Appendix I
Symbols Used in Engineering Drawings

In engineering drawings we need to convey information about some of the manufacturing processes that are to be used and the required results from these processes. Two examples of this are the surface finish symbols and the welding symbols. These sets of symbols are both conveying information which will tell the production/manufacturing something about the processes that have to be used in order to produce the finished item. This information helps them plan the processes and their sequence, and consequently, also the flow of material through the production plant. It will enable them to make estimates of manufacturing cost in the form of manhour and machine-hour consumption.

Another area where symbols are used is in diagramatic (schematic) drawings. These are drawings showing how, for instance, an electrical system is to be put together, showing all the different electrical devices and how they are to be connected. Hence, we need electrical device symbols. Piping systems are also drawn in diagramatic (schematic) form. Hence, we also need symbols that indicate different types of valves, pumps, filters etc.

This appendix gives a brief introduction to some of these symbols. The coverage is only meant to introduce some of the most commonly used symbols. For a more complete coverage I suggest that you consult the appropriate standards for symbols in the different areas of application.

Surface Finish Symbols:

The surface texture of a component often affects its performance. Therefore, one has to specify the surface finish that is required for acceptable performance. Figure 11.12 illustrates the surface texture features, and how the finish mark is used for communicating the desired finish.

The different terms used in describing the surface finish can be explained as follows:

Surface Texture is the variation in the surface in the form of roughness, waviness, lay, and flaws.

Roughness refers to the finest of the irregularities in the surface. They are caused by the process(es) used to smooth the surface.

Figure 11.12 Surface texture characteristics

Roughness Height is the average deviation from the mean plane of the surface (micrometres ĉm, or microinches ĉin).

Roughness Width is the width between succesive peaks and valleys of the roughness.

Roughness width cutoff is the largest spacing of irregularities including average roughness height.

Waviness is the widely spaced variation (millimetres, or inches) exceeding the roughness width cutoff. It is assumed that the roughness is superimposed on a surface that is wavy in nature.

Waviness height is the crest to trough height variation of the waves.

Waviness width is the wave length i.e. distance from crest to crest or from trough to trough.

Lay is the orientation of the surface pattern. This is determined by the manufacturing processes used.

Flaws are defects, or irregularities, that occur more or less at random over the surface. These defects can be such things as cracks, blow holes, ridges, scratches etc.

Contact Area is the surface that will make contact with a mating surface.

The Surface Texture Symbol is shown in figure 11.12 at the bottom indicating where one may indicate the magnitude of the different surface texture characteristics plus the orientation of the Lay. Figure 11.13 shows, in the upper left hand corner, the way the surface texture symbol should be drawn relative to the text's character height. How one may indicate different surface finish requirements is indicated by some the other versions of the symbol:

The Basic Surface Texture Symbol indicates that the surface may be produced by any method.

The Material Removal by Machining symbol (with the horizontal bar closing the V opening) indicates that machining is required to achieve the desired surface finish. This means that a material allowance has to be made to make this possible.

The Material Removal Allowance symbol indicates the amount of stock that one should remove by machining. A tolerance on this number may be indicated in a note.

The Material Removal Prohibited symbol with circle in the V, indicates that the surface must be produced by processes like casting, forging, hot finishing, cold finishing, die casting, injection moulding, or powder metallurgy.

It should be noted that numbers can be added as indicated in figure 11.12 to clarify the surface texture requirements. An average or a maximum and minimum roughness can be specified in the location indicated. The maximum waviness height and spacing may be specified. The direction of lay may be indicated. The roughness sampling length (roughness width cutoff) may be indicated (0.80 mm is assumed if nothing is specified). The maximum roughness may also be specified right next to the Lay symbol. The lay symbol may be:
Figure 11.13 Surface control symbols

Welding Symbols:

In this section you will find a very brief introduction to the use of welding symbols. The figures 11.14A through 11.14C show the most commonly used welding symbols for butt and fillet welds. These are the types of welds used most commonly for welding steel and aluminum structures. There are other types of welds like slot (or plug) and resistance spot and seam welds that are not shown here. Slot welds are generally used in cases where access for performing fillet welds is impossible or so awkward that the result would be a totally unsatisfactory weld. Resistance welding is usually used for thin sheet material like that used in autobodies etc. For these symbols and more detail on welding methods and the use of welding symbols I suggest that you refer to one of the many comprehensive textbooks available on engineering graphics.

It should be noted that there are several pieces of information that can be conveyed through the welding symbols shown in figure 11.14A which is not illustrated here. The included angle(s) of the V and X notches. This is done by indicating the angle(s) in the opening(s) of the V (X).

With V notch, or Bevel, one requires there to be a little bit of material left beyond the bottom of the V or Bevel. This is indicated by a number to the right of the V or Bevel symbol. The reason for leaving this nose is to avoid getting the material at the bottom of the groove from being overheated during cutting and welding.

One may also want there to be a slight opening between the plates being joined. This gap is indicated by a number just below the V or Bevel symbol.

For both butt and fillet welds one may want to indicate its surface contour. This done by using an arc with its centre outward from the V, Bevel, X, or inclined part of fillet symbol, to indicate a concave surface. The centre of the arc would be roughly at bottom of the V or Bevel symbol, or at the centre of the X symbol, or at the opposite corner for to indicate a convex surface contour of weld. A flush or plane surface contour is indicated with a straight line over the mouth of the V, Bevel, X symbol, or over the inclined portion of the fillet symbol.

Figure 11.14A Symbols for different butt weld notches

Figure 11.14B Symbols indicating fillet welds

Electrical Engineering Symbols:

Figure 11.14C Intermittent fillet welds