The ISO standards recommend that abbreviations and symbols are used wherever possible to avoid a link to any particular language. Examples of the use of symbology and English language abbreviations are as follows (BS 8888:2000):

'<})' or 'DIA' or 'D' or'd' = diameter 'n' = arc 'CL' = centre line 'CRS' = centres 'CSK' = countersunk 'CYL' = cylinder 'DRG' = drawing 'HEX' = hexagonal 'MMC' = maximum material condition

'PCD' = pitch circle diameter 'R' or 'RAD' = radius 'SP' = spherical diameter 'SQ' or □ (a small square) = a square feature

'SR' or '<))' = spherical radius 'THD' = thread 'THK' = thick 'TOL' = tolerance 'VOL' = volume

Symbols can also be used to dimension holes. Figure 4.8 gives examples of the use of symbology with respect to specifying holes. Alongside these are standard dimensions. The first is an M10 threaded hole, second is a countersunk hole and the third is a stepped hole. If symbology is used, there is no need to add any dimensions using the conventional extension lines and arrowheads since symbols render dimensioning unnecessary. There is a whole o

010x3 M10x12/14



p12x8U 08x17U 02


Tapped Hole Drawing Bs8888

Figure 4.8 Symbology for dimensioning holes

ISO standard (ISO 15786:2003) devoted to the simplified representation and dimensioning of holes.

With regards to the movable jaw detailed drawing in Figure 3.2, there are four holes which are dimensioned using symbology. The meaning of each symbol is as follows. The threaded hole has the symbology:

M8 x 10/12

This means that there is a single hole threaded to metric 8mm diameter which is 10mm deep. The full details of this metric thread are covered in the standard ISO 68-1:1998. The drilled hole, which was produced prior to tapping, is to be 12mm deep. The single large counter-bored hole has the symbology:

015 x 7,5U 010

This means that there is one hole like this. The 15mm diameter portion of the hole is 7,5mm deep with a flat bottom (shown by the 'U') and the remainder is 10mm diameter.

The symbology for the two-off small counter-bored holes is as follows:

This means that there are two-off holes like this, hence the '2 x' (two times). Both these holes have an upper portion which is 8mm in diameter and 5mm deep with a flat-bottomed hole (shown by the 'U'). The remainder of the hole is 5mm in diameter.

Symbology is also used to define welds. The relevant standard is ISO 2553:1992. Figure 4.9 shows the basic 'arrow' symbol used to define welds. There are three basic parts, an arrow line which points to the joint itself, a welding symbol and a horizontal reference line that represents the joint surface. In the case of Figure 4.9a, the weld symbol is for a fillet weld and there is only one weld that is on the arrow side of the joint. If there are welds on both sides of the joint, the symbology in Figure 4.9b is used. In this case there are two joint reference lines. One is continuous and the other is dotted. The solid line represents the arrow side joint whereas the dotted line represents the opposite side. Welding symbols are placed above the continuous line and below the dotted line. In many instances additional information is given, such as the weld dimensions, inspection rules and operating conditions like the welding rod specification. An example of additional information is shown in Figure 4.9c. The symbol in this case represents a low penetration single vee butt weld (the capital 'Y'). The 's5' refers to the fact that the weld depth is 5mm (see Figure 4.9d). The other numbers ('3 x 10(5)') mean that

Single Vee Butt Weld Sketch

'Arrow-side' joint

^Opposite side joint Weld symbol for side opposite to arrow

Weld symbol for'arrow-side' of joint \ 'Arrow-side' joint

'Arrow-side' joint

Step Weld Symbol
If] |c^2xl(e)

Figure 4.9 Basic arrow symbol for representing welds

Figure 4.9 Basic arrow symbol for representing welds there are 3 welds, each of 10mm length with a 5mm gap between them. Had it been a fillet weld, the starting letter would be either an 'a' or a 'z' (see Figure 4.9e). Had it been a seam weld, the starting letter would be a 'c' (see Figure 4.9f). In Figure 4.9c, the reference line has a fish tail end. Additional information is placed here. In this case the '111' is the code given in ISO 4063:1990 for metal-arc welding with a covered electrode. The next reference (ISO 5817:1992) refers to the acceptable weld quality level of imperfections. The ISO standard gives examples of other, highly specific information which can be referred to after the fish tail.

The table in Figure 4.10 shows numerous examples of welding symbology. The first column shows 10 types of weld and this is a selection from the 20 basic types. In each case the symbol used is representative of the actual weld. The symbol column shows two methods of representing the weld, one in which the arrow refers to the top face of the joint and one in which the arrow refers to the bottom face of the joint. In all cases there are two horizontal lines,

Weld Type

Square bun weld

Single V butt weld

Single bevel butt weld

Single U butt weld

Single J butt weld


Single V butt weld with broad rootface


Weld Type

Double V butt weld

Double V butt weld with broad root face

Double bevel butt weld

Single V butt weld with backing run



Flat single V

Convex double V

Flat V fillet & flat backing

Concave lillet weld

Smooth blended fillet weld both sides

Smooth blend


Figure 4.10 Examples of the use of welding symbols one continuous and one dotted. In all instances the continuous line refers to the weld on the 'arrow face' of the joint and the dotted line to the opposite face. Considering the shallow penetration depth single vee butt weld of row 4, the welding symbol is 'Y'. When the arrow points to the top face, the 'Y' is on the continuous line and when the arrow points to the under side, the Y symbol is on the dotted line. Although it doesn't matter whether the continuous line is above or below the dotted, I think it is more logical for the symbols to actually reflect the layout of the weld itself. Therefore, I would suggest that when the arrow points to the upper face, the continuous line is above the dotted line whereas when the arrow points to the under side of the joint, it is more logical to have the dotted line above the continuous line.

With regard to the second column in Figure 4.10, the first four rows show how the previous set of symbols apply to welds on both sides of the joint. If the welds are the same, there is no need to have both a continuous line and a dotted line. In this case one continuous line is all that is necessary. The lower six rows of this right-hand column show the weld finishing symbology. Welds are normally finished machined by grinding. Welds can be flat, concave, convex or smoothly blended.

The various examples above show that symbology can save a significant amount of time, effort and therefore money in engineering drawing.

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  • Maurice
    What is the symbol for a threaded hole?
    8 years ago

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