Surface Finish

INTRODUCTIONThe modern demands of the automobile, the airplane, and other modern machines (heavier loads, higher speeds with less friction) need for accurate control of surface.

Not all surfaces can be finished because of cost constrains. Few surfaces requires special surface finish and should indicate on drawing by using surface finish symbol.

The ideal surface finish is the roughest that will do the job satisfactorily.

ANSI/ASME recommended a system of surface texture symbols.

MEASUREMENT OF SURFACE FINISHSURFACE TEXTURE: 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

24/03/2012*Lec # 27 & 28

Lec # 27 & 28

SURFACE TEXTURE Repetitive or random deviations from the nominal surface which forms the pattern on the surface. Surface texture includes Roughness, Waviness, Lays and flaws.Factors affecting surface roughness:VibrationMaterial of the work pieceType of machiningRigidity of the systemType, form, material and sharpness of cutting toolCutting condition i.e. speed, feed, depth of cutType of coolant usedReasons for controlling surface texture:To improve service life of the components.To reduce initial wear of parts.To improve fatigue resistanceTo reduce frictional wearFor good appearance

Irregularities on surface can also be grouped into following two categories.1.PRIMARY TEXTURE (ROUGHNESS/ MICRO GEOMETRICAL)The surface irregularities of small wavelength are called primary texture and caused by direct action of cutting elements on the material i.e. cutting tool shape, tool feed rate and other disturbances like friction, wear or corrosion. It includes irregularities of third and fourth order.2.SECONDARY TEXTURE (WAVINESS/ MACRO GEOMETRICAL)The surface irregularities of considerable wavelength of a periodic character are called secondary texture. These result due to inaccuracies of slides, wear of guides, misalignment of centers, nonlinear feed motion, deformation of work under the action of cutting forces, vibration of any kind etc. These include irregularities of first and second order.

*Objectives of Machining Symbols (Finish Marks)Define the surface to be machines or finished. For die-maker a finish mark means that allowance of extra metal in the rough workpiece must be provided for the machining. Specify a particular surface finishDetermine the manufacturing process

Old finish marks symbol

SURFACE ROUGHNESS Surface roughness symbols: Surface texture symbols are used to define surface texture, roughness and lay.Surface may be produced by any method Material removal by machining is required to produce the surfaceMaterial removal prohibited 3.5Material removal allowance. 3.5 is the amount of stock to be removedMore surface characteristics are specified in these spaces.

SURFACE ROUGHNESS Application of surface roughness symbol on drawing:Normal to the surface. The point of the symbol should be directed inward toward the body metal, not upside down. Could be vertical or horizontal as the surface, but not at any angle. Could be provided as a general note.

Symbol

Approx. 3X1.5X3X60Letter height = X

SURFACE ROUGHNESSDefinitions:Surface texture: includes roughness, waviness, lay and flaws.Profile: is the contour (shape) of a surface in a plane perpendicular to the surface.Micrometer [] : 1*10-6 m = 1/1000000 mRoughness: small peaks and valleys found in the surface (fine irregularities)Roughness height: arithmetical average deviation [].

SURFACE ROUGHNESSDefinitions:Roughness width: distance between two peaks that make the roughnessWaviness: larger deviation from the nominal surface on which the roughness is superimposed Waviness height: distance from peak to valleyWaviness width: distance between successive wave peaks or successive wave valleys.Lay: directions of tool marks, or grains of surfaceFlaw: surface defects.

SURFACE ROUGHNESSLocation of surface specifications on surface finish symbol:Maximum values are specified

SURFACE ROUGHNESSLay symbols:If it is necessary to specify the direction of the tool marks, it must be applied to the surface texture symbol:

SURFACE SPECIFICATIONSSurface finish symbol (machining symbol) can be used to specify the surface finish and machining/manufacturing process.

Surface finish:-is usually specified as a roughness value in micrometers printed above the finish symbol.

-The roughness value is related to the machining process. See Table.-For more exact machining the upper and lower limits of roughness are specified.

Specifying process: Could be directly specified on the finish symbol.

EXAMPLES:Application of surface texture values to symbol

ADDITIONAL INFORMATION

Usually all information are not required. Only roughness height and machine allowance is important. In this case basic symbol is used.

Roughness height in [m] or in [ inch]. All other in mm or inch.

EXAMPLE

Centre Line Average (C.L.A.) Method (Ra): In this method surface roughness is measured as the average deviation from the nominal surface.C.L.A. is defined as the average value of the ordinates from the mean line, regardless of the arithmetic signs of the ordinates.C.L.A. value or Ra =

R.M.S method: In this method the roughness is measured as the average deviation from the nominal surface.R.M.S value is defined as the square root of the arithmatic mean of the values of the squares of the ordinates of the surface measured from a mean line.Let, the sample length L is divided into n equal parts and h1,h2,h3..hn are the heights of the ordinates erected at those points.Then,

hrms=

Ten Point Height Method (Rz):In this method the average difference between the five highest peaks and five lowest valleys of surface texture within the sampling length, measured from a line parallel to the mean line and not crossing the profile is used to denote the amount of surface roughness.So, Rz= Ten point height of irregularities

This method is relatively simple Method of analysis and measures the total depth of surface irregularities within the sample length. But, it does not provide Sufficient information about the surface as no account is taken of frequency of the irregularities and the profile shape.

Direct methods enable to determine a numerical value of the surface finish of any surface.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 work piece.Skid or ShoeFinely pointed Stylus or ProbeAn amplifying device for magnifying the stylus movement and indicatorRecording device to produce a traceMeans for analyzing the trace

Advantages:The electrical signal available can be processed to obtain any desired roughness parameter or can be recorded for display or subsequent analysis. Disadvantages:These instruments are bulky and complexThey are relatively fragileInitial cost is highNeeds skilled operatorDistance between stylus and skid and shape of the skid introduce error in measurement for wavy surfaces.

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