Objectives
•
•
•
Understand description and control of
variability through tolerancing
Understand various classes of fits
Introduce multiple part tolerancing
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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
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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.
1. Direct Limits and Tolerance Values
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1. Direct Limits and Tolerance Values –
Plus and Minus Dimensions
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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
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3. Tolerance Specifications in Title Block
General tolerance note
specifies the tolerance
for all unspecified
toleranced dimensions.
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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
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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
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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
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Fitting Multiple Parts
Tolerance of B
Fit Tolerance:
Clearance or
Interference
Part B
Tolerance of A
Part A
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Fitting Multiple Parts
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Shaft and Hole Fits
Clearance
Interference
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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)
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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?
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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
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Basic Hole System
Calculate Maximum and
Minimum Clearance
Clearance = Hole – Shaft
.490
.485
Cmax = Hmax – Smin
Cmax = .510 – .485 = .025
.510
.505
Cmin = Hmin – Smax
Cmin = .505 – .490 = .015
What type of fit is this?
Cmax > Cmin > 0 Clearance
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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
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Metric Preferred Hole Basis System of
Fits
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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
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