Surface finish Metrology
- 1. Made by:- Udit J. Modi B.Tech, Mechanical Engineering, Nirma University Basic terms of surface finish, Lay, Reasons for controlling surface roughness, factors affecting surface roughness Topic
- 2. Basic terms/Elements of Surface Texture Surface:- Surface is confined boundary which separates that part from another part, substance or space. Actual Surface:- It refers to the surface of a part which is actually obtained after manufacturing. Nominal Surface:- It is a theoretical, geometrically perfect surface which does not exist in practice. It is an average of the irregularities that are superimposed. Profile:- It is defined as the contour of any section through surface. Roughness:- It is relatively finely spaced microgeometrical irregularities. It is called primary texture and have fourth and third order irregularities.
- 3. Roughness Height:- It is arithmetical average deviation, normal to imaginary centre line, expressed in μm. Roughness Width:-It is distance parallel to normal surface between successive peaks or ridges that constitutes predominant pattern of roughness. Roughness Width cutoff:- This is maximum width of surface irregularities that is included in the measurement of roughness height. Waviness:-They are those irregularities which are of greater spacing in form of waves. It is macro geometrical errors of first and second order. This may be due to misalignment, vibrations, warping, deflections, etc. Effective profile:- It is real contour of surface obtained by instrument. Flaws:- They are surface irregularities which occur at infrequent and random intervals. E.g. holes, cracks, porosity, scratches, etc. They may be detected with penetrating dye or visualization. Surface texture:- Repetitive or random deviations from nominal surface which forms pattern on surface. E.g. roughness, waviness, flaws, lays, etc. Sampling length:- It is length of profile necessary for irregularities evaluation to be considered. It is “cut-off” length. It is measured in a direction parallel to general direction of the profile. These standard lengths are 0.08, 0.25, 0.8, 2.5 and 25 mm.
- 4. LAY The Lay is the direction of the predominant surface pattern produced by tool marks or scratches. It is usually determined by the production method used to process the surface. It should be noted that surface roughness is measured at 90˚ to the direction of lay. LAY AND ITS TYPES
- 5. Symbols || • parallel = Lay parallel to boundary line of the nominal surface. • It is lay parallel to the line representing surface to which symbol is applied. Example • E.g. parallel shaping , end view of turning and O.D. grinding • Perpendicular = Lay perpendicular to the boundary line of nominal surface. • It is lay perpendicular to the line representing surface to which symbol is applied. Example • E.g. end view of shaping , longitudinal view of turning and O.D. grinding
- 6. X • Angular = Lay is angular in both directions to the line representing the surface to which symbol is applied Example • e.g. Traversed end mill, side wheel grinding M • Lay is Multidirectional (can be any directions possible) Example • e.g. lapping, Honing, Super-finishing
- 7. C • Circular = Lay is approximately circular relative to the centre of the surface to which the symbol is applied. Example • e.g. Facing operation in a lathe machine R • Radial = Lay is approximately radial relative to the centre of the surface to which the symbol is applied. Example • e.g. Surface ground on a turntable, fly cut and indexed on end mill P • This Lay indicates that lay is non-directional, particulate and protuberant. Example • e.g. Reaming, Finishing processes, Rubbing
- 8. Reasons for controlling Surface Texture The main reasons to control surface texture are: 1) To reduce initial wear of parts increasing efficient & consistent performance and reasonable life. 2) To improve the service life of the components increasing Durability and Reliability with integrating ,developing and improving Quality Control. 3) To have a close dimensional tolerance on the parts due to which the job is not allowed to go away from accuracy and perfectness without causing any functional trouble, when assembled with its mating part and put it into actual service. 4) To reduce frictional wear and tear on the assembly parts. 5) To have aesthetic look and attractiveness or for better appearance. 6) To reduce corrosion on the surface by minimising depth of fined spaced micro- geometrical irregularities. 7) If the surface is not smooth enough, a turning shaft may act or brhave like a reamer and the piston rod like a broach. The moving parts can heat up, bind and freeze. 8) To improve the fatigue resistance as when the surface roughness increases, the crack may propagate resulting in fatigue failure.
- 9. Examples Different requirements may demand different surface texture. Excessive surface roughness on shafts and bearings say in an electrical household appliances require more power. So, here smooth surface finish is needed. Brake Drums and Clutch plates etc. work best with some allowable degree of surface roughness. Examples Surface finish on shaft
- 10. Heat Exchanger tubes transfer heat better when their surfaces are slightly rough rather than highly finished. For components which are subjected to load reversals, sharp irregularities act as stress raisers constituting the greatest potential source of fatigue cracks, therefore, the surface of the parts which are subjected to high stresses and load reversals are finished highly smooth.
- 11. Factors affecting Surface Roughness Plenty of factors plays an important role in determining Surface Roughness:- (1)Vibrations: Wear on the component, loose mounting, misalignment ,etc. are causes of vibration which will be further vibrate the work-piece or cutting tool. That is why Oscillations or Vibrations affect the surface texture. (2)Cutting Speed Condition: It is found that an increase of cutting speed generally improves surface quality. (3)Material of work-piece: Higher work material hardness results in better surface finish. To predict actual surface roughness, first compute ideal surface roughness and then multiply with ratio from the graph.
- 12. (4) Depth of cut: Increasing the depth of cut increases the cutting resistance and the amplitude of vibrations. As a result, cutting temperature also rises. Therefore, it is expected that surface quality will deteriorate. (5) Type of Machining: Irregularities will still be present after doing turning, grinding, lapping, boring, shaping, drilling, etc. to finish the surface. Lapping and Honing produce a texture irregular and multidirectional Grinding, most used as finishing process, may give texture irregular and unidirectional. Turning, facing, boring, shaping, etc. tend to give spaced and unidirectional.
- 13. (6)Engagement of the cutting tool: This factor acts in the same way as the depth of cut. (7)Type, form, material and sharpness of cutting tool: The irregularities of the cutting edge due to wear and material of tool are reproduced on the machined surface. Apart from that, as tool wear increases, other dynamic phenomena such as excessive vibrations will occur, thus further deteriorating surface quality. Increasing the tool rake angle improves surface finish. (8)Feed Condition: Experiments show that as feed rate increases surface roughness also increases due to the increase in cutting force.
- 14. (9) Type of Coolant used: Cutting fluids affect the surface finish by changing cutting temperature. It absorbs the heat that is generated during cutting by cooling mainly the tool point and the work surface. In addition to this, the cutting fluid is able to reduce the friction between the rake face and the chip as well as between the flank and the machined surface. Lastly, the washing action of the cutting fluid is considerable, as it consists in removing chip fragments and wear particles. Therefore, the quality of a surface machined with the presence of cutting fluid is expected to be better than that obtained from dry cutting. (10) Rigidity of the system consisting of machine tool, fixture of cutting tool and work: The improper mounting of the above mentioned may create additional force or gives rise to vibrations or oscillations to worsen the surface finish.
- 15. Contact pressure of bodies 1μm roughness and 5 μm roughness from ANSYS 13.0 is shown below:- It is clear that maximum contact pressure of 1 μm roughness is 0.899980 MPa while that of 5 μm roughness is 0.90086 MPa.The contact is having 0.9001 MPa contact pressure which is higher than maximum contact pressureo1 1 μm roughness contact. So, With increase in surface roughness, contact pressure increases along with rise in thermal & electrical resistance. Experimental data shows graph of heat transfer co-efficient V/S surface roughness
- 16. International Journal for Research in Applied Science & Engineering Technology (IJRASET) N. Dhar, M. Kamruzzaman(2006),Cutting temperature, toolwear, surface roughness and dimensional deviation in AISI4037 steel Dimension, Tolerances and Related Attributes of Surfaces and Effect of Manufacturing Processes Textbook of Metrology By M. Mahajan Mitutoyo America Corporation (www.mitutoyo.com) Whitehouse, David (2012). Surfaces and their Measurement. Boston: Butterworth- Heinemann J. a. J.B.P. Wiliamson, “contact of nominally flat Surfaces”, in Proceeding of Royal Society, London, 1966. REFERENCES