Page 219 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

TABLE 1
Roughness Values R Roughness Grade
2
µm symbols
50 N12
25 N11
12.5 N10
6.3 N9
3.2 N8
1.6 N7

0.8 N6
0.4 N5
0.2 N4
0.1 N3

0.05 N2
0.025 N1
The unit of roughness is a micron (0.001) and greater this number is higher the roughness. The roughness number
described here is called the Ra value of the surface.
The surface roughness for a given surface is indicated by a symbol. (Fig 5) Here 1.3 means that the average
depth of the tool marks is not to exceed 0.0013 mm. Any value less than this is acceptable. In case we need to
control the maximum and minimum permissible value of roughness it is to be indicated as shown in Fig 6 & 7.

Fig 5 Fig 6

Fig 7

Roughness symbols
These symbols indicate the practice followed in the industry and are given here for guidance only. B.I.S 3073-1967
(Table 2) states this as assessment of surface roughness.

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CITS :Engineering Drawing (Mechanical) - Exercise 9