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The most frequently used thread form is the 60° thread form found in the metric ISO thread and in the UN-series. However, there are other thread standards as well. These are either based on foreign standards or are used for special applications. E.G. in the medical technology, in aeronautical engineering or astronautics. The most common threads are:
• ISO Thread (metric)
• UN (Unified National) series Thread
• Whitworth Thread
• Pipe Thread
• Trapezoidal Thread
• Knuckle Thread
• Buttress Thread
The thread abbreviation includes the thread code letter and the nominal thread diameter or the thread size. Additional values for pitch or TPI, tolerance, multi-lead, taper and left-handedness are added. Often threads that are according to DIN Standard have the major DIN number put in front of the thread abbreviation.
Talking of screws the major diameter for screws is determined by the thread tips.
The minor diameter by the groove of the thread.
The pitch diameter is the distance of two opposite flanks or the distance of the center line of the profile.