Picking the right screw size depends on both the material and the weight it will support. The right diameter is important, too – an overly long screw can cause damage.
Screw sizes are typically indicated by three numbers on the package – gauge, threads per inch and length. Each has a unique meaning and considerations.
Length
Using the wrong screw size can cause damage to the material being worked on and lead to an insecure connection. Fortunately, measuring the length of a screw is relatively simple, equipping you with the knowledge to ensure the correct screws are used for any job.
Most commonly, metric screws will be listed on their packaging with the diameter followed by the length, for example “M5 x 70mm”. In some cases, imperial screw sizes may also be used, and a conversion chart can help you determine the equivalent metric screw size.
The most important thing to remember when measuring a screw is that the measurement should be taken from below the head to the tip of the screw. This is because different screw types sink into materials in different ways. For example, flat-head wood screws will sink completely into the material, while drywall screws will only partially penetrate the surface. This is why it’s essential to know the type of material you will be working with before choosing a screw length.
Diameter
The diameter of a screw is one of the most important factors to consider when choosing a fastener. The correct diameter allows a screw to fit into the material being held securely without breaking or damaging it. A screw that is too large can easily break or protrude from the material, which could lead to a poor and unsafe connection.
The screw diameter is determined by the size of its thread. The diameter is measured from the crest (top) of the thread to the trough (bottom) of the thread. It is sometimes referred to as the major diameter or full diameter. A rounded number called the nominal diameter is often used as a shorthand reference for the major diameter.
A good screw gauge is an essential tool for measuring the diameter of screws and other fasteners. Using a screw gauge accurately allows you to compare the size of different screws, so you can choose the best one for your job.
Threads
A screw thread is a spiral-shaped pattern that runs around the cylinder of a fastener. Its diameter and pitch affect how well the screw fits in a nut or bolt head.
There are many ways to generate threads, including the subtractive methods of cutting (single-point, taps and dies, milling) and molding; casting; forming and rolling; grinding; and abrasion. These processes change the dimensions of the material being generated, and a number of standards exist to define thread depth, pitch, and other characteristics.
Most screws are created to one of 2 main standardizations: the Unified (Inch) Thread Standard, or ISO metric threads. Both have been standardized to achieve certain classes of fit between male and female threads, which ensures predictably successful mating and interchangeability. This is facilitated by defining tolerance ranges for thread size, form, and finish.
Pitch
The pitch of a screw determines the distance between each thread. Screws are typically categorized as either standard, fine or extra fine based on their thread pitch. This measurement is typically made using a caliper, and is often more challenging than determining the major diameter of a screw as it requires very close attention to detail due to the minute spacing involved.
The thread pitch of a screw is also related to its lead, which is the distance that the screw will travel after a full revolution. The higher the lead, the more distance the screw will cover in one revolution.
The pitch of a screw can also affect the type of screw it is, as some types of threads have different pitches than others. For example, a coarse thread has a larger pitch than a fine thread, and it is important to match the appropriate threads when creating a product. This will help to prevent thread mismatches, which can cause screws to come loose.3/16 to decimal