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Faculty of Mechanical Engineering

Ghulam Ishaq Khan Institute of Engineering Sciences & Technology

ME-102 Engineering Graphics

Lecture #: 10

Geometric Tolerance

Lecture By: Muftooh Ur Rehman

Research Associate

Office # G-17 - FME

Email: muftooh@giki.edu.pk

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Geometric Tolerance

No component can practically be manufactured to exactdimensions (sizes).

Tolerances are used to control the variation that exists on allmanufactured parts.

It is the amount, each part is allowed to vary depending upon the

function of the part and assembly. Toleranced dimensions control the amount of variation on each

part of an assembly.

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Interchangeable Parts

Modern mass production manufacturing systems is based onproducing large quantities of interchangeable parts.

When different parts are assembled, they must fit together andfunction correctly.

All the parts are produced within a certain limits set by engineers.

A limit is the amount a part can be oversize or undersize and stillbe usable.

Tolerance is the amount of error permitted in making a product orits parts. The tolerance are specified in the engineering drawings.

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Relation b/w Tolerance and Cost

The more accuracy needed in a machined part, the higher themanufacturing cost.

Tolerances must be specified in such a way that a productfunctions as it should at a cost that is reasonable.

Tolerance should be as wide as possible, as the satisfactory

design function permits.

Approximaterelationship between

production cost andmanufacturing tolerance

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Important Terms

Nominal Size: A dimension used to describe the general size.

Basic Size: The theoretical size used as a starting point for theapplication of tolerance.

Actual Size: The measured size of the finished part aftermachining.

Limits: The maximum and minimum sizes shown by the tolerancedimension.

Allowance: The minimum clearance or maximum interferencebetween parts, or the tightestfit b/w two mating parts.

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Important Terms

Maximum material condition (MMC):

The condition of the part when it contains the greatest amount ofmaterial.

The MMC of an external feature (such as shaft), is the upper limit.

The MMC of an internal feature (such as a hole), is the lower limit.

Why??

Least material condition (LMC):

The condition of a part when it contains the least amount of materialpossible.

The LMC of an external feature is the lower limit.

The LMC of an internal feature, is the upper limit.

Why??

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Important Terms

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LMC Condition MMC Condition

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Bilateral Tolerance

A bilateral tolerance varies in both direction from the basic size. If thevariation is equal in both direction, then the variation is preceded by

symbol.

The approach is used when the two variations are equal.

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Unilateral Tolerance

When the tolerance value is specified in only one direction from thebasic size it is know as unilateral tolerance.

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Other Approaches

Tolerance can be specified

in a general way to cover fora wide range of dimensions

Tolerance specifiedin a tabulated

manner.

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Types of Fits

The degree of tightness between matingparts is called fit.

Clearance Fit:

In which the shaft is always smaller than thehole into which it fits. A clearance fit alwayshas a gap between the two mating parts.

Interference Fit:

In which the shaft is always bigger than thehole into which it fits. Interference fits alwaysoverlap and are used mainly for press fits

where the two parts are pushed together, andrequire no other fasteners.

Clearance fits - allowancealways positive

Interference fits - allowancealways negative, Why??

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Types of Fits

Transition Fit:

In which the shaft may be either bigger or smaller than the holeinto which it fits it will therefore be possible to get interference orclearance fits in one group of assemblies.

This type of fit may result in interference, or clearance.

This type of fit can be used for items such as snap fits, What arethey??Transition fitallowance may

be positive or negative

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Types of Fits

The two bases for a system of limits and fits:

(a) The hole basis

(b) The shaft basis

Hole Basis:

Hole diameter constant.

Shaft diameter varies.

Economical as only a single drill willbe uses

Shaft Basis:

Hole diameter varies.

Shaft diameter constant.

Tends to be costly, as morethen one drill is required.

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Determining Fits

The loosest fit is the differencebetween the smallest feature A and the

largest feature B.

The tightest fit is the difference b/wthe largest feature A and the smallest

feature B.

Study BOOK

Only Projection of lines (04),

Dimensioning (05), Engineeringgeometry included (07) included in final

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An engineer creates that never was.

So it is all about creativity.

How will you help the society throughEngineering????