This standard specifies the tolerances and designations of general purpose metric screw threads. The basic profile and diameter and pitch series of general purpose metric screw threads meet the requirements of GB/T 192 and GB/T 193 respectively.
This standard is applicable to general purpose fastening threads.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 192 General purpose metric screw threads - Basic profile (GB/T 192-2003, ISO 68-1: 1998, MOD)
GB/T 193 General purpose metric screw threads - General plan (GB/T 193-2003, ISO 261: 1998, MOD)
GB/T 3934 Specification of gauges for general purpose screw threads (GB/T 3934-2003, ISO 1502: 1996, MOD)
GB/T 14791 The terms of screw threads (GB/T 14791-2013, ISO 5408: 2009, IDT)
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purpose of this document, the terms and definitions specified in GB/T 14791 apply.
3.2 Symbols
For the purposes of this document, the following symbols apply.
D——basic major diameter (nominal diameter) of internal thread;
D1——basic minor diameter of internal thread;
D2——basic pitch diameter of internal thread;
d——basic major diameter (nominal diameter) of external thread;
d1——basic minor diameter (on basic profile) of external thread;
d2——basic pitch diameter of external thread;
d3——root minor diameter of external thread (on design profile, see Figure 5);
P——pitch;
Ph——lead;
H——height of fundamental triangle;
S——short thread engagement group;
N——normal thread engagement group;
L——long thread engagement group;
T——tolerance;
TD1——tolerance for minor diameter of internal thread;
TD2——tolerance for pitch diameter of internal thread;
Td——tolerance for major diameter of external thread;
Td2——tolerance for pitch diameter of external thread;
EI——lower limit deviation (basic deviation) of internal thread diameter;
ei——lower limit deviation of external thread diameter;
ES——upper limit deviation of internal thread diameter;
es——upper limit deviation (basic deviation) of external thread diameter;
R——root arc radius of external thread (see Figure 5);
C——root truncation depth of external thread (see Figure 5).
4 Tolerance system
The tolerance system consists of tolerance grades and tolerance positions. The tolerance grades are expressed by number, such as 4, 6 and 8. The tolerance positions are expressed by letter, such as H, G, h and g. The tolerance class designation shall be the combination of the number and the letter, for example 6H and 6g.
5 Tolerance positions
The tolerance positions of internal and external threads are selected according to the following requirements:
For internal threads: G——with positive fundamental deviation (EI), shown in Figure 1;
H——with zero fundamental deviation (EI), shown in Figure 2.
