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Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 1Autumn Quarter
Fits and Tolerances
Lecture 20
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 2Autumn Quarter
Tolerancing – Control of Variability
• Goals– Understand the description and control of
variability through tolerancing.– Use standard tables for tolerancing and
control of fit• Reference (BTG)– P. 312-317 – Dimensioning for Interchangeable
Parts– P. 349-354 – Standard Tables for Fits– P. 358-369 – Geometric Tolerancing
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 3Autumn Quarter
Definition of Tolerance
• Tolerance is the total amount a dimension may vary. It is the difference between the maximum and minimum limits.
• There is no such thing as an "exact size".• Tolerance is key to interchangeable parts.
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 4Autumn Quarter
Ways to Express Tolerance
• Direct limits or as tolerance limits applied to a dimension
• Geometric tolerances• Notes referring to specific conditions• A general tolerance note in title block
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 5Autumn Quarter
Direct Limits and Tolerance Values
Can be:
Limits: Upper limit – 3.53
Lower limit – 3.49
Unilateral – vary in only one direction
3.49
0 -.0X
+.0X - 0
Bilateral – vary larger or smaller (may or may not be same amount)
3.50
+.05 -.01, +.10 -.20 +/- 0.05
+.04 0
+.03-.01
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 6Autumn Quarter
Geometric Tolerance System
• Geometric dimensioning and tolerancing (GDT) is a method of defining parts based on how they function, using standard ANSI symbols.
• (More about this in a couple of weeks.)
Feature Control Frame
Concentricity Symbol
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 7Autumn Quarter
Notes and Title Block
ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005"
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 8Autumn Quarter
Important Terms – Single Part
• Nominal Size – a general size, usually expressed as a common fraction (1/2”)
• Basic Size – theoretical size used as starting point (.500”)
• Actual Size – measured size (.501”)• Limits – maximum and minimum sizes shown by
tolerances• Tolerance – total allowable variance in
dimensions (upper limit – lower limit)
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 9Autumn Quarter
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 leave a space– Interference Fit – tolerance of mating parts
always interfere– Transition Fit – sometimes interfere,
sometimes clear• Tolerance – total allowable variance in
dimensions (upper limit – lower limit)
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 10Autumn Quarter
Fitting Two Parts
Part A
Tolerance of A Part B
Tolerance of B Tolerance: Clearance or Interference
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 11Autumn Quarter
Shaft and Hole Fits
Clearance Interference
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 12Autumn Quarter
Shaft and Hole Fits
Transition
CLEARANCE FIT
+ .003
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 13Autumn Quarter
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)
See Tables in the Appendix (pp. A11-A23)
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 14Autumn Quarter
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"
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 15Autumn Quarter
Fit Calculations
• Clearance = Hole – Shaft• Cmax = Hmax – Smin• Cmin = Hmin – Smax
Both Cmax and Cmin >0 – Clearance fit
Both Cmax and Cmin <0 – Interference fit
Cmax > 0, Cmin < 0 – Transition fit• Allowance = Hmin - Smax (i.e., Cmin)
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 16Autumn Quarter
Fit Calculations
• System Tolerance = Cmax - Cmin (Sometimes called Clearance Tolerance)
• Also, System Tolerance = Σ Ti• So, System Tolerance, or Ts , can be written as:
Ts = Cmax - Cmin = Σ Ti• Thus, you always have a check value
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 17Autumn Quarter
Example
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 18Autumn Quarter
Metric Limits and Fits
• Based on Standard Basic Sizes – ISO Standard, see the Appendix material (Appendices 8 - 12)
• Note that in the Metric system:
Nominal Size = Basic Size• Example: If the nominal size is 8, then the basic
size is 8
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 19Autumn Quarter
Metric Preferred Hole Basis System of Fits
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 20Autumn Quarter
Metric Tolerance Homework – Example TOL-1B
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 21Autumn Quarter
Good Review Material
• BTG Chapter 7– Dimensions and Tolerances– Pages 290-335
• BTG Chapter 8– Dimensions For Production– Pages 340-375
Engineering H191 - Drafting / CAD
Gateway Engineering Education Coalition Lect 20 P. 22Autumn Quarter
Assignments
• Dwg 39 – G27 – Tolerances – Single Fits– Calculate the missing values for each
situation.– Use the tables for preferred limits and fits for
cylindrical parts.
• Dwg 40 – TOL–1A – Metric Tolerances– Using the given nominal sizes and fit
specifications, calculate remaining values.