Why Use Datum Dimensioning Over Continuous Dimensioning

Chain Dimensioning

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.

There are several exceptions to this as seen in the drawing in Figure 4.3:

1. Dimension values of the running dimensions are shown close to the arrowheads and not in the centre of the dimension line. This applies to the '19', '13' and '-20' horizontal dimension, i.e. any running dimension value.

2. Dimension values can be placed above the extension of the dimension line beyond one of the terminators if space is limited. This is the case with respect to the '1' horizontal dimension of the 30° chamfer.

3. Dimension values can be at the end of a leader line that terminates at a dimension line. This applies when there is too little space for the dimension value to be added in the usual way between the extension lines. This is the case with the horizontal '2' dimension for the O-ring groove on the outer diameter of the spool valve.

4. Dimensional values can be placed above a horizontal extension to a dimension line where space does not allow placement parallel to the dimension line. This is the case with the '21' diameter of the O-ring groove. This dimension is also different in that the dimension line and the arrowhead are in the opposite direction to the '14' and '24' diameters seen on the left of the spool valve. Furthermore, in this case, the line has only one terminator (the other one is assumed).

All the above descriptions apply to linear dimensions. However, some dimensions are angular and are dimensioned in degrees or radians. The dimensioning of angles is just as important as the dimensioning of linear dimensions if a component is to be dimensioned correctly for manufacture. Angular dimensions use the same four elements as described above for linear dimensions.

In the case of the spool valve in Figure 4.3, two of the 14 dimensions are angles. These are the angles of the two chamfers. The 45° dimension has the two arrowhead terminators on the inside of the dimension arc whereas the 30° dimension has the arrowheads on the outside of the angular dimension arc. The latter is used because space is limited.

All dimension values, graphical symbols and annotations should normally be positioned such that they can be read from the bottom and from the right-hand side of the drawing. These are the normal reading directions. However, in some instances, reading from the bottom-right is not always possible if the requirements stated above are to be met. With reference to Figure 4.3, it is not possible to meet these requirements with respect to the 45° chamfer and the 2.5 dimension value. In these cases, the reading direction is the left-hand side and the bottom of the drawing. Figure 4.4 shows the common positions of linear dimensions and angular dimension values as given in the latest ISO standard.

Figure 4.5 shows three different methods of dimensioning-related features. With parallel and running dimensioning, the position of the hole centres as well as the right-hand plate edge are related to the datum left-hand edge. The advantage of parallel and running dimensioning is that every feature is related back to the

Figure 4.4 Dimensioning different angular features

Parallel dimensioning

Running dimensioning

Chain dimensioning

Figure 4.5 Parallel, running and chain dimensioning same datum. Running and parallel dimensioning are identical methods. Chain dimensioning is an entirely different dimensioning methodology. In this case, only the left-hand hole is directly related to the left-hand datum surface. The right-hand hole is only related to the datum surface in a secondary manner and the right-hand edge is only related to the left-hand edge in a tertiary manner. The very name 'chain' illustrates the disadvantages in that the dimensions are chained together and not individually related back to a datum.

The dimensioning convention shown in Figure 4.3 is the convention given in the latest ISO 129-1:2003 standard. Previous ISO standards have used slightly different dimensioning conventions and of course it is very likely that some old drawings will conform to these conventions. Figure 4.6 shows a stepped shaft which has been dimensioned in a manner which is different from the convention in Figure 4.3 but which would have been recommended and allowed in previous ISO standards.

With reference to Figure 4.6, the dimensions on the drawing which do not conform to the current ISO 129-1:2003 convention but which do conform to previous versions of the standard are as follows:

I	>-
" " 30
10 I 30		10 20 ^_--;-te.	10 ^_te.

Parallel dimensioning

Running dimensioning

Figure 4.5 Parallel, running and chain dimensioning o rf o

Figure 4.6 Dimensioning practice according to previous ISO standards

1. Projection lines do not touch the outside outline of the part. There is a small space between the part and the extension line. It makes the part stand-alone and away from the dimensioning and therefore easier to read.

2. Dimension values may be placed not parallel to the dimension line but perpendicular to it, e.g. the '({>30'.

3. Dimension values may interrupt the dimension line, e.g. the '<J>30' and the '35'.

4. Diameters can be shown in half their full form, e.g. the '<|)20' and the '(|>40'.

5. Dimension values can be placed away from the centre of the dimension line and off to one side, e.g. the '45°' and the '2'.

6. Short dimensions using reversed arrows do not need to have a continuous dimension line, e.g. the '2' width of the chamfer.

Continue reading here: Unacceptable dimensioning practice

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