Thread Standard Types and Drawing Specification

 Threads are used both in internal and external applications for fastening two parts and creating a secure mechanical joint with the appropriate hardware combination which will vary with application and process conditions. For holes you use internal threads and for shafts you use external threads. When making an engineering drawing, you have to give the thread size, thread pitch, drill size, thread and drill depths, and thread class.

There are a variety of thread standards used in the world, but the most common include: American and Metric (ISO) thread standards. The American standard is better known as the Unified National Thread which includes UNC (coarse) and UNF (fine) that distinguishes the pitch type. Metric thread which is simply known as "M" also has fine and coarse types.

There are even special threads used for special purposes such as pipe thread standards which are used to join pipes and fittings. The most commonly used pipe thread standards are NPT (National Pipe Threaded), BSPP (British Standard Pipe Parallel) and BSPT (British Standard Pipe Tapered). These are used to define both straight and tapered threads. The British pipe standard BSPP and BSPT have become obsolete recently and have changed to be known as G and R/Rp/Rc designations, respectively. 

Another common thread in industry is the SAE thread type, which uses a straight thread sealing boss. It makes use of a BUNA-N rubber O-ring to ensure sealing of fittings.

When specifying a threaded hole callout there is a common method followed in engineering drawings. Typically you first specify a counterbore depth and size first (if needed), then the drill size and depth, and finally the tap size, pitch, and depth. For example, a threaded hole that is specified in the following way: 1/4"-20 UNC  1"  DP; Indicates that a hole has a thread for a 1/4" bolt that has a Coarse thread pitch that is 20 threads per 1 inch and will be tapped to a depth of 1".