Whether with a hobby craftsman or an experienced professional - the subject of screws always raises many questions. Which screw types are best suited for which application? What effect do the thread types have on the benefit? And what distinguishes a wood screw from a metal screw?In order to ensure the best result for each construction it is important to use the right screws. This is the only way to create the best connection and a stable component for workpieces of all kinds. In the following guide you will get a comprehensive overview of screw types, the fields of application and the structure of the individual screws. This makes it easy to choose the best screws for each project.
Only the area of the screw with the spiral notch is designated as thread. If there is a smooth area above it, it is called the shank.
Screws with metric thread were standardized in 1996 via ISO 1502 (DIN: Metric ISO thread general application). They are described in the DIN standards DIN 13 and DIN 14.
The screws with metric thread are offered as a pairing of screw and nut. Within the screws, a distinction is still made between models with standard thread and fine thread. The flank angles and the thread pitch of the fine thread are smaller than those of the standard thread. Screws with fine thread are very well suited for workpieces where there is little space for screwing in the screw. The reduced flank distance increases the number of existing thread cuts and thus the holding of the screw.
In general usage, screws with metric threads are also called screws with metal threads. This is also the case if the screws are made of plastic in part or in whole. The designation metal thread indicates that these screws are only used together with a preceding nut. In comparison, universal screws or self-tapping screws, for example, cut their own threaded hole into the material.
In addition to the different types of thread, there are also several types of flank. These can each be used within the thread forms. For example, there are screws with metric threads that have different flank shapes. The flank shapes often limit the individual application area:
Globally, metric screws are the most common utility screws. There are some markets, where inch screws still hold their ground. Especially in the USA, inch screws are widely used. In comparison to a metric screw, the main difference between a screw with inch thread is the underlying unit of measurement.
One inch corresponds to 2.54 cm. If you want or need to work with inch screws, it is important to make the correct calculation before buying screws. This applies to screws with fine and coarse thread.
There are various types of threads for inch screws. The following are the most common on the worldwide market:
UNC and UNF threads also have a flank angle of 60°, but have a different diameter than the metric screws. Therefore, metric screws, American inch screws and British inch screws are not compatible with each other.
Regardless of the thread type of the inch screws, the length of the screws is always indicated in inches or mm. The thread diameter, on the other hand, is only given as breakage. Inch is described with the special character ". A thread with "¼" therefore has a diameter of 6.35 mm (= 1/4∙2.54).
The cap screws, also known as socket head screws, are used in areas with special design requirements. If the screw head is difficult to access due to the construction of the workpiece, hexagon socket head screws are the right choice. In most cases, the workpiece has identical dimensions as the screw head. The specific product requirements that the cylinder head screw must meet depend on the component.
The range of cylinder head screws is correspondingly extensive. Models with a low head height or versions with a slotted drive are available. In addition, cylinder head screws are available in accordance with the metric DIN standard. You also have the choice between full thread and partial thread.
It is possible to have special designs manufactured. The dimensions of the screw are then perfectly adapted to the workpiece to be machined. However, this offer is rarely used in the private sector. The screws are offered in usual standard lengths:
They are versatile and are particularly suitable for joining different metals such as galvanized and ungalvanized steel and stainless steel.
A further speciality in the screw assortment are screws with drill tip. These screws have a pronounced tip, which - as the name suggests - is suitable for drilling into a workpiece. The basic shape is similar to that of a classic drill bit. Usually the drill tip has two cutting edges. You can use a screw with drill tip very well for joining metals or plastics. This type of screw is regularly used for fixing window fittings.
The screws with drill point can be found in various design combinations:
The starting point for the tool, the bit, is named as the screw drive. The best known forms are the slot drive (bad when used with electrical screwdrivers, because the centering is missing and the screw wobbles when screwed in) and the cross slot drive. Furthermore, there are among others the hexagon socket, the external hexagon, the TX (or Torx) inner star or the square drive.
The wrench screws also fall into the category of special screws. These are screws that can be fixed with a wrench or a socket wrench. The wrench screws correspond to DIN 571 and are also in use under other names:
The thread of a wrench screw does not start directly below the screw head. There is a smooth shank. The partial thread ensures that a high pressure is created between the components to be connected. If, for example, two boards are screwed together, the lower wooden component is pulled securely against the upper one.
The key screw is a classic wood screw. The hexagonal screw head makes retightening easier. The screws are also available in stainless steel. When used correctly, key screws must always be pre-drilled in two steps in the wood.
You can get the key screws among others in galvanized and hot-dip galvanized steel.
The screws therefore differ from commercially available screws primarily by their special drive. You can buy the matching bits in a hardware store, should it be necessary to open a special screw. The most common screw drives for special screws are the following:
A disposable screw cannot be loosened with a normal tool. The drive flanks are flattened in the return stroke and do not provide a support surface for a tool. If it should be necessary to disassemble it, there are special tools such as a left-hand screwdriver.
Special screws are always used when loosening of the screw connection should be prevented as far as possible. A classic example are vehicles in public transport. Companies that manufacture electrical appliances increasingly rely on special screws to prevent customers from carrying out dangerous repairs themselves. Children's toys that use potentially dangerous components inside also use special screws.
Depending on the type of safety screw, models are available with lens head, flat head or countersunk head.
The choice of the suitable screw head determines the functionality and the appearance. The selection is extensive, and the individual heads have very different advantages and disadvantages to offer:
The screw diameter is measured at the thread. The value indicates the widest thread diameter of the screw. The diameter of the screw is relevant in many areas. If a pilot hole is important, you can select the correct size for the drill using the screw diameter.
Especially with wood screws, an incorrectly selected diameter can cause problems. If the screw is too thick, the danger of splitting in the wood is too high. Therefore, in general, always pre-drill in wood for wider screws, even if the thread is self-tapping, unless the edge distance and the wood thickness and width are large enough.
The offer for different screw materials is constantly growing. There are even mixed types that use several materials. However, since certain conditions such as tensile strength or corrosion protection usually play a role, some materials are used particularly frequently:
Stainless steel screws and steel bolts provide high strength. Stainless steel screws are extremely weather resistant. In addition, they are inexpensive to manufacture. Brass screws are nice to look at and offer even better corrosion protection than e.g. galvanized steel screws. Steel screws and other metal screws are used in private and industrial environments.
Plastic screws are often marked PA: polyamide. These screws are light and cheap to produce. They also have a high flexibility. Unfortunately, they are not particularly temperature-resistant and have little load-bearing capacity.
If you want to use dowels, screws and dowels must be matched. Although it is possible to buy pairs of screws and plugs ready to use, it is often easy to put together the two counterparts yourself. The choice of plug is determined by whether you drill or fix to a wall or ceiling. The material of the substrate is also important. For aerated concrete, for example, you need a different plug than in concrete. Basically, you can get universal plugs, special plugs and cavity plugs for the combination with screws.
You get the correct screw length by adding the plug length, the thickness of the work piece to be fastened and the screw diameter. You can also easily determine the appropriate screw diameter:
OSB boards are available in the standard dimensions 2500 mm x 625 mm or 2500 x 1250 mm. In addition, the boards are available in different thicknesses. They are easy to install and are very suitable as a vapour barrier, among other things. If the boards are screwed to a substructure, the center distances between the beams or uprights should not exceed 62.5 cm. Some boards (so-called installation boards) have a tongue and groove all around. If the panels do not have a static load-bearing function for the stability of the building, a general screw spacing of 10 - 15 cm can be provided at each linear intersection with beams or uprights of the substructure.
The strength class of a screw is indicated for metric screws made of hardened steel by a numerical combination of two values. The two numbers are simply separated by a dot. The first number denotes the index value of the tensile strength / 100. The second number denotes the index value of the yield point as a ratio of the yield point to the tensile strength:
If the screw drive is worn out, it is not easy to loosen a screw. But with a few simple tricks you can also remove stuck screws: