Self-tapping screws provide a useful and versatile range of fastenings. They are usually made from mild steel and, after the thread has been rolled, they are case-hardened to produce a hard surface at the thread with a tough core. Nickel or cadmium plating is the most common finish, but zinc, brass, copper and chromium platings are also used and plain steel and stainless steel screws are available.
There are two basic kinds of self-tapping screw: thread-forming and thread-cutting. Thread-forming Designed for use in materials which are softer than the screw material, this type forms or squeezes the material into which it is inserted to produce the internal thread. Thread-cutting As the name implies, this type cuts a thread in the material into which it is inserted. It can be identified by a flute which runs a short way back from the point along the side of the thread.
The screw sizes are described by screw gauge numbers in the same way as ordinary screws. The sizes most likely to be held by local stockists are from No 4 to No 10 gauge, in lengths of 9-63mm
Self-tapping screws are available in a variety of head styles. The most common are countersunk, round and pan head, while other styles include raised, mushroom and hexagon. All styles have either conventional slotted heads or ‘cross slot heads (known as Pozidriv or Supadriv). There are several advantages to the cross slot head; for example, the risk of the screwdriver slipping is reduced and a greater tightening torque can be applied to the screw because the head design is stronger than the normal slotted design. A conventional screwdriver should not be used on cross slot screws because it will not be fully effective and the advantages of the design will be lost. Screwdrivers to suit cross slot heads are sold by driver point number and the correct driver point depends on the screw gauge being used.
Screw gauge Driver point
Up to No 4 No I
No 4—No 10 No 2
Over No 10 No 3
Self-tapping screw types
Many stockists do not carry a fully comprehensive range of self-tapping screws because there are so many types, head styles and finishes available. If you intend to use these screws a great deal, it may be worth buying a kit which is available from some suppliers. This consists of a box with a range of screw sizes and types stored in separate compartments; in some cases the kit contains a set of appropriate screwdrivers and a leaflet which specifies the correct hole size and type of screw to be used in particular materials. Alternatively you can buy screws loose or prepacked in small quantities.
For many purposes, the choice of screw type is not critical; but if you are working with a variety of materials it is an advantage to use a screw type designed for a particular application.
This is a thread-forming screw with widely spaced threads and a sharp gimlet point which makes it easy to insert in a drilled pilot hole. Its main use is for joining light sheet metal, plywood and soft plastics. For metal sheet up to 0.91mm (20 SWG, 0.036in) use No 6 screws; for sheet metal up to 1.22mm (18 SWG, 0.048in) use larger screws.
Like type AB, this is a thread-forming screw with widely spaced threads, but the point is blunt with a slight taper to help insertion into a pilot hole. It can be used for heavier gauge sheet metal up to 4.88mm (6 SWG, 0.192in) thick, as well as for non-ferrous castings, plastics and plywood.
A very secure fastening can be obtained by using type B self-tapping screws and spring steel nuts. These nuts come in various shapes and have projections into which the screw is inserted; tightening the screw flattens the nut against the surface on which it bears.
This is a screw with a fine pitch and a flute to permit chip clearance as it cuts the thread in the material. It is suitable for cutting threads in materials which have low strength, such as aluminium. It should not be used in thin materials because not enough threads will be cut to hold the screw in position.
The main difference between type T and type D is the flute provided. They are used for the same purposes, but type D provides less clearance for the chips produced during cutting. Type D is useful for rethreading previously tapped holes and both types give a high clamping force between the parts being joined.
This has widely spaced threads with a blunt point. It is intended for use in materials which are likely to crumble, such as some of the brittle plastics. Type BT removes very little material while cutting the thread and is particularly useful for producing long threads in blind holes. The wide flute helps to prevent chips binding in the hole as the thread is cut.
A very coarse pitch and spiral flute make this type suitable for use in brittle materials such as cast iron and brittle plastics.
These screws have a very deep spiral thread and usually a round head which has no slot or cross slot; so they have to be pressed into a pre-drilled hole or driven in with a hammer. They are intended for permanent fixings since once fitted they are not easily removed. A typical example of their use is to secure the hardboard panel which forms the base of a record player plinth; they are not, however, suitable for fixing very thin panels to a frame. As a general guide, the thickness of the panel to be fixed should be greater than the diameter of the screw being used.
In general the thinner the metal, the smaller the hole diameter required for a self-tapping screw. A rough guide to the hole size is to use a drill which is about 80 percent of the screw diameter for sheet metal up to 1.22mm (18 SWG, 0.048in) thick. For thicker sheets, non-ferrous metals and plastics the hole should be drilled to a diameter which is approximately 90 percent of the screw diameter.