5th topic

Graphic techniques (EN ISO 128)

Graphic techniques should follow the recommendations in EN ISO 128, in particular:

(a) line thickness should be not less than 0.25 mm on drawings to be reproduced without reduction or to be microfilmed;

(b) if different line thicknesses are used, each thickness should be at least twice the next thinner line;

(c) the space between lines should not be less than 0.7 mm;

(d) inclined lines should be at an angle of 15° or a multiple of 15° ;

(e) lettering (including numbers) should have a capital height of:

(1) not less than 2.5 mm for drawings as in (a) above;

(2) not less than 3.5 mm for drawings as in (b) above.

While these guidelines originally refer to manual drafting, the same principles should ensure clarity when applied to output from 2D CAD and 3D modelling systems.

Title block (ISO 9431:1990, reviewed and confirmed in 2022). Lettering (EN ISO 3098-1:2015)

The main information block contains essential information about the object and its details. It is usually located in the bottom right-hand corner of the drawing. This block includes the following information: what the drawing is for, who it is for, the part number, a short description, and information about the material and finish. Information specific to a project can be inserted into the title block of the drawing sheets, for example (see Pic. 5.2):

• the name and logo of the client,

• the name and logo of the originator,

• the project name, and

• the project number.

A project number can be added to the drawing template as a company project number, but it is not part of the file name.

Lettering (Pic. 5.2, b) is described in the standard EN ISO 3098-1. Two fonts are used as standard fonts: isocpeur and Ariel Nova Cond Light ©.

Drawing scales (EN ISO 5455:1979)

The internationally agreed and recommended range of scales for use in the construction industry is given in Table 1.

Where two or more scales are used on the same sheet, these should be clearly indicated. The main units to be used are shown in Table 2.

Linetypes (ISO 128-23)

General information on views, sections and cuts (ISO 128-3:2020)

The most informative view is used as the principal view. 

Below (Pic. 5.4), you can see how to mark the view direction, the cutting plane, and the location of an object part on the detail view.

To save time and space, a partial view (Pic. 5.6, a) and a symmetrical object (Pic. 5.6, b-c) can be drawn as a fraction of the whole. When the scale of a technical drawing does not allow all features to be clearly shown or dimensioned, an unclear feature should be encircled and designated with a capital letter. The feature should then be shown at an enlarged scale and labelled with the same letter (Pic. 5.6, d).

Cuts and sections

Sectional views are also called sections, and the process of creating sections is referred to as sectioning. Sections are used to show the hidden interior of an object. Interior features are easier to visualize when exposed as visible features. The sectional view also allows you to expose hidden features for dimensioning. Note: it is not good practice to add dimensions directly to hidden features.

If objects are complex, sections and cross-sections can be used to reveal their structure. A section is an image in a plane obtained by making an imaginary cutting of the object.

The position of the cutting plane is indicated with a dashed-dotted wide line (Pic. 5.7). If the cutting plane changes direction, the cutting line should only be drawn where the cutting plane changes direction (Pic. 5.8). The direction of viewing for the relevant cut and section is indicated with reference arrows. Each cut and section plane is clearly indicated with the same capital letter shown beside each of the cut and section arrows. The letters should be positioned for reading from the bottom of the technical drawing. Hatching consists of parallel continuous narrow lines at an angle chosen to make the lines distinguishable from principal lines. 

A section/cut on two intersecting planes, one rotated onto the plane of projection, is shown in Picture 5.9.

A direct orthographic projection of an object is obtained from the intersection of a projection line with a plane at right angles (Pic. 5.9). The view shows the side of the object which faces the observer. Orthographic projection is the method generally used.

Mirrored orthographic projection is a mirror image an object when the mirror is parallel to the horizontal planes of the objects (Pic. 5.10)

Successive sections can be arranged as shown in Picture 5.12 when it suits the layout and makes the drawing easier to understand. Unless they help to clarify the drawing, outlines and edges beyond the cutting plane may be omitted.

Presentation of Dimensions (EN ISO 129-1:2018)

Only dimensions necessary to define the nominal geometry (nominal model) unambiguously are shown. Each dimension is shown only once using a dimension line, a dimension value, and if necessary, a property indicator. If it is necessary to repeat a dimension, auxiliary dimensions may be used. Dimensional values indicated in decimal notation use a comma as the decimal marker. Unless otherwise specified, dimensions are indicated for the finished state of the dimensioned feature.

The text of all dimensions, graphical symbols and annotations are indicated above the dimension line and read from the bottom. When the text of a dimension, symbol, or annotation is shown vertically, it is read from the right. The determination of orientation is based on the center of the dimension, symbol, or annotation.

Property indicator symbols are described in Table 3 below. 

Dimensions are placed on a view or section showing the relevant features most clearly (Pic. 5.13)

The thickness of a part may be indicated by the symbol "t" followed by an equals sign and the thickness dimension with a leader line terminating in a point on the surface of the part (Pic. 5.14).

The square graphical symbol shown before the dimensional value indicates the dimensions of a square in the projection plane. If the tolerances are the same for both sides of the square, the square can be dimensioned by only dimensioning one side of the square (Pic. 5.15). 

If it is necessary to show the length of a workpiece before bending, it may be shown using auxiliary dimensions (Pic. 5.16).