Page 282 - Aeronautical Engineer Data Book

228 Aeronautical Engineer’s Data Book
it is simply a different way of describing the
same thing.
12.6.1 Choice of surface finish: approximations
Basic surface finish designations are:
• Rough turned, with visible tool marks:
(12.5 µm or N10)
500 µin R a
• Smooth machined surface:
(3.2 µm or N8)
125 µin R a
• Static mating surfaces (or datums):
(1.6 µm or N7)
63 µin R a
• Bearing surfaces:
(0.8 µm or N6)
32 µin R a
• Fine ‘lapped’ surfaces:
(0.025 µm or N1)
1 µin R a
Figure 12.10 shows comparison between the
different methods of measurement.
Finer finishes can be produced but are more
suited for precision application such as instru­
ments. It is good practice to specify the surface
finish of close-fitting surfaces of machine
elements, as well as other ASME/ANSI Y
14.5.1 parameters such as squareness and paral-
lelism.

Fine finish Rough finish
R, (µm) 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50
BS1134
R, (µinch) 1 2 4 8 16 32 63 125 250 500 1000 2000
ANSI B46.1
N-grade N1 N2 N3 N4 N5 N6 N7 N8 N9 N10 N11 N12
DIN ISO 1302
Ground finishes Smooth Medium
turned turned
Seal-faces and Rough turned finish
running surfaces

16
A prescribed surface finish is shown on a drawing as
– on a metric drawing this means 1.6µm R a
Fig. 12.10 Surface measurement