surface roughness for mdcg
TRANSCRIPT
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Surface Finish An engineering component may be cast,
forged, drawn, welded or stamped, etc.
All the surfaces may not have functionalrequirements and need not be equallyfinished
Some surfaces (owing to their functionalrequirements) need additional machiningthat needs to be recorded on the drawing
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Surface RoughnessSurface Roughness
The geometrical characteristics of a surface include,
1. Macro-deviations,
2. Surface waviness, and
3. Micro-irregularities.
The surface roughness is evaluated by the height, Rt and mean
roughness index Ra of the micro-irregularities.
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Surface roughness number
Represents the average departure of the surface
from perfection over a prescribed samplinglength, (usually selected as 0.8 mm)
Surface roughness number (Ra
) is expressed inmicrons.
Ra = (h1+h2+-----+hn)/n
The measurements are usually made along a line,running at right angle to the general direction oftool marks on the surface.
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Actual profile, AfIt is the profile of the actual surface obtained by finishingoperation.
Reference profile, RfIt is the profile to which the irregularities of the surface isreferred to. it passes through the highest point of the actualprofile.
Datum profile, DfIt is the profile, parallel to the reference profile .it passesthrough the lowest point B of the actual profile
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Mean Profile, Mf
It is that profile, within the sampling length chosen (L)such that the sum of the material-filled areas enclosedabove it by the actual profile is equal to the sum of thematerial void area enclosed below it by the profile.
Peak to valley height, Rt It is the distance from the datum profile to the referenceprofile.
Mean roughness index, Ra
It is the arithmetic mean of the absolute value of thehighest hibetween the actual and mean profile.
Ra = 1/Lx=0 |hi| dx , where L is sampling lengthx=L
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Readings
Either arithmetic average
roughness height (Ra) or
root mean square (Rq)
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Average surface roughness producedby standard machining processes
Microinches Micrometers
Turning 100250 2.56.3Drilling 100200 2.55.1
Reaming 50150 1.33.8
Grinding 20100 0.52.5
Honing 520 0.130.5Lapping 110 0.0250.254
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Surface Roughness expected from manufacturing processes
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Surface Roughness expected from manufacturing processes
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Measurement of roughnessThe roughness may be measured, using any of the following :
1. Straight edge
2. Surface gauge3. Optical flat
4. Tool markers microscope
5. Profilometer
6. Profilograph7. Talysurf
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INDICATION OF SURFACE TEXTURE
The basic symbol consists of two legs ofunequal length inclined at approximately
60 to the line representing the considered
surfaceThe symbol must be represented by thin line
If the removal of material by machining isrequired, a bar is added to the basic symbol,
If the removal of material is not permitted,
a circle is added to the basic symbol.
When special surface characteristics have to
be indicated, a line is added to the longer arm of
any of the above symbols,
Basic symbol : only be used alone when its meaning is explained by a note
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Indication of Surface Roughness
The value or values defining the principal criterion of roughness
are added to the symbols
a- surface roughness value
If it is necessary to impose maximum
and minimum limits of the principal criterionof surface roughness, both values shall be
shown
maximum limit (a1) ;minimum limit (a2).
Roughness a obtained byany production process
Roughness a obtained byremoval of material bymachining
Roughness a shall beobtained without removalof any material
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If it is required that the required surface texture be produced by one
particular production method, this method shall be indicated in plain
language on an extension of the longer arm of the symbol
Indication of machining allowance where it is necessary to specify the value
of the machining allowance, this shall be indicated on the left of the symbols.
This value shall be expressed in millimeters.
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Generally to indicate the surface roughness, the symbol is used instead of value.
The relation is given in following table.
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Machining Symbols
This symbol may also be used in a drawing, relating to a production process , to indicate thata surface is to be left in the state ,resulting from a preceding manufacturing process,
whether this state was achieved by removal of material or otherwise
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Position of the Specifications of the Surface Texture in the Symbol - The
specifications Of surface texture shall be placed relative to the symbol as
shown in figure.
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symbol Interpretation
Parallel to the plane of projectionof the view in which the symbol is used
Perpendicular to the plane of projectionof the
view in which the symbol is used
Crossed in two slant direction relative tothe plane of projection of the view inwhich the
symbol is used
Multidirectional
Approximately circular relative to thecentre of the surface to which the symbolis applied
Approximately radial relative to thecentre of the surface to which the symbolis applied
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Symbols with Additional Indications.
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If it is necessary to define surface texture
both before and after treatment, this shall
be explained in a suitable note or inaccordance with figure
The direction of lay is the direction of the
predominant surface pattern, ordinarily determined
by the production method employed.
If it is necessary to control the direction
of lay, it is specified by a symbol added to thesurface texture symbol
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Ways to represent roughness
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Surface finish grade is shown
Surface finish value is shown
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Separate note is written
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Surface finish grades specified
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