Post on 18-Dec-2015
transcript
Objectives
• Understand description and control of variability through tolerancing
• Understand various classes of fits• Introduce multiple part tolerancing
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ToleranceTolerance 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
dimension2. Geometric tolerances3. A general tolerance note in title block4. 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.
2. Geometric Tolerance System
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Geometric Dimensioning and Tolerancing (GD&T) is a method of defining parts based on how they function, using standard ANSI symbols.
Feature Control Frame
Concentricity Symbol
3. Tolerance Specifications in Title Block
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General tolerance note specifies the tolerance for all unspecified toleranced dimensions.
4. Notes Referring to Specific Conditions
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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
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
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Fitting Multiple Parts
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Part A
Tolerance of A Part B
Tolerance of B Fit Tolerance: Clearance or Interference
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____
Fill in the subscripts (min, max) in the equations above.
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SMAX
SMIN
HMAX
HMIN
Basic Hole System
• Clearance = Hole – Shaft• Cmax = Hmax – Smin• Cmin = Hmin – Smax
Both Cmax and Cmin <0 – _________ fitBoth Cmax and Cmin >0 – _________ fitCmax > 0; Cmin < 0 – ___________ fit
What types of fits are these?
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SMAX
SMIN
HMAX
HMIN
Basic Hole System
• Clearance = Hole – Shaft• Cmax = Hmax – Smin• Cmin = Hmin – Smax
Both Cmax and Cmin <0 – Interference fitBoth Cmax and Cmin >0 – Clearance fitCmax > 0; Cmin < 0 – Transition fit
• System Tolerance = Cmax – Cmin• Allowance = Min. Clearance = Cmin
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SMAX
SMIN
HMAX
HMIN
Basic Hole System
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.490
.485
.510
.505
Calculate Maximum and Minimum Clearance
Clearance = Hole – Shaft
Cmax = Hmax – Smin
Cmax = .510 – .485 = .025
Cmin = .505 – .490 = .015
What type of fit is this? Cmax > Cmin > 0 Clearance
Cmin = Hmin – Smax
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 Tolerance Homework – TOL-1
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????
????
9.960
9.924
Free Running H9/d9Basic Size: 10
(1) Nominal Size: ?(1) Nominal Size: 10
(2) Shaft Limits:
(7) MinimumClearance: ????
(8) MaximumClearance: ????
(3) Shaft Tolerance: ????(3) Shaft Tolerance: 0.036
(4) Hole Limits: ????
????
10.036
10.000(5) Hole Tolerance: ????(5) Hole Tolerance: 0.036
(6) Ts: ????(6) Ts: 0.072
(7) MinimumClearance: 0.040
(8) MaximumClearance: 0.112
CHECK: Ts = Cmax – Cmin?CHECK: 0.072 = 0.112 – 0.040 = 0.072