Screws

Mechanical fasteners, such as screws, have threaded shafts and are designed to screw into parts.  Wood and self-tapping screws, which have a tapered shaft and sharp threads designed to cut a matching thread in the part they are attached to, fall into this category.  It also includes machine screws, which much more closely resemble bolts, but their entire shaft is normally threaded.

They are sometimes delineated by their use.  They mate with a threaded hole in one of the parts being fastened.  Fasteners that are either self-tapping or have their entire shaft threaded are normally called screws.

To remain securely fastened, they generally reply on friction at the threads. In the case of the self-tapping version, radial expansion of the hole causes much of this friction.  For the machine type, friction is mostly caused by axial force resulting from the torque applied to the head and the subsequent tension in the screw combined with compression of the parts.  Machine screws may alternatively, or additionally, use a locking nut or thread-locking adhesive to prevent loosening.

Machine screws

Machine screws have a standard thread on a parallel shaft, designed to be screwed into a tapped hole in a part, they may also be used with a nut. The most common type is the hex screw — which is very similar to a hex bolt, but without the shank. There are many types of machine screws for specific applications, generally differing in the shape of the head:

• Countersunk head screw
• Cap head screw
• Pan head screw
• Grub screw

Socket-headed screws may have many different socket types.  Some of the most common socket types are:

• Hex socket or ‘Allen key’
• Crosshead
• Torz
• Pozidriv

Self-tapping and Wood screws

Self-tapping is a term generally used for screws designed to cut their own thread in sheet metal, although in a general sense wood and plastic screws are also self-tapping. These types often have a sharp pointed end and sharp threads on a tapered shaft.