Requirements of engineering drawings

Engineering drawings need to communicate information that is legally binding by providing a specification. Engineering drawings therefore need to met the following requirements:

■ Engineering drawings should be unambiguous and clear. For any part of a component there must be only one interpretation. If there is more than one interpretation or indeed there is doubt or fuzziness within the one interpretation, the drawing is incomplete because it will not be a true specification.

■ The drawing must be complete. The content of an engineering drawing must provide all the information for that stage of its manufacture. There may be several drawings for several phases of manufacture, e.g. raw shape, bent shape and heat-treated. Although each drawing should be complete in its own right, it may rely on other drawings for complete specification, e.g. detailed drawings and assembly drawings.

■ The drawing must be suitable for duplication. A drawing is a specification which needs to be communicated. The information may be communicated electronically or in a hard copy format. The drawing needs to be of a suitable scale for duplicating and of a sufficient scale such that if is micro-copied it can be suitable magnified without loss of quality.

■ Drawings must be language-independent. Engineering drawings should not be dependent on any language. Words on a drawing should only be used within the title block or where information of a non-graphical form needs to be given. Thus, there is a trend within ISO to use symbology in place of words.

■ Drawings need to conform to standards. The 'highest' standards are the ISO ones that are applicable worldwide. Alternatively standards applicable within countries may be used. Company standards are often produced for very specific industries.

1.6.1 Sizes and layout of drawing sheets

The standard dealing with the sizes and layout of drawing sheets is ISO 5457:1999. If hard copies of drawings are required, the first choice standard sizes of drawings are the conventional 'A' sizes of drawing paper. These sizes are illustrated in Figure 1.9. Drawings can be made in either portrait or landscape orientation but whatever orientation is used, the ratio of the two sides is 1:^2, (1:1.414). The basic 'A' size is the zero size or '0', known as 'AO'. This has a surface area of lm2 but follows the 1 :V2 ratio. The relationship is that A1 is half AO, A2 is half Al, etc.

A blank drawing sheet should contain the following things (see Figure 1.10). The first three are mandatory, the last four are optional.

1. Title block.

2. Frame for limiting the drawing space.

3. Centring marks.

Drawing Sheet Sizes

Figure 1.9 The AO to A4 range of standard drawing sheets sizes

Figure 1.9 The AO to A4 range of standard drawing sheets sizes

4. Orientation marks.

5. Metric reference graduation.

6. Grid reference system.

7. Trimming marks.

The title block is a specially designated area of the drawing sheet containing information for identification, administration and interpretation of the whole drawing. Irrespective of whether landscape or portrait orientation is used, the title block is normally located in the bottom right-hand corner of the drawing. The information included in the title block can range from the very simple to the exceedingly complex. The manual of British Standards in Engineering Drawing and Design (Parker, 1991) recommends that the following basic information always be included in a title block:

■ Name of company or organisation, drawing number, title, date, name of the draughtsman, scale, copyright, projection symbol, measurement units, reference to standards, sheet number, number of sheets and issue information.

The following supplementary information can be provided if necessary:

■ Material and specification, heat treatment, surface finish, tolerances, geometrical tolerances, screw thread forms, sheet size, equivalent part, supersedes, superseded by, tool references, gauge references and warning notes.

A border should be used to define the edge of the drawing region. It should have a minimum width of 20mm for AO and A1 sizes and 10mm for A2, A3 and A4. The border shows the edge of the drawing area and would therefore reveal the fact that the drawing had, say, a torn-off corner. The drawing frame is the area within the border (see Figure 1.10).

Trimming marks may be added at the edge of the drawing within the border to facilitate trimming of the paper. There should be four trimming marks at each corner. They can be of two types. The first type is in the form of a right-angled isosceles triangle as shown in the top left-hand corner in Figure 1.10. The second alternative trimming mark is an 'L' shape shown on the top right-hand side of the drawing shown in Figure 1.10.

Centring marks should be provided on the four sides of a drawing to facilitate positioning of the drawing. They take the form of dashes that extend slightly beyond the border as shown in Figure 1.10. They are placed at the centre of each of the four sides.

Orientation marks may be provided on two sides of the drawing sheet (see Figure 1.10). These consist of arrows which coincide with the centring marks. Two such orientation marks should be provided

Trimming marks (x4)

Alternative trimming mark \

Drawing frame

Grid reference system

Centring marks (x4) -

Title box

- Border

Reference graduations

Edges of drawing sheet

Figure 1.10/1 typical blank sheet used for engineering drawing on each drawing, one of which points towards the draughtsman's viewing position.

Areference metric graduation scale may be provided with a minimum length of 100mm that is divided into 10mm intervals (see Figure 1.10). The reference graduations consist of 10 off 10mm graduations together making a total length of 100mm. From this graduation scale one can conclude that the drawing size is A3. This calculation shows the usefulness of the reference graduation scale and that it still permits scaling of a drawing when it is presented at a different scale than the original.

An alphanumeric grid reference system is recommended for all drawings to permit the easy location of things like details, additions and modifications. The number of divisions should be a multiple of two, the number of which should be chosen with respect to the drawings. Capital letters should be used on one edge and numerals for the other. These should be repeated on the opposite sides of the drawing. ISO 5457:1980 suggests that the length of any one of the reference zones should be not less than 25mm and not more than 75mm.

1.6.2 Types of drawings

There are a number of different types of engineering drawings, each of which meets a particular purpose. There are typically nine types of drawing in common use, these are:

1. A design layout drawing (or design scheme) which represents in broad principles feasible solutions which meet the design requirements.

2. A detail drawing (or single part drawing) shows details of a single artefact and includes all the necessary information required for its manufacture, e.g. the form, dimensions, tolerances, material, finishes and treatments.

3. A tabular drawing shows an artefact or assembly typical of a series of similar things having a common family form but variable characteristics all of which can be presented in tabular form, e.g. a family of bolts.

4. An assembly drawing shows how the individual parts or subassemblies of an artefact are combined together to make the assembly. An item list should be included or referred to. An assembly drawing should not provide any manufacturing details but merely give details of how the individual parts are to be assembled together.

5. A combined drawing is a combination of detail drawings, assembly drawings and an item list. It represents the constituent details of the artefact parts, how they are manufactured, etc., as well as an assembly drawing and an accompanying item list.

6. An arrangement drawing can be with respect to a finished product or equipment. It shows the arrangement of assemblies and parts. It will include important functional as well as performance requirements features. An installation drawing is a particular variation of an arrangement drawing which provides the necessary details to affect installation of typically chemical equipment.

7. A diagram is a drawing depicting the function of a system, typically electrical, electronic, hydraulic or pneumatic that uses symbology.

8. An item list, sometimes called a parts list, is a list of the component parts required for an assembly. An item list will either be included on an assembly drawing or a separate drawing which the assembly drawing refers to.

9. A drawing list is used when a variety of parts make up an assembly and each separate part or artefact is detailed on a separate drawing. All the drawings and item lists will be cross-reference on a drawing list.

Figures 1.11 and 1.12 show an assembly drawing and a detailed drawing of a small hand vice. The assembly drawing is in orthographic third-angle projection. It shows the layout of the individual parts constituting the assembly. There are actually 14 individual parts in the assembly but several of these are common, such as the four insert screws and two-off hardened inserts such that the number of identifiable separate components numbers 10. On the drawing each of the 10 parts is numbered by a balloon reference system. The accompanying item list shows the part number, the number required and its description. Separate detailed drawings would have to be provided for non-standard parts. One such detailed drawing is shown in Figure 1.12, which is the detailed drawing of the movable jaw. This is shown in third-angle orthographic projection with all the dimensions sufficient for it to be manufactured. Tolerances have been left off for convenience.

Engineers Vice Orthographic Drawing
Figure 1.11 An assembly drawing of a small hand vice
Manual Hand Engineering Drawing
Figure 1.12 A detailed drawing of the movable jaw of a small hand vice

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