Requirements

Figure 14.43 shows mandatory positions of associated surface texture requirements. Note that in accordance with BS EN ISO 1302: 2002 the position 'X' is no longer used. Only positions a, b, c, d and e should be used. Position 'a' indicates one single surface texture requirement. Positions 'a' and 'b' indicate two or more surface texture requirements. Position 'c' indicates manufacturing method, treatment, coating or other process. Position 'd' indicates surface lay. Position 'e' shows machining allowance.

Three principal groups of surface texture parameters have been standardized in connection with the complete symbol and are defined as R, W and P profiles. The R profile series relates to roughness parameters. The W profile series relates to waviness parameters. The P profile series relates to structure parameters. The Ra value is the most commonly specified value throughout the world and examples follow in this chapter. Figure 14.44 illustrates an application with a single Ra requirement. Fig. 14.45 shows an application with an

Ground

Ground

8888 Surface Texture Symbols

Fig. 14.44

Ground

Ground

Fig. 14.45

upper and lower call out requirement. Figure 14.46 shows examples of position and orientation on various surfaces. Note that the general rule when reading drawings is to read from the bottom or the right hand side of the drawing. Figure 14.47 is a typical application in connection with features of size of mating parts. Figures 14.48 and 14.49 show examples of typical tolerance frames for geometrical tolerance applications. Further information on this specialized area of

Bs8888 Surface Finish Frame

Ra 0,05

Fig. 14.46

Ra 0,05

Fig. 14.46

0 80h7

0 80h7

0 80H7

0 80H7

Fig. 14.47

Fig. 14.50

Fig. 14.50

Ra 0,2

c

0 0,1

C

D

K

)

0,05

Fig. 14.49

Metrology is given in BS EN ISO 1302 - 2002. Indication of Surface Texture in Product Documentation. The examples in Fig. 14.50 show methods of indicating four typical examples of surface lay.

(a) The lay is parallel to the plane of projection of the view where the symbol is drawn. The 'equals' symbol is added to the graphical symbol in the position shown. An arrow is indicated on the workpiece for clarification.

(b) The lay is perpendicular to the plane of projection and the symbol drawn is an inverted letter 'T'.

(c) The lay crosses the workpiece in two oblique directions indicated by the letter 'X'.

(d) The lay consists of concentric circles and the symbol used is the letter 'C'.

The standard also states that where a surface pattern is required which is not covered by BS 8888, then an explanatory note shall be added to the drawing.

Figure 14.51 shows approximate Ra surface roughness ranges for components manufactured by some common production methods. This information can only be approximate, since finish depends on many factors, such as the skill of the machinist, the accuracy and condition of the machine, the speeds and feeds selected for the operation, and the quality and condition of the cutting tools.

The approximate relationship between surface roughness and the cost of producing such a finish is shown in Fig. 14.52. The cost of rough machining can be considered as the zero datum on the y axis of the graph, and other processes can be compared with it. For example, a finish of 6.3 micrometres produced by grinding may well cost four times as much as rough machining. Many factors contribute towards production costs, and this information again can be only approximate.

Requirements Engineering Cartoon

22

20

18

16

st

14

o

c

>

12

13

o DC

10

8

6

4

2

Surface roughness - micrometres

Fig. 14.52 Approximate relationship between surface roughness and cost

Chapter 15

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Responses

  • Siro
    How to show surface roughness on detail drawing?
    8 years ago
  • Kimmo
    Which sign show ground position in drawing?
    3 years ago

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