The American National form (Figure I-290) is now obsolete and has evolved into the Unified form (Figure I-291). The basic geometry of the two systems is similar.
Taps and dies are marked with letter symbols to designate the series of the threads they form. For example, the symbol for American Standard taper pipe thread is NPT, for Unified coarse thread it is UNC, and for Unified fine thread the symbol is UNF. Thread Depth The American Standard for Unified threads (Figure I-291) is similar to the American National Standard with certain modifications. The thread forms are practically the same, and the basic 60-degree angle is the same. The depth of an external American National thread is and the depth of the Unified thread is The constant for American National thread depth may be rounded off from .6495 to .65, and the constant for Unified thread depth may be rounded to .613. The thread depth and the root truncation (tool flat) of the American National thread is fixed or definite, but these factors are variable within limits for Unified threads. A rounded root for Unified threads is desirable whether from tool wear or by design. A rounded crest is also desirable but not required. The constants .613 for thread depth and .125 for the flat on the end of the tool were selected for calculations on Unified threads in this unit. Unified and American National Standard form threads are interchangeable. An NC bolt will fit a UNC nut. The principal difference between the two systems is that of tolerances. The Unified system, a modified version of the old system,allows for more tolerances of fit. Thread fit classes 1, 2, 3, 4,and 5 were used with the American National Standard, 1 being a loose fit, 2 a free fit, 3 a close fit, 4 a snug fit, and 5 a jam or interference fit. The Unified system expanded this number system to include a letter, so the threads could be identified as class 1A, 1B, 2A, 2B, and so on. A indicates an external thread and ban internal thread. Because of this expansion in the Unified system, tolerances are now possible on external threads and are 30 percent greater on internal threads. These changes make easier the manufacturer’s job of controlling tolerances to ensure the interchangeability of threaded parts. See Machinery’s Handbook for tables of Unified thread limits. Limits are the maximum and minimum allowable dimensions of a part—in this case, internal
and external threads.Threads are designated by the nominal bolt size or major diameter, the threads per inch, the letter series, the thread tolerance, and the thread direction. Thus in.-12 UNF-2BLH would indicate a Unified nut with 12 threads per inch, a class 2 thread fit, and a left-hand helix.Unified screw thread systems are the American Standard for fastening types of screw threads. Manufacturing processes where V threads are produced are based on the Unified system. Many job and maintenance machine shops, on the other hand, still use the American National thread system when chasing a thread with a single-point tool on an engine lathe. The flat on the crest of the thread on both the Unified and American National systems is or The root flat (flat on the end of the external threading tool) is calculated as or for the Unified thread system.
To cut 60-degree form threads, the tool is fed into the work with the compound set at 30 degrees (Figure I-292).However, in practice, the compound is actually set to 29 degrees to provide a light finishing cut on the trailing side of the threading tool. The 1-degree difference will make only a slight difference in calculations. The in feed depth along the flank of the thread at 30 degrees is greater than the depth at 90 degrees from the work axis. This depth may be calculated for the Unified form by the following formula: For 29-degree compound angle cutting Unified form threads: