Screws are essential tools for building, repair and many other projects. Using the right screws will help ensure that the project is completed correctly and will avoid potential problems such as loose screws or snapping screw heads.
The American Industrial Revolution in the 1700s and 1800s brought increased demand for fasteners and created a need for an American thread standard. William Sellers’ system improved upon the Whitworth British standard and boosted productivity, simplified machine repairs and united diverse machinery makers and users coast to coast.
Length
Unlike metric screw sizes which list both diameter and thread pitch, imperial screws typically only list the gauge (or “gauge number”) and length on their packaging. The length is usually specified at the end of the callout, such as in the example of a Duraspin #8 x 1-1/4″ screw:
Length measures the distance from directly under the head to the tip of the screw. Screws with heads that can be driven into a surface without needing to go all the way in are known as countersinking screws; examples include hex-, pan-, and button-head screws. Some screws like hex-, oval-countersunk, and bugle-head screws can be countersunk partially, or not at all; these are called non-countersinking screws. Occasionally, you will find a screw that cannot be countersunk at all and is considered to be fully flush with the workpiece; these are called non-countersinking wood screws. Choosing the correct length is important, since driving a screw into a surface too deep can split or damage the material.
Diameter
For machine screws that belong to the inch series, there are two standards commonly used for size designation: the Unified Screw Thread System (which uses fractions) and the ISO metric screw thread system (which uses millimeters). In each case, a one- or two-digit number is expressed to indicate a diameter, followed by the number of threads per inch, and then the length in inches.
While the metric system is gradually displacing the imperial inch-based system in the US, most of the rest of the world still uses the former standard. As such, it’s important to understand both specifications if you’re planning on using screws in a global project.
The simplest way to understand screw diameter is to look at the gauge. This number indicates the thread’s diameter in inches, and it increases by a certain amount for each successive size. MISUMI carries over 60 different types of screws, and each one has its own unique gauge number.
Threads
The threads of a screw are its most distinctive feature. They can be square, triangular, trapezoidal, or even helical, but most are based on an isosceles triangle called a V-thread. The symmetry of the V shape helps to generate a tighter fit and greater strength than would be possible with more irregular shapes.
The majority of US screws have a coarse thread; there is also a less common fine thread standard. In either case, the diameter and pitch (the space between threads) are indicated by a number with a multiplication symbol at the end, for example, #8-32 screws have an outer diameter of.32″ and a pitch of 32 threads per inch.
These US sizes are standardized internationally as ISO metric screw threads preferred series, which largely displaced older systems like Whitworth and the BA system (British Association). Despite the fact that these unified thread standards were created to facilitate more reliable inter-national interchangeability, many hardware products are still labelled or cataloged with old American National or SAE specifications.
Tolerances
The essential principal is that the actual thread profiles of a bolt and nut must never cross or transgress each other. Practically this requires tolerances to be applied which have nothing to do with the degree of quality or craftsmanship of either the nut or screw.
These tolerances are indicated by a combination of a tolerance band (signed as an alphabetic letter) and a tolerance class (signed as a number). Generally speaking the higher the class the tighter the tolerance.
The letter also indicates whether the threads are internal or external. For example H signifies an internal thread while g signifies an external thread. A table is provided that relates the tolerance bands and classes to their positions on the pitch diameter, thus determining the fundamental deviation of the threads. This is important because it affects the tolerance that must be added or subtracted from a given major diameter or pitch to produce the desired thread profile.