Fits and tolerances
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Fits and Tolerances. Lecture 20. 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

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Fits and Tolerances

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Fits and Tolerances

Lecture 20

Autumn 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

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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.

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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

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+.04

0

+.03

-.01

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

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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.)

Geometric Tolerance System

Feature Control Frame

Concentricity Symbol

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Notes and Title Block

ALL DECIMAL DIMENSIONS THAT ARE THREE PLACE ACCUARCY (.XXX) TO BE HELD TO +/-.005"

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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)

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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 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)

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Tolerance of B

Tolerance: Clearance or Interference

Part B

Tolerance of A

Part A

Fitting Two Parts

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Shaft and Hole Fits

ClearanceInterference

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CLEARANCE FIT+ .003

Shaft and Hole Fits

Transition

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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)

    See Tables in the Appendix (pp. A11-A23)

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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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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)

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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

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Example

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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

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Metric Preferred Hole Basis System of Fits

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Metric Tolerance Homework – Example TOL-1B

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Good Review Material

  • BTG Chapter 7

    • Dimensions and Tolerances

    • Pages 290-335

  • BTG Chapter 8

    • Dimensions For Production

    • Pages 340-375

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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.

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