Surface Quality and Roughness Values

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Introduction

Types of surfacesNomenclature of surface texture

Surface roughness value

Machining symbolsRoughness grades and symbols

Surface roughness indication

Direction of lay

Roughness symbol on drawing

Surface roughness for various

machining processes

SURFACE QUALITY & MACHINIG SYMBOLS

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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

INTRODUCTION

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For high class of accuracy, surface finish

requirements are high and hence it is moreexpensive

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

INTRODUCTION

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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

TYPES OF SUFRACES

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(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.

NOMENCLATURE OF SURFACE TEXTURE

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(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


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(5)Roughness-width cut off: It is a particular length of

the profile decided to evaluate the arithmetic

average deviation. It is also calledsampling 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


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(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


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(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.


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(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


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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

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SURFACE ROUGHNESS VALUE

In Centre Line Average (C.L.A.) method;

Centre Line Averagey1+y2+y3+.+yn

n= In Root Mean Square(R.M.S.) method;

Root Mean Square =n

y

2

n

2

3

2

2

2

As per BIS, the surface roughness is assessed interms of C.L.A. value and is denoted by Ra

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The quality of surface texture is indicated by

certain symbols on technical drawings

The basic symbol consists of two unequal legs at

600 included angle to the line representing the

surface under consideration with vertex touching

as shown in figure. Here,

MACHINIG SYMBOLS

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

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The principal criterion of surface roughness, Ra

may be indicated by the corresponding

roughness grade number, as shown in Table

ROUGHNESS GRADES & SYMBOLS

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The value or values defining the principal

criterion of roughness are added to the symbol as

shown in figure

SURFACE ROUGHNESS INDICATION

a = Roughness value, Rain micrometres

b = Production method/ treatment of coatingc = Sampling length

d = Direction of lay

a

b

de

(f)

e = Machining allowance and

f = Other roughness values

= Roughness limits or grade numbers N1to N12

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DIRECTION OF LAYThe direction of lay is the direction of the

predominant surface pattern ordinarily determined

by the production method and is shown in table.

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DIRECTION OF LAY

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INDICATION OF ROUGHNESS SYMBOLSON DRAWING

The symbols should be oriented such that they

may be read from the bottom or the right handside 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).

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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)

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In accordance with the general principles of

dimensioning, the symbol is only used once for agiven surface and, if possible, on the view which

carries the dimension, defining the size or position

of the surface.

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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).

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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.

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SURFACE ROUGHNESS FOR VARIOUSMACHINING PROCESSES

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SURFACE QUALITY

MACHINING SYMBOLS

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Surface Quality and Roughness Values