MARYLAND METRICS
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SURFACE ROUGHNESS TABLES
AND U.S. TO METRIC COMPARISON CHARTS
General guidelines for feasible roughness R for different processing methods
METRIC SURFACE ROUGHNESS VALUES ARE INDICATED BY THE a
� m
UNIT: MICROMETER ( ). Material removing or roughness Ra in um
R
a
separating operations 0.012 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 13 25 50
SYMBOLS: There are a number of symbols in use, all of which have a
flame cutting
specific meaning. A symbol generally consists of 2 lines, one of which
sawing
is longer than the other. The angle between the lines is 60 degrees.
planing
punching
Basic symbol (without top line) should not be used alone.
chemical treatment
Either the processing method or the surface roughness
spark erosion machining
should be listed with it.
drilling
milled
3.2
If written as per this example: it means
boring
machining optional. The surface roughness
milling
� m
should be 3.2 (0.0032 mm)
turning
broaching
Symbol (with top line) means: machining mandatory
3.2
reaming
� m).
(surface roughness 3.2
filing
3.2
Symbol (with circle in the vee) means: machining
grinding
� m
prohibited (surface roughness 3.2 ).
barreling
brushing
electrolytic grinding
honing
polishing
lapping
The data in the right hand table is furnished only for practical
superfinishing
information and to provide an idea of the achievable roughness
Non material
for different processing methods.
removing operations
sandcasting
This data is primarily for metal surfaces. Other materials may show
hot rolling
differences.
die forging
gravity die casting
AVERAGE ACHIEVABLE FINER COARSER
investment casting
ROUGHNESS
extruding
cold rolling
Roughness values in micro-inches
die casting
are 40 times the values in micrometers
R R Roughness Roughness Old American
a a R
**Use this symbology on
R
t (
a) Style standard
micrometer micro-inch Grade Numbers Grade Numbers
drawings for international
�m
�in
(New)** (Old)***
suppliers, and for new designs.
50 2000 N12
***Old surface roughness
25 1000 N11
symbols which are still found
12.5 500 N10
on older metric drawings.
32
6.3
They should not be used on
6.3 250 N9 32 250
3.2
new designs.
3.2 125 N8 16 125
Examples of how the
8
1.6
1.6 63 N7 8 6
new system is used:
0.8
32
0.8 32 N6 4
N9 6.3
2
0.4 =
16
0.4 16 N5 2
OLD
NEW
0.2
8
0.2 8 N4 1
N6 0.8
0.5
0.1
=
0.1 4 N3 0.5 4
0.25
0.05
0.05 2 N2 0.25 2
In certain circumstances, it
0.025 1 N1
will be necessary to
R
Notes: 1. Triangles, , or Rt on a drawing indicates peak to valley roughness measurements
z
indicate the method of
in micrometers (microns). (one micron = 0.001 mm)
manufacturing:
2. , Ra on a drawing indicates AVERAGE roughness rather than peak to valley values.
precision ground
Example:
(Ra is the unit of measurement in micrometers)
N7
3. is the American standard per ASA B46.1. (average roughness in micro-inches)
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filename: http://mdmetric.com PAGE: VERSION: P1A/T3J
SURFRUFF.CDR8 SECTION: TECHDATA
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