# Fits and Tolerances - PowerPoint PPT Presentation

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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

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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

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### Shaft and Hole Fits

ClearanceInterference

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CLEARANCE FIT+ .003

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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### 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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