Fits and Tolerances

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# Fits and Tolerances - PowerPoint PPT Presentation

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

Lecture 20

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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.

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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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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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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