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Unit4 Surface Texture
 

Unit4 Surface Texture

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    Unit4 Surface Texture Unit4 Surface Texture Document Transcript

    • 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.
    • 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)
    • 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,
    • 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.
    • 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.
    • 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
    • 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
    • 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
    • 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
    • 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.
    • 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.
    • 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 ).
    • 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)
    • 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.
    • 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
    • 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)
    • 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.
    • 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
    • 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.
    • 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
    • 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 J3103/4/21 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 OF SELF-ASSESSMENT 4 1. Surface Texture is defined as a degree of finish conveyed to the machinist by 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 rough surface, which is shown in figure below. The arithmetic mean value, Ra, is defined as A f g h i j k B a b c d e Centre line (datum line) 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, formerly identified as RMS is defined as a 2 + b 2 + c 2 + d 2 +... Rq = n
    • 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 J3103/4/22 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 (iii) 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 = 5 h1 h3 h6 h5 h7 h9 h2 h4 h8 h10 Maximum roughness height (Rt) (b) To ensure proper functioning, long life and interchangeability of machine parts.
    • 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 J3103/4/23 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 2. 4. 9. Flaw Waviness height Lay direction Roughness height Roughness Roughness width Spacing cutoff Waviness width 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 is applied)