meng 3324: manufacturing processes, utpb, ch05

7/27/2019 MENG 3324: Manufacturing Processes, UTPB, ch05

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2012 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 5/e

DIMENSIONS, TOLERANCES,

AND SURFACES

1. Dimensions, Tolerances, and Related Attributes

2. Conventional Measuring Instruments and Gages

3. Surfaces

4. Measurement of Surfaces

5. Effect of Manufacturing Processes


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Dimensions and Tolerances

Factors that determine the performance of a

manufactured product, other than mechanical and

physical properties, include : Dimensions - linear or angular sizes of a

component specified on the part drawing

Tolerances - allowable variations from the

specified part dimensions that are permitted inmanufacturing


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Dimensions (ANSI Y14.5M-1982)

A dimension is "a numerical value expressed in

appropriate units of measure and indicated on a drawing

and in other documents along with lines, symbols, andnotes to define the size or geometric characteristic, or

both, of a part or part feature"

The dimension indicates the part size desired by the

designer, if the part could be made with no errors orvariations in the fabrication process


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Bilateral Tolerance

Variation is permitted in

both positive and negative

directions from the nominaldimension

Possible for a bilateral

tolerance to be unbalanced

Ex: 2.500 +0.010, -0.005


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Unilateral Tolerance

Variation from the

specified dimension is

permitted in only onedirection

Either positive or

negative, but not both


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Limit Dimensions

Permissible variation in a

part feature size consists of

the maximum and minimumdimensions allowed


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Measurement

Procedure in which an unknown quantity is compared

to a known standard, using an accepted and

consistent system of units Measurement provides a numerical value of the

quantity of interest, within certain limits of accuracy

and precision


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Accuracy and Precision

Accuracy - the degree to which a measured valueagrees with the true value of the quantity of interest

A measurement procedure is accurate when it avoidssystematic errors (positive or negative deviations thatare consistent from one measurement to the next)

Precision - the degree of repeatability in themeasurement process

Good precision means that random errors in themeasurement procedure are minimized


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Conventional Measuring

Instruments and Gages

Precision gage blocks

Measuring instruments for linear dimensions

Comparative instruments

Fixed gages

Angular measurements


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Precision Gage Blocks

Standards against which other dimensional

measuring instruments and gages are compared

Usually square or rectangular blocks Surfaces are finished to be dimensionally accurate

and parallel to several millionths of an inch and

are polished to a mirror finish

Precision gage blocks are available in certainstandard sizes or in sets, the latter containing a

variety of different sized blocks


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Measurement of Linear

Dimensions

Measuring instruments are divided into two types:

Graduated measuring devices include a set of

markings on a linear or angular scale to which theobject's feature of interest can be compared for

measurement

Nongraduated measuring devices have no scale

and are used to compare dimensions or to transfera dimension for measurement by a graduated

device


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Micrometer

External micrometer, standard one-inch size withdigital readout (photo courtesy of L. S. Starret Co.)


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Mechanical Gages:

Dial Indicators

Mechanical gages are designed to mechanically

magnify the deviation to permit observation

Most common instrument in this category is the dialindicator, which converts and amplifies the linear

movement of a contact pointer into rotation of a dial

The dial is graduated in small units such as 0.01 mm

or 0.001 inch Applications: measuring straightness, flatness,

parallelism, squareness, roundness, and runout


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As part is rotated about its center, variations in outside

surface relative to center are indicated on the dial

Dial Indicator Setup to Measure

Runout


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GO/NO-GO gages

So-named because one gage limit allows the part to be

inserted while the other limit does not

GO limit - used to check the dimension at itsmaximum material condition

Minimum size for internal feature such as a hole

Maximum size for external feature such as an

outside diameter NO-GO limit - used to inspect the minimum material

condition of the dimension in question


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Gaging the diameter of a part (difference in height of

GO and NO-GO gage buttons is exaggerated)

Snap Gage


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Plug Gage

Gaging of a hole diameter (difference in diameters of

GO and NO-GO plugs is exaggerated)


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Measurement of Angles

Bevel protractor

with Vernier

scale (courtesyL. S. Starrett

Co.)


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Surfaces

Nominal surfacedesigners intended surface contour

of part, defined by lines in the engineering drawing

Nominal surfaces appear as absolutely straightlines, ideal circles, round holes, and other edges

and surfaces that are geometrically perfect

Actual surfaces of a part are determined by the

manufacturing processes used to make them Variety of processes result in wide variations in

surface characteristics


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Why Surfaces are Important

Aesthetic reasons

Surfaces affect safety

Friction and wear depend on surface characteristics

Surfaces affect mechanical and physical properties

Assembly of parts is affected by their surfaces

Smooth surfaces make better electrical contacts


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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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Metallic Part Surface

Magnified cross section of a typical metallic part

surface


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Surface Texture

The topography and geometric features of the

surface

When highly magnified, the surface is anything butstraight and smooth

It has roughness, waviness, and flaws

It also possesses a pattern and/or direction

resulting from the mechanical process thatproducedit


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Surface Texture

Repetitive

and/or random

deviations fromthe nominal

surface of an

object


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Four Elements of Surface

Texture

1. Roughness - small, finely-spaced deviations from

nominal surface

Determined by material characteristics andprocesses that formed the surface

2. Waviness - deviations of much larger spacing

Waviness deviations occur due to work

deflection, vibration, tooling, and similar factors Roughness is superimposed on waviness


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Texture

3. Lay - predominant direction or pattern of the surfacetexture


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Texture

4. Flaws - irregularities that occur occasionally on the

surface

Includes cracks, scratches, inclusions, and similardefects in the surface

Although some flaws relate to surface texture, they

also affect surface integrity


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Surface Roughness and

Surface Finish

Surface roughness - a measurable characteristic

based on roughness deviations

Surface finish - a more subjective term denotingsmoothness and general quality of a surface

In popular usage, surface finish is often used as a

synonym for surface roughness

Both terms are within the scope of surface texture


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Surface Roughness

Average of vertical deviations from nominal surface

over a specified surface length


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Surface Roughness Equation

Arithmetic average (AA) based on absolute values of

deviations, and is referred to as average roughness

where Ra = average roughness; y= vertical deviation

from nominal surface (absolute value); and Lm =specified distance over which the surface deviations

are measured

dxL

yR

mL

ma

0

=


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Alternative Surface Roughness

Equation

Approximation of previous equation is perhaps easier

to comprehend

where Ra has same meaning as above; yi= vertical

deviations (absolute value) identified by subscript i;and n = number of deviations included in Lm

n

i

ia

n

yR

1


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Cutoff Length

A problem with the Ra computation is that waviness

may get included

To deal with this problem, a parameter called thecutoff length is used as a filter to separate waviness

from roughness deviations

Cutoff length is a sampling distance along the surface

A sampling distance shorter than the wavinesseliminates waviness deviations and only includes

roughness deviations


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Surface Roughness Specification

Surface texture symbols in engineering drawings: (a)

the symbol, and (b) symbol with identification labels


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Surface Integrity

Surface texture alone does not completely describe a

surface

There may be metallurgical changes in the alteredlayer beneath the surface that can have a significant

effect on a material's mechanical properties

Surface integrity is the study and control of this

subsurface layer and the changes in it that occurduring processing which may influence the

performance of the finished part or product


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Surface Changes Caused by

Processing

Surface changes are caused by the application ofvarious forms of energy during processing

Example: Mechanical energy is the most commonform in manufacturing

Processes include forging, extrusion, andmachining

Although its primary function is to change

geometry of workpart, mechanical energy can alsocause residual stresses, work hardening, andcracks in the surface layers


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Energy Forms that Affect

Surface Integrity

Mechanical energy

Thermal energy

Chemical energy

Electrical energy


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Mechanical Energy

Residual stresses in subsurface layer

Example: bending of sheet metal

Cracks - microscopic and macroscopic Example: tearing of ductile metals in machining

Voids or inclusions introduced mechanically

Example: centerbursting in extrusion

Hardness variations (e.g., work hardening) Example: strain hardening of new surface in

machining


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Thermal Energy

Metallurgical changes (recrystallization, grain size

changes, phase changes at surface)

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

Heat-affected zone in welding (includes some of the

metallurgical changes listed above)

Hardness changes


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Surface Changes by Caused

Chemical Energy

Intergranular attack

Chemical contamination

Absorption of certain elements such as H and Cl inmetal surface

Corrosion, pitting, and etching

Dissolving of microconstituents

Alloy depletion and resulting hardness changes


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Electrical Energy

Changes in conductivity and/or magnetism

Craters resulting from short circuits during certain

electrical processing techniques such as arc welding


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Measurement of Surfaces

Two parameters of interest:

Surface texture - geometry of the surface,

commonly measured as surface roughness Surface roughness

Surface integrity - deals with the material

characteristics immediately beneath the surface and

the changes to this subsurface that resulted fromthe processes that created it


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Measurement of Surface

Roughness

Three methods to measure surface roughness:

1. Subjective comparison with standard test

surfaces Fingernail test

2. Stylus electronic instruments

3. Optical techniques


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Stylus Instruments

Similar to the fingernail test, but more scientific

In these electronic devices, a cone-shaped diamond

stylus is traversed across test surface at slow speed As the stylus head is traversed horizontally, it also

moves vertically to follow the surface deviations

The vertical movement is converted into an electronic

signal that can be displayed as Profile of the actual surface

Average roughness value


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Stylus Instruments

Stylus instrument

traversing a standard

test surface (courtesy

George E. Kane

Manufacturing

Technology

Laboratory, Lehigh

University)



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Stylus head traverses horizontally across surface, while

stylus moves vertically to follow surface profile

Stylus Traversing Surface


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Tolerances and Manufacturing

Processes

Some manufacturing processes are inherently more

accurate than others

Most machining processes are quite accurate,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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2012 John Wiley & Sons Inc M P Groover Fundamentals of Modern Manufacturing 5/e

Surfaces and Manufacturing

Processes

Some processes are inherently capable of producing

better surfaces than others

In general, processing cost increases withimprovement in surface finish because additional

operations and more time are usually required to

obtain increasingly better surfaces

Processes noted for providing superior finishesinclude honing, lapping, polishing, and

superfinishing

meng 3324: manufacturing processes, utpb, ch05

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