At Component Supply we are privileged to work with thousands of people in all stages of their careers, from engineers and researchers in all different industries to college students just getting started in their field. As we work with a variety of customers, we’ve noticed that just because certain information is common knowledge in one discipline doesn’t mean it’s common knowledge in other industries. One product that we have found this to be true of is fasteners, so we want to provide helpful information about the head styles, drive styles and sizing of fasteners, specifically SAE fasteners.
The most common head styles are flat head and pan head. Flat head fasteners are typically used when a flat surface is a desired result because they will be flush with the surface being fastened. Pan head screws are used when this is not necessarily a concern. Other head styles include binding, socket and fillister head.
Many drive systems existing today have been developed for automation purposes. The hex drive, the slotted drive and the Phillips drive are the most common. The slotted drive is simply a slot in the head of the screw where a slotted driver can be inserted. The Phillips drive has a cross pattern. Finally there is the hex drive style, which uses a hexagonal key as the driver. Other common drive styles are Torx and Star drive.
Sizing for fasteners is broken down into two a pairs of numbers. The first number indicates the diameter of the material before threads are cut. This number can either be a fraction of an inch or a wire size number. The wire size numbers are preceded by a pound or number sign, such as #2-56. The second number indicates the number of threads per inch and typically gets smaller as the diameter gets larger. So, in smaller sizes it is a wire size and in larger sizes it is a fractional size. For example, #2-56 is a #2 wire size, and there are 56 threads per inch. The 1/4-20 is a quarter of an inch in diameter, and there are 20 threads per inch.
One more piece of trivia: the difference between a screw and a bolt is how the fastener is used. In very general terms, if the fastener uses a nut, it is considered a bolt. The same fastener would be considered a screw if threaded into something that had internal threads.