The Ultimate Guide to Types of Screws

Screws seem simple. Bust out your screwdriver, line up the driver with the screw head, give the screwdriver a couple turns, put some extra muscle into it to get the screw really tight, and boom, two things that were once separate are now held together. Of course there are more to screws than this – as you may have experienced if you’ve ever stripped a screw head, used the wrong sized screw or tried to drive a metal screw into wood. This ultimate guide to types of screws will make you an expert (or close enough to one) on the different types of screws and how to choose the right one.

​Basics of a Screw

​A screw is a type of fastener. Fasteners hold two things together mechanically, and include nails, bolts, clamps, clips and pins. Contrast this to something that holds two things together chemically, like glue or chewing gum. Or something that holds things together magnetically, like a refrigerator magnet.

​While “standalone” screws are used for fastening two other distinct objects with the screw, screw threads can be built into one of the distinct objects, as in the case of a garden hose and faucet, obviating the need for a separate standalone screw. A third object isn’t needed to fasten the hose and faucet, the screw threads are already built into both of those.

​Mechanics of a Screw and Its Uses

​(Want image of helical device here)

Besides being a fastener a screw has other uses. Screw mechanics translates rotation into lateral motion. Looking in a shop, you might see a screw used in your wood clamps. If you were Archimedes you invented a way to move water uphill by turning a large screw. Screws are used for gear reduction and in submarine propellers (go watch The Hunt for Red October again). You might find a screw doing work in a machine as a leadscrew or power screw. All of these uses of a screw are worth studying, especially if you're mechanically inclined, but beyond the scope of this particular guide.

Parts of a Screw

(Want image of screw here showing parts)

​A screw has a head that's used to drive the screw with a tool like a screwdriver, wrench, impact driver, fingers or teeth (don't pretend like you've never unscrewed something with your teeth). The head diameter is usually greater than the diameter of the overall screw to prevent the screw from being driven too far into the opposing object or material being fastened, and causing the point of the screw to come out the other side. The underside of the head provides a bearing surface that contacts with the material being fastened to secure it in place.

Below the head is a cylindrical shaft, called the shank. We'll use shaft and shank pretty interchangeably in this guide. Shanks can be tapered or non-tapered, depending if the screw needs to cut into the material it's being driven into or not.

Along the shank, or shaft, is a corkscrew-like, twisty, helical ridge, called the thread, that runs fully or partially the length of the shank.

Head, shank and thread. Those are the three parts of a screw.

Bolts, which we'll discuss later, have these same parts. A bolt will typically need to be paired with something like a nut, which has the opposite threads as the bolt so that they'll join. We discuss threads more in the next section.

(image of bolt)

​More Details Than You Ever Wanted to Know About Screw Threads

The thread running along the shank is the male, or external, thread. In some cases, as with a bolt or some metal parts, the screw will drive into a pre-existing reciprocal female, or internal, thread. The male or female (external or internal) property is unsurprisingly called the gender of the thread. The internal thread might be in a nut or cast into the metal of your mailbox. Non-tapered screws and bolts need to be used when paired with an internal thread.

Many times the screw will cut its own internal thread as it's driven into the material with which it's being joined. For example, wood screws do this when you drive them into a block of wood. This is part of the reason why it's difficult to straighten a wood screw when it's been driven in at an angle even if you back it out- the internal thread has already been created in a slanted position. Better to fill that hole and try again. Screws need to be tapered to cut their own threads.

Threads Per Inch

Threads per inch is the number revolutions of the twisty, helical ridges - threads - there are per inch of length of the screw. You need to understand the concepts of lead and pitch to completely understand threads per inch.

Screw lead

Lead is the axial travel, or linear advancement (or lateral advancement), for one revolution of the screw. Remember, the mechanics of a screw translates rotation into linear or lateral motion. To be clear, axial travel, linear motion, and lateral motion are used in this article to mean the same thing. Travel and movement are used to mean the same thing. Rotational motion, twisting, revolution and turning motion are used to mean the same thing.

(image of rotational motion and lateral motion and lead)

OK, now we're going to get in to the stuff you'll overhear during class changes in the mechanical engineering department. Axial travel is travel along the length of an axle. In a screw, the axle is the threaded shank. When you turn the screw one revolution, how far into the material it's being screwed does the screw move? That distance of movement into the material is the lead. This assumes that the material you're screwing into doesn't give, causing the screw to slip - but assume perfect threads for this definition. You have an innate sense that when you turn a screw, it moves axially back and forth. Now you know the definition of what the length of that movement is - the lead.

So converting, or translating, rotational motion into linear motion is the purpose of the screw. The screw moves linearly when you turn it. But why go to the trouble of designing a fastener that you have to turn, instead of simply using a nail and hammering the fastener in with a linear striking motion? Because the screw prevents linear motion back out without a turning motion. The screw won't pull or out of the material (again assuming the material doesn't fail or give) without first turning it. An advantage of screws is that they won't pop out like nails will. Alas, normal screws are not as strong as nails when resisting shearing forces - those that try to rip the screw in two through its cross-section.

Back to our lead discussion, if you increase the lead the screw will drive deeper and longer per revolution. How would you physically increase the lead by changing the threads on the shank? You would move the ridges of the threads farther apart. In a screw with a single thread, this would make the angle of the thread down the shank get steeper. Changing this angle will cause more force to be required to drive the screw. You could in fact increase the steepness of the ridges all the way straight down, turning it into a ridged nail, and negating its resistance to popping out.

Screw pitch

Pitch is the distance from crest to crest of the ridges of the screw threads measured along a straight line down the shank. You can think of pitch as the wavelength of a wave, the distance between the tops of the waves. Screws with a fast pitch have more threads per inch, and those with a slow pitch fewer.

So far we've only been talking about single start screws, meaning that there is a single thread running down the shank. If you could unravel the metal thread from the screw you'd have a single piece of thread. In single start screws there is a single thread, and the lead is equal to the pitch (wavelength). That makes sense. If you make one revolution of the screw, you go the axial distance from crest to crest, so the lead and pitch are the same.

Now let's talk multi-start screws. This means there are multiple separate threads on the screw. How are multi-start screws constructed? Let's take an example of a 3-start screw. Each thread begins 1/3 of the way around the circumference of the shank from each other. The angle of the threads down the shank will have to be different than in a single start screw, otherwise the threads would cross each other. You can help visualize this with pencil and cotton thread. With one piece of cotton thread, you can wrap the pencil from eraser to point tightly. Now try to wrap the pencil with a piece of red, a piece of white and a piece of blue cotton thread. The red, which and blue threads aren't allowed to cross, so they're angled down the shankless than the single piece. The threads can still be wrapped tightly, but each individual thread is spaced farther apart to allow the other two threads to fit in between it. Same concept with a screw.

In a multi-start screw the pitch is still the distance from one crest to the next crest, but you you're measuring crests from different threads, thread 1 to thread 2, or thread 2 to thread 3. The lead is not equal to the pitch though, because there are multiple threads on the shank. Back to our 3-start screw. When you rotate the screw one revolution it's axial travel is the width of the 3 threads. This is a simplification, but in a multi-start screw lead is equal to number of starts multiplied by the pitch.

Why go to the trouble of making a multi-start screw instead of simply increasing the pitch in a single start screw? Multi-start screws are more complicated to make. But a multi-start screw can provide the same lead with a smaller pitch as a single start screw with a greater pitch, while being stronger and providing more thread contact area to reduce pull out.

(lead and pitch image)

Metric Screws

A quick note about metric screws. Their measurements are expressed by their pitch, as opposed to threads per inch that you see in Imperial standards. Pitch and threads per inch are reciprocals of each other - if the pitch, the distance between crests, is 5mm (about .2 inches), then you could fit about 5 threads per inch.

Coarse and Fine

Coarse threads have a larger pitch and fine threads have a smaller pitch. Coarse threads have a thicker thread and fine threads have a thinner thread.

Thread Form

Look at a screw thread close up. Extending our cotton thread example of the thread winding down the screw shaft, imagine cutting through the cotton thread and looking at the cross-section. It would be a circle. The cross-sectional shape of the screw thread is called the thread form. It could be any shape, with triangular being popular as a triangular thread form moves easily through the female thread.

A triangular thread form usually doesn't form a complete triangle extending up from the shaft. The top of the triangle is lopped off. If the triangular thread form weren't lopped off there would be no clearance between male and female thread and slight imperfections or even dirt would make it very difficult to drive the screw. The tip of the triangular thread form would also be very brittle.

(picture of triangular thread form here)

The thread angle of the thread form is the angle of the two sides of the trough between the threads.

(thread angle picture)

Screw Diameters

The three diameters of a screw to concern yourself with are the major diameter, the minor diameter, and the pitch diameter. The major diameter is the diameter to the top of the thread form - the crest of the wave made by the thread. The diameter is twice the radius from the center of the screw shank to the top of the thread. The major diameter is also called the thread diameter.

The minor diameter is the diameter to the trough of the wave or valley made by the thread. The diameter is twice the radius from the center of the screw shank to the bottom of the thread. The minor diameter is also called the root diameter.

The height of the screw thread can be calculated by subtracting the minor diameter from the major diameter and dividing by 2.

The pitch diameter is a little trickier to explain. It's the diameter to the point on the thread where the lateral thickness of the thread is equivalent to the lateral distance between threads. Think of ocean waves and imagine the point on the curve of the wave where the wave depth equals the distance to the wave coming up being it. Measure from that point to the axial center, which is a radius, and then double it to get the diameter.

Screw Fit

In order to fit, the diameters, major, minor and pitch, of the external thread of the screw must be less than the diameters of the internal thread. A screw and nut won't be able to fit tightly if only the major and minor diameters are close - the pitch diameters must be close too. Simply put, you can't fit a larger screw or bolt into a smaller nut. At the same time, the external thread diameters can't be too much smaller than the internal thread diameters or the external screw will fall out.

Clearance fit is the situation where it's relatively easy to screw into the nut - you can do it with your fingers. Interference fit is the situation where you have to use a tool like a wrench to tighten into the nut. The Unified Thread Standard used in the U.S. and Canada defines tolerance standards that relate to fit and range from 1 being the loosest to 3 being the tightest.

Handedness of Screw Threads

Threads are manufactured in a twisty spiral up and down the screw shank. They can spiral along the shank in two directions. The direction of spiral is the handedness of the screw, right-handed or left-handed. The easiest way to determine handedness is to hold a screw upright, with the head pointing up and the tip pointing down. Look at the way the threads spiral up the shank. If they spiral up from left to right you're holding a right-handed screw. If they spiral down from left to right you're holding a left-handed screw.

With a right-handed screw if you twist it right it drives axially down and into the material or female threads and if you twist it left it drives axially up and out of the material or female threads. Left-handed screws are the opposite - twist them left to go down and right to go up. Righty-tighty lefty-loosey only applies to right-handed screws.

Look down the shank to see why screws drive this way. Remember in a right-handed screw the threads spiral up from left to right, so the upper right of the tip is being pushed farther in when you twist the screw right. A point on the thread appears to move away from you when twisted in that right or clockwise fashion.

(image of right and left handed threads)

Most screw threads are right-handed. There are some useful applications of left-handed screws. A couple that we've seen around the house include the left pedal on a bike. When pedaling a bike the left pedal tends to move counterclockwise, or to the left. You want the pedaling movement to actually tighten the pedal, so left-handed threads are used for lefty-tighty. Our weed torch that connects to the propane tank uses left-handed threads to prevent the wrong things being hooked up to it - like an oxygen tank or garden hose - which would be extremely dangerous.

Screw Measurements

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Screws have three basic measurements: gauge; threads per inch (covered above); and shank length in inches. You will find the measurements written as 'gauge - threads per inch x shank length in inches'. An example measurement could be 8 - 20 x 2". The 8 means the screw has a #8 gauge diameter (which is a 0.164" major diameter); the 20 means it has 20 threads per inch - if the middle number is missing then the screw has the standard number of threads per inch (you can find this information online) , which is 15 threads per inch on a #8 wood screw; the 2" means the screw is 2 inches long. Length is independent of gauge.

Metric screws specify measurements as: 'major diameter (in millimeters) - pitch (in mm) x length (in mm or cm)'.

You can find handy charts of wood screw sizes many places. You can find handy charts of machine screw sizes too.

Screw Gauge

Screw gauge specifies screw major diameter. Screw diameters increase as the gauge increases, but a #4 gauge isn't twice the diameter of a #2 gauge. There is a linear equation for screw gauge though. y = 13x + 0.060", where x is the screw gauge number translated to thousandths and y is the diameter.

For a #2 gauge: 13 x (#)2 + 0.060" = 0.026" + 0.060" = 0.086"

For a #8 gauge: 13 x (#)8 + 0.060" = 0.104" + 0.060" = 0.164"

After you get above about #16 screw gauge starts to be presented as the actual measurement, so you might see 1/2" or 1" in the screw gauge field.

Screw Length

The length of screw is taken from the bearing surface of the head to the point of the screw.

(image of screw lengths)

Types of Drives

The drive is the shape in the screw head that you place your driving tool into so you can turn the screw. To make it as confusing as possible, screw designers have tried to make as many different drive shapes as imaginable. Just kidding. There are several different types of drives that we'll go over.

Slotted/Flathead

Like the Rose Bowl, this is the grandaddy of all the drives. This is the original screw drive. Very popular, and terrible to use. It is very easy to cam out when driving a flathead screw. To cam out means for the driving tool, in this case a flathead screwdriver, to slip out of the drive in the head when too much torque is applied. This is easy to do on flathead screws and makes them difficult to tighten. Camming out also tends to damage the screw.

Phillips/Crosshead

This drive type is also very popular. It's a plus. Phillips drivers self-center within a Phillips head. A Phillips head provides better protection from camming out than a flathead, but not much. Camming out on these screws tends to really tear up the screw. Why are these screws (and flatheads) made? Because they're cheaper to make. You will see sizes of these like Phillips #2. Each size of Phillips screwdrivers fits a couple of sizes of Phillips screws, but there is not a direct correspondence in the numbers.

Square

We love this drive type. It's very resistant to cam out and you can readily hold the screw on the driver at a 90 degree angle without it falling off. You can't do that with a Phillips or some of the other specialty drives. The popular sizes are Square #2 and #3.

Pozidriv

This drive is more common in Europe. It's similar to a Phillips, with an additional, smaller plus set at a 45 degree angle to the big plus, giving additional torque points to prevent cam out.

Torx

Torx drives make the most contact with your driver, which reduces slippage and improves driving power. They require compatible driving tools in sizes specified by a Torx number such as T10.

Hex

Hex drives have flat sides, so they’re easy to tighten with a wrench. Hex size is measured from flat to flat.

Tamper-resistant

Tamper-resistant drives add a level of security. They come in a variety of nonstandard shapes that are difficult to remove without special tools. A one-way head is the ultimate in tamper protection. They're intended to be able to be driven in with a standard driver but made not possible to drive back out.

Screw Head Types/Shapes

Screw head type is the shape of the screw head. Shape is important not only for aesthetics, but also for the mechanical properties of the screw. A screw head type with a greater head diameter to shank thread diameter provides a greater bearing surface.

Countersunk/Flat

This head type is meant to be driven flush with the connecting surface. You will commonly find this in wood screws. Countersinking drill bits will carve out the space in the connecting material to allow the screw to be countersunk.

Oval Countersunk

These are countersunk just like a flat head type, but leave part of the rounded head above the connecting surface for decorative purposes.

Round

A round head provides a decorative look with a good bearing surface.

Pan

Pan head types have a large diameter and a low height, like a frying pan, not rounded like a bowl. The outer edge sides are straighter than in a round head which makes them easier to drive than a round head.

Truss

The truss head type is also called oven head or stove head. This is wide head type that gives hold in thin materials.

Button/Dome

This is a cylindrical head with a rounded transitioning to flat top.

Fillister and Cheese Head

Fillister and cheese head types are tall and narrow, making a cylindrical shape.

Binding

The binding head type is used to bridge wires and other electrical connections.

Hex

Hex heads have 6 sides and can be driven with a wrench.

Screw Point Types

Screw points can be pointy, blunt, tapered, non-tapered and have other characteristics. Tapered screws are self-tapping screws.

Self-tapping screws come in two types - thread forming and thread cutting. Thread forming screws leave the material being driven into in place, requiring more torque to drive the screw, but allowing the screw to be very difficult to pull out. Type A, Type AB and Type B are thread forming screws.

Thread cutting screws are self-tapping screws that move some of the material being driven into out of the way, making the screw easier to drive and putting less internal force on the material. Type 25, Type F and Type 23 are thread cutting screws.

Self drilling screws allow screws to be driven into a material without the need to first drill a pilot hole.

Screw Material Composition and Coating

Do you like your screws to be magnetic so they'll hold to a magnetic drill bit? Are you worried about your screws conducting electricity? What about rust? A screw's material composition affects its properties.

Steel is a very common material out of which to make screws. With the proper iron content in the steel, the screw will be magnetic. You'll find steel screws are often zinc-plated so they'll resist rusting, with resist being the operative word. Zinc screws used in outdoor applications will rust. A water resistant coating, like what you find in deck screws, is needed to make a steel screw waterproof. Stainless steel resists rusting and corrosion better than zinc plating.

Brass screws resist rusting well. They are an excellent conductor of electricity, as they contain copper. Aluminum screws also resist rusting and are good electrical conductors. We mention conductivity to make you aware of what will happen if you screw a conducting screw through an electric line. Brass and aluminum aren't magnetic.

Ceramic screws can be manufactured to be heat resistant. Ceramic isn't conductive and can be used as an insulator.

As for coatings, we mentioned zinc plating earlier. Chrome plating on screws is an option to make the screw shiny. Black oxide coating makes the screw more resistant to rust/oxidation/corrosion. Coatings are used for decorative purposes in addition to corrosion resistance.

Types of Screws

We've reviewed the mechanics and characteristics of a screw, so let's talk about real world screws and their applications. There are many types of screws ("but this one is mine" - email us if you catch the reference). Each type is best suited to a particular job.

Wood Screws

Wood screws are used to join one material with a piece of wood. The one material is usually another piece of wood, but it could be almost anything.

Wood screws may have threads that don't run the entire length of the shank - a portion of the shank starting at the head may be smooth and threadless. Wood screws create their own internal threads. If there is a gap between the materials being joined and the internal threads that are created don't match up the materials being joined will maintain the gap between them. Having a portion of the shank be smooth allows the upper material being joined to the lower material to be pulled tight into the lower material.

Drywall Screws

Drywall screws are used to fasten drywall to wood studs or metal studs. Coarse threads are used for wood studs and fine threads for metal studs.

Machine Screws

Machine screws are used with nuts or with materials that already have external threads created and are often used to secure metal parts. Machine screws and bolts are similar. Their ends are typically blunt.

Sheet Metal Screws

Sheet metal screws are used to join one material with a piece of thin sheet metal - a metal brace to a garage door for example. The one material is usually another piece of metal, but it could be any material.

Socket Screws

Socket screws are used when there is little space to fit a wrench in to turn a screw. They usually have a hex shape cut out of the round head into which you can insert a hex wrench.

Set Screws

Set screws don't have heads. They are used to fasten objects when you don't want to have a head sticking out of one side and to prevent separation of moving parts - machinery being an example. The bearing surface is not the underside of the screw head since that doesn't exist, but rather the tip of the set screw.

Bolts

Bolts are threaded fasteners similar to screws. Like a screw, a bolt will hold two things together, but a bolt requires a nut with an internal thread to be attached and tightened from the opposite end of the shank from the head to complete the fastening. A screw drives into an internal thread in the things being fastened, and may cut those internal threads itself as it's being driven.

Bolts can be turned and driven into a nut, or a nut can be turned and driven onto a bolt. Contrast that with the convention that screws are turned and driven into the material - in almost no cases is the material being turned and driven into the screw; only the screw moves.

Hex Bolts

Hex bolts are very common bolts that have hexagonal heads and are driven by wrenches.

Carriage Bolts

Carriage bolts have round heads without slots for any kind of driver. Immediately under the bearing surface of the head is a square neck that fits into a square pocket of one of the materials being fastened. The nut is turned and driven onto the carriage bolt, not the other way around.

Lag Bolts

Lag bolts or screws are thick bolts (large diameter) with hex heads. They are designed to withstand greater loads and shearing forces than regular screws and bolts. A lag bolt is usually driven with a hex shaped driver like a ratchet. One application is to fasten a heavy item into wall studs.

Eye Bolts

An eye bolt has a round loop instead of a traditional head. Hooks can be attached to the round loop.

Eye Lags

An eye lag is a lag bolt or screw with a round loop at the head. They are sturdier eye bolts.

J-Bolts

A j-bolt is a bolt with its shank formed into a j shape at the head end. Hooks can be hung off the j end.

U-Bolts

U-bolts have their shanks shaped like a u, and both ends are threaded instead of having a head. U-bolts can be used to secure a pipe, which fits under the u shape.

Shoulder Bolts

Shoulder bolts have a long portion of their shaft that's unthreaded and of a greater diameter than the threaded section. Pulleys or gears are attached and rotate around the unthreaded section. Engines may have shoulder bolts to support their moving parts. And if you've assembled a skateboard you put the skateboard wheels on shoulder bolts.

Elevator Bolts

Elevator bolts have a large flat head and a square neck under the head like a carriage bolt. They're called elevator bolts because they were originally used in grain elevators. Today you might use them to attach a skateboard deck to the trucks.

Sex Bolts and Mating Screws

A sex bolt is actually a nut with a barrel like a traditional nut and a head attached to the end of the barrel. A mating screw fits into the sex bolt, so you end up with kind of a barbell-shaped fastener when the two are joined.

Hanger Bolts

Hanger bolts are threaded at both ends and have no head. On one end the threading is designed to be driven into wood (self-tapping) and on the other end a nut is designed to be attached.

Flange Bolts

Flange bolts have a flanged neck directly under the bolt head. They look like a washer has been welded directly under the head. The flange is indeed designed to act like a washer increasing the bearing surface and spreading the fastening load over a greater area.

Break Off Screw

A break off screw is a variable length screw. If you don't know how long of a screw you need you can use a break off screw. The shaft can be broken off at certain points without damaging the threading.

Bibliography

Machinery's Handbook
http://www.radax.com/
http://diy.stackexchange.com/questions/13824/understanding-screw-sizes
http://www.technologystudent.com/
http://store.curiousinventor.com/
http://www.wisegeek.org/
http://www.roton.com/