Measurement of surface finish

Measurement of surface finish, surface texture, Methods of measuring surface finish, surface inspection by comparison, touch inspection, visual inspection, scratch inspection, microscopic inspection, surface photographs, Wallace surface dynamometer, reflected light intensity, Intersection method, microkarato surface roughness tester, Tomlinson surface roughness tester, Forster surface roughness tester, talysurf roughness tester, perthometer, numerical evaluation of surface texture, Indication of surface roughness symbol, symbols for direction of lay, position of the specification of roughness symbol,

1. Measurement of Surface Finish Prepared by : Ratnadeepsinh jadeja

2. Surface Texture The Surface texture may be classified as: “The characteristics quality of an actual surface due to small departures from its general geometrical form which, occurring at regular or irregular intervals, tend to form a pattern or texture on the surface.” Surface texture vary according to the machining processes used in producing it. These differences are apparent by visual examination and can be felt readily by passing a finger nail over the surface.

3. Surface Texture Any material being machined by chip removal process can’t be finished perfectly. Due to conditions not being idle, the surface produced will have some irregularities; and these geometrical irregularities could be classified into the following four categories: First order: due to inaccuracies in the machine tool itself (lack of straightness of guideways on which tool post is moving) Second order: due to vibrations of any kind such as chatter marks. Third order: due to machining process itself. This also includes the feed marks of the cutting tool. Fourth order: due to the rapture of the material during the separation of the chip.

4. Surface Texture These irregularities of four orders can be grouped into the following two groups: Primary texture (or Roughness) Secondary texture. Primary Texture: Irregularities of small wavelength caused by direct action of cutting element on the material or by some other disturbance such as friction, wear, or corrosion. These errors are mainly caused due to tool chatter i.e., it includes irregularities of third and fourth order and constitutes that micro-geometrical errors.

5. Surface Texture Secondary Texture: Irregularities of considerable wavelength of a periodic character resulting from mechanical disturbances in the generating setup. These errors are termed as macro-geometrical errors and include irregularities of first and second order and are mainly due to misalignment of centers, lack of straightness of guideways and nonlinear feed motion. Any surface could be considered to be combination of two form of wavelengths (waviness and roughness)

6. Surface Texture

7. Methods of Measuring Surface Finish The surface finish of machined surface can be measured by the following two methods: 1. Surface inspection by comparison methods 2. Direct instrument measurements.

8. Surface Inspection by comparison Touch Inspection: One can feel the distance between the peaks of the surface irregularities by touching it with finger, on conditions that this distance is greater than 3 mm. The surface having maximum depth of roughness of about 0.1 μm can be can be compared when the sliding speed of the edge is between 3 and 5 cm/sec. The main limitation of this method is that the degree of surface roughness can’t be assessed. Also minute flows cannot be detected. This method can simply tell which surface is more rough.

9. Surface Inspection by comparison Visual Inspection: One can visualize and differentiate between surface having a roughness variation of not less than 5 % due to the fact that the brightness of these surfaces varies proportionally. Visual inspection by naked eye is always likely to be misleading particularly when surfaces having high degree of surface finish are inspected. The method is limited to rougher surfaces and results vary from person to person.

10. Surface Inspection by comparison Scratch Inspection: A soft 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 visualized. Microscopic Inspection: In this method a master finished surface is placed under the microscope and compared with the surface under inspection. Surface Photograph: Magnified photographs of the surface are taken with difference type of illumination. In case we use vertical illumination then defects like irregularities and scratched appear as dark spots and flat portion of the surface as bright area. In case of oblique illumination, reverse is the case. Photograph with different illumination are compared and the results assessed.

11. Surface Inspection by comparison Micro-interference: An optical flat is placed on the surface to be inspected and illuminated by a monocromatic source of light. Interference bands are studied through a microscope. Scratches in the surfaces appears as interference lines extending from the dark bands into the bright bands. The depth of the defects is measured in terms of the fraction of the interference band.

12. Surface Inspection by comparison Wallace surface dynamometer: It is a short of friction meter. It consists of a pendulum in which the testing shoes are clamped to the bearing surface and a predetermined spring pressure can be applied. The pendulum is lifted to its initial starting position and allowed to swing over the surface to be tested. If the surface is smooth, then there will be less friction and pendulum swings for a longer period. Thus time of swing is a direct measure of surface roughness. Reflected light intensity: A beam of light of known quantity is projected upon the surface. This light is reflected in several directions as beams of lesser intensity and the change in light intensity in different directions is measured by a photo cell. The measured intensity changes are already calibrated by means of readings taken from the surface of known roughness by some other suitable method.

13. Surface Inspection by comparison Comparison with standard specimens: For qualitative assessment of the surface roughness, the roughness comparison specimens are used. Roughness of the finished product is evaluated by visual or tactile comparison. The specimens used for the comparison are available either in flat of segmented cylindrical form. The flat specimens have the size: length = 25 mm; width = 16 mm and thickness = 1.6 to 10 mm. cylindrical specimens will have either convex surface or concave surface having a surface radius about 125 mm.

14. Direct Instrument Measurement The quantitative measurement of surface roughness can be done by the following methods: Intersection method: Interface method Stylus method Pneumatic measuring method Perthen condenser method

15. Intersection Method Cross-section method: A specimen is cut perpendicular to its surface after applying a protective layer of approximately the same hardness is applied to the surface either by chromate plating or by casting of a thin film layer of white metal on the surface. The specimen, after cutting, has to be polished and the actual surface texture can be observed under a measuring microscope. The destruction of a component for measuring its surface hardness can be avoided by making a surface replica from celluloid or gelatine . After drying the replica, it can be sliced and observed under the microscope.

16. Intersection Method Light-section method: A narrow light band is made to intersect the test surface and intersected contour of the surface can be observed by microscope. When i = o, then arrangement is known as bright field When i ≠ o, then arrangement is known as dark field

17. Microkarato surface roughness tester This instrument be directly applied for measuring the roughness of flat surfaces. It uses a stylus with a mechanical magnification based upon the microkator principle. The instrument is supported on three fixed skids and an adjustable one. The diamond stylus traces the test surface.

18. Tomlinsion tester

19. Tomlinsion tester When surface finish is to be measured, the body is transversed across the surface by a screw rotated by a synchronous motor. Any vertical movement of the stylus caused by the surface irregularities, causes the horizontal lapped steel cylinder to roll. By its rolling, the light arm attached to its end provides a magnified movement on smoked glass plate. This vertical movement equipped with horizontal movement produces a trace on the glass magnified in the vertical direction and there is no magnification in the horizontal direction. The smoke glass trace is then, further projected at ×50 or ×100 magnification for examination.

20. Forster surface roughness tester Forster surface roughness tester with cam Forster surface roughness tester with electric magnet

21. Forster surface roughness tester In this instrument the stylus is lifted and made to fall on the surface under test either by a cam rotating at a constant speed or by an electric magnet. The nose radius of the stylus is 10 mm and the static measuring force is 1g so that compressive stress of 6.2 KN/mm2 will be achieved. The impulse tracing method has the advantage that the frictional and traditional forces on the stylus are minimized and grease layer that might cover the test surface does not affect the measurement. This tester allows tracing a 100mm long surface but the weight of the component should not exceed 3 kg. The distance moved by the component during two successive impulses is about 1 mm and the measuring range is between 0 and 125 μ m.

22. Talysurf surface roughness tester

23. Talysurf surface roughness tester A downward movement of the stylus results in decreasing the air gap of the primary coil and in an equal increase of the air gap at the secondary coil. The impedance of the coils will be changed according to the variation of air gap and an additional alternating current flows in the secondary coil whose magnitude is governed by the variation of the impedance of the coil and is proportional to the displacement of the stylus. The maximum length that can be traced is 10 mm. The measuring force of the stylus is 0.1 g. The vertical magnification can be varied in steps from 1000 to 50000 times.

24. Perthometer The perthometer is equipped with different tracer systems that can be selected according to the measuring conditions. 1. Pendulum type 2. Semi-rigid type 3. Free supported type with slip gauge 4. Free supported type with reference datum plane.

25. Perthometer The two inductive coils are connected in a Wheatstone bridge circuit which can be electrically balanced when the iron plate, i.e., the stylus is in the middle position. The displacement of the stylus results in upsetting the balance of the circuit and in generating a current that flows in a diagonal connection of the Wheatstone bridge circuit which can be amplified and recorded. The magnitude of this current is proportional to the displacement of the stylus. Perthometer can be used for measuring all parameters of the surface texture. The length of the transverse of the tracer head can be selected from 2 to 25 mm according to the requirements. It is provided with six measuring ranges with six different amplification is given below: Measuring Range (mm) 25 10 2.5 1 0.25 0.1 Amplification 400:1 1000:1 4000:1 10000:1 40000:1 100000:1

26. Numerical Evaluation of Surface Texture C.L.A. = ‘Centre Line Average’ R.M.S. = ‘Root Mean Square’ n = Total number of ordinates L = Length of pen record

27. Indication of surface roughness symbol

28. Symbols for direction of lay Parallel to the plane of projection of the view in which the symbol is used Perpendicular of the plane of projection of the view in which the symbol is used Crossed in two slant directions relative to the plane of projection of the view in which the symbol is used Multi-directional Approximately circular relative to the centre of the surface to which the symbol is applied Approximately radial relative to the centre of the surface to which the symbol is applied

29. Position of the specification of roughness in symbol