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Fits and Tolerances

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Fits and Tolerances

Lecture 20

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- 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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- 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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- 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

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.)

Feature Control Frame

Concentricity Symbol

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

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- 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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- 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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ClearanceInterference

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CLEARANCE FIT+ .003

Transition

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- 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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- 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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- 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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- 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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- 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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- BTG Chapter 7
- Dimensions and Tolerances
- Pages 290-335

- BTG Chapter 8
- Dimensions For Production
- Pages 340-375

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- 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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