Drawing Conventions

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Fig. 18/17 Boh heeds end Kiev, heads

Types of Bolt« and Scraw»

There are many types of heads for bolts and screws apart from the standard hexagonal head Some are shown in Fig 18/17.

Fig. 18/17 shows only e few types of bolt end screw heeds that are in use. There are Wedged-Shaped Heads, Tommy Heads. Conical Heads. Hook Bolts and Eye Bolts. There ere Small. Medium and Large Headed Square screws. 60". 120* and 140* countersunk screw heads with straight slots, cross slots and hexagonal slots. There are Instrument Screws and Oval Cheese-Headed Screws to name only a few. The dimensions for all these screws can be obtained from any good engineering handbook.

DIMENSIONING When an engineenng drawing is made, dimensioning is of vital importance All the dimensions necessary to make the erticles drawn must be on the drawing and they must be presented so that they can be easily read, easily found, and not open to misinterpretation A neat drawing can be spoilt by bad dimensioning

In British drawing practice the decimal point is shown in the usual way. i.e. 15.26. On the Continent, however, the decimel point in Metric units is a comma. i.e. 15,26 or 0.003.

Also, in the Metric system a space is left between every three digits, U. 12 056.0 or 0.002 03. Note that values less than unity are prefixed by a nought Engineering drawings are ususlty dimensioned in millimetres, inespective of the sae of the dimension, but the centimetre end metre ere also sometimes used

There are many rules about how to dimension a drawing properly, but it is unlikely that two people will dimension the same drawing in exactly the same way. However, remember when dimensioning that you must be particularly neat and concise, thorough and consistent. The following rules must be adhered to when dimensioning.

1. Protection lines should be thin lines and should extend from about 1 mm from the outline to 3 mm to 6 mm past the dimension line.

2. The dimension line should be a thin line and terminate with arrowheads et least 3 mm long and these errow-heads must touch the protection lines

3 The dimension may be inserted within a breek in the dimension line or be placed on top of the dimension line.

4. The dimensions should be placed so that they are read from the bottom of the pa por or from the right-hand side of the paper.

5. Dimension lines should be drawn outside the outline. whenever possible, and should be kept well clear of the outline.

6. Overall dimensions should be placed outside the intermediate dimensions.

DIMENSION UNE

PROJECTION UNE

R» 18/18 Illustrating lulu 1-6

7. Centre lines must never be used as dimension lines, They may be used as projection lines.

8. Diameters mey be dimensioned in one of two ways Either dimension directly across the circle (not on e centre line), or project the diameter to outside the outline Diameter' is denoted by the symbol 4 placed in front of the dimension.

9. When dimensioning e radius, you must, if possible, show the centre of the radius. The actual dimension for the redius may be shown either side of the outline but should, of course, be kept outside if possible The word radius must be abbreviated to R and placed in front of the dimension.

10. When a diameter or e radius is too small to be dimensioned by any of the above methods, a leader may be used. The leader line should be a thin line and should terminate on the detail that it ts pointing to with an arrowhead or, within an outline, with a dot. Long leeder lines should be avoided even if it means inserting another dimension The leader line should always meet another line at an acute angle

11. Dimensions should not be repeated on a drawing. It is necessary to put a dimension on only once, however many views ere drawn. There is one exception to thia rule. If. by inserting one dimension, it saves adding up lots of small dimensions then this is allowed. These types of dimensions are called auxiliary dimensions and are shown to be so either by underlining the dimensions or putting it in brackets.

Bs308 Dimensioning Slot

12. Unlets una voidable, do not dimension hidden detail. It is usually possible to dimension the same detail on another view.

13. When dimensioning angles, draw the dimension lines with a compass; the point of the compass should be on the point of the angle. The arrowheads may be drawn either side of the dimension lines, and the dimension may be inserted between the dimension lines or outside them. Whatever the angle, the dimension must be placed so that it can be read from either the bottom of the paper or from the right • hand side.

14. If a lot of parallel dimensions are given, it avoids confusion if the dimensions are staggered so that they are all easier to reed.

16. If a lot of dimensions are to be shown from one projection line (often referred to as a datum Una), either of the methods shown in Fig. 18/20 may be used. Note that in both methods, the actual dimension is close to the arrowhead and not at the centre of the dimension line.

16. If the majority of dimensions on a drawing are in one unrt. it is not necessary to put on the abbreviation for the units used, i.e. cm or mm. In this case, the following note must be printed on your drawing.

UNLESS OTHERWISE STATED. DIMENSIONS ARE IN MILLIMETRES

Fig. 18/20 Illustrating rules 12-16

Fig. 18/20 Illustrating rules 12-16

17. If e very larg« radius is drawn who«« centre is off th« drawing, th« dimension line it drawn with a tingle zig-zag in it.

18. Dimensioning small spaces raises its own problems and solutions Some examples are shown in Fig. 18/21

DIMENSIONS IN rrm

There ere one or two more rules that do not require illustrating.

19. If the drawing is to scale, the dimensions put on the drawing are the actual dimensions of the component and not the size of the line on your drawing.

DIMENSIONS IN rrm

Drawing Scale

Fig. 18/21 lllustrsting rules 17 and 18

The above nineteen rules do not cover all aspects of dimensioning (there are a whole new set on toleranced dimensions a lone) but they should cover all that is necessery up to 'O' level G.C.E. Dimensioning properly is a matter of applying common sense to the rules because no two different drawings can ever raise exactly the same problems Each drawing that you do needs to be studied very carefully before you begin to dimension It.

Examination questions often ask for only ftva or six 'important" dimensions to be inserted on the finished drawing. The overall dimension»—length, breedth and width—are obviously important but the remaining two or three are not ao obvious. The component or assembled components need to be studied in order to atcerteln the function of the object If. for instance, the drawing it of a bearing, then the size of the beering is vitally important because something has to fit into that bearing. If the

Fig. 18/21 lllustrsting rules 17 and 18

drawing it of a machine vice, then the aize of the vice jaws should be dimensioned so that the limitations of the vice are immediately apparent. These are the types of dimensions that should make up the total required.

CONVENTIONAL REPRESENTATIONS There are many common engineering details that are difficult and tedius to draw. The screw thread is an example of this type of detail and it has been shown earlier in this part of the book that there ere conventional ways of drawing screw threads which ere very much simpler then drawing out helical »crew threads in full.

Fig. 18/22 shows tome more engineering details and alongside the detailed drawing is shown the conventional representation for that detail. These conventions are designed to save time and should be used wherever and whenever possible.

These are not all the standard conventions but the rest are beyond the scope of this book. The interacted student can find the rest in BS308.

Bs308 Drawing Conventions
H». 11/22 to)

COMPESSION SPRINGS

TENSION SPRINGS

SPUNEO SHAFTS

SERRATED SHAFTS

HOLES ON CIRCULAR PITCH

HOLES

ON LINEAR PITCH

fr

Urheberrechtlich geschütztes Materie

Machining Symbols

The shape of an engineering component can be determined in several ways. The component may be forged, cast drawn, etc After one or more of these processes, it is quite likely that some machining will have to be done It is therefore important that these machined faces be indicated on the drawing. The method recommended by BS 308 is shown in Fig. 18/23 but this is not the only method in use. Sometimes, the letter T is written over the face to be machined. Thia letter T stands for 'finish'.

The small tick shows only that that particular face has to be machined. It does not show how it is to be machined, nor does it show how smooth the finish is to be. The method of machining—turning, milling, grinding, etc. —is not normally put on a drawing but the standard of finish is very smooth or the bearing will overheat and eeue'. On the other hand, smooth finishes are expensive to produce and should be kept to a sensible minimum. The moving parts of an internal combustion engine can be so well finished that it is not necessary to 'run in' the car but this is an expensive process applied only on very expensive motor cars The mass-produced car needs several hundred miles of careful driving while the surfaces 'wear smooth'.

If the surface of a piece of machined metal is magnified it will look like a range of very craggy mountains. The surface roughness is the distance from the highest 'peek' to the lowest 'valley1. This roughness is measured in micrometres and one micro- metre is one millionth part of a metre. Not only can a surface be made smooth to one micro-metre but it can also be measured to one micro -

Surface roughneaa

This short section on roughness symbols is beyond the G.C.E. '0' level syllabus, but it is well worth looking at.

The standard of finish, or roughnees of a surface, is of vital importance in engineering. The degree of roughness permitted depends on the function of the component. When two pieces of metal slide against each other, as in the case of a bearing, the finish on both parts must be

The British Standard index numbers of surface roughness are 0.025: 0.05: 0.1: 0.2: 04: 0 8: 1.8: 3.2: 6.3: 12.5 and 25.0. A surface roughness of from 0.026 to 0.2 cen be obtained by lapping or honing. 0.4 can be obtained by grinding and 0.8 by careful turning, rough grinding, etc. The surface roughnese number is shown within the vee of the machining symbol A tolerance on surface roughness is shown as a fraction, with the maximum

MACHINING SYMBOL

Surface Finish Symbols Drawings

1ST ANCLE PROJECTION

Machining Symbols

Fig. 18/23 Application of machining symbol

1ST ANCLE PROJECTION

Fig. 18/23 Application of machining symbol x 594 mm. The aver age student will do the majority ol his drawing on size A2. 594 mm x 420 mm end this is also the size that most examination questions are answered on.

If it it possible, an engineering drawing should be so positioned that it makes the maximum use of the evaileble spec«. The positions of the elevation« to be drawn must be calculated before the drawing is started. The calculations ere simple enough and are dependent upon the overall size of the component.

Assuming that these sizes are A. B end C for the maximum length, breadth and height respectively, end assuming that the spaces between the three elevations to be drawn and the edge of the peper are to be equal, a specimen layout is shown in Fig. 18/25.

It is not necessary to use exact figures for dimensions A. B and C. They should be approximated so that the calculations are simplified.

If the aire of the component that is drawn is such that the drawn views fit the paper neatly without large gaps between the elevations, then the frame around the drawing should be at least 15 mm from the edge of the peper all the way round

The distance between the elevations should not be larger than would be required to fully dimension the drawing neatly. If the peper is obviously much larger than is necessary, and this often happens in examinations, do not attempt to fill the peper and thus have large spaces between the drawn views. Position the elevations so that they «re not too far apart and draw the frame round the

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Responses

  • penelope ricci
    How to state dimensions bs 308?
    3 years ago
  • uta fleischer
    What is conventions in engineering drawings?
    2 years ago
  • marko
    What are conventions in technical drawing?
    1 year ago

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