The latest ISO standard concerned with dimensioning is ISO 129"1985. However, it is known that a new one is soon to be published which is ISO 129 Part 1 (the author sits on BSI/ISO committees and has seen the draft new standard). It has been through all the committee approval stages and has been passed for publication. However, it is being held back, awaiting the approval of a Part 2 so that they can be published together. The BSI estimates the publication date will be 2003, hence it will be ISO 129-1:2003.
There have been versions prior to 1985 and each has defined slightly different dimensioning conventions. Needless to say, the 2003 convention gives a slightly different convention to the 1985 one! Throughout this section, the 2003 convention will be presented so that readers are prepared for the latest version. Figure 4.3 shows a hypothetical spool valve that is defined by 14 dimensions in which 12 are linear and two angular. The valve is shown using the ISO principles of line thickness described in
Chapter 3. Note that the valve outline uses the ISO type 'A' thickness whereas the other lines (including the dimension lines) are the ISO type 'B' thin lines. The outline thus has more prominence than the other lines and hence the valve tends to jump out of the drawing page and into the eye of the reader. The valve dimensions
follow the dimensioning convention laid down in the future ISO 129-1:2003 standard. Tolerances have been left off the figure for convenience. In this case there are two datum features. The first is the left-hand annular face of the largest cylindrical diameter, i.e. the face with the 30 ~ chamfer. Horizontal dimensions associated
with this datum face use a terminator in the form of a small circle.
The other datum feature is the centre rotational axis of the spool valve represented by the chain dotted line. All the extension lines touch the outline of the spool valve. The dimension values are normally placed parallel to their dimension line, near the middle, above and clear of it. Dimension values should be placed in such a
way that they are not crossed or separated by any other line.
Engineering Drawing for Manufacture
by Brian Griffiths
Publisher: Elsevier Science & Technology Books
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