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
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
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
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
Figure 11.13 Surface control symbols
1. the Perpendicular symbol shown in figure 11.12 indicating
that the lay direction is perpendicular to the line to which the symbol
2. the = symbol which indicates that the lay direction is parallel
to the line;
3. the X symbol indicates that the Lay is angular in two
directions relative to the line representing the surface;
4. the M symbol indicates a multidirectional Lay;
5. the C symbol indicates that the lay is approximately
circular relative to the centre of the surface;
6. the R symbol indicates that the lay is approximately
radial relative to the centre of the surface;
7. the P symbol indicates that the Lay is particulate,
non-directional, or protuberant.
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
Some examples of symbols used in electrical diagrammatic (schematic)
drawings are shown in figure 11.15.