workshop technology 2, chapter 3
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J3103/4/1 SURFACE TEXTURE
General Objectives: To understand the importance of surface texture inengineering.To understand the methods of calculating the surfaceroughness.
Specific Objectives : At the end of this unit you will be able to:
Ø Identify the surface finish symbols that appear on adrawing.
Ø Identify the surface texture terms/ definitions.
Ø Calculate the arithmetic mean value, Ra.
Ø Calculate the root-mean-square average, Rq.
Ø Calculate the maximum roughness height, Rt.
Ø Compare Ra and Rq.
UNIT 4 SURFACE TEXTURE
OBJECTIVES
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J3103/4/2 SURFACE TEXTURE
4.0 DEFINITION
Surface Texture is defined as a degree of finish conveyed to the machinistby a system of symbols devised by a Standards Association, eg. ASA – AmericanStandards Association, BS – British Standards
Modern technology has demanded improved surface finishes to ensureproper functioning and long life of machine parts. Pistons, bearings, and gearsdepend to a great extent on a good surface finish for proper functioning and
therefore, require little or no break-in period. Finer finishes often requireadditional operation, such as lapping or honing. The higher finishes are notalways required on parts and only result in higher production costs. To preventoverfinishing a part, the desired finish is indicated on the shop drawing.Information specifying the degree of finish is conveyed to the machinist by asystem of symbols devised by Standards Associations, eg. ASA American
Standards Association and BS British Standards. These symbols provide astandard system of determining and indicating surface finish. The inch unit forsurface finish measurement is microinch (µin), while the metric unit ismicrometer (µm)
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J3103/4/3 SURFACE TEXTURE
4.1. SURFACE TEXTURE TERMS AND DEFINITIONS
Surface profile Error of form Waviness Roughness
Regardless of the method of production, all surfaces have their owncharacteristics, which are collectively referred to as surface texture, Fig. 4.1.Certain guidelines have been established to identify surface texture in terms ofwell-defined and measurable quantities (Figure 4.2)
4.1.1. Flaws
Flaws or defects, are random irregularities, such as scratches,cracks, holes, depression, seams, tears or inclusions. These defects can becaused during the machining or production process such as moulding,
Figure 4.1. Standard terminology and symbols to describe surface finish
Laydirection
Wavinesswidth
Roughnessspacing
RoughnessHeight, Rt
Wavinessheight
Flaw
Roughnesswidth cutoff
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J3103/4/4 SURFACE TEXTURE
drawing, forging, machining, eg, holes cause by air bubbles duringcasting, crack and tears by forging and drawing process.
4.1.2. Lay
Lay or directionality, is the direction of the predominant surfacepattern caused by the machining process and it is usually visible to thenaked eye.
4.1.3. Roughness
Roughness is defined as closely spaced, irregular deviation on ascale smaller than that of waviness. It is caused by the cutting tool or theabrasive grain action and the machine feed. Roughness may besuperimposed on waviness.
4.1.3.1. Roughness height
Roughness height, Ra is the deviation to the centre line inmicro inches or micrometers.
4.1.3.2. Roughness width
Roughness Width is the distance between successiveroughness peaks parallel to the nominal surface in inches ormillimeters.
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J3103/4/5 SURFACE TEXTURE
4.1.4. Waviness
Waviness is a recurrent deviation from a flat surface, much like
waves on the surface of water. It is measured and described in terms ofthe surface between adjacent crests of the waves (waviness width) andheight between the crests and valleys of the waves (waviness height).Waviness can be caused by:
a) deflection of tools, dies or the work pieceb) force or temperature sufficient to cause warping
c) uneven lubricationd) vibratione) any periodic mechanical or thermal variations on the system
during manufacturing operations.
4.1.5. Profile
The contour of a specified section through a surface.
4.1.6. Microinch and micrometer
The unit of measurement used to measure surface finish. The
microinch is equal to 0.000 001 inch and the micrometer equals to0.000 001 meter.
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J3103/4/6 SURFACE TEXTURE
4.2. STANDARD SYMBOLS TO DESCRIBE SURFACE TEXTURE/FINISH
0.02 – 26.31.6 0.01
Figure 4.2 A sample of a surface texture/finish designation
Symbols’ definition:0.02 – Maximum waviness height (mm)2 - Maximum waviness width (mm)6.3 - Maximum roughness height (mm)
1.6 - Minimum roughness height (mm)
0.01 - Maximum roughness width (mm)- Lay symbol (Lay perpendicular to the line
representing the surface to which the symbol isapplied)
Sometimes, the roughness number is used as a substitute for theroughness value eg. N7 is equals to 1.6 µm, (Table. 4.1). Table 4.2 shows anaverage surface roughness produced by standard machining processes.
Table 4.1. Roughness number and value
µm 50 25 12.5 6.3 3.2 1.6 0.8 0.4 0.2 0.1 0.05 0.025Roughnessnumber N12 N11 N10 N9 N8 N7 N6 N5 N4 N3 N2 N1
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J3103/4/7 SURFACE TEXTURE
Table 4.2 Average surface roughness produced by standard machiningprocesses
PROCESS MICROINCHES MICROMETERSTurning 100 - 250 2.5 - 6.3Drilling 100 - 200 2.5 - 5.1Reaming 50 - 150 1.3 - 3.8Grinding 20 - 100 0.5 - 2.5Honing 5 - 20 0.13 - 0.5Lapping 1 - 10 0.025 - 0.254
4.3. SYMBOLS FOR SURFACE ROUGHNESS
The following symbols indicate the direction of the lay (Table 4.3)
LaySymbol
Interpretation Examples
=Lay parallel to the line representing the surfaceto which the symbol is applied
Lay perpendicular to the line representing thesurface to which the symbol is applied.
Lay angular and both direction to linerepresenting the surface to which symbol isapplied
Lay multidirectional
X
M
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J3103/4/8 SURFACE TEXTURE
LaySymbol
Interpretation Examples
Lay approximately circular relative to the centreof the surface to which the symbol is applied
Lay approximately radial relative to the centreof the surface to which the symbol is applied
Pitted, protuberant, porous, or particulate non-directional lay
Figure 4.3. Standard lay symbols for engineering surfaces
C
R
P
C
P
R
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J3103/4/9 SURFACE TEXTURE
TEST YOUR UNDERSTANDING BEFORE CONTINUING TO THENEXT INPUT…!
4.1. Explain why present-day standards relating to surface texture arevery important to industry .
4.2. List and explain the types of defects found on surfaces.
4.3. Explain the following terms:
a) roughnessb) wavinessc) lay
ACTIVITY 4A
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J3103/4/10 SURFACE TEXTURE
4.1. The present-day standards such as American StandardsAssociation (ASA) and British Standards Association (BSA)
specifies the degree of finish on the drawing which is then conveyedto the machinist. This is to prevent over finishing a component orpart and to be more cost-effective.
.4.2. Types of defects found on surfaces are scratches, cracks, holes,
depression, seams, tears or inclusions. These defects can be caused
during the machining or production process such as moulding,drawing, forging, machining, eg, holes in casting can be caused byair bubbles during casting, crack and tears by forging and drawingprocesses.
4.3. a) Roughness
Roughness is defined as closely spaced, irregular deviation on ascale smaller than that of waviness. It is caused by the cutting toolor the abrasive grain action and the machine feed. Roughness maybe superimposed on waviness.
FEEDBACK ON ACTIVITY 4A
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J3103/4/11 SURFACE TEXTURE
b) WavinessWaviness is a recurrent deviation from a flat surface, much likewaves on the surface of water. It is measured and described in
terms of the surface between adjacent crests of the waves (wavinesswidth) and height between the crests and valleys of the waves(waviness height).
c) LayLay or directionality, is the direction of the predominant surfacepattern caused by the machining process and it is usually visible tothe naked eye.
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J3103/4/12 SURFACE TEXTURE
4.4 SURFACE ROUGHNESS
Surface roughness is generally described in two methods: arithmetic meanvalue and root-mean-square average.
4.4.1 The Arithmetic Mean Value, Ra.Ra, formerly identified as AA for arithmetic average or CLA for
centre-line average is based on the schematic illustration of a roughsurface, which is shown in (Figure 4.4). The arithmetic mean value, Ra, isdefined as
Ra =n
fedcba ...++++++ (4.4.1)
Where, all ordinates, a, b, c, …, are absolute values, and n is the numberof readings
4.4.2. The Root-Mean-Square Average, Rq.Rq, formerly identified as RMS is defined as
Rq =n
dcba ...2222 ++++ (4.4.2)
The datum line AB in figure 4.4 is located so that the sum of thearea above the line is equal to the sum of the areas below the line. Theunits generally used for surface roughness are µm (micrometer, or micron)or µin (microinch). ( Note, 1µm = 40 µin and 1µin = 0.025 µm ).
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J3103/4/13 SURFACE TEXTURE
3.4.3. Maximum Roughness Height, Rt
Maximum roughness height, Rt, is defined as the height from thedeepest trough to the highest peak. It indicates how much material has tobe removed in order to obtain a smooth surface by polishing or othermeans
Rt =5
)()( 10864297531 hhhhhhhhhh ++++-++++ x (4.4.3)
h1 h3 h6 h5 h7 h9
h2 h4 h8 h10
A f g h i j k B
a b c d e
Centre line (datum line)
Figure 4.3. Coordinates used for surface – roughness using equations 4.4.1 &4.4.2
Figure 4.4. Maximum roughness height (Rt)
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J3103/4/14 SURFACE TEXTURE
Where,h1, h2…......hn - height of ordinates in mm
M - magnification
4.5. COMPARISON OF Ra AND Rq
The arithmetic mean value, Ra was adopted internationally in the mid-1950s and is used widely in engineering practice. Equations 4.4.1 and 4.4.2
show that there is a relationship between Rq and Ra, as shown by the ratioRaRq .
The table 4.4 below gives this ratio for various surfaces:
Table 4.4 Ratio for various surfaces
SurfaceRaRq
Sine Curve 1.1Machining by cutting 1.1Grinding 1.2Lapping and honing 1.4
In general, a surface cannot be describe by its Ra and Rq value alone,
since these values are averages. Two surfaces may have the same roughnessvalue but have actual topography which is very different. A few deep troughs onan otherwise smooth surface, for example, do not affect the roughness valuessignificantly. However, the type of surface profile can be significant in terms offriction, wear and fatigue characteristics of a manufactured product.
It is therefore, important to analyze the surface in great detail,
particularly for parts used in critical applications. Some 130 parameters havebeen identified thus far for measuring surface roughness.
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J3103/4/15 SURFACE TEXTURE
4.6 USING A SURFACE TEXTURE COMPARATOR
A more elaborate device for measuring surface finish is the surface
analyzer (prolimeter surface measuring instrument). It utilizes a recordingdevice to reproduce the surface irregularities on a graduated chart, providing anink-line record.
Although the surface indicator is the most common, other methods may beused to measure surface finish with reasonable accuracy during machiningprocesses, including:
4.6.1. Comparison blocks.
Comparison blocks which are use for comparing the finish on thework piece with the calibrated finish on a test block. Surface roughnesscan be measured by comparison inspection or by direct measurement. In
comparison inspection the surface roughness can be estimated bymatching a work piece surface with blocks that display a series ofincreasingly smooth surfaces, Fig. 4.5.
4.6.2. Commercial sets of standard finished specimens.
Commercial sets of standard finished specimens, which have up to
25 different surface finish samples. They consist of blocks or plateshaving surface varying from the smoothest to the roughest likely to berequired.
These specimens ( Fig. 4.5. ) are used to check the finish of themachined part against the sample finish to determine approximately thefinish produced of the part. It is often difficult to determine the finish
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J3103/4/16 SURFACE TEXTURE
visually. In such cases, the surfaces may be compared by moving the tip ofyour fingernail over the two surfaces.
Figure 4.4. Maximum roughness height (Rt)
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J3103/4/17 SURFACE TEXTURE
TEST YOUR UNDERSTANDING BEFORE CONTINUING WITH THENEXT INPUT…!
4.4. What do Ra, Rq and Rt stand for?
4.5. Describe how you would use the surface roughness comparatorgauge.
ACTIVITY 4B
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J3103/4/18 SURFACE TEXTURE
4.4
( i). The Arithmetic Mean Value, Ra.The arithmetic mean value, Ra, is defined as
Ra =n
fedcba ...++++++
where all ordinates, a, b, c, …, are absolute values, and n is thenumber of readings
( ii). The Root-Mean-Square Average, Rq.
Rq, is defined as
Rq =n
dcba ...2222 ++++
The datum line AB in the figure above is located so that the sum ofthe area above the line is equal to the sum of the areas below theline. The units generally used for surface roughness are µm
A f g h i j k B a b c d e
Centre line (datum)
FEEDBACK ON ACTIVITY 4B
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J3103/4/19 SURFACE TEXTURE
(micrometer, or micron) or µin (microinch). Note, for example 1µm= 40µin and 1µin = 0.025µm.
(iii). Maximum Roughness Height, Rt
Maximum roughness height, Rt, is defined as the height from thedeepest through to the highest peak. It indicates how muchmaterial has to be removed in order to obtain a smooth surface bypolishing or other means
Rt =5
)()( 10864297531 hhhhhhhhhh ++++-++++
h1 h3 h6 h5 h7 h9
h2 h4 h8 h10
4.5. Comparison blocks which are use for comparing the finish on the
workpiece with the calibrated finish on a test block. Surface roughnesscan be measured by comparison inspection or by direct measureement. Incomparison inspection the surface roughness can be estimated bymatching a workpiece surface with blocks that display a series ofincreasingly smooth surfaces.
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J3103/4/20 SURFACE TEXTURE
1. What is meant by surface texture of a component?
a. Name three (3) method of indicating quality or texture number of asurface.
b. State the importance of surface texture in engineering.
2. Sketch a surface profile of a machined surface. Label five (5) main parts.
3. Define the symbol on figure below.
0.03 – 1.5 3.2 1.6
SELF-ASSESSMENT 4
0.01
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J3103/4/21 SURFACE TEXTURE
1. Surface Texture is defined as a degree of finish conveyed to the machinistby a system of symbols.
(i) The Arithmetic Mean Value, Ra.Ra, formerly identified as AA for arithmetic average or CLA for
centre-line average is based on the schematic illustration of a roughsurface, which is shown in figure below. The arithmetic mean value, Ra, isdefined as
Ra =n
fedcba ...++++++
Where, all ordinates, a, b, c, …, are absolute values, and n is the numberof readings
.(ii) The Root-Mean-Square Average, Rq.Rq, formerly identified as RMS is defined as
Rq =n
dcba ...2222 ++++
FEEDBACK OF SELF-ASSESSMENT 4
A f g h i j k B a b c d e
Centre line (datum line)
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J3103/4/22 SURFACE TEXTURE
(iii) Maximum Roughness Height, Rt
Maximum roughness height, Rt, is defined as the height from thedeepest trough to the highest peak. It indicates how much material has tobe removed in order to obtain a smooth surface by polishing or othermeans
Rt =5
)()( 10864297531 hhhhhhhhhh ++++-++++
h1 h3 h6 h5 h7 h9
h2 h4 h8 h10
Maximum roughness height (Rt)
(b) To ensure proper functioning, long life and interchangeability of machineparts.
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J3103/4/23 SURFACE TEXTURE
2.4. 9.
3.. Symbols’ definition:0.02 – Maximum waviness height (mm)2 - Maximum waviness width (mm)6.3 - Maximum roughness height (mm)
1.6 - Minimum roughness height (mm)
0.01 - Maximum roughness width (mm)- Lay symbol (Lay perpendicular to the line
representing the surface to which the symbol isapplied)
Flaw
Waviness height Lay direction
Roughnessheight
Roughness Roughness width Spacing cutoff
Waviness width
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