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The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal. For example, a screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in and a 1 ⁄ 13 in pitch (13 threads per inch) has a pitch of 0.077 in.
Self-tapping screws can be divided into two classes: [3] those that displace material (especially plastic and thin metal sheets) without removing it, known as "thread-forming" self-tapping screws, and self-tappers with sharp cutting surfaces that remove the material as they are inserted, termed "thread-cutting" self-tapping screws.
Pilot holes may be used when driving a screw, typically in wood, concrete, or plastic where the screw cuts its own threads. When a screw is driven into a material without a pilot hole, it can act as a wedge, generating outward pressure which can cause many materials to split. By drilling a small pilot hole into the material, into which a screw ...
The tapped hole (or nut) into which the screw fits, has an internal diameter which is the size of the screw minus the pitch of the thread. Thus, an M6 screw, which has a pitch of 1 mm, is made by threading a 6 mm shank, and the nut or threaded hole is made by tapping threads into a hole of 5 mm diameter (6 mm − 1 mm).
[2] [3] This feature enables a bottoming tap to cut threads to the bottom of a blind hole. A bottoming tap is usually used to cut threads in a hole that has already been partially threaded using one of the more tapered types of tap; the tapered end ("tap chamfer") of a bottoming tap is too short to successfully start into an unthreaded hole.
Bases for plant formed in concrete, brickwork or blockwork, to a scale of not less than 1:20 Attendant builders work, holes, chases, etc. for conduits, cables and trunking etc. and any item where access for a function of the installation is required to a scale of not less than 1:100