lecture 8 tolerance

7/30/2019 Lecture 8 Tolerance

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GEOMETRICTOLERANCE


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Definition of Tolerance

Tolerance is the total amount a dimension mayvary. It is the difference between the maximumand minimum limits.


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TolerancingControl of Variability

Objectives

Understand the description and control of

variability through tolerancing.

Use standard tables for tolerancing and control of

fit


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Important Terms

Nominal Sizeused for the purpose of general

identification

Basic Sizetheoretical size used as starting point to derived

the limit for that dimension

Actual Sizemeasured size

Limitsmaximum and minimum sizes permissible for a

specific dimension (shown by tolerances)

Tolerancetotal allowable variance in dimensions

(upper limitlower limit)


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Important TermsMultiple Parts

Allowancethe minimum clearance or maximum

interference between parts

Fitdegree of tightness between two parts

* Clearance Fittolerance of mating parts always leave aspace

*Interference Fittolerance of mating parts always interfere

* Transition Fitsometimes interfere, sometimes clear

Bilateral Tolerancevariation is permitted in both direction

from the specified dimension

Unilateral Tolerancevariation is permitted in only one

direction from the specified dimension


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Key Concept

TERMINOLOGY EXAMPLE EXPLANATION

BASIC SIZE 1.500

BASIC SIZE WITH 1.500.004 HALF OF TOTALTOLERANCE ADDED TOLERANCE

LIMITS OF SIZE 1.504 LARGEST AND SMALLEST

1.496 SIZE PERMITTED

TOLERANCE .008 DIFFERENT BETWEEN

LIMITS OF SIZE


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Can be:

Limits: Upper limit = 3.53

Lower limit = 3.49

Unilateral

vary in only one direction

Bilateral

vary larger or smaller (mayor may not be same amount)

Direct Limits and Tolerance Values


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Geometric Tolerance System

Geometric

dimensioning and

tolerancing (GDT) is amethod of defining

parts based on how

they function, using

standard ANSIsymbols.


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Notes and Title Block

ALL DECIMAL

DIMENSIONS THAT

ARE THREE PLACEACCUARCY (.XXX) TO

BE HELD TO +/-.005"


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Fitting Two Parts


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Fit Calculations

Clearance = Hole Shaft

Cmax = HmaxSmin

Cmin = HminSmax

Both Cmax and Cmin >0Clearance fit

Both Cmax and Cmin 0, Cmin < 0Transition fit

Allowance = Hmin - Smax (i.e., Cmin)


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Fit Calculations

System Tolerance = Cmax - Cmin(Sometimes called Clearance Tolerance)

Also, System Tolerance = Ti

Ti = Hole tolerance + Shaft tolerance

So, System Tolerance, or Ts , can be write

as:Ts = Cmax - Cmin= Ti

Thus, you always have a check value


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Example

Example:

Clearance = HoleShaft

Cmax = HmaxSmin

= 0.510 - 0.485 = 0.025

Cmin = HminSmax

= 0.5050.490 = 0.015

Since Cmax and Cmin are both

positive. It is a Clearance Fit.

Check!Clearance tolerance = CmaxCmin

= 0.0250.015 = 0.010

Hole Tolerance = 0.5100.505 = 0.005

Shaft Tolerance = 0.4900.485 = 0.005

System Tolerance = 0.005 + 0.005 = 0.010


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Shaft and Hole Fits

Clearance Interference


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Shaft and Hole Fits

Transition


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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 inthe system will be 0.500"


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Metric Preferred Hole Basis

System of Fits


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Exercise!!!


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Do it your self !

Can you read this drawing ?


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ANY QUESTION ?

lecture 8 tolerance

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