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Lect 20 P. 1
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Fits and Tolerances
Lecture 20
Lect 20 P. 2
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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
Lect 20 P. 3
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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.
Lect 20 P. 4
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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
Lect 20 P. 5
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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
Lect 20 P. 6
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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
Lect 20 P. 7
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Notes and Title Block
ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005"
Lect 20 P. 8
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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)
Lect 20 P. 9
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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)
Lect 20 P. 10
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Fitting Two Parts
Part A
Tolerance of A Part B
Tolerance of B Tolerance: Clearance or Interference
Lect 20 P. 11
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Shaft and Hole Fits
Clearance Interference
Lect 20 P. 12
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Shaft and Hole Fits
Transition
CLEARANCE FIT
+ .003
Lect 20 P. 13
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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)
Lect 20 P. 14
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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"
Lect 20 P. 15
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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)
Lect 20 P. 16
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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
Lect 20 P. 17
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Example
Lect 20 P. 18
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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
Lect 20 P. 19
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Metric Preferred Hole Basis System of Fits
Lect 20 P. 20
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Metric Tolerance Homework – Example TOL-1B
Lect 20 P. 21
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
Good Review Material
• BTG Chapter 7– Dimensions and Tolerances– Pages 290-335
• BTG Chapter 8– Dimensions For Production– Pages 340-375
Lect 20 P. 22
Engineering H191 - Drafting / CAD
Autumn Quarter Gateway Engineering Education Coalition
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.