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                                               UNIT 4                      SURFACE TEXTURE




                                                                            OBJECTIVES




                                           General Objectives:        To understand the importance of surface texture in
                                                                      engineering.
                                                                      To understand the methods of calculating the surface
                                                                      roughness.

                                          Specific Objectives :       At the end of this unit you will be able to:

                                                                  Ø   Identify the surface finish symbols that appear on a
                                                                      drawing.


                                                                  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.
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                                                                    INPUT




                                          4.0   DEFINITION


                                                Surface Texture is defined as a degree of finish conveyed to the machinist
                                          by a system of symbols devised by a Standards Association, eg. ASA – American
                                          Standards Association, BS – British Standards
                                                Modern technology has demanded improved surface finishes to ensure
                                          proper functioning and long life of machine parts. Pistons, bearings, and gears
                                          depend to a great extent on a good surface finish for proper functioning and
                                          therefore, require little or no break-in period.   Finer finishes often require
                                          additional operation, such as lapping or honing. The higher finishes are not
                                          always required on parts and only result in higher production costs. To prevent
                                          overfinishing a part, the desired finish is indicated on the shop drawing.
                                          Information specifying the degree of finish is conveyed to the machinist by a
                                          system of symbols devised by Standards Associations, eg. ASA American
                                          Standards Association and BS British Standards.       These symbols provide a
                                          standard system of determining and indicating surface finish. The inch unit for
                                          surface finish measurement is microinch (µin), while the metric unit is
                                          micrometer (µm)
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                                          4.1.   SURFACE TEXTURE TERMS AND DEFINITIONS

                                                                              Lay
                                                                           direction


                                                                          Flaw
                                                              Waviness
                                                               height




                                                 Roughness
                                                 Height, Rt

                                                                   Roughness                           Roughness
                                                                   spacing                             width cutoff
                                                                                 Waviness
                                                                                  width




                                                 Surface profile         Error of form      Waviness             Roughness

                                                  Figure 4.1. Standard terminology and symbols to describe surface finish


                                                 Regardless of the method of production, all surfaces have their own
                                          characteristics, which are collectively referred to as surface texture, Fig. 4.1.
                                          Certain guidelines have been established to identify surface texture in terms of
                                          well-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 be
                                                 caused during the machining or production process such as moulding,
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                                              drawing, forging, machining, eg, holes cause by air bubbles during
                                              casting, crack and tears by forging and drawing process.


                                                4.1.2.      Lay


                                                     Lay or directionality, is the direction of the predominant surface
                                              pattern caused by the machining process and it is usually visible to the
                                              naked eye.


                                                4.1.3.      Roughness


                                                     Roughness is defined as closely spaced, irregular deviation on a
                                              scale smaller than that of waviness. It is caused by the cutting tool or the
                                              abrasive grain action and the machine feed. Roughness may be
                                              superimposed on waviness.


                                                     4.1.3.1.       Roughness height


                                                            Roughness height, Ra is the deviation to the centre line in
                                                     micro inches or micrometers.


                                                     4.1.3.2.       Roughness width


                                                            Roughness Width is the distance between successive
                                                     roughness peaks parallel to the nominal surface in inches or
                                                     millimeters.
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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 of
                                              the surface between adjacent crests of the waves (waviness width) and
                                              height between the crests and valleys of the waves (waviness height).
                                              Waviness can be caused by:
                                                    a) deflection of tools, dies or the work piece
                                                    b) force or temperature sufficient to cause warping
                                                    c) uneven lubrication
                                                    d) vibration
                                                    e) 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 to
                                                    0.000 001 meter.
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                                          4.2. STANDARD SYMBOLS TO DESCRIBE SURFACE TEXTURE/FINISH



                                                                           0.02 – 2
                                                                     6.3
                                                                     1.6         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 is
                                                                           applied)


                                                Sometimes, the roughness number is used as a substitute for the
                                          roughness value eg. N7 is equals to 1.6 µm, (Table. 4.1). Table 4.2 shows an
                                          average 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 0.025
                                          Roughness
                                                    N12 N11 N10            N9     N8    N7    N6    N5    N4    N3    N2     N1
                                          number
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                                                   Table 4.2 Average surface roughness produced by standard machining
                                                                                processes

                                                         PROCESS          MICROINCHES          MICROMETERS
                                                         Turning             100 - 250             2.5 - 6.3
                                                         Drilling            100 - 200             2.5 - 5.1
                                                         Reaming              50 - 150             1.3 - 3.8
                                                         Grinding             20 - 100             0.5 - 2.5
                                                         Honing                5 - 20             0.13 - 0.5
                                                         Lapping               1 - 10           0.025 - 0.254




                                          4.3. SYMBOLS FOR SURFACE ROUGHNESS

                                                       The following symbols indicate the direction of the lay (Table 4.3)

                                          Lay                       Interpretation                          Examples
                                          Symbol



                                                   Lay parallel to the line representing the surface

                                           =       to which the symbol is applied



                                                   Lay perpendicular to the line representing the
                                                   surface to which the symbol is applied.


                                                   Lay angular and both direction to line
                                                   representing the surface to which symbol is
                                           X       applied



                                           M       Lay multidirectional
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                                          Lay                       Interpretation                        Examples
                                          Symbol


                                                   Lay approximately circular relative to the centre
                                           C       of the surface to which the symbol is applied                     C



                                                                                                                         R
                                                   Lay approximately radial relative to the centre
                                           R       of the surface to which the symbol is applied



                                                   Pitted, protuberant, porous, or particulate non-
                                           P       directional lay
                                                                                                                      P




                                                   Figure 4.3. Standard lay symbols for engineering surfaces
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                                                                     ACTIVITY 4A


                                              TEST YOUR UNDERSTANDING BEFORE CONTINUING TO THE
                                              NEXT INPUT…!



                                              4.1.   Explain why present-day standards relating to surface texture are
                                                     very important to industry .


                                              4.2.   List and explain the types of defects found on surfaces.


                                              4.3.   Explain the following terms:
                                                     a) roughness
                                                     b) waviness
                                                     c) lay
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                                                                  FEEDBACK ON ACTIVITY 4A



                                              4.1.   The    present-day   standards    such   as   American     Standards
                                                     Association (ASA) and British Standards Association (BSA)
                                                     specifies the degree of finish on the drawing which is then conveyed
                                                     to the machinist. This is to prevent over finishing a component or
                                                     part 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 by
                                                     air bubbles during casting, crack and tears by forging and drawing
                                                     processes.


                                              4.3.   a) Roughness
                                                     Roughness is defined as closely spaced, irregular deviation on a
                                                     scale smaller than that of waviness. It is caused by the cutting tool
                                                     or the abrasive grain action and the machine feed. Roughness may
                                                     be superimposed on waviness.
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                                                    b) 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 of the surface between adjacent crests of the waves (waviness
                                                    width) and height between the crests and valleys of the waves
                                                    (waviness height).


                                                    c) Lay
                                                    Lay or directionality, is the direction of the predominant surface
                                                    pattern caused by the machining process and it is usually visible to
                                                    the naked eye.
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                                                                               INPUT




                                          4.4 SURFACE ROUGHNESS

                                                Surface roughness is generally described in two methods: arithmetic mean
                                          value 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 rough
                                                surface, which is shown in (Figure 4.4). The arithmetic mean value, Ra, is
                                                defined as

                                                            a + b + c + d + e + f + ...
                                                      Ra =                                      (4.4.1)
                                                                        n
                                                Where, all ordinates, a, b, c, …, are absolute values, and n is the number
                                                of readings

                                                4.4.2. The Root-Mean-Square Average, Rq.
                                                       Rq, formerly identified as RMS is defined as


                                                           a 2 + b 2 + c 2 + d 2 +...
                                                      Rq =                                    (4.4.2)
                                                                       n



                                                       The datum line AB in figure 4.4 is located so that the sum of the
                                                area above the line is equal to the sum of the areas below the line. The
                                                units 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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                                                         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


                                                3.4.3. Maximum Roughness Height, Rt

                                                         Maximum roughness height, Rt, is defined as the height from the
                                                deepest trough to the highest peak. It indicates how much material has to
                                                be removed in order to obtain a smooth surface by polishing or other
                                                means


                                                                (h1 + h3 + h5 + h7 + h9 ) - (h2 + h4 + h6 + h8 + h10 )
                                                         Rt =                                                          x (4.4.3)
                                                                                          5




                                                    h1             h3                h6           h5               h7                     h9

                                                             h2              h4                    h8             h10




                                                                  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
                                                                                                                           Rq
                                          show that there is a relationship between Rq and Ra, as shown by the ratio          .
                                                                                                                           Ra
                                          The table 4.4 below gives this ratio for various surfaces:


                                                                 Table 4.4 Ratio for various surfaces

                                                                                                Rq
                                                                        Surface
                                                                                                Ra
                                                             Sine Curve                         1.1
                                                             Machining by cutting               1.1
                                                             Grinding                           1.2
                                                             Lapping 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 roughness
                                          value but have actual topography which is very different. A few deep troughs on
                                          an otherwise smooth surface, for example, do not affect the roughness values
                                          significantly. However, the type of surface profile can be significant in terms of
                                          friction, 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 have
                                          been identified thus far for measuring surface roughness.
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                                          SURFACE TEXTURE
      C




                                                                                                                                      C
      w                        om                                                                                                     w                        om
  w




                                                                                                                                  w
          w.                                                                                                                              w.
               A B B Y Y.c                                                                                                                     A B B Y Y.c




                                          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 recording
                                          device to reproduce the surface irregularities on a graduated chart, providing an
                                          ink-line record.
                                                Although the surface indicator is the most common, other methods may be
                                          used to measure surface finish with reasonable accuracy during machining
                                          processes, including:


                                                4.6.1. Comparison blocks.


                                                       Comparison blocks which are use for comparing the finish on the
                                                work piece with the calibrated finish on a test block. Surface roughness
                                                can be measured by comparison inspection or by direct measurement. In
                                                comparison inspection the surface roughness can be estimated by
                                                matching a work piece surface with blocks that display a series of
                                                increasingly 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 plates
                                                having surface varying from the smoothest to the roughest likely to be
                                                required.
                                                       These specimens ( Fig. 4.5. ) are used to check the finish of the
                                                machined part against the sample finish to determine approximately the
                                                finish produced of the part. It is often difficult to determine the finish
F T ra n sf o                                                                                                              F T ra n sf o
          PD                   rm                                                                                                    PD                   rm
      Y                                                                                                                          Y
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                he




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      C




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      w                        om                                                                                                w                        om
  w




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          w.                                                                                                                         w.
               A B B Y Y.c                                                                                                                A B B Y Y.c




                                              visually. In such cases, the surfaces may be compared by moving the tip of
                                              your fingernail over the two surfaces.




                                                            Figure 4.4. Maximum roughness height (Rt)
F T ra n sf o                                                                                                         F T ra n sf o
          PD                   rm                                                                                               PD                   rm
      Y                                                                                                                     Y
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                                                                                                                       Y
                                er




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ABB




                                                                                                                      ABB
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                       bu




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                     to




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                he




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           k




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          lic




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                                          SURFACE TEXTURE
      C




                                                                                                                            C
      w                        om                                                                                           w                        om
  w




                                                                                                                        w
          w.                                                                                                                    w.
               A B B Y Y.c                                                                                                           A B B Y Y.c




                                                                      ACTIVITY 4B


                                              TEST YOUR UNDERSTANDING BEFORE CONTINUING WITH THE
                                              NEXT INPUT…!



                                              4.4.   What do Ra, Rq and Rt stand for?

                                              4.5.   Describe how you would use the surface roughness comparator
                                                     gauge.
F T ra n sf o                                                                                                                F T ra n sf o
          PD                   rm                                                                                                      PD                   rm
      Y                                                                                                                            Y
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                                                                                                                              Y
                                er




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ABB




                                                                                                                             ABB
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                       bu




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                                    2.0




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                he




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           k




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          lic




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                                          SURFACE TEXTURE
      C




                                                                                                                                   C
      w                        om                                                                                                  w                        om
  w




                                                                                                                               w
          w.                                                                                                                           w.
               A B B Y Y.c                                                                                                                  A B B Y Y.c




                                                                           FEEDBACK ON ACTIVITY 4B


                                          4.4


                                                       A                 f g h i j k                                   B
                                                           a      b c d e

                                                                                             Centre line (datum)



                                          ( i). The Arithmetic Mean Value, Ra.
                                          The arithmetic mean value, Ra, is defined as


                                                               a + b + c + d + e + f + ...
                                                      Ra =
                                                                           n
                                                      where all ordinates, a, b, c, …, are absolute values, and n is the
                                                      number of readings


                                          ( ii). The Root-Mean-Square Average, Rq.
                                                      Rq, is defined as


                                                                a 2 + b 2 + c 2 + d 2 +...
                                                      Rq =
                                                                            n

                                                      The datum line AB in the figure above is located so that the sum of
                                                      the area above the line is equal to the sum of the areas below the
                                                      line. The units generally used for surface roughness are µm
F T ra n sf o                                                                                                                                    F T ra n sf o
          PD                   rm                                                                                                                          PD                   rm
      Y                                                                                                                                                Y
 Y




                                                                                                                                                  Y
                                er




                                                                                                                                                                                 er
ABB




                                                                                                                                                 ABB
                          y




                                                                                                                                                                           y
                       bu




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                                    2.0




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                     to




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                he




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      C




                                                                                                                                                       C
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          w.                                                                                                                                               w.
               A B B Y Y.c                                                                                                                                      A B B Y Y.c



                                                          (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 the
                                                          deepest through to the highest peak. It indicates how much
                                                          material has to be removed in order to obtain a smooth surface by
                                                          polishing or other means
                                                                 (h1 + h3 + h5 + h7 + h9 ) - (h2 + h4 + h6 + h8 + h10 )
                                                          Rt =
                                                                                           5




                                                     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 roughness
                                                can be measured by comparison inspection or by direct measureement. In
                                                comparison inspection the surface roughness can be estimated by
                                                matching a workpiece surface with blocks that display a series of
                                                increasingly smooth surfaces.
F T ra n sf o                                                                                                                 F T ra n sf o
          PD                   rm                                                                                                       PD                   rm
      Y                                                                                                                             Y
 Y




                                                                                                                               Y
                                er




                                                                                                                                                              er
ABB




                                                                                                                              ABB
                          y




                                                                                                                                                        y
                       bu




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                                    2.0




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                     to




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                  re




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                he




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           k




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          lic




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                                          SURFACE TEXTURE
      C




                                                                                                                                    C
      w                        om                                                                                                   w                        om
  w




                                                                                                                                w
          w.                                                                                                                            w.
               A B B Y Y.c                                                                                                                   A B B Y Y.c




                                                                        SELF-ASSESSMENT 4




                                          1.      What is meant by surface texture of a component?

                                                    a. Name three (3) method of indicating quality or texture number of a
                                                       surface.

                                                    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
                                                                                0.01
                                                                      1.6
Unit4 Surface Texture
Unit4 Surface Texture
Unit4 Surface Texture

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Unit4 Surface Texture

  • 1. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/1 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c UNIT 4 SURFACE TEXTURE OBJECTIVES General Objectives: To understand the importance of surface texture in engineering. To understand the methods of calculating the surface roughness. Specific Objectives : At the end of this unit you will be able to: Ø Identify the surface finish symbols that appear on a drawing. 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.
  • 2. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/2 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c INPUT 4.0 DEFINITION Surface Texture is defined as a degree of finish conveyed to the machinist by a system of symbols devised by a Standards Association, eg. ASA – American Standards Association, BS – British Standards Modern technology has demanded improved surface finishes to ensure proper functioning and long life of machine parts. Pistons, bearings, and gears depend to a great extent on a good surface finish for proper functioning and therefore, require little or no break-in period. Finer finishes often require additional operation, such as lapping or honing. The higher finishes are not always required on parts and only result in higher production costs. To prevent overfinishing a part, the desired finish is indicated on the shop drawing. Information specifying the degree of finish is conveyed to the machinist by a system of symbols devised by Standards Associations, eg. ASA American Standards Association and BS British Standards. These symbols provide a standard system of determining and indicating surface finish. The inch unit for surface finish measurement is microinch (µin), while the metric unit is micrometer (µm)
  • 3. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/3 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 4.1. SURFACE TEXTURE TERMS AND DEFINITIONS Lay direction Flaw Waviness height Roughness Height, Rt Roughness Roughness spacing width cutoff Waviness width Surface profile Error of form Waviness Roughness Figure 4.1. Standard terminology and symbols to describe surface finish Regardless of the method of production, all surfaces have their own characteristics, which are collectively referred to as surface texture, Fig. 4.1. Certain guidelines have been established to identify surface texture in terms of well-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 be caused during the machining or production process such as moulding,
  • 4. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/4 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c drawing, forging, machining, eg, holes cause by air bubbles during casting, crack and tears by forging and drawing process. 4.1.2. Lay Lay or directionality, is the direction of the predominant surface pattern caused by the machining process and it is usually visible to the naked eye. 4.1.3. Roughness Roughness is defined as closely spaced, irregular deviation on a scale smaller than that of waviness. It is caused by the cutting tool or the abrasive grain action and the machine feed. Roughness may be superimposed on waviness. 4.1.3.1. Roughness height Roughness height, Ra is the deviation to the centre line in micro inches or micrometers. 4.1.3.2. Roughness width Roughness Width is the distance between successive roughness peaks parallel to the nominal surface in inches or millimeters.
  • 5. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/5 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 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 of the surface between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height). Waviness can be caused by: a) deflection of tools, dies or the work piece b) force or temperature sufficient to cause warping c) uneven lubrication d) vibration e) 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 to 0.000 001 meter.
  • 6. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/6 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 4.2. STANDARD SYMBOLS TO DESCRIBE SURFACE TEXTURE/FINISH 0.02 – 2 6.3 1.6 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 is applied) Sometimes, the roughness number is used as a substitute for the roughness value eg. N7 is equals to 1.6 µm, (Table. 4.1). Table 4.2 shows an average 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 0.025 Roughness N12 N11 N10 N9 N8 N7 N6 N5 N4 N3 N2 N1 number
  • 7. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/7 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Table 4.2 Average surface roughness produced by standard machining processes PROCESS MICROINCHES MICROMETERS Turning 100 - 250 2.5 - 6.3 Drilling 100 - 200 2.5 - 5.1 Reaming 50 - 150 1.3 - 3.8 Grinding 20 - 100 0.5 - 2.5 Honing 5 - 20 0.13 - 0.5 Lapping 1 - 10 0.025 - 0.254 4.3. SYMBOLS FOR SURFACE ROUGHNESS The following symbols indicate the direction of the lay (Table 4.3) Lay Interpretation Examples Symbol Lay parallel to the line representing the surface = to which the symbol is applied Lay perpendicular to the line representing the surface to which the symbol is applied. Lay angular and both direction to line representing the surface to which symbol is X applied M Lay multidirectional
  • 8. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/8 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Lay Interpretation Examples Symbol Lay approximately circular relative to the centre C of the surface to which the symbol is applied C R Lay approximately radial relative to the centre R of the surface to which the symbol is applied Pitted, protuberant, porous, or particulate non- P directional lay P Figure 4.3. Standard lay symbols for engineering surfaces
  • 9. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/9 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c ACTIVITY 4A TEST YOUR UNDERSTANDING BEFORE CONTINUING TO THE NEXT INPUT…! 4.1. Explain why present-day standards relating to surface texture are very important to industry . 4.2. List and explain the types of defects found on surfaces. 4.3. Explain the following terms: a) roughness b) waviness c) lay
  • 10. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/10 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c FEEDBACK ON ACTIVITY 4A 4.1. The present-day standards such as American Standards Association (ASA) and British Standards Association (BSA) specifies the degree of finish on the drawing which is then conveyed to the machinist. This is to prevent over finishing a component or part 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 by air bubbles during casting, crack and tears by forging and drawing processes. 4.3. a) Roughness Roughness is defined as closely spaced, irregular deviation on a scale smaller than that of waviness. It is caused by the cutting tool or the abrasive grain action and the machine feed. Roughness may be superimposed on waviness.
  • 11. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/11 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c b) 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 of the surface between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height). c) Lay Lay or directionality, is the direction of the predominant surface pattern caused by the machining process and it is usually visible to the naked eye.
  • 12. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/12 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c INPUT 4.4 SURFACE ROUGHNESS Surface roughness is generally described in two methods: arithmetic mean value 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 rough surface, which is shown in (Figure 4.4). The arithmetic mean value, Ra, is defined as a + b + c + d + e + f + ... Ra = (4.4.1) n Where, all ordinates, a, b, c, …, are absolute values, and n is the number of readings 4.4.2. The Root-Mean-Square Average, Rq. Rq, formerly identified as RMS is defined as a 2 + b 2 + c 2 + d 2 +... Rq = (4.4.2) n The datum line AB in figure 4.4 is located so that the sum of the area above the line is equal to the sum of the areas below the line. The units generally used for surface roughness are µm (micrometer, or micron) or µin (microinch). ( Note, 1µm = 40 µin and 1µin = 0.025 µm ).
  • 13. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/13 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 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 3.4.3. Maximum Roughness Height, Rt Maximum roughness height, Rt, is defined as the height from the deepest trough to the highest peak. It indicates how much material has to be removed in order to obtain a smooth surface by polishing or other means (h1 + h3 + h5 + h7 + h9 ) - (h2 + h4 + h6 + h8 + h10 ) Rt = x (4.4.3) 5 h1 h3 h6 h5 h7 h9 h2 h4 h8 h10 Figure 4.4. Maximum roughness height (Rt)
  • 14. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/14 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 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 Rq show that there is a relationship between Rq and Ra, as shown by the ratio . Ra The table 4.4 below gives this ratio for various surfaces: Table 4.4 Ratio for various surfaces Rq Surface Ra Sine Curve 1.1 Machining by cutting 1.1 Grinding 1.2 Lapping 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 roughness value but have actual topography which is very different. A few deep troughs on an otherwise smooth surface, for example, do not affect the roughness values significantly. However, the type of surface profile can be significant in terms of friction, 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 have been identified thus far for measuring surface roughness.
  • 15. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/15 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 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 recording device to reproduce the surface irregularities on a graduated chart, providing an ink-line record. Although the surface indicator is the most common, other methods may be used to measure surface finish with reasonable accuracy during machining processes, including: 4.6.1. Comparison blocks. Comparison blocks which are use for comparing the finish on the work piece with the calibrated finish on a test block. Surface roughness can be measured by comparison inspection or by direct measurement. In comparison inspection the surface roughness can be estimated by matching a work piece surface with blocks that display a series of increasingly 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 plates having surface varying from the smoothest to the roughest likely to be required. These specimens ( Fig. 4.5. ) are used to check the finish of the machined part against the sample finish to determine approximately the finish produced of the part. It is often difficult to determine the finish
  • 16. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/16 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c visually. In such cases, the surfaces may be compared by moving the tip of your fingernail over the two surfaces. Figure 4.4. Maximum roughness height (Rt)
  • 17. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/17 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c ACTIVITY 4B TEST YOUR UNDERSTANDING BEFORE CONTINUING WITH THE NEXT INPUT…! 4.4. What do Ra, Rq and Rt stand for? 4.5. Describe how you would use the surface roughness comparator gauge.
  • 18. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/18 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c FEEDBACK ON ACTIVITY 4B 4.4 A f g h i j k B a b c d e Centre line (datum) ( i). The Arithmetic Mean Value, Ra. The arithmetic mean value, Ra, is defined as a + b + c + d + e + f + ... Ra = n where all ordinates, a, b, c, …, are absolute values, and n is the number of readings ( ii). The Root-Mean-Square Average, Rq. Rq, is defined as a 2 + b 2 + c 2 + d 2 +... Rq = n The datum line AB in the figure above is located so that the sum of the area above the line is equal to the sum of the areas below the line. The units generally used for surface roughness are µm
  • 19. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/19 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c (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 the deepest through to the highest peak. It indicates how much material has to be removed in order to obtain a smooth surface by polishing or other means (h1 + h3 + h5 + h7 + h9 ) - (h2 + h4 + h6 + h8 + h10 ) Rt = 5 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 roughness can be measured by comparison inspection or by direct measureement. In comparison inspection the surface roughness can be estimated by matching a workpiece surface with blocks that display a series of increasingly smooth surfaces.
  • 20. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/4/20 he he k k lic lic SURFACE TEXTURE C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c SELF-ASSESSMENT 4 1. What is meant by surface texture of a component? a. Name three (3) method of indicating quality or texture number of a surface. 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 0.01 1.6