246 Machine Drawing
dharm
d:\N-Design\Des16-1.pm5
(a)
a 2
a 1
(b)
a
Milled
(d)
c
(e)
T
a (^2) a
1
Chromium plated
(c)
f
Fig. 16.4
The principal criterion of surface roughness, Ra may be indicated by the corresponding
roughness grade number, as shown in Table 16.2.
Table 16.2 Equivalent surface roughness symbols
Roughness values Roughness Roughness
Ra μm grade number grade symbol
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
16.4.1 Indication of Special Surface Roughness Characteristics
In certain circumstances, for functional reasons, it may be necessary to specify additional
special requirements, concerning surface roughness.
If it is required that the final surface texture be produced by one particular production
method, this method should be indicated on an extension of the longer arm of the symbol as
shown in Fig. 16.4b. Also, any indications relating to treatment of coating may be given on the
extension of the longer arm of the symbol.
Unless otherwise stated, the numerical value of the roughness, applies to the surface
roughness after treatment or coating. If it is necessary to define surface texture, both before
and after treatment, this should be explained by a suitable note or as shown in Fig. 16.4c.
