know exactly what to look for. A good online retailer offers:

• Slotted Head Machine Screws
• Phillips Head Machine Screws
• Torx Head Machine Screws
• Robertson Machine Screws
• Tamper-Resistant Machine Screws

They will offer these in the full range of diameters, lengths, thread coarseness, and reputable manufacturers will offer the correct complimentary nuts or inserts to match your brass machine screws.

The Importance of Quality

As with any online purchase, quality is paramount, because inferior machine screws, especially when made from a softer alloy, such as brass, do not carry much strength, and hardened steel drivers can destroy the heads, especially if you use automated tools.

This has to be married with price; how many people long for the days when you could bulk-buy machine screws by the pound, at very cheap rates. Sadly, modern retailers now insist on charging you a couple of bucks for a tiny packet of ten screws – a good online retailer will still allow you to buy screws in bulk, useful for contractors or DIY enthusiasts with a big project in mind.

Buy Brass Machine Screws Online With Amazon and SmallParts

Amazon and its subsidiary, SmallParts.com, offer a huge range of brass machine screws, nuts and brass inserts, making shopping easy. Alongside Amazon’s commitment to quality, the well-honed Amazon delivery network ensures that you will receive your hardware quickly, and at a very reasonable price.

Pozidriv Screws – Buy a Pozidriv Screwdriver Online

Pozidriv screws, often incorrectly named Pozidrive screws, are a derivative of the Phillips screw, designed to negate many of the disadvantages associated with that screw type. Superficially, the Pozidriv screw resembles the cruciform Phillips screw, but closer inspection shows that the Pozidriv screw incorporates four extra tick marks at 45-degree angles to the main recess, and Pozidriv screws do not have the rounded corners common to Phillips screws.

Unlike Phillips screws, Pozidriv screws do not cam out under conditions of high torque, a trait that was advantageous in the early days of mass production, because it prevented over-tightening of the screws and reduced the resulting risk of stripping the screw or damaging the product.

With the advent of power tools that automatically disengage if torque exceeds desired levels, this is now seen as a disadvantage, and the Pozidriv seeks to negate this. Despite this, the Pozidriv screw never became popular in North America, although they are very common in Europe.

The Advantages of Pozidriv Screws

• Pozidriv screws do not cam out under high torque, making it easier to drive self-tapping screws into hardwood or prevent machine bolts from loosening under conditions of high vibration
• Unlike star drive and hex heads, you can use a slightly larger or slightly smaller Pozidriv screwdriver, ensuring that you need fewer drivers in your set
• The screw can be engaged in four positions, making them a better option than slotted screws in restricted spaces

The Disadvantages of Slotted Head Screws

• Pozidriv heads are very hard to find outside Europe, and they are not available in a wide range of styles or types
• It is very easy to mistake Phillips heads for Pozidriv heads, and vice versa. To a certain extent, Pozidriv and Phillips head screwdrivers are interchangeable, but using the wrong type can damage screw or driver
• If the user overtightens the screw, this can damage the screw recess, strip the thread, or damage the driver

Buy Pozidriv Screws Online

Despite the restricted availability of Pozidriv screws in North America, they are still common in Europe. For repairing European-made electronics and older vehicles, a Pozidriv screwdriver is essential, but Pozidriv screwdrivers and bits are generally unavailable at your local hardware store.

Fortunately, online retailers are the saviors, offering Pozidriv screwdriver sets at a competitive price and delivering them to your doorstep. Amazon and its sister site, SmallParts.com, are amongst the leading online hardware retailers, with a highly efficient delivery network and a strong focus towards good customer service. If you need to buy Pozidriv screwdrivers online, they are a great option.
.