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Surface texture
- 1. Measurement of SurfaceFinish •Basic understanding of surface roughness. •Indication of finish on drawing. •Parameters of Surface roughness. •Measurement methods: contact and non contact type.
- 2. INTRODUCTION •Engineering components aremanufactured by casting, forging, welding etc… •These components are then subjected to various machining operations for getting required geometrical surfaces . •It is not practically possible to produce a component having a geometrically ideal surface. •The surface finish after machining depends upon the material, vibration, deflection, speed, feed and other working conditions. •The surface finish requirements depend upon the functional requirements of the components to be assembled.
- 3. Why Surface finish? •Surface finish, also known as surface texture or surface topography, is the nature of the surface as defined by characteristics such as lay, surface roughness and waviness. • Surface texture is one of the important factors that control friction during sliding. • To improve look and feel of the product. • Fit between parts. • To control the process that results in the surface needed.
- 4. Importance of surfacefinish • Surface roughness, is important from the point of view of fundament problems such as friction and wear, surface contact, lubrication, and tightness of joints. • It affects conduction of heat, cleanliness and reflectivity of the surface. • It also affects the positional accuracy of mating parts, resistance to corrosion.
- 5. Factors influencing surfacefinish • The machining variables that includes: a) Cutting speed b) Depth of cut c) Feed • Geometry of tool. • Work piece and tool materials, and their mechanical properties. • Quality and type of machine tool used. • Lubrication used. • Vibrations between work piece, cutting tools and machine tools.
- 6. Classification of GeometricalIrregularities The geometrical irregularities can be classified as •First order •Second order •Third order •Fourth order 1. FIRST ORDER: These irregularities are formed in the machine, due to inaccuracies, this irregularity also contains lack of straightness of guide ways, this is caused due to metal weight and cutting force. 2. SECOND ORDER: Irregularities such as chatter marks, etc. are formed due to vibrations. This type of irregularities is called second- order irregularities. 3. THIRD ORDER: This is formed due to the machining process, even if the machine is perfect and vibration free are called third order irregularities. 4. FOURTH ORDER: This is formed due to the separation of the chip, this is caused due to rupture of material are fourth order irregularities.
- 7. Types of SurfaceFinish Surface of a body is its boundary which separates it from another body. Generally there are two types of surfaces namely: Nominal Surface: A theoretically geometrically perfect surface is called as a nominal surface. Rough Surface: A magnified view of an actual surface is shown. This surface has numerous small peaks and valleys that deviate from the nominal surface. It is seen that the surface has a certain degree of roughness and hence it is called a rough surface. Wavy Surface: A surface having waviness as shown in the figure is called wavy surface. Waviness is caused by: 1. Deflections of tools and work piece. 2. Uneven lubrication. 3. Vibrations
- 8. Nomenclature of Surfacetexture 1. Surface Roughness: It is the micro-irregularities in a surface resulting from the action of production process. 2. Profile: It is the shape of any section through a surface. 3. Flaw: It is an irregularity that occurs in a surface due to cracks, blow holes, scratches, etc., Unless otherwise specified, the effect of flaws should not be accounted in the measurement of roughness. 4. Sample or Cut-off length: The particular length that is taken for sample measurement on the surface is called as cut of length. It is also known as roughness width cut off. 5. Lay: It is the direction of the predominant top surface grooves that are produced by machining.
- 9. 6. Waviness: Surfaceirregularities that are of greater spacing than roughness.
- 10. Analysis of surfacefinish The assessment of average roughness is carried out by a) Centre line average (CLA) value (Ra). b) Root mean square (RMS) value (Rs). c) Ten point method. (Rz)
- 11. Analysis of surfacefinish a) Centre line average (CLA) value (Ra): Ra is the average height from a mean line of all ordinates of the surface, regardless of sign. Let y1, y2, …., yn be the height of the equally spaced ordinates at positions 1,2,…n. In Centre Line Average (C.L.A.) method; Centre Line Average= ( y1+y2+y3+…….+yn)/ n
- 12. Another way ofcalculating Ra is shown in the figure. As per BIS, the surface roughness is assessed in terms of C.L.A. value and is denoted by Ra
- 13. b) Root meansquare (R.M.S) value (Rs): R.M.S Rs is the square root of the mean of the squares of the surface measured from the mean line. As y1 , y2 , ……yn are the heights of the equally spaced ordinates at positions 1,2,3,…n, In Root Mean Square(R.M.S.) method; Root Mean Square = c) Ten point height method: The average difference between five highest peaks and five lowest valleys of surface is taken and irregularities are calculated by
- 14. Direction of lay Thedirection of lay is the direction of the predominant surface pattern ordinarily determined by the production method and is shown in table.
- 16. Roughness grade andsymbols The principal criterion of surface roughness, Ra may be indicated by the corresponding roughness grade number, as shown in Table
- 17. Roughness grade forvarious machining processes
- 18. Representation of surfaceroughness The value or values defining the principal criterion of roughness are added to the symbol as shown in figure a = Roughness value, Ra in micrometres b = Production method/ treatment of coating c = Sampling length d = Direction of lay e = Machining allowance and f = Other roughness values
- 19. Methods of measurementof Surface roughness There are basically two methods for measuring surface finish, namely 1. Comparative or Qualitative Methods 2. Direct Measurement or Quantitative Methods Comparative or Qualitative Methods In these methods, the surface texture is assessed by observation of the surface. The surface to be tested is compared with known value of roughness specimen and finished by similar machining process. The various methods which are used for comparison are 1. Touch Inspection. 2. Visual Inspection. 3. Microscopic Inspection. 4. Scratch Inspection. 5. Micro Interferometer. 6. Surface photographs.
- 20. Touch Inspection • Themain limitation of this method is that the degree of surface roughness can’t be assessed. • Also the minute flaws can’t be detected. • This method can simply tell which surface is more rough. In this method, the finger-tip is moved along the surface at a speed of about 25 mm per second and the irregularities as small as 0.01 mm can be easily detected. Visual Inspection In this method, the surface is inspected by naked eye and this measurement is limited to rough surfaces . Microscopic Inspection • This is probably the best method for examining the surface finish but suffers due to limitation that only a small portion of the surface can be inspected at a time. • Thus several readings are required to get an average value. • In this method, a master finished surface is placed under the microscopic and compared with the surface under inspection. • In another method a straight edge is placed on the surface to be inspected and abeam of light projected at about 60° to the work. • Thus the shadows cast into the surface scratches are magnified and the surface irregularities can be studied.
- 21. Scratch Inspection • Inthis method, a softer material like lead babbit or plastic is rubbed over the surface to be inspected. • By doing so it carries the impression of the scratches on the surfaces which can be easily visualised. Surface Photographs • In this method magnified photographs of the surface are taken with different types of illumination. • In case we use vertical illumination, then defects like irregularities and scratches appear as dark spots and flat portion of the surface appears as bright area. • In case of oblique illumination, reverse is the case. • Photographs with different illumination are compared and the results assessed. Micro-Interferometer • In this method, an optical flat is placed on the surface to be inspected and illuminated by a monochromatic source of light. • Interference bands are studied through a microscope. • Defects, i.e. scratches in the surface appear as interference lines extending from the dark bands into the bright bands.
- 22. Direct Methods ofSurface roughness measurement Instructor Mr. Gaurav Bharadwaj Assistant Prof. Department of ME GLA University
- 23. Direct Measurement orQuantitative Methods Some of the direct measuring instruments are 1. Stylus probe instruments 2. Tomlinson surface meter Stylus probe instrument Principle When the stylus is moved over the surface to be measured, the irregularities in the surface texture are measured and it is used to assess the surface finish of the workpiece. Working • The stylus type instruments consist of skid, stylus, amplifying device and recording device. • The skid is slowly moved over the surface by hand or by motor drive. • The skid follows the irregularities of surface and stylus moves along with skid. • When the stylus moves vertically up and down, the stylus movements are magnified, amplified and recorded to get a trace. • Then the trace is analyzed by an automatic device.
- 24. Advantage: 1. Any desiredroughness parameter can be recorded. Disadvantages: 1. Fragile material cannot be measured. 2. High Initial cost. 3. Skilled operators are needed to operate.
- 25. Tomlinson Surface meter Principle:This instrument uses mechanical-cum-optical means for magnification. Construction: •In this, the diamond stylus on the surface finish recorder is held by spring pressure against the surface of a lapped cylinder. • The lapped cylinder is supported one side by probe and other side by rollers. •The stylus is also attached to the body of the instrument by a leaf spring and its height is adjustable to enable the diamond to be positioned and the light spring steel arm is attached to the Lapped cylinder. • The spring arm has a diamond scriber at the end and smoked glass is rest on the arm.
- 26. Working • When measuringsurface finish, the body of the instrument moves across the surface by a screw rotation. • The vertical movement of the probe caused by the surface irregularities makes the horizontal lapped cylinder to roll. • This rolling of lapped cylinder causes the movement of the arm, So, this movement induces the diamond scriber on smoked glass. • Finally, the movement of scriber together with horizontal movement produces a trace on the smoked glass plate and this trace is magnified by an optical projector.
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