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Metric threads are designed along similar lines to UN threads. The profiles supported by this software are M and
the latter being the metric analog of UNJ. For a metric thread, a tolerance class consists of four parts :
a tolerance grade and position for pitch diameter, and a tolerance grade and position for crest diameter.
The tolerance grades are single digit numbers, whereas the positions are letters, capital for internal threads,
lowercase for external threads. A lowercase "h" indicates no allowance for external threads, capital "H" for internal
threads. Unlike UN threads, metric internal threads can have allowances, indicated by position "G"
(or "E" using the nonmandatory Appendix F in ASME B1.13M-2005).
For external threads, the valid allowance positions are the letters "e" through "h".
The valid tolerance grades for external pitch diameter are the digits 3 through 9; for external major diameter, the
grades can be 4, 6, or 8. The valid internal tolerance grades for both pitch diameter and minor diameter are the
digits 4 through 8. A smaller number indicates a smaller tolerance.
4g6g pitch diameter tolerance grade = 4, allowance position = g, major diameter tolerance grade = 6, allowance position = g.When both diameters have the same grade and position, it is customary to abbreviate the class by writing the grade and position just once. For example, 6g6g is normally written as just 6g.
6H is the same as 6H6H, pitch and minor diameter grades are both 6, their allowance positions are both H.
For M product threads, see ASME B1.13M-2005; for gages, see ANSI/ASME B1.16M-1984.
For MJ product threads, see ASME B1.21M-1997; for gages, see ANSI/ASME B1.22M-1985.
Working Thread Rings are used to gage external product threads. The "GO" ring should assemble with the product it is gaging, whereas the "NOT GO" ring should not. Generally, the minimum major diameter is specified, without a tolerance.
Working Thread Plugs are used to gage internal product threads. The "GO" plug should assemble with the product it is gaging, whereas the "NOT GO" plug should not. Generally, the maximum minor diameter is specified, without a tolerance.
Setting Rings are used to set indicating gages for internal product threads. GO rings are used to set gages to the maximum material size, NOT GO for the minimum material size. Generally, the minimum major diameter is specified, without a tolerance.
Setting Plugs are used to set indicating gages for external product threads, in addition to adjustable working ring gages. GO plugs are used to set gages to the maximum material size, NOT GO for the minimum material size. Generally, the maximum minor diameter is specified, without a tolerance.