Few people know and understand the difference between metal screws and wood screws. In practice, these screws differ widely and can’t be used interchangeably. Why is this, and what’s the difference?
What is a Metal Screw?
Metal screws are typically used to clamp thick, uniform metal sections together. These are not self-tapping and thus require a pilot hole to be drilled. Usually, the pilot hole should be tapped (threading operation) as well.
Three Types of Metal Screws
1. Machine Screws
Machine screws are generally used for thick metal pieces and are not self-tapping. Here, the entire shank is typically threaded, and the screw fastens into a pre-tapped hole that matches the screw’s thread pattern. For added security, machine screws may be fastened using a nut.
2. Cap Screws
Cap screws are typically used to fasten metal plates used in dies, molds, machine parts, parts found in appliances, and consumer electric devices. These screws are usually fastened without a nut and have a large head attached to a threaded cylindrical shaft. The head provides a mechanical stop when tightening the screw, generating a large clamping force.
Cap screws require you to drill a pilot hole equal to the core diameter of the screw and the threads need to be machined as they are not self-tapping.
Generally, the metal screws have finer threads (more threads per inch) when compared to wood screws.
3. Sheet Metal Screws
Sheet metal screws also have a fully threaded shank, but the tip is sharp, unlike other metal screws. This is because sheet metal screws are usually self-tapping.
I will explain the design and working of the sheet metal screws in the following section.
What is a Sheet Metal Screw?
Sheet metal screws are highly versatile screws meant to use in thin materials. These are typically self-tapping but can also be self-drilling (more on that later). They have a sharp tip and full-length threads, allowing them to cut their own thread into most materials. Thus they don’t require a pre-drilled pilot hole.
Sheet metal screws are versatile and can be used in metal, plastic, and even wood.
Screw Head
Sheet metal screws have various types of heads, usually referred to as pan, round, flat, or oval. The head type determines what the screw looks like on the finished product – whether it protrudes, is flush with the surface, or sunk. Pan or round head screws typically protrude above the surface after installation. In contrast, flat or oval countersunk screws are usually flush with the surface.
Drive Socket
Then, there’s the drive. This refers to the type of screwdriver needed to install the screw. Typically, screw drives are designated as Philips, flat, or combination, but several customized drives are also available. These customized drives are usually found on anti-tamper screws, and you need a special screwdriver to install them.
What is a Sheet Metal Screw Made of?
Carbon steel is most commonly used to manufacture sheet metal screws since it’s economical. The downside here is that carbon steel is prone to rust when exposed to moisture and harsh chemicals. For this reason, carbon steel sheet metal screws are best suited for indoor use.
An alternative to carbon steel sheet metal screws is galvanized or stainless steel screws. These are more corrosion-resistant and can be used outdoors. However, they’re generally more expensive than their carbon steel counterparts.
As in some aesthetic projects, zinc- and nickel-coated sheet metal screws are often used when an alternative surface finish is desired.
Self-Tapping vs. Self-Drilling Screws
Sheet metal screws are typically self-tapping or self-drilling.
Self-Tapping Screws
Self-tapping screws are equipped with a sharp tip (like a wood screw) that can penetrate through metal to create the grooves needed for its thread. It still needs a pilot hole, though. This pilot hole has the same diameter as the core diameter of the screw shank, so the threads will still cut into the sheet metal or other material.
What is a Self-Drilling Screw?
Self-drilling screws have a drill point tip that will cut through the sheet metal without a pre-drilled hole. These screws typically have a sharp, grooved tip. This allows the tip to “bite” into the material when drilling, thus eliminating the need for a pilot hole. When used in wood, the tip shape prevents the timber from splitting.
Metal self-drilling screws have a broad tip, similar to a shovelhead. These screws also don’t require a pilot hole since the screw tip drills through the metal, creating its own pilot hole before the thread cuts into the material.
What is a Wood Screw?
Wooden screws consist of four parts: the head, shank, threads, and tip. The tip is usually sharp and pointed to guide the screw precisely as it cuts into the material. The shank forms the screw’s core, with the thread wrapped around it. The thread cuts grooves into the material as the screw is driven deeper, allowing it to grip the material securely. This grip prevents the screw from being pulled out, securing the workpieces together.
Thread & Shank
Screws are either partially threaded or fully threaded. Partial threading indicates that the thread stops at some point along the shank before reaching the head. In contrast, fully threaded screws have thread covering the entire shank.
Wooden screws can have coarse threads or fine threads. Coarse threads are better for softwood and plywood, while fine threads are better suited to hardwood. The general rule of thumb is that coarser threads for with softer materials since these are more likely to chip or crack.
Wood screws vary in length and diameter, with longer screws typically having a wider diameter.
Wood Screw Drive Types
Screw heads have two components: shape and drive. The drive determines the type of screwdriver needed to install the screw. Some wood screws use a slotted drive, often called a flathead, referring to the type of screwdriver needed. Installing these screws requires a lot of patience since they don’t work well with drills or impact drivers. They’re still widely used, though, since historically, these were the first types of screws widely circulated, and they’re easy to manufacture.
Then, there’s the Philips drive, patented by Henry Philips in 1930. This screw drive is shaped like a + and comes in various sizes. When installing one of these screws, ensure that your driver bit matches the screw’s size since they don’t work well when using the incorrect drive size.
Next, there’s the square drive. This screw head has a square-shaped hole at the top, which fits a square-shaped screwdriver tip. Here, the screw and screwdriver sizes must match perfectly for this to work well. The benefit of these square drives is that they work well with drills and impact drivers since they rarely slip.
Lastly, the stardrive or Torx is similar to the square drive, except that the hole in the screw head is star-shaped. These are ideal for projects that require extremely tight fastening since they can handle a lot of torque. Stardrive screws are typically available in premium quality, ensuring that they won’t snap when overtightened.
Wood Screw Head
Screw heads typically come in one of two shapes: flat or rounded. Screws with a flat head aren’t necessarily the same as flathead screws (slotted screws that fit with a flathead screwdriver).
Countersunk screws with flat head can have any drive and are the most common screws for woodworking since they offer a clean, neat finish. These screws typically have a beveled head that sinks into the wood, allowing the head’s surface to be flush with the wood’s surface. This flush finish is achieved in one of two ways: driving the screw into the wood until it’s flush or countersinking the surface of a pilot hole. The latter tends to be neater, but usually, the former option is sufficient. This depends on the desired aesthetic since the second option is generally neater.
Panhead or roundhead screws have shallow or domed heads, and they don’t sink into the wood like flat head screws. These are typically used when attaching other material, like plastic or metal, to wood, in a way that you won’t countersink.
Other Wood Screws
Some other types of screws that can be used for woodwork, but don’t necessarily offer the same quality finish, include drywall screws and multi-purpose screws.
Drywall screws are generally less expensive and are often used in temporary projects or don’t require a high-quality aesthetic finish. However, they are not strong and can break especially if you try to drive them into hardwood.
In contrast, multi-purpose screws are generally more expensive, but they’re self-drilling, high-quality, and highly versatile. These screws will work for nearly all projects.
There are also lag screws, carriage bolts, and structural screws used for heavy-duty carpentry and construction work.
Wood Screws vs. Sheet Metal Screw
Wood screws and sheet metals screws are both typically self-tapping, eliminating the need for a pilot hole.
Wood screws typically have a coarser pitch and thread than sheet metal screws. This allows for a better penetration in brittle wood that’s prone to cracking. In contrast, sheet metal screws typically have a fine thread, allowing better grip in the malleable metal they’re used in.
Sheet metal screws tend to be shorter than wood screws, although some tiny wood screws are available on the market. While wood screws generally have flat, beveled heads that allow for a flush surface finish, sheet metal screws have dome heads that protrude from the surface.
The reasoning here is practical: in woodworking, you generally want a neat, clean aesthetic, hence the countersunk screws. Sheet metal is typically much thinner than wood, so there isn’t much space to countersink screws, hence the protruding dome head.
Can you use sheet metal screws for wood?
Yes, you can use sheet metal screws for wood. In some cases, they may not be long enough for the task at hand, but they will generally do the job in a pinch. When you’re working with softwood, take care when opting for sheet metal screws, though. The sheet metal screws’ fine thread might cause soft, brittle wood to crack.
Dan Miller is a professional toolmaker with extensive experience in the metalworking and tooling industry.
An avid DIY enthusiast and passionate hobbyist woodworker, Dan shares his expert knowledge here.