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Dimensions Tolerances and Surfaces
The quality
o
a part is not only dependent on the physical and mechanical
properties o the material from which it
is
manufactured, but also upon the
geometrical attributes too. These attributes include the dimensions, tolerances,
flatness, roundness, angularity and surface finishes.
The American National Standards Institute (ANSI) defines a dimension as a
numerical value expressed in appropriate units
o
measure and indicated on a
drawing and in other documents along with lines, symbols and notes to define
the size or geometric characteristic, or both,
o
a part or part feature.
The dimension represented on a part drawing
is
an almost ideal state that
is
generally unattainable
in
practice because
o
inaccuracies ... deviations ....
introduced by imperfections in the manufacturing processes. Tolerances
imposed on a dimension are specified to accommodate these imperfections, and
to specify the maximum admissible excursion from the desired perfect state.
Therefore parts with absolute dimensions lying outside this state are
unacceptable, they are rejected as scrap, and a financial penalty is incurred.
ANSI defines a tolerance as the total amount by which a specific dimension is
permitted to vary. The tolerance is the difference between the maximum and
minimum limits.
Surfaces
The quality
o
a part's surface
is
dictated by the manufacturing process employed
to create the two or three dimensional shape. Furthermore the surface quality
varies widely among processes and this part attribute is
o
importance for the
engineer because
o
a variety
o
technical and commercial reasons.
- Aesthetics
- Safety
- Friction and wear
- Assembly operations
- Electrical conductivity
- Mechanical and physical properties
When magnified, using n optical microscope, surface irregularities and
imperfections become clearly evident. Flaws, roughness, waviness and patterns
imposed by the manufacturing process responsible for creating the surface are
readily identifiable.
urface finish is a subjective term concerning the surface characteristics while
surface roughness can be defined as the average
o
the vertical deviations from
the nominal surface over a specified length. An arithmetic average can be
employed or a root-mean-squared average.
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The decision of which manufacturing processes to employ to create a part in a
specific material with specific surface qualities and embodying specific
dimensions within prescribed geometrical tolerances is
n important engineering
consideration especially when it must be accomplished cost-effectively. To
facilitate this activity manufacturing processes can be codified based on the
dimensional accuracy and surface finish they can attain.
Generally the more accurate the part and the more refined the surface attributes
the more expensive the manufacturing processes needed to achieve this.
Therefore one should always specify the roughest surface with the largest
tolerances for a prescribed set of operating conditions.
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Crater (flaw)
Roughness height
Roughness width
Surface texture
.......
Altered layer
/ ~ : I - - -
Substrate
Lay
direction
Crack (flaw)
FIGURE 5.5 Deviat ions
from nominal surface used
in
the
two
definitions
o
surface roughness.
Waviness spacing
Vertical deviations
y;)
Nominal surface
x
~ - - - - - - - - - - - - - - - L m - - - - - - - - - - - - - - - -
FIGURE 5.2 A magnif ied cross
section
o
a typical metallic part
surface.
Waviness
heigh1
FIGURE 5.1
Three ways to
specify tolerance limits or a
nominal dimension o
J
T
T
.500:
a)
bilateral,
b)
2.505
. 5 0 0 ~ : ~
Y 2 . 5 0 0 : g : g ~
nilateral, and c) limit
2.495
dimensions.
a)
b)
(c)
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T BLE 5.4 Typical tolerance limits,
based
on process capabili ty Section
44 2lt
for various manufacturing processes.
Typical Tolerance Limits Typical Tolerance Limits
Process
mm
inches
Process
rom
inches
Sand casting
Cast iron
±1.3
±0.050
Abrasive processes:
Grinding
±0-008
±0.0003
Steel
±1.5
±0.060
Lapping
±0.OO5
±0.0002
Aluminum
±O.5
±0.020
Honing
±0.005
±O.0002
Die casting
Plastic molding:
Polyethylene
Polystyrene
Machining:
Drilling, diameter:
6 rom (0.250 in)
25 rom (1.000 in)
, ±0.12
±0.3
±0.15
+0.08, 0.03
+0.13, -O.OS
±0.005
±0.01O
±0.006
+0.003,
0.001
+0.006, 0.002
Nontraditional processes:
Chemical machining
Electric discharge
Electrochem. grind
Electrochem. machine
Electron beam cutting
Laser beam cutting
Plasma arc cutting
±0.08
±0.025
±0.025
± O ~
±O
•
±0.08
±1.3
±O.003
±O.OOl
±O.OOl
±0.002
±O.003
±0.003
±O.OSO
Milling
±0.08
±0.OO3
fuming
±0.05
±0.002
Compiled from
[4],
[5], and
other
sources.
T BLE 5.5 Surface roughness values produced by the various manufacturing processes.
a
Typical
Surface
Range
o Typical Surface Range
of
Process Fmish
Rougbness
b
Process
Finish
Rougbness
b
Casting:
Abrasive:
Die casting
Good 1 2
(30-65)
Grinding Very good
0.1-2 (S-75)
Investment Good 1.5-3 (SO-l00)
Honing Very good
0.1-1 (4-30)
Sand casting Poor 12-25 (SOO-l000) Lapping Excellent 0.05-0.5 (2-1S)
Metal forming:
Polishing Excellent
O.l-O.S (5-15)
Cold rolling
Good 1 3 (25-125)
Superfinish Excellent
0.02-0.3 (1-10)
Sheet metal draw
Good
1 3 (25-125)
Nontraditional:
Cold extrusion
Good 1 4
(30-1S0)
Chemical milling Medium
1.5-5 (50-200)
Hot
rolling Poor
12-25 (500-1000)
Electrochemical
Good
0.2-2 (10-100)
Machining:
Electric discharge Medium
1.5-1S (50-500)
Boring Good
O.S-6
(15-250)
Electron beam Medium
1.5-15 (50-500)
Drilling Medium
1.5-6 (60-2S0)
Laserbearn Medium
15 1S
(50-500)
Milling Good
1 6
(30-250)
Thermal:
Planing Medium
1.5-12 (60-S00)
Arc welding
Poor
5 25 (250-1000)
Reaming
Good
1 3
(30-12S)
Flame cutting
Poor
12-25 (500-1000)
Shaping Medium
1.5-12 (60-500)
Plasma
arc
cutting
Poor
12-25 (500-1000)
Sawing Poor
3 25
(100-1000)
Turning
Good
0.5-6 (15-250)
• Compiled from
m
[2J,
and o ther sources.
b Subjective description and typical range of surface roughness values are given, porn
po-in).
Roughness can vary significantly for a given process,
depending
on
process parameters.
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Crater flaw)
Roughness height
Roughness width
~
Surface texture
~ ~ . : . . . ~ L . i J . . o I I - - - -
Altered layer
~ ~ ~ ~
/ 4 Substrate
Lay direction
Crack flaw)
FIGURE 5.5 Deviations
from nominal surface
used
in the two definitions o
surface roughness.
Waviness spacing
Actual surface
y
Vertical deviations
Nominal surface
~ - - - - - - - - - - - - - - - L m - - - - - - - - - - ~
FIGURE
5.2 A magnified cross
section o a typical metallic part
surface.
FIGURE
5.1
Three ways to
specify tolerance limits for a
nominal dimension
o
T
T
.500: a) bilateral, b) T
5 0 0 ~ g : ~
y
2 505
unilateral, and c) limit 2 500
Y
2 495
dimensions.
a)
b)
c)
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T BLE
5.4 Typical tolerance limits, based on process capabili ty (Section 44.2), for various manufacturing processes.
Typical Tolerance Limits
1YPical Tolerance Limits
Process
mm
inches
Process
mI
inches
Sand casting
Abrasive processes:
Cast iron
±1.3
±0.050
Grinding
±0.008
±OJX)03
Steel
±1.5
±0.060
Lapping
±0.005
±0.0002
Aluminum
±O.S
±0.020
Honing
±O.OOS
±O.OOO2
Die casting
±0.12
±O.OOS
Nontraditional processes:
Plastic molding:
Chemical machining
±0.08
±O.003
Polyethylene
±0.3
±0.010
Electric discharge
±O.025
±0.00l
Polystyrene
±0.1S
±0.006
Electrochem. grind
±0.025
±0.001
Machining:
Drilling, diameter:
6 mm
0.250 in)
2S mm 1.000 in)
+0.08, 0.03
+0.13,
-O.OS
+0.003,
0.001
+0.006, 0.002
Electrochem. machine
Electron
beam
cutting
Laser
beam
cutting
Plasma arc cutting
±O.OS
±0.08
±0.08
±1.3
±0.002
±O.003
±O.003
±O.OSO
Milling
±0.O8
±0.003
Turning
±0.05
±0.002
Compiled from
[4],
[5], and other sources.
T BLE
5.5 Surface roughness values produced by the various manufacturing processes.
Typical Surface
Range
of
Typical Surface
Range
of
Process Finish
Roughness
b
Process Finish
Roughness
b
Casting:
Abrasive:
Die casting
Good
1 2 30-65)
Grinding
Very good
0.1-2 5-75)
Investment
Good
1.5-3 50-100)
Honing Very good
0.1-1 4-30)
Sand casting
Poor
12-25 500-1000)
Lapping
Excellent
0.05-0.5 2-15)
Metal forming: Polishing Excellent 0.1-0.5 5-15)
Cold rolling Good
1 3
25-125)
Superfinish Excellent
0.02-0.3 1-10)
Sheet metal draw
Good 1 3 25-125)
Nontraditional:
Cold extrusion Good
1 4
30-150)
Chemical milling Medium
1.5-5 50-200)
Hot rolling Poor 12-25 SOO-looo)
Electrochemical
Good 0.2-2 10-100)
Machining:
Electric discharge Medium
1.5-15 50-500)
Boring Good 0.5-6 15-250)
Electron beam Medium 1.5-15 50-500)
Drilling Medium
1.5-6 60-250)
Laser
beam
Medium
1.5-15 50-SOO)
Milling
Good
1 6 30-250)
Thermal:
Planing Medium
15 12 60-500)
Arc
welding Poor
5-25 250-1000)
Reaming Good 1 3 30-125)
Flame cutting Poor 12-25 500-1000)
Shaping Medium
1.5-12 60-500)
Plasma arc cutting
Poor 12-25 500-1000)
Sawing ,Poor
3-25 100-1000)
Thrning
Good
0.5-6 15-250)
• Compiled from 1], [2], and other sources.
b
Subjective description and typical range
of
surface roughness values a re given,
Lm
JL-in).
Roughness can vary significantly for a given process,
depending on process parameters.
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