A primer on how heads, threads, tapers, and drives differ depending on application
Given the variety of screws and bolts available (collectively called fasteners), you could easily write a book on the topic, and in fact, someone has!
Fastener geeks the world over rushed to pick up their copies of this fascinating book, which was published just a few years ago: One Good Turn: A Natural History of the Screwdriver and the Screw by Witold Rybczynski
Why so many types of fasteners?
Essentially, there’s a fastener for every category of material (wood, metal, masonry, etc.), and for every subcategory (hardwood, softwood, engineered wood, OSB/chipboard, MDF, drywall, etc.).
Isn’t a fastener just a fastener? How many features can one have?
Rather than review all fastener types, which would be an exhaustive study, let’s look at the key features available, and with those in mind, you can make an informed decision about what fastener is right for your job.
Figure 1. There are more than a dozen types of fastener head types, including the common ones like flat, oval, pan, truss, hex, socket-cap, and button.
In most fasteners, the head is larger than the shaft or screw body. The exception is set screws, where the head is purposely smaller than or the same size as the shaft. When the head is smaller than the shaft, the fastener can offer no bearing surface and the screws entire performance is delivered by the holding power of the threads through friction (as you see with dowel screws).
There are easily a dozen types of fastener head types, including the common ones like flat, oval, pan, truss, hex, socket-cap, and button (see Figure 1).
Figure 2. Each head has a type of drive. Exotics like 12-point flange, Bristol, and Pentalobe are curiosities, where the commons ones are Phillips, slot, and square torx.
Each head has a type of drive. Exotics like 12-point flange, Bristol, and Pentalobe are curiosities, where the ones we usually encounter are Phillips and slot, as well as the square and the torx, a six-pointed star-shaped screw that reduces blowout, because there is so much more surface area of contact between the drive and the fastener head (see Figure 2).
Taper and threads
OK, put aside all the details you just learned about the head and the drive, because the real differentiator among fastener types come with the thread and the taper (or non-taper) of the fastener shaft. Let’s focus just on common wood screws.
A traditional wood screw has a tapered shaft. It is tapered so it can be started more easily in the wood. As with most screws, the thread is at a 60-degree angle. The potential problem with tapered wood screws is that as the shaft expands, it can split the wood, especially with hardwoods.
Another type of wood screw can solve that problem. It has the same sharp point but its shaft is not tapered, except at the point. This is sometimes called a “modern” wood screw. This same non-tapered design is also what you commonly see in a masonry screw (where a taper would crack the masonry), or with structural wood screws, especially longer versions of these types of fasteners (see MiTek Hex Head structural wood screws).
The class isn’t over, students! We have simply run out of space, and we haven’t even covered thread coarseness, thread pitch, coatings, and anti-corrosions treatments. Let’s save that for another article.