SURFACE QUALITY & MACHINIG SYMBOLS Introduction Types of surfaces Nomenclature of surface texture Surface roughness value Machining symbols Roughness

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2 SURFACE QUALITY & MACHINIG SYMBOLS Introduction Types of surfaces Nomenclature of surface texture Surface roughness value Machining symbols Roughness grades and symbols Surface roughness indication Direction of lay Roughness symbol on drawing Surface roughness for various machining processes 2

3 INTRODUCTION Engineering components are manufactured 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

4 INTRODUCTION For high class of accuracy, surface finish requirements are high and hence it is more expensive Surface quality or surface texture or surface finish is the amount of geometric irregularity produced on the surface Surface texture notes on a drawing refer the required finish that must be machined on any particular surface 4

5 TYPES OF SUFRACES Surface of a body is its boundary which separates it from another body. Surface may be; (i) Normal surface (ii) Rough surface (iii) Wavy surface and (iv) Wavy surface roughness superimposed 5

6 NOMENCLATURE OF SURFACE TEXTURE (1) Profile: It is the contour of a surface in a plane which is perpendicular to the surface itself. (2) Surface roughness: It is the micro irregularities in a surface resulting from the action of production process. It is also called primary texture roughness. 6

7 NOMENCLATURE OF SURFACE TEXTURE (3) Roughness height: It is the arithmetic average of the roughness deviation and is measured in micrometers (4) Roughness width: It is the distance parallel to nominal surface between the adjacent peaks or ridges which constitute the predominant pattern of the roughness 7

8 NOMENCLATURE OF SURFACE TEXTURE (5) Roughness-width cut off: It is a particular length of the profile decided to evaluate the arithmetic average deviation. It is also called sampling length It is the greatest spacing of repetitive surface irregularities to be included in the measurement of average roughness height It must be greater than roughness width It may be taken as 0.8mm if not specified 8

9 NOMENCLATURE OF SURFACE TEXTURE (6) Waviness: It is the irregularities in a surface having wider spacing than the roughness-width cut off resulting from machine deflection, vibrations, heat treatment, warping etc (7) Waviness height: It is the distance from the peak to the valley of the wave and is measured in millimetres 9

10 NOMENCLATURE OF SURFACE TEXTURE (8) Waviness width: It is the distance between the adjacent peak or valley of the wave and is measured in milimetres. (9) Lay: It is the predominant direction of tool that makes the surface pattern on the machined surface. 10

11 NOMENCLATURE OF SURFACE TEXTURE (10) Flaw: It is an irregularity that occurs in a surface due to cracks, blow holes, ridges, scratches etc There are no finish symbols for flaws Special note should be included on the drawing where no flaw is permitted 11

12 SURFACE ROUGHNESS VALUE Figure shows hypothetical profile of a surface Centre line is a line about which the roughness is measured The roughness may be calculated by; (1) Centre Line Average (C.L.A.) method and (2) Root Mean Square (R.M.S.) method In both methods, surface roughness is measured as average deviation from the nominal surface 12

13 SURFACE ROUGHNESS VALUE In Centre Line Average (C.L.A.) method; y 1 +y 2 +y 3 +.+y n Centre Line Average = n In Root Mean Square(R.M.S.) method; Root Mean Square = y1 y2 y3... yn n As per BIS, the surface roughness is assessed in terms of C.L.A. value and is denoted by R a

14 MACHINIG SYMBOLS The quality of surface texture is indicated by certain symbols on technical drawings The basic symbol consists of two unequal legs at 60 0 included angle to the line representing the surface under consideration with vertex touching as shown in figure. Here, a = Surface may be produced by any method b = Removal of material is not permitted c = Removal of material by machining d = If special surface characteristic is to be indicated 14

15 ROUGHNESS GRADES & SYMBOLS The principal criterion of surface roughness, R a may be indicated by the corresponding roughness grade number, as shown in Table 15

16 SURFACE ROUGHNESS INDICATION The value or values defining the principal criterion of roughness are added to the symbol as shown in figure b (f) a e d a = Roughness value, R a in micrometres = Roughness limits or grade numbers N 1 to N 12 b = Production method/ treatment of coating c = Sampling length d = Direction of lay e = Machining allowance and f = Other roughness values 16

17 DIRECTION OF LAY The direction of lay is the direction of the predominant surface pattern ordinarily determined by the production method and is shown in table. 17

18 DIRECTION OF LAY 18

19 INDICATION OF ROUGHNESS SYMBOLS ON DRAWING The symbols should be oriented such that they may be read from the bottom or the right hand side of drawing Figure (a) If it is not practicable to adopt this general rule, the symbol may be drawn in any position provided that it does not carry any indications of special surface texture characteristics as in Figure (b). 19

20 INDICATION OF ROUGHNESS SYMBOLS ON DRAWING The symbol may be connected to the surface by a leader line, terminating in an arrow. The symbol or the arrow should point from outside the material of the piece, either to the line representing the surface, or to an extension of it as shown in Figure (a) 20

21 INDICATION OF ROUGHNESS SYMBOLS ON DRAWING In accordance with the general principles of dimensioning, the symbol is only used once for a given surface and, if possible, on the view which carries the dimension, defining the size or position of the surface. 21

22 INDICATION OF ROUGHNESS SYMBOLS ON DRAWING If the same surface roughness is required on all the surfaces of a part, it is specified, either by a note near a view of the part near the title block, or in the space devoted to general notes (as shown in figure), or following the part number on the drawing. 22

23 INDICATION OF ROUGHNESS SYMBOLS ON DRAWING If the same surface roughness is required on majority of the surfaces of a part, it is specified with the addition of the notation, except where otherwise stated (Figure a), or a basic symbol (in brackets) without any other indication (Figure b), or the symbol or symbols (in brackets) of the special surface roughness or roughnesses (Figure c). 23

24 INDICATION OF ROUGHNESS SYMBOLS ON DRAWING To avoid the necessity of repeating a complicated specification a number of times, or where space is limited, a simplified specification may be used on the surface, provided that its meaning is explained near the drawing of the part, near the title block or in the space devoted to general notes. 24

25 SURFACE ROUGHNESS FOR VARIOUS MACHINING PROCESSES 25

26 SURFACE ROUGHNESS FOR VARIOUS MACHINING PROCESSES 26

27 SURFACE ROUGHNESS FOR VARIOUS MACHINING PROCESSES 27

28 SURFACE QUALITY & MACHINING SYMBOLS 28