Dimensioning
• Orthographic and Isometric Views define the
shape and general features of the object
• Dimensioning adds information that specifies
– Size of the object
– Location of features (e.g. holes)
– Characteristics of features (e.g. depth and diameter
of hole)
• Dimensions also communicate the tolerance
(or accuracy) required
2
Some General Guidelines
• Start by dimensioning basic outside
dimensions of the object.
• Add dimension for location and size of
removed features
• Add general and specific notes – such as
tolerances
3
Dimensioning Basic Shapes –
Assumptions
• Perpendicularity
• Symmetry
4
Dimensioning Shows:
A) Size B) Location and Orientation
•ALWAYS give DIAMETER
" " for full circles (360
degrees) and RADIUS "R"
for arcs (less than 360
degrees)
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Dimensioning – Terminology
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Principles of Good Dimensioning
• The overriding principle of dimensioning is
CLARITY
• Principles – not an infallible rule set, need
to apply good judgment.
7
Principles of Good Dimensioning
• Each feature dimensioned
once and only once
• Dimensions should suit the
function of the object
8
Objectives
•
•
•
Understand description and control of
variability through tolerancing
Understand various classes of fits
Introduce multiple part tolerancing
10
Tolerance
Tolerance is the total amount a dimension may vary.
It is the difference between the maximum and
minimum limits.
Ways to Express:
1. Direct limits or as tolerance limits applied to a
dimension
2. Geometric tolerances
3. A general tolerance note in title block
4. Notes referring to specific conditions
11
ANSI/ASME Standard
ANSI/ASME Standard Y14.5
Each dimension shall have a tolerance,
except those dimensions specifically
identified as reference, maximum, minimum,
or stock. The tolerance may be applied
directly to the dimension or indicated by a
general note located in the title block of the
drawing.
Variation is Unavoidable
• No two manufactured objects are
identical in every way. Some
degree of variation will exist.
• Engineers apply tolerances to
part dimensions to reduce the
amount of variation that occurs.
14
Tolerances
Three basic
tolerances that occur
most often on
working drawings
are: limit
dimensions,
unilateral, and
bilateral tolerances.
Tolerances
Three basic
tolerances that occur
most often on
working drawings
are: limit
dimensions,
unilateral, and
bilateral tolerances.
Limit Dimensions
Limit dimensions are
two dimensional values
stacked on top of each
other. The dimensions
show the largest and
smallest values allowed.
Anything in between these
values is acceptable.
Limit Dimensions
These are limit dimensions,
because the upper and
lower dimensional sizes are
stacked on top of each other.
Unilateral Tolerance
A unilateral
tolerance exists when
a target dimension is
given along with a
tolerance that allows
variation to occur in
only one direction.
Unilateral Tolerance
This tolerance is
unilateral, because
the size may only
deviate in one direction.
Bilateral Tolerance
A bilateral
tolerance exists if
the variation from a
target dimension is
shown occurring in
both the positive and
negative directions.
1. Direct Limits and Tolerance Values
22
1. Direct Limits and Tolerance Values –
Plus and Minus Dimensions
23
2. Geometric Tolerance System
Feature Control Frame
Geometric
Dimensioning and
Tolerancing (GD&T) is a
method of defining
parts based on how
they function, using
standard ANSI symbols.
Concentricity
Symbol
AU 2008
24
3. Tolerance Specifications in Title Block
General tolerance note
specifies the tolerance
for all unspecified
toleranced dimensions.
AU 2008
25
4. Notes Referring to Specific
Conditions
General Tolerances could be in the form of a note
similar to the one shown below:
ALL DECIMAL DIMENSIONS TO BE HELD TO .002"
Means that a dimension such as .500 would be
assigned a tolerance of 0.002, resulting in a upper
limit of .502 and a lower limit of .498
26
Important Terms – Single Part
• Nominal Size – general size, usually expressed in common fractions (1/2" for
the slot)
• Basic Size – theoretical size used as starting point (.500" for the slot)
• Actual Size – measured size of the finished part (.501" for the slot)
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Important Terms – Single Part
• Limits – maximum and minimum sizes shown by tolerances (.502 and .498 –
larger value is the upper limit and the smaller value is the lower limit, for the
slot)
• Tolerance – total allowable variance in dimensions (upper limit – lower limit) –
object dimension could be as big as the upper limit or as small as the lower
limit or anywhere in between
28
Important Terms – Multiple Parts
• Allowance – the minimum clearance or
maximum interference between parts
• Fit – degree of tightness between two parts
– Clearance Fit – tolerance of mating parts always
leaves a space
– Interference Fit – tolerance of mating parts always
results in interference
– Transition Fit – sometimes interferes, sometimes
clears
29
Fitting Multiple Parts
Tolerance of B
Fit Tolerance:
Clearance or
Interference
Part B
Tolerance of A
Part A
AU 2008
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Fitting Multiple Parts
31
Shaft and Hole Fits
Clearance
Interference
32
Shaft and Hole Fits
Transition
CLEARANCE FIT
+ .003
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Standard Precision Fits: English Units
•
•
•
•
•
Running and sliding fits (RC)
Clearance locational fits (LC)
Transition locational fits (LT)
Interference locational fits (LN)
Force and shrink fits (FN)
34
Basic Hole System or Hole Basis
• Definition of the "Basic Hole System":
The "minimum size" of the hole is equal to the
"basic size" of the fit
• Example: If the nominal size of a fit is 1/2",
then the minimum size of the hole in the
system will be 0.500"
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Basic Hole System – G28-A
• Clearance = Hole – Shaft
• Cmax = H____ – S____
• Cmin = H____ – S____
SMAX
SMIN
HMAX
Fill in the subscripts (min, max) in the
equations above.
HMIN
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Basic Hole System
• Clearance = Hole – Shaft
• Cmax = Hmax – Smin
• Cmin = Hmin – Smax
Both Cmax and Cmin <0 – _________ fit
Both Cmax and Cmin >0 – _________ fit
Cmax > 0; Cmin < 0 – ___________ fit
SMAX
SMIN
HMAX
HMIN
What types of fits are these?
AU 2008
37
Basic Hole System
• Clearance = Hole – Shaft
• Cmax = Hmax – Smin
• Cmin = Hmin – Smax
Both Cmax and Cmin <0 – Interference fit
Both Cmax and Cmin >0 – Clearance fit
Cmax > 0; Cmin < 0 – Transition fit
SMAX
SMIN
HMAX
HMIN
• System Tolerance = Cmax – Cmin
• Allowance = Min. Clearance = Cmin
AU 2008
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Basic Hole System
Calculate Maximum and
Minimum Clearance
Clearance = Hole – Shaft
Cmax = Hmax – Smin
Cmax = .510 – .485 = .025
.490
.485
.510
.505
Cmin = Hmin – Smax
Cmin = .505 – .490 = .015
What type of fit is this?
Cmax > Cmin > 0 Clearance
39
Metric Limits and Fits
• Based on Standard Basic Sizes – ISO Standard
• Note that in the Metric system:
Nominal Size = Basic Size
• Example: If the nominal size is 8, then the
basic size is 8
40
Metric Preferred Hole Basis System of
Fits
AU 2008
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Metric Tolerance Homework – TOL-1
Free Running H9/d9
Basic Size: 10
(1) Nominal Size: 10
?
(2) Shaft Limits:
(3) Shaft Tolerance: 0.036
????
9.960
????
????
9.924
(4) Hole Limits:
(5) Hole Tolerance: 0.036
????
10.036
????
????
10.000
(6) Ts: 0.072
????
(7) Minimum
Clearance: 0.040
????
(8) Maximum
Clearance: 0.112
????
CHECK: Ts
0.072
= Cmax
= 0.112
– Cmin?
– 0.040 = 0.072
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Today's Assignment
• Tolerance Yellow Packet
– All problems.
– Due 4/16
43
Objectives
• Introduce Surface Control terms and symbols
• Introduce Geometric Dimensioning and
Tolerancing
45
Surface Control
• Why do we need to control surface
characteristics?
– Rough surfaces cause friction and wear
– It is difficult to make accurate measurements from
rough surfaces
46
Surface Characteristics
• Roughness
– Small hills and valleys found on a surface
– Defined as the arithmetic average of the
deviations above and below a mean height of a
surface
– Expressed in microinches or micrometers.
47
Surface Characteristics
• Waviness
– Surface irregularities greater than roughness
– Expressed in inches or millimeters
– See Figure 7.21 on page 7-14 of TG
• Lay
– Direction of tool marks on a machined surface.
– See Figure 7.20 on page 7-13 of TG.
48
Surface Texture Symbols
Surface Control Symbol
.125 in
Material removal not specified
Material must be removed
Material must not be removed
Autumn 2009
49
.25 inches
Lay Symbols
Parallel to line representing surface
Perpendicular to the line representing the surface
Bar added
Both directions to the line representing the surface
Symbol
location
50
M
Multidirectional marks
C
Circular
R
Radial
P
Lay particulate, non-directional, protuberant
Examples with Roughness and Lay
24
Average roughness is 0.000024 inches, which is
often referred to as 24 microinches or µ inches.
Material must be removed.
28
14
Maximum average roughness is 28 microinches.
Minimum average roughness is 14 microinches.
Material must not be removed.
45
R
51
Average roughness is 45 microinches
Lines on the surface are radial with respect
to the center of the surface
Geometric Tolerancing
• Geometric Tolerancing includes specifications
of form, profile, orientation, location, and
runout.
52
Geometric Tolerancing
What features of
PART 1 need
some constraints
so that the
assembly will
work properly?
Discuss with the
people at your
table.
What happens if
the two shafts are
not parallel?
53
Geometric Tolerancing – Definitions
• Basic Dimension – A numerical value for theoretical exact size or
location
• True Position – The theoretically exact location of a feature
established by basic dimensions
• Datum – A theoretically exact point, axis, or plane used as the origin
from which location or geometric characteristics of features are
located
• Datum Target – A specified point, line, or area on a part used to
establish a datum
• Datum Feature – An actual feature of a part used to establish a
datum
54
Geometric Tolerancing – Definitions
• Maximum Material Condition (MMC) – The condition in which a feature of
size contains the maximum amount of material with the stated limits of
size. For example, minimum hole diameter and maximum shaft diameter
• Least Material Condition (LMC) – Opposite of MMC, the feature contains
the least material. For example, maximum hole diameter and minimum
shaft diameter
• Virtual Condition – The envelope or boundary that describes the collective
effects of all tolerance requirements on a feature (See Figure 7-25 TG)
Autumn 2009
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Virtual Condition Envelope
All Required Tolerances
20.06" Maximum
Envelope
20.00"
Maximum
Allowable
Diameter
56
0.06"
Maximum
Allowable
Curvature
Basic Symbols for Geometric
Characteristics
Individual Features
Tolerance of Form
Straightness
Flatness
Circularity (roundness)
Cylindricity
Individual or Related
Tolerance of Profile
Profile of a line
Profile of a surface
Related Features
Tolerance of Orientation Angularity
Perpendicularity
Parallelism
Tolerance of Location
Position
Concentricity
Tolerance of Runout
Circular Runout
Total Runout
57
Modifying Symbols for Geometric
Characteristics
•
•
•
•
•
•
•
•
•
58
At maximum material condition
At least material condition
Projected tolerance zone
Diameter
Spherical diameter
Radius
Spherical Radius
Reference
Arc length
M
L
P
S
R
SR
( )
Geometric
Characteristic
Symbol
Feature Control Frame
Tolerance
Datum
Reference
.007
M
B
Material condition
B
.007
This feature must be parallel to Datum B within .007 at MMC
(largest cylinder) as measured on the axis
59
0.03
Tolerance of Form –
This cylinder must be straight
Straightness
within 0.03 mm.
19.89
19.86
What it means -
0.03 Tolerance Zone
60
Concentricity Tolerance Note
.007 A
A
XX
YY
This cylinder (the right cylinder) must be concentric
within .007 with the Datum A (the left cylinder)
as measured on the axis
61
Concentricity Tolerance Note – What It
Means
.007 Tolerance Zone
62