Measurement OF SURFACE FINISH Prof. M. M. Joke Chapter 7 Measurement OF SURFACE FINISH

Prof. M. M. Joke Introduction The surface texture greatly influences the functioning of the machined parts. The properties such as appearance, corrosion resistance, wear resistance, fatigue resistance, lubrication, initial tolerance, ability to hold pressure, load carrying capacity, noise reduction in case of gears,etc. are influenced by the surface texture.

Prof. M. M. Joke Introduction Whatever may be the manufacturing process used, it is not possible to produce perfectly smooth surface. The imperfections and irregularities are bound to occur.

Factors Affecting Surface Roughness Prof. M. M. Joke Factors Affecting Surface Roughness Vibrations. Material of the workpiece. Type of machining. Rigidity of the system consisting of machine tool, fixture, cutting tool and work. Type, form, material and sharpness of cutting tool. Cutting conditions i.e. speed, feed and depth of cut. Type of coolant used.

Reasons for controlling Surface Roughness Prof. M. M. Joke Reasons for controlling Surface Roughness It is seen that different requirement demand different surface texture. Heat exchanger tubes transfer heat better when their surfaces are slightly rough rather than highly finished. Brake drums and clutch plates, etc. work best with some degree of surface roughness. The surface of the parts which are subjected to high stresses and load reversals are finished highly smooth.

Reasons for controlling Surface Roughness Prof. M. M. Joke Reasons for controlling Surface Roughness The principal reasons for controlling the surface texture are: To improve the service life of the components. To improve the fatigue resistance. To reduce initial wear of parts. To have a close dimensional tolerance on the parts. To reduce frictional wear. To reduce corrosion by minimizing depth of irregularities. For good appearance. If the surface is not smooth enough, the moving parts can heat up, blind and freeze.

Orders of Geometrical Irregularities Prof. M. M. Joke Orders of Geometrical Irregularities 1. First Order: The irregularities caused by inaccuracies in the machine tool itself. e.g: i) irregularities caused due to lack of straightness of guideways on which the tool must move. ii) surface irregularities arising due to deformation of work under the action of cutting forces. iii) due to weight of the material itself.

Orders of Geometrical Irregularities Prof. M. M. Joke Orders of Geometrical Irregularities 2. Second Order: The irregularities caused due to vibrations of any kind. e.g: i) chatter marks on the surface of the parts. 3. Third Order: The irregularities caused by machining due to characteristic of the process. i) feed mark of the cutting tool. 4. Fourth Order: The irregularities caused by the rupture of the material during the separation of the chip.

Orders of Geometrical Irregularities Prof. M. M. Joke Orders of Geometrical Irregularities The irregularities on the surface of the part produced can also be grouped into two categories: Primary Texture (Roughness) – Surface irregularities of small wavelength. These are caused by direct action of cutting elements on the material. Includes irregularities of third and fourth order.

Orders of Geometrical Irregularities Prof. M. M. Joke Orders of Geometrical Irregularities The irregularities on the surface of the part produced can also be grouped into two categories: 2. Secondary Texture (Waviness) – Surface irregularities of considerable wavelength of a periodic character. These are caused by inaccuracies of slides, wear of guides, misalignment of centres, vibrations, deformation of work, etc. Includes irregularities of first and second order.

Orders of Geometrical Irregularities Prof. M. M. Joke Orders of Geometrical Irregularities Primary Texture (Roughness) and Secondary Texture (Waviness)

Prof. M. M. Joke Terminology

Sampling Length and Lay Prof. M. M. Joke Sampling Length and Lay Sampling Length – It is the length of the profile necessary for the evaluation of the irregularities to be taken into account. It is also known as ‘cut-off’ length. It is related to the process employed for finishing. The standard lengths are 0.08, 0.25, 0.8, 2.5 and 25 mm.

Sampling Length and Lay Prof. M. M. Joke Sampling Length and Lay Lay – It is the direction of predominant surface pattern produced by tool marks or scratches. It is determined by the method of production used.

Prof. M. M. Joke Types of Lay

Evaluation of Surface Finish Prof. M. M. Joke Evaluation of Surface Finish Three methods of evaluating primary texture (roughness) of a surface: Peak to valley height method The average roughness Form factor or bearing curve

Evaluation of Surface Finish Prof. M. M. Joke Evaluation of Surface Finish The Average Roughness – For assessment of average roughness, the following three statistical criteria are used: C.L.A. Method (Centre Line Average) R.M.S. Method (Root Mean Square) Ten point height Method (Rz)

Evaluation of Surface Finish Prof. M. M. Joke Evaluation of Surface Finish C.L.A. Method (Centre Line Average)

Evaluation of Surface Finish Prof. M. M. Joke Evaluation of Surface Finish C.L.A. Method (Centre Line Average)

Evaluation of Surface Finish Prof. M. M. Joke Evaluation of Surface Finish 2. R.M.S. Method (Root Mean Square)

Evaluation of Surface Finish Prof. M. M. Joke Evaluation of Surface Finish 3. Ten Point Height Method (Rz) Rz

Statement of Surface Finish Prof. M. M. Joke Statement of Surface Finish Surface Roughness Value – It is expressed as Ra value in microns (μm). If a single Ra value is stated, it is understood that any Ra value from zero to that stated is acceptable. Limiting Values – When both minimum and maximum Ra values needed to be specified, these shall be expressed as follows: Ra 8.0/16.0 or alternatively, Ra 8.0 – 16.0

Roughness Grade Symbols Prof. M. M. Joke Roughness Grade Symbols

Symbol to designate Surface finish on drawing Prof. M. M. Joke Machining Method Sampling Length Roughness value in μm Machining Allowance Direction of Lay

Symbol to designate Surface finish on drawing Prof. M. M. Joke Milling 2.5 6.3 = 1.2

Measurement of Surface Finish / Surface Texture Prof. M. M. Joke Inspection by comparison (Qualitative Analysis) Direct Instrument Measurement (Quantitative Analysis)

Measurement of Surface Finish / Surface Texture Prof. M. M. Joke Inspection by comparison (Qualitative Analysis) Visual inspection Touch inspection Scratch inspection Microscopic inspection Surface photographs Micro-Interferometer Wallace surface Dynamometer Reflected light intensity

Measurement of Surface Finish / Surface Texture Prof. M. M. Joke 2. Direct Instrument Measurement (Quantitative Analysis) Principle : Stylus probe type surface texture measuring instruments – If a finely pointed probe or stylus be moved over the surface of a workpiece, the vertical movement of the stylus caused due to the irregularities in the surface texture can be used to assess the surface finish of the workpiece.

Stylus Probe Instruments Prof. M. M. Joke Stylus Probe Instruments

Stylus Probe Instruments Prof. M. M. Joke Stylus Probe Instruments

Tomlinson Surface Meter Prof. M. M. Joke Tomlinson Surface Meter