Section 13. qxd

SURFACE QUALITY VERSUS TOLERANCES
13-75
Another categorization is by 
contact methods
and 
noncontact methods.
Contact methods include stylus methods (tracer-point analysis) and
capacitance methods. Noncontact methods include light section
microscopy, optical reflection measurements, and interferometry.
Replicas of typical standard machined surfaces provide less accurate
but often adequate reference and control of rougher surfaces with 
R
a
over 16 
m
in.
The United States and 25 other countries have adopted the 
roughness
average
R
a
as the standard measure of surface roughness. (See ANSI/
ASME B46.1-1985.)
PRODUCTION
Various production processes can produce surfaces within the ranges
shown in Table 13.5.6. For production efficiency, it is best that critical
areas requiring surface texture control be clearly designated on drawings
so that proper machining and adequate protection from damage during
processing will be ensured.
SURFACE QUALITY VERSUS TOLERANCES
It should be remembered that surface quality and tolerances are dis-
tinctly different attributes that are controlled for completely separate
purposes. 
Tolerances
are established to limit the range of the size of a
part at the time of manufacture, as measured with gages, micrometres,
Table 13.5.5
Lay Symbols
Table 13.5.3
Preferred Series Roughness Average Values
R
a
,
m
m and 
m
in
m
m
m
in
m
m
m
in
m
m
m
in
m
m
m
in
m
m
m
in
0.012
0.5
0.125
5
0.50
20
2.00
80
8.0
320
0.025
1
0.15
6
0.63
25
2.50
100
10.0
400