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7/31/2019 Lecture 13 Geometric Tolerance
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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: [email protected]
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Muftooh Ur Rehman Siddiqi, RA-FME, GIK Institute Page 2 of 17
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
Muftooh Ur Rehman Siddiqi, RA-FME, GIK Institute Page 7 of 17
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????