Dimensions, Tolerances, And Surfaces

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2007 John Wiley & Sons, Inc. M P Groover, Fundamentals o f Mod ern Manufac tur ing 3/e

5. Dimensions, Tolerances, and Surfaces

1. Dimensions, Tolerances, and Related Attributes

2. Surfaces

3. Effect of Manufacturing Processes


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ME 3631

Manufacturing Processes I

Lecture Objective

Dimensions, Tolerances, and Surfaces

Dimensions and TolerancesSurfaces

Characteristics of Surfaces

Surface Texture

Surface Integrity

Tolerances and m anufactur ing Processes

Surfaces and manufactur ing Processes

Lecture No. 03


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2. Surfaces



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Dimensions and Tolerances

Factors that determine the performance of amanufactured product, other than mechanicaland physical properties, include :

Dimensions - linear or angular sizes of acomponent specified on the part drawing

Tolerances - allowable variations from thespecified part dimensions that are permitted

in manufacturing


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Dimensions (ANSI Y14.5M-1982):

A dimension is "a numerical value expressed inappropriate units of measure and indicated on adrawing and in other documents along with lines,symbols, and notes to define the size or

geometric characteristic, or both, of a part or partfeature"

Dimensions on part drawings represent nominalor basic sizes of the part and its features

The dimension indicates the part size desired bythe designer , if the part could be made with noerrors or variations in the fabrication process


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Tolerances (ANSI Y14.5M-1982):

A tolerance is "the total amount by which aspecific dimension is permitted to vary. Thetolerance is the difference between themaximum and minimum limits"

Variations occur in any manufacturing process,which are manifested as variations in part size

Tolerances are used to define the limits of the

allowed variation


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Bilateral Tolerance

Variation is permittedin both positive andnegative directionsfrom the nominal

dimension Possible for a

bilateral tolerance tobe unbalanced; for

example, 2.500+0.010, -0.005

Figure 5.1 Ways to specifytolerance limits for anominal dimension of 2.500:(a) bilateral


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Unilateral Tolerance

Variation from thespecified dimension ispermitted in only onedirection

Either positive ornegative, but not both

Figure 5.1 Ways to specifytolerance limits for anominal dimension of 2.500:(b) unilateral


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Limit Dimensions

Permissiblevariation in a partfeature size consistsof the maximum and

minimumdimensions allowed

Figure 5.1 - Ways to specifytolerance limits for anominal dimension of 2.500:(c) limit dimensions


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Other Geometric Attributes

Angularity. The extent to which a part featuresuch as a surface or axis is at a specified anglerelative to a reference surface

Circularity. Circularity is the degree to which allpoints on the intersection of the surface and aplane perpendicular to the axis of revolution areequidistant from the axis.

Concentricity . The degree to which any two (ormore) part features, such as a cylindrical surfaceand a circular hole have a common axis.


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Flatness. The extent to which all points on a surfacelie in a single plane.

Parallelism. The degree to which all points on a partfeature, such as a surface, line, or axis, areequidistant from a reference plane or line or axis.

Perpendicularity. The degree to which all points on apart feature, such as a surface, line, or axis, are 90o

from a reference plane or line or axis.

Straightness. The degree to which a part featuresuch as a line or axis is a straight line.


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2. Surfaces



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Why Surfaces are Important

Aesthetic reasonsSurfaces affect safety

Friction and wear depend on surface

characteristicsSurfaces affect mechanical and physicalproperties

Assembly of parts is affected by their surfaces

Smooth surfaces make better electricalcontacts


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Surfaces

Nominal surface designers intended surfacecontour of part, defined by lines in theengineering drawing

The nominal surfaces appear asabsolutely straight lines, ideal circles,round holes, and other edges andsurfaces that are geometrically perfect

Actual surfaces of a part are determined bythe manufacturing processes used to make it

Variety of processes result in widevariations in surface characteristics


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Surface Technology

Concerned with:Defining the characteristics of a surface

Surface texture

Surface integrity

Relationship between manufacturing processes and characteristics of resulting

surface


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Surface Texture

The topography and geometric features of thesurface

When highly magnified, the surface is anythingbut straight and smooth

It has roughness, waviness, and flaws

It also possesses a pattern and/or directionresulting from the mechanical process that

produced it


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Metallic Part Surface

Figure 5.2 A magnified cross-section of a typical metallic part surface


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Surface Texture

Repetitive and/or random deviations from thenominal surface of an object

Figure 5.3 Surface texture features


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Four Elements of Surface Texture

1. Roughness - small, finely-spaced deviationsfrom nominal surface

Determined by material characteristics and processes that formed the surface

2. Waviness - deviations of much larger spacing

Waviness deviations occur due to workdeflection, vibration, heat treatment, andsimilar factors

Roughness is superimposed on waviness


20/382007 John Wiley & Sons, Inc. M P Groover, Fundamentals o f Mod ern Manufac tur ing 3/e


3. Lay -predominantdirection orpattern of thesurface texture

Figure 5.4 Possiblelays of a surface


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4. Flaws - irregularities that occur occasionally onthe surface

Includes cracks, scratches, inclusions, andsimilar defects in the surface

Although some flaws relate to surfacetexture, they also affect surface integrity


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Surface Texture

Repetitive and/or random deviations from thenominal surface of an object

Figure 5.3 Surface texture features


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Surface Roughness

Average of vertical deviations from nominalsurface over a specified surface length

Figure 5.5 Deviations from nominal surface used in the twodefinitions of surface roughness.


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Surface Roughness Equation

Arithmetic average (AA) based on absolutevalues of deviations, and is referred to asaverage roughness

where R a = average roughness; y = verticaldeviation from nominal surface (absolutevalue); and L m = specified distance over whichthe surface deviations are measured

dx Ly

R

mL

ma 0=


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Alternative Surface Roughness Equation

Approximation of previous equation is perhapseasier to comprehend

where R a has the same meaning as above; y i =

vertical deviations (absolute value) identified bysubscript i ; and n = number of deviationsincluded in Lm

n

i

i a

n

y R

1


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Cutoff Length

A problem with the R a computation is thatwaviness may get included

To deal with this problem, a parameter calledthe cutoff length is used as a filter to separatewaviness from roughness deviations

Cutoff length is a sampling distance along thesurface

A sampling distance shorter than thewaviness eliminates waviness deviationsand only includes roughness deviations


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Surface Roughness Specification

Figure 5.6 Surface texture symbols in engineeringdrawings: (a) the symbol, and (b) symbol with

identification labels


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Surface Integrity

Surface texture alone does not completelydescribe a surface

There may be metallurgical changes in thealtered layer beneath the surface that can havea significant effect on a material's mechanicalproperties

Surface integrity is the study and control of thissubsurface layer and the changes in it thatoccur during processing which may influencethe performance of the finished part or product


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Surface Changes Caused by Processing

Surface changes are caused by the applicationof various forms of energy during processing

Example: Mechanical energy is the mostcommon form in manufacturing

Processes include forging, extrusion,and machining

Although its primary function is to change

geometry of workpart, mechanical energycan also cause residual stresses, workhardening, and cracks in the surface layers


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Energy Forms in Surface Integrity

Mechanical energyThermal energy

Chemical energy

Electrical energy


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Surface Changes by Mechanical Energy

Residual stresses in subsurface layerExample: bending of sheet metal

Cracks - microscopic and macroscopic

Example: tearing of ductile metals inmachining

Voids or inclusions introduced mechanically

Example: centerbursting in extrusion

Hardness variations (e.g., work hardening)

Example: strain hardening of new surface inmachining


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Surface Changes by Thermal Energy

Metallurgical changes (recrystallization, grainsize changes, phase changes at surface)

Redeposited or resolidified material (e.g.,welding or casting)

Heat-affected zone in welding (includes someof the metallurgical changes listed above)

Hardness changes


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Surface Changes by Chemical Energy

Intergranular attackChemical contamination

Absorption of certain elements such as H and

Cl in metal surfaceCorrosion, pitting, and etching

Dissolving of microconstituents

Alloy depletion and resulting hardness changes


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Surface Changes by Electrical Energy

Changes in conductivity and/or magnetismCraters resulting from short circuits duringcertain electrical processing techniques suchas arc welding


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2. Surfaces



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Tolerances and Manufacturing Processes

Some manufacturing processes are inherentlymore accurate than others

Examples:

Most machining processes are quiteaccurate, capable of tolerances = 0.05 mm( 0.002 in.) or better

Sand castings are generally inaccurate, and

tolerances of 10 to 20 times those used formachined parts must be specified


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Surfaces and Manufacturing Processes

Some processes are inherently capable ofproducing better surfaces than others

In general, processing cost increases withimprovement in surface finish because

additional operations and more time areusually required to obtain increasingly bettersurfaces

Processes noted for providing superiorfinishes include honing, lapping, polishing,and superfinishing


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Thats All for Today

2007 J h Wil & S I M P G F d t l fM d M f t i 3/

Dimensions, Tolerances, And Surfaces

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