Unit4 Surface Texture

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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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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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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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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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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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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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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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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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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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.

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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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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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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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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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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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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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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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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)

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(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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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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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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(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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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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Unit4 Surface Texture

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Transcript of Unit4 Surface Texture