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This standard is developed in accordance with the rules given in GB/T 1.1-2009.
This standard replaces GB/T 18685-2002 The bar diameters for rolling general purpose metric screw threads. In addition to editorial changes, the following main technical changes have been made with respect to GB/T 18685-2002:
——22 thread specifications (M8×0.75, M10×0.75, M18×1, M20×1, M22×1, M24×1, M27×1, M30×1, M39×4, M42×4, M45×4, M45×3, M45×2, M45×1.5, M48×4, M48×3, M48×2, M48×1.5, M52×4, M52×3, M52×2 and M52×1.5) are added in this standard (see Table 1);
——When the blank is processed by turning process, this standard uses "twice the arithmetic mean deviation value of surface roughness profile (2Ra)" to correct the blank diameter; the 2002 edition standard uses "the maximum height of surface roughness profile (Rz)" to correct the blank diameter (see 7.1 of this edition, paragraph 1 of Clause 7 in 2002 edition);
——This standard recommends that the ratio of the blank diameter tolerance to the pitch diameter tolerance of the thread should not be less than 0.20; the maximum diameter of Grade 4 thread blank of 11 specifications that do not meet this condition is adjusted (the shrinkage factor fT of the pitch diameter of the through-end thread ring gauge is not considered in the calculation) (M18×2.5, M20×2.5, M22×2.5, M30×3.5, M33×3.5, M36×4, M39×4, M42×4, M45×4, M48×4, M52×4); the 2002 edition standard specifies the condition that the blank diameter tolerance is not less than 0.01mm, and it has not adjusted the maximum blank diameter (see Clause 7 and Table 1 of this edition, A.3 and Table 1 of 2002 edition);
——The method for determining the thread length (B) of the through-end thread ring gauge in this standard is different from that in the 2002 edition (see A.1 of this edition, A.1 of 2002 edition).
This standard was proposed by and is under the jurisdiction of SAC/TC 108 National Technical Committee on Screw Threads of Standardization Administration of China.
The previous edition of this standard is as follows:
——GB/T 18685-2002.
The blank diameters for rolling general purpose metric screw threads
1 Scope
This standard specifies the blank diameters for rolling general purpose metric screw threads. The nominal diameter of general purpose metric screw threads ranges from 1mm to 52mm (the first and second series diameter; the maximum pitch is 4mm); there are five thread tolerances: 4h, 6h, 6g, 6f and 6e.
This standard is applicable to the general purpose metric screw threads of carbon steel and alloy steel materials processed with full-profile extrusion process.
Note: "Full-profile extrusion process" refers to the processing method in which the thread materials completely fill the thread bottom space of the cutting tool when the thread is extruded.
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 971-2008 Thread rolling cylindrical dies
GB/T 972-2008 Thread rolling flat dies
GB/T 3934-2003 Specification of gauges for general purpose screw threads
GB/T 14791 Screw threads - Vocabulary
3 Terms and definitions
For the purpose of this document, the terms and definitions specified in GB/T 14791 apply.
4 Calculation formula of blank diameter
The calculation formula of blank diameters for rolling screw threads shall meet the requirements of Annex A.
5 Limit dimensions of blank diameter
The limit dimensions of blank diameters for rolling general purpose metric screw threads shall meet the requirements of Table 1.
6 Precision grade of thread cutting tool
The precision grade of thread rolling flat dies or thread rolling cylindrical dies shall be selected according to the corresponding relation of thread tolerance grade and precision grade of cutting tool specified in Table 2.
The dimensions of thread rolling flat dies and the thread rolling cylindrical dies of general purpose metric screw threads shall meet the requirements of GB/T 972-2008 and GB/T 971-2008 respectively.
7 Adjustment of blank diameter
7.1 It is recommended to use wire drawing or grinding process to process the blank diameter. If the blank is processed by turning process, its diameter shall be the dimension specified in Table 1 plus twice the arithmetic mean deviation value of surface roughness profile (2Ra).
7.2 When the thread is processed by non-full-profile extrusion process, the minimum blank diameter can be smaller than the value specified in Table 1.
7.3 When the blank diameter tolerance calculated by the formula is too small (the ratio of the blank diameter tolerance to the corresponding pitch diameter tolerance of thread is less than 0.20), the following methods are recommended to increase the maximum blank diameter:
a) Improve the precision of the pitch and flank angle of cutting tool, and calculate the maximum blank diameter according to the actual error value of the pitch and flank angle of cutting tool used;
b) Recalculate the maximum blank diameter after ignoring the internal shrinkage of the through-end pitch diameter of the new thread ring gauge (deleting the fT item in the formula).
7.4 When using the thread rolling flat dies, thread rolling cylindrical dies and thread gauge specified in other standards, the blank diameter shall be recalculated according to the parameter precision of the corresponding standard.
Foreword II
1 Scope
2 Normative references
3 Terms and definitions
4 Calculation formula of blank diameter
5 Limit dimensions of blank diameter
6 Precision grade of thread cutting tool
7 Adjustment of blank diameter
Annex A (Normative) Calculation formula of blank diameter
