Lect 20P. 1

1
Fits and Tolerances

• Lecture 20

2
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

3
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.

4
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

5
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
6
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
7
Notes and Title Block
ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE
ACCUARCY (.XXX) TO BE HELD TO /-.005"
8
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)

9
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)

10
Fitting Two Parts
11
Shaft and Hole Fits
Clearance Interference
12
Shaft and Hole Fits
Transition
13
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)

14
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"

15
Fit Calculations

• Clearance Hole Shaft
• Cmax Hmax Smin
• Cmin Hmin Smax
• Both Cmax and Cmin gt0 Clearance fit
• Both Cmax and Cmin lt0 Interference fit
• Cmax gt 0, Cmin lt 0 Transition fit
• Allowance Hmin - Smax (i.e., Cmin)

16
Fit Calculations

• System Tolerance Cmax - Cmin (Sometimes called
  Clearance Tolerance)
• Also, System Tolerance S Ti
• So, System Tolerance, or Ts , can be written as
• Ts Cmax - Cmin S Ti
• Thus, you always have a check value

17
Example
18
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

19
Metric Preferred Hole Basis System of Fits
20
Metric Tolerance Homework Example TOL-1B
21
Good Review Material

• BTG Chapter 7
• Dimensions and Tolerances
• Pages 290-335
• BTG Chapter 8
• Dimensions For Production
• Pages 340-375

22
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 TOL1A Metric Tolerances
• Using the given nominal sizes and fit
  specifications, calculate remaining values.