The screw size that you need depends on the material you’re working with and the amount of weight it must hold. Choosing the correct screw diameter is important for safe, accurate application.
Screw sizes are determined by a combination of major diameter and thread pitch (or the distance between adjacent thread peaks). Most packages of screws list their major diameter first, followed by the length in inches.
Length
A screw’s length is important because it affects the load capacity. Ideally, a fastener should be long enough to fully embed itself into the substrate material. The best way to measure the length of a screw is to place it in a hole and use a caliper to determine its exact length.
For wood screws, the length that you choose will depend on the thickness of the substrate and the load that the screw will support. If you’re working with thin materials, go with a thinner screw size such as #2 or #3; for heavier applications, select a thicker screw size such as #8.
In some cases, the length of a screw may also be listed on the package using the metric system (for example, 5.0 x 40mm). However, imperial measurements are more common and are used to indicate both the diameter and the length of a screw. For this reason, it’s a good idea to familiarize yourself with the metric system before buying screws in that format.
Diameter
The diameter of a screw is the measurement from the base or ‘root’ of each thread. The more threads a screw has, the smaller the diameter will be.
The length of a screw is the third number listed in sizing charts and usually follows the head diameter and threads per inch measurements (for example, 6-32 x 1 1/2″). The shaft portion of the screw is measured from where the flat bottom of the head meets the top to the tip.
The diameter of the screw determines how much energy is supplied by each flight during injection molding. Larger shot sizes require more flights to supply the same amount of energy as a smaller shot size. This affects both the melt and the transition zone, which is an important part of achieving high quality parts. Some screws, such as pan heads or flat heads, will be denominated by an industry Numeric Size and do not show a number of threads per inch designation.
Threads
The threads of a screw affect its load capacity, holding power and performance. Depending on the material and application, it is important to use the correct thread type and size to avoid stripping or damage.
Threads are characterized by their major diameter (Dmaj) and their pitch (P), which is the distance between adjacent thread peaks. They are also characterized by their lead angle (a), which is the distance from the thread centerline to a point halfway between the tip and root of the thread.
In addition to these three characteristic dimensions, industry standards specify minimum and maximum limits for external and internal thread sizes. These are called the Basic Size or Limits.
For instance, the ANSI standard for screw sizes states that an external hex bolt should be no smaller than an internal hex nut. Additionally, the hex standard specifies that a coarse thread should not be smaller than a fine thread. This is because a coarse thread has a higher pitch and therefore offers less strength.
Load Capacity
There are several factors that affect the weight capacity of a screw. These include head size, thread type, and material type. A larger head size can distribute the load more evenly and increase the holding strength. It can also help prevent screw heads from stripping. Coarse threads are generally stronger than fine ones and can handle more pressure, but the exact amount of stress depends on the material being drilled.
To determine the correct screw size, you need a ruler or tape measure and a screw gauge. You also need to know the number of threads per inch, or TPI, on your screw. To do this, simply count the number of thread peaks along one inch of the screw. Then, multiply the TPI by the major diameter to get the screw size. For example, a screw with 20 threads per inch and a major diameter of 1/4” will have a screw size of 1/4” x 20.15mm in inches
