All About Screws

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I'd like to go back to basics and talk about something that we encounter everyday and probably don't think about all that much: the screw. It is a pretty amazing invention when you think about it. No more than a simple lever in a helix pattern, the screw allows us to bind and unbind different materials firmly, quickly, and easily. The power of this little invention has literally changed the world.

There are many types of screws, heads, drivers, and materials. The sheer variety can be over-whelming in scope, and I'd like to help you navigate this landscape. Most of the common mistakes that end up stripping a screw head are because you are using the wrong type of driver and are easily avoided with a little care. By the end you'll know which type of driver to reach for and why. Let's get started!

But wait, what about Bolts?

While closely related, a bolt is not a "screw" in the traditional sense. The common differentiation is that a bolt passes through the materials it is binding together and has a fastener (nut) on the other end.

I'll be dealing with bolts in a follow up article, so stay tuned.

A screw is actually two simple inventions in one small package. It is an inclined plane, wrapped around a cylinder which gives you a mechanical advantage over what you are trying to bind by forcing them together via mechanical advantage. It is also a wedge in that the thread's edge is a cutting edge that forces itself into the substrate you are screwing into.

A screw has four main parts: a head, a shank, the threads, and the point of the screw. The head fits the driver and holds the screw in place. It also provides a contra for the force of the screw on the outer surface of the substrate you are screwing into. The shank can be short or long, and connects the head of the screw to the threaded rod. It is the point and the threaded rod that do the real work of the screw. As you push the driver down, the point forces an opening into your substrate. Into this hole your thread will catch on the substrate and lever its way further along. Some threads are "self-tapping" which means that they will cut their own way down. Some screws like machine screws require that your hole be pre-tapped, or already have its matching thread cut into the hole which you are screwing into.

Tapping is its own whole article.

Over the last 150 years, screw heads have been engineered to fit just about every use case from the simple slotted screw to the modern Torx. Screws will come in inch or metric depending on where you are. If you are unlucky, you'll have to deal with both.

The most common types of screws are split into a few general families:

1. Slotted
2. Cruciform (just means some sort of cross)
3. Internal polygon (some of internal shape)
4. Hexalobular (has six points)

There are many (many!) more besides these, but you aren't likely to encounter them outside of engineering or heavy industry. If you're curious, the Wikipedia page is fun reading.

In general, if you purchase a set of screw drivers or a good bit set. You'll at least need three sizes of Standard (Slotted) and three sizes of Philips. With these you will have most of your bases covered.

Robertson

In Canada you'll have to add Robertson type screw drivers or bits to your set. They are quite common there and don't have any real overlap with a normal set. You'll be happy to have them when you need them.

Pozidriv

In Europe and Asia, you'll have to add a Pozidriv screwdriver set and to your standard bit set. They are similar enough to Philips but if you try to use a Philips set you will likely strip your screws as they don't fit properly. The angles of the flanges are different. You can identify Pozidriv by the cross on the screw. The bits themselves will have an extra set of flanges, as you can see in the image above.

Torx

Torx is the weirdo in the room, and is becoming more and more common. They have a big advantage when it comes to not stripping, and I highly recommend them. I built my shed with them recently and it was a pleasure to be able to have screws stay where I put them (on my bit) and doubly-so when I didn't have to toss a bit halfway through a project because it was busted. The biggest downside is, of course, you'll need a separate set of bits and drivers.

There are many, many types of screw heads. Of these types, some you'll only see matched with some specific types of screw drives, while others will be mixed and matched according to the type of screw and its use.

See the image above for a good overview of what Flat, Oval, Pan, Truss, Round, Hex, Socket Cap, and Button heads look like.

Now we are down into the nitty-gritty of it, and we can talk a little about some of the different types of screws and how to identify them.

Before we get started, I want to define to important concepts: TPI or threads per inch, and the "pitch" which is how aggressive the incline of the thread is.

Wood Screws

Wood screws, in their simplest form, are a single threaded screw type. They generally have a flat or oval head. These screws will have a smaller TPI and a more aggressive pitch.

Machine Screws

Machine screws are used in pre-tapped holes, or pre-drilled holes for machine screws which are self-tapping. These screws will have a higher TPI and a less aggressive pitch for each thread.

Plastic Screws

Coarse, sharply angled threads with round shank are used in plastics such as polypropylene, polycarbonate, acetal, polystyrene, and ABS to provide maximum holding strength with minimal stress.

Drywall Screws

Finally, drywall screws are a twin-fast (double-helix) type screw which has a medium TPI with an aggressive pitch. These are commonly used in plaster work, but can also be used in wood. They tend to be hard, brittle types of screws that are not good against corrosion.

Screws come in many different materials, from steel to plastic, to aluminum. The material used depends on the use defined. If you are working out-doors, you will want a corrosion resistant screw like stainless steel, or indoors where you can use galvanized screws which cost less.

Some common materials are:

1. Silicon bronze - used in marine environments. Highly corrosion resistant, very expensive.

2. Stainless steel - used where weather is a factor. Highly corrosion resistant, expensive.

3. Zinc coated steel - there are many variations and two methods (mechanical and electrical) for applying the coating. In general these should not be used outdoors as they are not very corrosion resistant, but they are relatively cheap.
4. Brass - these are soft metal and generally found as decorative or ornamental additions to wood projects.

A driver is a tool used to turn a screw into a substrate. There are four types of drivers for screws.

Manual Screw Driver

Most people will recognize these immediately, and are what you generally have on-hand. For any beginner, these will be your go-to as they are cheap and any set of tools will usually have them. They come in many different sizes, formats, and lengths. I'll get into the different screw types soon, but any beginner should start with a simple set like this which includes Philips and Standards in 3 different sizes, as it will include most of what you will encounter in day to day work.

Torqued Screw Driver

Torqued drivers have a clutch in the driver which can be set to a specific, required torque. The clutch will engage when to ensure that you don't over-tighten the screw. These are available in manual, electric, and pneumatic types. Torque screwdrivers can exert torques from 6 inch ounces (0.04Nm) to at least 27Nm. Although no single tool covers the entire range, low-, mid-, and high-torque ranges are available. These are a specialty tool and most beginners won't need them. They can be quite expensive!

Ratcheted Screw Driver

What you will likely want to get sooner rather than later is a good ratcheting screw driver. For tight spots or screws that you'd like to do by hand, these will save you a significant amount of effort. They have a ratcheting mechanism that allows you to set the screw direction, and when you screw in your screw you will be able to turn back your hand without turning back the screw! Very handy! Most ratcheting screw drivers come with a set of bits that are also handy when you have to deal with small bolts or lags. I highly recommend this set of ratcheting drivers and bits, as it will have almost everything you could possibly want for a good price.

Mechanized Screw Driver

Finally, we get to the fun stuff! Mechanized screw drivers come in many flavors, from the hefty, heavy-duty types to the light and easy ones that a home-owner might want to have handy. These drivers make quick work of jobs that used to be tedious and time-consuming.