Drawings provide most of the technical information you need to undertake projects in the engineering workshop. The information covered in this chapter will help you:
Successful completion of an engineering task will depend on your ability to read information from a technical drawing and apply that information accurately to your work. To help you achieve this, you will need some basic knowledge of the types of drawings used in the Engineering Industry as well as standard drawing practice.
Standards Australia has recommended standards for drawing practice in all fields of engineering. These standards are set out in documents titled AS1100 and AS1102. Your teacher or school library should be able to provide a copy for your perusal. You should find most of the information you need for your course in AS1100.101.
Engineering drawings are prepared according to these standards so that technical information is always presented in a clear and consistent manner. In other words, when you have learned to interpret a technical drawing you should be able to interpret all drawings in the same way.
[[ sh /n ][ Selecting the Correct Drawing ]]In a typical engineering workshop there could be many different drawings in use at the same time. There might be several projects being machined or assembled, or a large project with a number of different components, each with its own drawing.
Sometimes components of a project can be quite similar, for example, bushes with the same outside diameter but different inside diameters. Their drawings would look much the same.
These are just a few examples of how a careless worker might use the wrong drawing and cause a production mistake.
Production mistakes can be very costly in terms of wasted time and materials. Always remember that you are responsible for the quality of your own work.
Also the work of other members of your group will often depend on your task being completed correctly and on time. So it is very important that you commence your engineering task by always selecting the correct technical drawing.
When you are given an engineering task your instructions will include requirements of the job. The drawing you select should be validated against the job requirements.
For example, let’s say the product your workgroup is manufacturing contains several different bushes which are described in terms of their outside diameter (OD), inside diameter (ID) and length. Your allocated task is to machine the required quantity of one of these bushes, 24 OD x 11·5 ID x 55 Long.
Validating the drawing you select could be as simple as checking the title of the drawing, as in the title block illustrated below. Alternatively you might need to refer to the drawing itself to check dimensions or other information.
In some cases it might also be necessary to check that the part can be manufactured according to the drawing with the equipment, tools and machines that are available.
Title blocks vary considerably in their design but generally contain information similar to the example on the previous page. Many engineering firms use CAD programs to produce their own drawings and generally design title blocks to suit their own requirements.
Sometimes drawings are changed or revised before they are approved for use in the workshop. You need to check that you have the approved version of the drawing.
This information can generally be found in the title block. The title block on the previous page shows details of two revisions and the date the drawing was approved.
Another common method of showing that a drawing is approved for use in the workshop is to stamp it as shown in the example on the right.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mr /f ][ What information is contained in the title block of an engineering drawing? ][ The client's name. ][ The material made from. ][ The id of the drawer. ][ * The client's address. ][ The scale of the drawing. ][ The client's address isn't usually included. ]]
[[ mc /f ][ How should you check that you have the correct drawing for the job? ][ Match the title on the drawing against the job card. ][ * Ask your workmates to check for you. ][ * Phone the client and ask them to confirm the job details. ][ * Ask your supervisor to confirm the job details. ][ Matching the drawing title against the job card is necessary and sufficient in almost all cases. ]]
When you have selected a drawing and checked that it is the correct one, you then need to be able to interpret the drawing to find the information you require for your engineering task. Interpreting a drawing involves recognising features of the object and identifying dimensions, instructions, material requirements and symbols.
Engineering drawings usually consist of two or more two-dimensional views of the object, often accompanied by a three-dimensional pictorial view. You need to be familiar these drawing methods so you can recognise features and components of the project that are shown in different parts the drawing.
Pictorial Drawings
Three-dimensional pictorial drawings can help you get a clear picture in your mind of the overall shape, proportions and details of the project. There are several different types of pictorials that could be used in engineering drawing. The most common of these would be Isometric and Oblique views. Perspective is another pictorial method that could also be used.
The line drawing on the right is an oblique view of a simple bearing. In this type of pictorial drawing the horizontal length and vertical height measurements are full size but the width measurements that recede at 45° are drawn half size.
Width measurements are halved so the drawing appears in much the same proportions as the real object. In other words, if the receding edges were made full size the drawing would look much wider than the object was meant to be. Some measurements are distorted in all types of pictorials, so make sure you never try to take measurements off a pictorial drawing unless it is dimensioned.
Remember, pictorials are used in technical drawings simply to help you get a clear picture of the object in your mind. This will help you interpret the technical information in other parts of the drawing.
Pictorial views used in technical drawings are sometimes rendered or shaded to make them look a bit more realistic. Compare the line drawing on the right above with the rendered oblique view on the left. Rendering can help you get a better understanding of the three dimensional shape of the object.
The line drawing below is an isometric view of a machined part. In this type of pictorial drawing height measurements are made on vertical lines while length and width measurements are made on lines drawn at 30° to the right and 30° to the left to create the three-dimensional picture.
Only these isometric lines are true length. All other lengths and shapes are distorted. For example, the corners of the horizontal surface of the base are actually 90° but they appear as 60° and 120° in the isometric picture.
Once again, don’t make the mistake of trying to take measurements off a pictorial view except where dimensions are actually placed on the drawing.
The pictorial below is a rendered view of the same machined part. Here again we can see that the shading makes the drawing look much more realistic.
Engineering projects can sometimes be quite complicated and their technical drawings can seem a bit confusing at first. Interpreting the drawings can be made a lot easier when you become familiar with the project by studying a pictorial view.
Multi-View Drawings
Multi-View drawings are used to present most of the information required to complete an engineering project. The drawing usually consists of two or three views of the project looking in directions that are at right angles to each other.
The illustration below shows an oblique view of a simple bearing. The arrows point in the direction of the front view, side view and top view that could be included in a multi-view drawing of the bearing.
With the bearing in this position the front view shows its length and height, the top view shows its length and width and the side view shows height and width.
The illustration on the following page shows the relationship between the views. The top view is drawn above and vertically in line with the front view. The side view is drawn beside the front view and horizontally in line with it. A right side view would be drawn on the right side of the front view and a left side view would be drawn on the left side of the front view.
The light projection lines between the views show how corresponding points or features of the bearing line up from one view to the other.
As you can see it is easier to recognise features and shape of the bearing in the pictorial view than the multi-view drawing.
As you look at the pictorial view, try to imagine that you are looking down from above in the direction of the top view arrow.
Now look at the top view in the multi-view drawing. This is what you would see. When you feel you can properly recognise the top view, look at the pictorial again and imagine you are looking in the direction of the side view arrow. Now look at the side view in the multi-view drawing. Once again, this is what you would see.
In the example above the shape of the front view is quite obvious in the oblique pictorial and easy to recognise in the multi-view drawing.
In the isometric drawing of the machined part on the left the shape of the front view is not as obvious. Try to visualise the shapes of the front, side and top views of the machined part. When you are satisfied you can recognise them, try making a multi-view sketch with the views in their correct positions as in the example above.
The standard types of lines that should be used in engineering drawings are set out in AS 1100.101. Some examples are shown below.
Hidden Lines
Hidden lines (series of short dashes) indicate details or features that can’t actually be seen in the view.
In the drawing on the right the hidden detail lines in the top view show the large bearing hole passing through the casting.
The hidden detail lines in the front view show the bolt holes through the base. The hidden detail lines in the side view represent both the large bearing hole and the bolt holes through the base. Hidden lines are included in a drawing to help you recognise and understand features of the object.
For example, without the hidden lines there is nothing in the drawing above to show that the large hole goes right through the casting. How would the hidden detail appear if the large hole only went halfway through the casting? Try drawing a sketch of the top view or side view and show the hidden detail.
Centre Lines
Centre lines (alternate long and short lines) are used to show the centres of holes and circular shapes.
Centre lines are also used to show that a shape is symmetrical (the same both sides) about the centre line.
They can also be used to show the centre of a surface.
Section Lines And Sections
Sometimes it is helpful to see a cut-away view or section of the project you are making.
The position of a section is shown by a section line which is similar to a centre line but usually drawn a bit heavier.
In the drawing on the right a vertical section through the centre of the large hole is shown by the section line A-A.
Imagine that the right half of the casting is cut away and you are looking at the cut section in the direction of the side view. The metal that is ‘cut’ is shown in the side view by a series of 45° lines called cross hatching. All sections are cross hatched in an engineering drawing. Hidden detail is not usually shown in a sectional view.
Dimensions on a drawing show the overall sizes as well the size of all features and components necessary for successful completion of the project. Dimension lines can be drawn using any of the methods shown below.
The short lines give the limits of the dimension and the dimension lines show the distance from point to point by means of arrowheads, dots or small diagonal lines as illustrated below.
Depending on the CAD program used to produce technical drawings, the most common method used in the engineering industry would probably be arrowheads.
The dimensioned drawing is shown below. Overall length, width and height measurements are shown as well as other measurements that determine the shape and proportions of the object.
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Note also the way holes and circular shapes are dimensioned by showing their diameters using the diameter symbol, for example Ø60. This would be read ‘diameter sixty’. A large hole or diameter can be dimensioned from point to point. The diameter of small holes can be shown by an external dimension line pointing to the circumference and lining up with the centre.
Apart from dimensions, an engineering drawing is likely to contain other job instructions that are necessary for successful completion of the project. Job instructions could be printed on the drawing itself or in some cases, contained in the title block. Instructions in the title block are usually standard instructions that apply to all similar engineering projects.
Standard instructions in the title block are often preceded by the words unless otherwise specified. This means that the standard instruction could be over-ridden by an instruction specified in the drawing. For example, one of the standard instructions in the title block on page 210 says ‘Unless otherwise specified: dimensions are in millimetres’.
In most cases dimensions will be in millimetres. However, you need to be aware that some dimensions on a drawing could be imperial measurements (inches); for example, in a project that is to be machined to fit a part made in a country where imperial is the standard system of measurement. Always check a drawing carefully for notes or other indications that imperial measurements have been specified.
Another instruction in the title block on page 210 says ‘deburr and break sharp edges’. This would generally be a standard operating procedure for all engineering projects. Instructions such as this could also be printed on the drawing itself.
Tolerance Values
Tolerance values shown in a drawing are very important instructions about quality of the work. Tolerance values set acceptable limits for how much measurements can vary from the given dimensions as shown in the drawing of the bush below.
The dimensioned drawing shows that the tolerance values for both the inside and outside diameters of the bush are plus or minus one hundredth of a millimetre. This means that the outside diameter must be between 30·01 and 29·99 millimetres and the inside diameter must be between 24·99 and 25·01 millimetres.
Note also that standard tolerances for linear and angular measurements are shown on the drawing. Standard tolerances for various types of work would generally be set as part of the engineering workshop’s Quality Standards. Keep in mind that you are responsible for the quality of your own work and the finished product must meet all quality standards.
Tolerance values can be shown on an engineering drawing using the following methods where they apply:
Note: Where both size limits are specified, the larger limit is given first for outside dimensions and the smaller limit is given first for inside dimensions.
Material requirements for a simple engineering project are generally printed on the drawing itself or in the title block. The title block on page 210 shows that the project (24 OD x 11·5 ID x 55 Long BUSH) is to be made from 24mmOD CS1020 steel. In other words, for each bush you make you will require approximately 60mm of the Ø24 steel, allowing for facing the ends.
When a drawing shows a number of different parts to be assembled in the project, a material or parts list in the form of a table is included, generally at the top of the title block against the right hand border line.
A material or parts list would include information such as the part number, the part name, the quantity required, the material and its specification and where applicable, the drawing number of each individual part.
Symbols are used in engineering drawings as a ‘shorthand’ way of providing technical information about the job. You need to know the meanings of a range of basic symbols so you can correctly interpret workshop drawings.
Datum Surfaces
A datum surface is a reference surface from which measurements can be made. Datum surfaces or other datum features are shown on a drawing by a triangle symbol and pointer.
A solid triangle is often used so the datum symbol looks different from surface finish symbols which are also triangular in shape.
In the front view of the casting below the pointer from the datum symbol shows that the bottom or base of the casting is the datum surface.
The datum or reference surface should always be machined first so that other surfaces, such as the front and back, can be machined at right angles to it.
Also in this example, the centre height of the bearing hole would be measured accurately from the datum surface.
Surface Finish Symbols
Surface finish symbols and roughness values indicate the machined surfaces. If you are not familiar with these terms, refer back to the section titled Surface Finish commencing on page 189.
The table on page 190 gives roughness values for a range of machined surfaces and a general description of the surface finish for each roughness value.
The drawing of the casting on the right shows that the datum surface requires average machining, the front and back surfaces good machining and the inside bearing surface fine machining.
Note that the top surface of the casting is not to be machined. The symbol for surfaces that must not be machined is similar to the surface finish symbol but features a circle instead of the triangle.
Welding Symbols
Welders as well as machinists working in the engineering industry need to be able to interpret technical drawings.
In particular, there is a system of welding symbols used in drawings that a welder should be able to interpret. Symbols exist for the full range of welded joints.
However, only butt and fillet welds will be covered here because these are the joints that you are most likely to use in your workshop projects.
The table below shows the welding symbols, their meaning and an example of each.
The drawings on the next page are not intended to represent complete workshop drawings. Dimensions, material requirements and other information would be necessary for the welder or fabricator to be able to complete the work. They are simply examples of the parts of workshop drawings where welding symbols would be used to indicate the types of joints required.
The illustration on the left shows two views of a piece of hollow square section steel that is to be welded to a heavy mild steel base.
The welding symbol used indicates that fillet welds are required all round.
The arrow of the welding symbol shown in the front view is pointing to the location of the welded joint.
The triangular part of the welding symbol indicates that a fillet weld is to be used and the circular part indicates that the joint is to be welded all round.
Note that the weld itself is not shown in the drawing. The illustrations in the examples column of the table on the previous page only show the welds for explanation purposes.
The illustration on the right shows the front and top views of three pieces of hollow square section steel that are to be welded.
The welding symbol indicates that a fillet weld should be placed on both sides of the joint.
The ‘fish tail’ attached to the end of the welding symbol is used when any additional information about the weld is required in the drawing.
In this drawing the additional information means that similar welds are required for the four joints.
Before leaving this page, think of what you've just been reading, and test yourself with these questions:
[[ mm /f ][ Match each view to an appropriate description of the view: ][ Perspective ~ Length and width both shrink at a distance. ][ Isometric ~ Length and width both recede at 30° to the left and right. ][ Oblique ~ Length and height are square-on and full size; width is half-size at 45°. ][ Multi-View ~ Object seen square-on, from front, side and top. ][ Perspective => dimensions shrink at a distance; Isometric => edges recede at 30°; Oblique => width recedes at 45°; Multi-View => seen from front, side and top. ]]
[[ mr /f ][ Which views can be measured directly to provide engineering dimensions? ][ Isometric Views ][ Multi-View Diagrams ][ * Perspective Views ][ * Oblique Views. ][ Perspective and oblique views are not drawn with true dimensions. ]]
[[ mc /f ][ How are hidden detail lines drawn on an engineering diagram? ][ * They are clearly labelled as hidden detail. ][ * They are drawn as hatched areas. ][ They are drawn as dashed lines. ][ * They are drawn in heavy ink. ][ Hidden detail is drawn as light dashed lines. ]]
[[ mc /f ][ How are centre lines drawn on an engineering diagram? ][ * They are clearly labelled as centre lines. ][ * They are drawn as hatched areas. ][ They are drawn as alternate short and long dashed lines. ][ * They are drawn in heavy ink. ][ Centre lines are drawn as alternating short and long dashed lines. ]]
[[ mc /f ][ Which of the following tolerance statements is the odd-one-out? ][ * 21.3 over -0.2 ][ * 21.1 over +0.2 ][ * 21.3 over 21.1 ][ 21.2 over ±0.2 ][ 21.2 over ±0.2 implies limits of 21.0 to 21.4, rather than 21.1 to 21.3. ]]
[[ mc /f ][ "An upwards-bent arrow with a circle around the bend" symbol implies what welding instruction? ][ * Fillet weld on the arrow side of the joint. ][ * Square butt weld. ][ * Fillet weld on opp. side of the joint to the arrow. ][ Weld all around the joint. ][ * Fillet weld all around the joint. ][ The symbol implies to weld all around the joint. ]]