BS ISO 2904:2020 pdf download.ISO metric trapezoidal screw threads — Basic dimensions.
1 Scope
This document specifies the basic dimensions of ISO metric trapezoidal screw threads in accordance with ISO 2902. The values refer to the design profiles in accordance with ISO 2901.
This document is chiefly applicable to traversing threads for traversing motion on machines, tools, etc.
It can also be used for fastening threads.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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.
ISO 5408, Screw threads — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5408 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
4 Symbols
For the purposes of this document, the following symbols apply.
D 1 basic minor diameter of internal thread
D 2 basic pitch diameter of internal thread
D 4 basic major diameter of internal thread
d basic major diameter of external thread (nominal diameter)
d 2 basic pitch diameter of external thread
d 3 basic minor diameter of external thread
P pitch
a c clearance at major and minor diameters
R 1 radius on crest corners of external thread on design profile
R 2 radius on root corners of internal and external threads on design profile
5 Basic dimensions
Basic dimensions shown in Figure 1 are given in Table 1.
The values of D 1 , D 2 , D 4 , d 2 and d 3 , in Table 1, have been calculated from the following formulae and
rounded to the third decimal place.
D 1 = d − P
D 2 = d 2 = d – 0,5 P
D 4 = d + 2 a c
d 3 = d – P − 2 a c BS ISO 2904 pdf download.