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Screw Type Guide: How To Buy Screws Online |

Screw Type Guide

Buy Screws Online

Choosing the right screws from amongst the huge range of available sizes and types can be a herculean task. Apart from choosing between Phillips head or square drive, and hardened steel or brass, you need to know a little about screw sizes and functions. In this useful guide, you will learn a little about the various options and make the right choice, whether you want a handful of screws for a specific project or you want to take advantage of bulk discounts.

Screws are available in a huge array of styles and materials, each suiting a different task, from fixing door-hinges to assembling machines. The majority of everyday screws are fabricated from cheap, mild steel, but hardened steel is available for tough jobs and self-tapping screws, whilst brass and stainless steel are perfect for guarding against corrosion.

For many projects, screws are a much better choice than nails, adding strength to a joint and allowing a fine degree of precision, essential for metalwork and carpentry. Screws allow you to disassemble joints without damage and, if a little glue is added, they add permanence and strength. For applications where vibration or physical damage are risks, such as window frames or engines, screws are the only logical choice.

Screw Types Guide: Brass Slotted Head Screw

Screw Driver Types

The first consideration is the driver type, which is dictated by the exact use. There are five main screw driver types, each with its own distinct set of advantages and disadvantages.

Slotted Screws

Slotted screws are the oldest and most familiar type, designed to be driven with a flat screwdriver. They are common in woodworking, and the simple slot ensures that it is very easy to improvise if you don’t have the right screwdriver to hand. However, the driver head can slip out of the slot, potentially damaging the surface, and old or cheap screw heads often shatter under conditions of high torque.

Phillips Screws

The easily recognizable Phillips screws incorporate a distinctive cross-pattern, gripping the screwdriver head more securely than the slot head type. Under high torques, the Phillips head forces the driver to cam out, risking damage to the head and to the driver.

Pozidriv Screws

The Pozidriv screw is derivative of the Phillips head, popular in Europe, with an added four smaller spokes for added grip. This screw head has straight-sided slots, to reduce camming out, making these screws excellent for high-torque applications. The downsides are that they are exceptionally rare in North America, and they are often mistaken for Phillips screws; the driver or screw head can be damaged if the wrong screwdriver is used.

Square Drive Screws

The Robertson screw, often referred to as the square drive, incorporates a simple square hole in the head of the screw. These screws are superb under conditions of high torque and rarely can out, making them the screw of choice where strength is important.

Because of a historical issue with licensing rights, these screws are rare outside Canada, although retailers are increasingly offering them as a sound alternative to the Phillips head screw. In many parts of the world, hex heads became a decent alternative, carrying the same advantages, and both types can be driven with a wrench rather than screwdriver, making them a great choice for limited spaces.

Torx Screws

Torx screws, also known as Star Drive screws, carry a distinctive 6-pointed star shaped recess, gripping the driver head securely and preventing camming out. These screws are popular in the automotive industry, due to their ability to handle high-speeds and high-torques, and they are increasingly becoming a good option for the home.

Screw Types: Stainless Steel Machine Screw

Screws and Metal Type

Standard Mild Steel Screws

The most common type of screw, cheap, readily available, and adequate for most tasks, is fabricated from mild steel. Try to avoid hunting in the bargain basement and buying  low-quality options, because these tend to snap or strip, especially if you use power tools for driving.

Corrosion-Resistant Stainless Steel Screws

Corrosion-resistant stainless steel is the perfect choice for damp areas where rusting is a major issue. They are a good choice for exterior work, where the screw will be exposed to the elements, such as fixing guttering, or for damp areas, such as wet-rooms.

For woodworking, stainless steel is often used with high-tannin content woods, such as cedar and redwood, because the tannin can react with regular steel and cause black stains. Stainless steel isn’t a particularly durable alloy, so try to use slightly longer, heavier screws, with a finer thread.

There are many different grades of stainless steels used for screws and fasteners, and manufacturers include a Stainless Steel Grade SAE code to help you pick the right grade. These numbers come in two forms:

  • A three digit number beginning with ‘3’, such as 305 or 316
  • A number such as 18/6 or 18/10, referring to the percentage of chromium and nickel in the alloy

For most tasks, 305 or 306 are perfect, but 316-grade alloy may be better for precision engineering or in marine environments. Stainless steel is expensive, making bulk-discounts even more important.

Durable Hardened Steel Screws

Hardened steel screws are tough and durable, making them perfect for applications where strength and load-bearing capacity is paramount. They are a little more expensive than the mild steel versions, but their sturdy construction makes them perfect for working with masonry and concrete, and these screws can often be driven straight into softer wood with no need for pilot holes.

When driving hardened metal screws, especially with powered screwdrivers, make sure that you have high quality, toughened driver heads to prevent wear and tear. In addition, the high carbon content of the steel makes these screws particularly prone to rusting, so use coated screws in damp environments.

Elegant Brass Screws

Brass screws are extremely elegant and perfectly suited for decorative features such as door furniture. Brass does not corrode as easily as steel, but it is a soft alloy, best avoided where load bearing and durability are concerns. Brass can be expensive, so shopping around is always wise.

Corrosion-Beating Coated Screws

Coated screws are fabricated from steel and galvanized with a corrosion-proof layer, usually zinc or chrome. These screws are much stronger than stainless steel or brass, making them perfect for large projects, such as decking and fencing. The main disadvantage is that they are prone to chipping, exposing the steel core to the elements.

  • Hot-Dipped Galvanized Screws: Hot-dipped screws are cheap and easily available, with the study coating able to handle rough treatment. Where resistance to corrosion is a priority, these galvanized screws can last for years, even in the harshest climates. The main downside is that the coating is rough, making Phillips screws and Robertson screws difficult to engage, so you will have to reject a hefty proportion.
  • Mechanically Coated Screws: This option has a much thinner coating than the hot-dipped version, avoiding the problem of irregularity. However, they are more expensive and prone to chipping, exposing the body of the fastener to rusting.
  • Electroplated Screws: Electroplated screws are very cheap and offer some protection from corrosion. This coating is easily damaged, so they are best reserved for light, indoor work.
  • Coated Screws: For a cheap and practical alternative, screws coated with ceramic, paint, epoxy resin, or polymer are decent choices. The coating protects the screw from corrosion, and they are easier to engage than galvanized screws. The coating prevents rusting and epoxy coatings are particularly durable and resilient, making them a great alternative to stainless steel.

Buying coated screws always involves a balancing act: epoxy and ceramic are resilient, but the coating can chip and expose the bare steel. Conversely, paint and polymer are resistant to chipping but tend to wear out more quickly. Ceramic screws are also excellent for decorative features, such as porcelain door handles and ceramic address plaques.

Combination Heads

There are many other variations, including combination heads, which attempt to combine the best aspects of two different types of screw head. One common type, known as an electrician’s screw, is the hybrid Phillips/slot screw, which means that you always have the right screwdriver to hand. Robertson screws are often combined with Phillips or slotted heads, but combination heads are usually unsuitable for conditions of high torque, largely restricting them to the electronics industry.

Screws and Fasteners: Self-Tapping Screws

Types of Screw Heads

There are four main types of screw head shape, flat, round, oval, and pan

Flathead/Countersink Screws

These screws incorporate a perfectly flat head that fits flush to the surface. These screws require a pre-drilled countersunk hole, doubling the amount of work required. Flat heads are particularly common in woodworking, because they are discreet and easy to hide.

Roundhead Screws

These screws have a semicircular profile, and they are useful if the substrate is too thin for countersinking. They are popular for sheet metalwork, and decorative versions are often used for fitting door hardware and for cabinet making.

Pan Head Screws

Pan Head screws, with an oversized, flat head, are perfect where the substrate is extremely delicate or thin, giving plenty of grip without causing damage, making them a great choice for sheet metalwork and roofing.

Oval Head Screws

Oval head screws are a hybrid between countersink and rounded head screws, sitting comfortably into a countersunk hole but allowing decorative embellishment.

How to Size Screws

There are many conventions used for determining the size of screws, and these geographical differences can make buying screws a veritable nightmare.

One measurement generally common to all screws is the gauge, describing the thickness of the screw. This is usually given as a number between 1 and 20, with 4, 6, 8, and 10 the most common sizes for everyday use.

The length of the screw is given in millimeters or inches, and is measured from the top of the head in countersunk screws, and from just underneath the head in the other types.

The final measurement is the thread count, which will be expressed as threads per inch or, alternatively, as the space between the threads in millimeters. A high thread count ensures that the screw will bite firmly into the substrate, but it will need more turns to engage, and cross-threading can be an issue. Conversely, a low thread count, commonly found in self-tapping screws, is easy to drive, but sacrifices strength and grip.

Screw sizes will usually take the form:

8-15 4 ½ rh

This screw is 8-gauge, with 12 threads per inch, a 2 ½ inch length, and a rounded head

Screw Type Guide

As you can see, there is a little more to the humble screw than meets the eye, so you are now equipped to face the online quagmire and order screws online.



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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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