THREAD SPECIFICATION AND SYMBOLS

THREAD SPECIFICATION AND SYMBOLS

ANSI UNIFIED THREAD (UN/UNR) SPECIFICATION:

3/4-16UNF-2A-LH, DOUBLE, 1 3/4 LG

3/4: The major (nominal) diameter of the thread. Appendix 10 in textbook lists various diamaters available.
16UNF: 16 indicates number of threads per inch (n). UNF (unified fine); UNEF (uni. extra fine); UNC (uni. coarse)
2A: indicates the class of fit (1A, 2A, 3A : increasing precision order), A stands for external threads and B for internal threds.
LH: indicates left hand thread. When a thread is right-hand no notation is made in the thread specification.
DOUBLE: indicates factor of multiple thread.(L = 2*p). When a thread is single no notation is made in the thread specification.
1 3/4 LG: Length of thread depends on designer, but given as in even increments and depends on major diameter. May be given as a dimension or as an addendum, 1 3/4 LG. DP(deep) would be used for threaded holes.

Examples:

3/4-16UNC-2A
3/4-10UNC-2B-1 3/4 LG

METRIC THREAD SPECIFICATION:

M20 x 1.5 - 4g6g, 45 LG

M20: The major (nominal) diameter of the thread. Appendix 13 in textbook lists metric screw thread series.
1.5: is the pitch of the thread.
4g6g: is the tolerance class. It is a combination of tolerance grade and tolerance position. It indicates the allowance and tolerance for the pitch and major diameters of external threads, and pitch and minor diameters of internal threads. Tolerance grade (e.g. 4 close tolerance, 6 medium 8 coarce tolerance); Tolerance position (e.g. g,h for external threads; G,H forinternal threads ; h,H stands for zero allowance)

45 LG: Length of thread is the choice of designer.
M20 x 1.5, M16 x 2 are some examples. Such notations are sometimes called basic designation.

SYMBOLS FOR STRAIGHT THREADS:

Threads are generated on external and internal surfaces and figure 4 illustrates how threads are generate

A true representation of a thread is not needed on a working drawing. Symbols are used instead. Three types of symbols are in use: detailed, schematic and simplified. We will prefer simplified symbolic representation during our studies. Figure 5 presents the simplified thread symbols. As a common practice metric and UN/UNR threads are drawn in the same manner.