ch2_dim-tol&sur

7/28/2019 Ch2_Dim-Tol&Sur

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CHAPTER (2)

Dimensions, Tolerances, AndDimensions, Tolerances, AndSurfacesSurfaces

Dr. Ahmed Abou El-Wafa


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Dimensions (ANSI Y14.5M-1982):

Dimension is a numerical value expressed in appropriateunits of measure and indicated on a drawing and in other

documents along with lines, symbols, and notes to definethe size or geometric characteristic, or both, of a part or

part feature.

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Tolerances (ANSI Y14.5M-1982):

Tolerance is "the total amount by which a specific

dimension is permitted to vary. The tolerance is the

difference between the maximum 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- Probably the most common Type.

- Variation is permitted in both positive and negative

directions from the nominal dimension.-Two types are found; balanced (a) and unbalanced (b);

BalancedUnbalanced

(b)

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Unilateral Tolerance- Variation from the specified dimension is permitted inonly one direction

Either positive or negative, but not both.

(a)

+0.000

-0.007

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Limit Dimensions

- Permissible variation in a part feature size consistsof the maximum and minimum dimensions allowed.

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Typical tolerance limits for various manufacturing processes

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Surfaces

Nominal surface designers intended surface contour ofpart, defined by lines in the engineering drawing.

The nominal surfaces appear as absolutely straightlines, ideal circles, round holes, and other edges andsurfaces that are geometrically perfect

Actual surfaces of a part are determined by themanufacturing processes used to make it.

Variety of processes result in wide variations in surfacecharacteristics.

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Why surfaces are commercially and

technologically 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

A 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

but straight and smooth It has roughness, waviness, and flaws,

It also possesses a pattern and/or direction

resulting from the mechanical process that

produced it (lay).

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Surface TextureRepetitive and/or random deviations from the nominal

surface of an object.

Surface texture features.

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Four Elements of Surface Texture

1. Roughness - small, finely-spaced deviationsfrom 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, heat treatment, and

similar factors.

Roughness is superimposed on waviness

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3. Lay - predominant

direction or pattern

of the surface

texture.

Possible lays of a surface.15


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4. Flaws - irregularities that occur occasionally on

the surface

Includes cracks, scratches, inclusions, and

similar defects 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

denoting smoothness 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 surfacetexture.

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Surface RoughnessAverage of vertical deviations from nominal surface over a

specified surface length

Deviations from nominal surface used in the two definitions ofsurface roughness.

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Surface Roughness Parameters

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

where Ra = average roughness; y = verticaldeviation from nominal surface (absolute

value); and Lm = specified distance over whichthe surface deviations are measured.

dxL

yR

mL

0 m

a =

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Alternative Surface Roughness Equation

Approximation of previous equation is perhaps

easier to comprehend

where Ra has the same meaning as above; yi =vertical deviations (absolute value) identified bysubscript i; and N = number of deviations

included in Lm.

=

=

N

1i

ia

N

yR

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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

the cutoff 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

waviness eliminates waviness deviations

and only includes roughness deviations.

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Roughness parameter Rq(R

s)

Rq or Rs Root Mean Square Roughness

Older terms: RMS ...Root Mean Square, Still widelyused in some industries (i.e. optical)

Rq

is the Root Mean Square of the distance of thefiltered or unfiltered Roughness Profile from its meanline.

=

=

=

Ni

1i

2iq y

N1R

Where: N is the number of Data points

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Roughness parameter Rt(Rmax)

Rt or Rmax .. Maximum Peak to Valley Height

Rt is the distance between the highest Peak to the lowest Valley

in the assessment length.

Peak

Valley

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Roughness parameter Rz

Rz .. Average Peak to Valley Height

- is also known as the ISO 10 point height parameter.

- It is numerically the average height difference

between the five highest peaks and the five lowest

valleys within the assessment length.

Rz

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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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Measurement of Surface Roughness

1- Standard Test Surfaces

The surface is compared to the standard surface finish

blocks both visually and by the finger nail test. The userjudge which standard is closest to the specimen.

2- Stylus Instruments

A cone-shaped diamond (orhardened steel) stylus with pointradius of about 0.005 mm and 90tip angle is traversed across thetest surface at a constant slowspeed. The vertical movement isconverted into an electronic signalthat represents the topography of

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3- Optical Techniques

These techniques are based on light reflectance from

the surface, light scatter or diffusion, and lasertechnology.

Measurement of Surface Roughness

They are useful in applications where stylus contact withthe surface is undesirable. Some of the techniquespermit very high speed operation, thus making 100%

inspection feasible.27


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Some processes are inherently capable of producing better

surfaces than others;

Typical range of surface roughness values are given in m (in.).28


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Surface Integrity

Surface texture alone does not completely

describe a surface There may be metallurgical changes in the

altered layer 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

occur during processing which may influence

the performance of the finished part or product.

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- Evaluation of Surface Integrity

Surface integrity is more difficult to assess than surfaceroughness. Some techniques to inspect for subsurfacechanges are destructive to the material specimen.

Evaluation techniques include the following:

1- Surface texture: Surface roughness, designation of lay,

and other measures provide superficial data on surfaceintegrity.

2- Visual examination: Can reveal various surface flawssuch as cracks, craters, laps, and seams. It is augmented by

fluorescent and photographic techniques.

3- Microstructural examination: This involves preparingcross sections and obtaining photomicrographs forexamination of microstructure in the surface layerscompared with the substrate.

Surface Integrity

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Surface Integrity4- Microhardness profile: The part is sectioned, and

hardness is plotted against distance below the surface to

obtain a hardness profile of the cross section.

5- Residual stress profile: X-ray diffraction techniques can

be employed to measure residual stresses in the surface

layers of a part.

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