In this section, you will become familiar with technical drawing conventions and learn to interpret the meaning of mechanical drawings.
The work of the Standards Association of Australia towards standardisation of drawing practice has been of great value in recent years. Not only are we reaching the situation where all drawings produced in Australia are similar, but drawings produced overseas are also more readily interchanged with ours due to the Standards Association’s affiliation with the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).
This section is designed to make you familiar with the Australian standard drawing symbols and give you some practice in drawing and recognising them.
After completing this section, you should be able to:
In this section we will examine the basic types of mechanical drawings, and their purpose. Engineering drawings can be classified according to their style (type of projection), or according to the purpose that the drawing serves.
There are two common styles of mechanical drawing – pictorial and orthographic drawings.
In pictorial or three-dimensional drawings, the completed drawing looks like a three dimensional object. The viewpoint is chosen from a skew direction (not parallel to the object’s axes) in order to reveal the most information about the object in one view. Styles of pictorial representation include isometric, oblique, and perspective drawings, with isometric projection being the most common.
These types of drawings make it easy to imagine how the object will look, but they distort the dimensions of the object. Figure 1 below shows a pictorial drawing, specifically an isometric projection.
The other style shows different views of the object on one drawing, as seen from different directions. This style of drawing is known as an orthographic projection. The viewing directions are at right angles, and may include a top or plan view, a front view or elevation and an end view.
Figure 2 below shows an orthographic projection of the object in Figure 1.
In Australia, the third angle projection has been adopted as the standard method of projection for orthographic drawings. For this projection, you draw the view you see from the side you are looking. That means that the top view is drawn above the front view, a view from the right is drawn on the right side and a view from the left drawn on the left. It is as if the object is placed in a glass box, and the view from each side is traced is drawn on that face. The sides of the box are opened and laid flat to produce the orthographic views. Up to six views may be obtained using this method.
Figure 1: Pictorial drawing
Figure 2: Orthographic (third angle) projection
To indicate that the orthogonal drawing has been produced in 3rd angle projection, a symbol is placed either in the title block or in the top left corner of the sheet. This symbol consists of two views of a truncated cone drawn in the same projection as the drawing itself. Figures 3a, 3b and 3c show the derivation of the symbol.
Figure 3a: third angle symbol
Figure 3b: third angle symbol
Figure 3c: complete third angle symbol
When something is manufactured, a set of working drawings are produced to ensure that all the parts are produced and assembled correctly. Working drawings can be subdivided into detail drawings and assembly drawings.
Detail drawings give the information and instructions required to manufacture a single part. They usually include all details such as the shape, dimensions, materials, tolerances and surface finish of the part. Most commonly, detail drawings will be an orthographic projection. Figure 4 below shows an example of a detail drawing.
Figure 4: Shaft detail drawing
Assembly drawings show how the parts of a product can be fitted together. Many of the details present in the detailed drawing will be excluded from the assembly drawing. Assembly drawings show the relative position of the parts, and the most important dimensions for assembly and use of the product. Assembly drawings are often pictorial, but may also be orthographic.
Figure 5 shows an example of a complete assembly drawing. Here the parts are labelled directly, but often the labels are shown as numbers or letters, to avoid cluttering the diagram. In this case, a key would be provided for the names of each part.
Figure 5: Main assembly
Note that cylindrical objects such as the shafts and proprietary items such as nut and bolts are not sectioned in assembly drawings. Note also that dimensions are not shown unless they relate to the assembly procedure. This diagram is commonly referred to as a General Assembly drawing.
Figure 6: General assembly
Figure 7: Shaft and end cap assembly
Figures 7, 8, 9 and 10 show examples of sub-assembly drawings. A sub-assembly comprises a number of parts that are assembled separately as a unit, before being integrated into the complete assembly.
Sub-assembly drawings provide more detail than the general assembly drawing. Subassemblies may be manufactured and tested in isolation from the remainder of the completed product. Often, critical assembly notes will be provided on the drawing, as shown in figure 7. In this drawing, the matching and alignment of the radial holes at the join is very important for the operation of the machine.
Figure 8: Shaft and Pulley Sub-Assembly
Assembly drawings are often shown as exploded views, which show the parts aligned along their assembly axis but separated in space. This makes the shape, number and assembly sequence of the parts very clear. Figure 9 shows an exploded orthogonal projection, and Figure 10 shows an exploded pictorial view.
Figure 9: Shaft and pulley sub-assembly – exploded orthographic
Figure 10: Shaft sub-assembly – exploded pictorial
An exploded assembly drawing is very useful for repair or maintenance drawings and can be read easily by people who are not familiar with orthogonal drawings. They are used in spare parts listings, maintenance diagrams or in explaining assembly procedures.
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Activity 1 |
Referring to Figure 11 below, answer the following questions.
1 What is the name of the drawing?
2 On what size sheet is the drawing?
3 The what scale is the drawing made?
4 What revision has been made to the drawing?
5 What is the name of Item 2?
6 From what material is Item 2 manufactured?
7 On what date was the drawing made?
8 On what date was the drawing issued for manufacture?
9 In what zone is the dimension R4.5?
10 What bend radius is used for the material?
Figure 11: Conduit mounting assembly
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Activity 2 |
Figure 12 below shows another view of the conduit mounting assembly from figure 11. Answer the following questions.
1 What type of drawing is this?
2 What is the advantage of a drawing of this type?
3 What, if any, would be the disadvantage of drawing of this type?
4 Where would this type of drawing be most likely to be used?
5 How many parts are required in the assembly?
6 What is the specification for the bolts?
Figure 12: Conduit mounting assembly
Most technical drawings you encounter will have a standard presentation. They will be presented on paper of standard size, with a standard title block.
The standard drawing sizes in use in Australia are A0, A1, A2, A3, and A4. Normally it isn’t necessary to know the exact sheet sizes as you purchase drawing sheets to this specification. As long as you can recognise the difference between A3 and A2, for example, you will be all right.
Each size increase represents a doubling of the area. The smallest is A4 which is the size of the paper which this material is printed on. Two A4 sheets joined together down their long sides is the same size as an A3 sheet. An A2 sheet can be made by joining two A3 sheets together and so on.
The choice of which sheet size to use will depend on a number of factors. These are:
Title blocks are often designed to incorporate company logos and other information. The following information must appear as a minimum.
The drawing needs a name. The name may consist of the overall project name, the particular apparatus, and the type of drawing. For example:
Lithgow College of TAFE Refurbishment
Electrical Engineering Classroom/Workshop
Equipment Location Diagram.
The first line here is the project name, the next line is the particular apparatus (or, in this case, the part of the project shown on the drawing), and the last line describes what type of drawing it is.
Each drawing will have a number for filing purposes. Usually the number will have provision for recording the version number of the drawing. The original may be 1480 A1, for instance, and when later amended it may become 1480A1/A.
The date the drawing was authorised.
The name or initials of the person who drew the drawing.
This is more than just an advertisement. The drawing number may only be unique within the one company. Another company may possess a drawing with an identical drawing number.
Drawings are usually reduced-size versions of the real-life object, but may also be magnified. If we draw something at actual size, the scale is said to be ‘one to one’ (1:1).
If we draw something so that ten millimetres on the drawing represents one metre in actual size we have drawn at a scale of 1:100. The fact that the smaller number comes first tells us that we have reduced our drawing from real life. A scale of 100:1 is a magnification which may be appropriate for a very small component.
If a drawing is not to scale, or if some measurements are unknown at the time of drawing, the words: ‘not to scale’ or ‘do not scale’ can be added to the drawing.
These parts of a drawing are almost always in the form of tables. If it is a drawing showing the various parts of an automatic washing machine and how they fit together, the parts list would list all the parts shown on the drawing and relevant information about them. This information might be ‘Manufacturer’; ‘Description of Part’; and ‘Part Number’.
If the drawing was of something to be fabricated from raw materials such as sheet steel, the parts list would be called a material list.
If the drawing was a layout or site plan, or a circuit diagram, a legend should be included if there are departures from the commonly recognised symbols.
An example may be a refurbishment layout drawing for a set of offices. Walls, light fittings (luminaires), telephone points and many other items may need to be relocated, removed completely, or supplied new. While there is a standard symbol for a telephone point, for instance, it is cumbersome to write an explanatory note next to each point.
Symbols are often changed slightly to indicate the work required. As an example, a new telephone point might use the standard symbol but represent it filled in; an existing one to be removed might be drawn with dashed lines; a new point to be installed as part of the project may have a circle drawn around it; and an existing point which is to stay undisturbed might be shown as the standard symbol without modification. Obviously these specially designed symbols need to be defined on a legend.
Once you have put a legend on a drawing it is a good idea, especially if the drawing is part of an instruction to a contractor, to include all but the most widely understood symbols in a legend on the drawing itself. If a drawing forms part of a set, or if the number of parts of types of symbol is large, a separate drawing may be made for the purpose of a parts list or legend.
Generally speaking, the dimensions shown in a detailed drawing are the dimensions that will be needed to produce the object. Assembly drawings may contain no dimensions, or only those dimensions critical to the assembly or operation of the item.
Care should also be taken not to ‘over-dimension’ the object. If a dimension is shown twice on a drawing, confusion arises in the factory or on site as to which dimension to use, especially if a change is made to one of the dimensions.
As with any other part of a drawing, dimensions must be placed on the drawing in accordance with the current drawing standards AS100.
The basic rules of dimensioning are:
Sometimes it doesn’t matter how well you draw in third angle projection, there is something that you just can’t show in enough detail. An example is the adaptor fitting shown in Figure 15.
Figure 15: Sectioning
Although the internal drillings can be shown by broken lines on one of the normal views, too many broken lines can be difficult to follow. A clearer drawing results when the view is ‘cut open’ to show the internal details.
Normally a sectional view is included as an extra view on the drawing unless, as in the case of our example, a half sectional view can be included which will serve both the needs of a sectional view and that of an additional third angle view.
Note on Figure 15 the sectioning plane is shown on another view by the arrows A and A. The direction of the arrows indicates the direction that the section is being viewed from. Sometimes a shape is so complex that several sectional views are shown. They may be labelled A–A, B–B, C–C, and so on.
If necessary an isometric drawing can be sectioned. Figure 16 shows the adaptor in isometric representation with a 90 degree section taken from it. Notice that arrows labelled A–A or similar are not used to indicate sectioning in the isometric drawing, but lines showing where the removed piece came from are used instead.
Figure 16: Isometric half sectional view
In both orthogonal and isometric drawings the actual surface of the cut is indicated by a series of slanting lines, known as hatching.
You may have noticed in Figure 15 that holes are represented from the top and bottom (in third angle) by circles. From the side the holes are shown as broken lines (broken lines indicate that something is there but can’t be seen from where the view is being drawn). A faint chain line indicates the centre line of the hole.
When sectioned the section may expose some hidden parts, as it does in our illustration. Therefore in the sectioned portion, the hole is shown as full lines with no hatching between them.
Note that if there are hidden parts behind a section plane, they would not be shown on the section because broken lines would be hard to see amongst the hatching. It is good practice to choose a hatching plane which shows the maximum detail.
Drawing a screw thread promises to be a difficult task, however, technical drawings are designed to convey information as simply as possible. This means that there is a simplified way of representing threads without having to draw them all in.
Threads are simply drawn as concentric cylinders or holes with the larger diameter equal to the diameter across the tops of the threads, and the smaller diameter equal to the diameter across the bottom of the threads.
Looking down on the top or bottom of the thread it is shown in a similar way, but the larger diameter circle is not completed and is drawn with a thinner line than the hole.
Figure 17: Drawing convention for threads
Source: http://lrr.cli.det.nsw.edu.au/LRRDownloads/5103/1/5103_1.doc
Web site to visit: http://lrr.cli.det.nsw.edu.au/
Author of the text: NSW DET 2007 2006/060/06/2007 LRR 5103
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