Unified National Threads
Unified National Thread Note Components
After drawing a thread, we need to identify the size and thread form in a thread note.
The following items are included in a thread note.
- Major Diameter: The largest diameter.
- Threads per inch: Number of threads per inch for a particular diameter. This is equal to one over the pitch (1/P).
- Thread Form and Series: The shape of the thread cut.
- UNC = Unified National coarse. For general use.
- UNF = Unified National fine. Used when high degree of tightness is required.
- UNEF = Unified National extra fine. Used when length of engagement is limited (Example: Sheet metal).
- Thread Class: Closeness of fit between the two mating threaded parts. A thread class of 2 is assumed if none is stated.
- 1 = Generous tolerance. For rapid assembly and disassembly.
- 2 = Normal production
- 3 = High accuracy
- External or Internal Threads:
- A = External threads
- B = Internal threads
- Right handed or Left handed Thread:
- RH = Right handed (right handed threads are assumed if not stated.)
- LH = Left handed
- Depth of thread: The thread depth is given at the end of the thread note and indicates the thread depth for internal threads. This is not the tap drill depth.
